Cardiff Strategic Flood Consequences Assessment Phase … · Cardiff Strategic Flood Consequences...

Cardiff Strategic Flood Consequences Assessment

Phase 2 Part 1 Update Extend Development

Lifetime to 2110

Areas A, G, H and I

Final Report

Revised November 2011

/5097656-70-DG017-3.docx

Cardiff Strategic Flood Consequences Assessment

Notice

This report was produced by Atkins Limited for Cardiff Council for the specific purpose of undertaking a Strategic Flood Consequence Assessment for Cardiff.

Atkins accepts no liability for any costs, liabilities or losses arising as a result of the use of or reliance upon the contents of this report by any person other than Cardiff Council.

Atkins Limited

Document History

JOB NUMBER: 5097656 DOCUMENT REF: 5097656-DG-017

1 Final Issue to Client DMH KIO HR HR 16-08-11

2 Section 4 amended to be consistent with Nov 2009 SFCA report

DMH KIO KIO HR 18-08-11

3 Section 6 revised to reflect increased risk through the docks. Area G assessment added as Section 4.

DMH KIO/HDW HR HR 01-12-11

4 Client comments incorporated. Issue to Environment Agency.

DMH KIO HR HR 15-12-11

Revision Purpose Description Originated Checked Reviewed Authorised Date

5097656-70-DG017-3.docx i

Contents

Section Page

Glossary of Terms vi

1. Introduction 1

1.1 Background 1

1.2 Purpose of the SFCA 1

1.3 Approach to the SFCA 2

1.3.1 Phase 1 3

1.3.2 Phase 2 3

2. The Study Area & Assessment Methodologies 5

2.1 TAN15 Constraints 5

2.2 Development Sites 5

2.2.1 Site Areas 6

2.2.2 Lifetime of Development 6

2.2.3 Climate Change 7

2.3 General Climate and Hydrology 7

2.3.1 Hydrological Situation 7

2.3.2 Extreme Sea Levels 8

2.4 Sources of Flooding 8

2.5 Assessment Methodologies 9

2.5.1 Flood Risk from Tidal Sources 9

2.5.2 Flood Risk from Main Rivers 10

2.5.3 Flood Risk from Ordinary Watercourses 10

2.5.4 Flood Risk from Sewers 11

2.5.5 Flood Risk from Culverted Watercourses 11

2.5.6 Flood Risk from Groundwater 11

2.5.7 Flood Risk from Surface Water 12

3. Area A – Wentloog Levels 13

3.1 Site Description and Flood Risks other than by Tidal Inundation 13

3.1.1 General Site Description 13

3.1.2 Proposed Development 13

3.2 Flood Risk from Tidal Sources 15

3.3 Summary of Flood Risk Management 17

3.3.0 Key Access Routes across the Wentloog Levels 17

3.3.1 Study Site 76LGW 21

3.3.2 Study Site 22LGRM 22

3.3.3 Study Site 21LGR 24



3.3.6 Study Site 74LGEMW 26

3.3.7 Study Site 2LGRM (formerly Site 77) 27

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3.3.9 Study Site 106LGEM 29

3.3.10 Study Site GTechPk 30

3.3.11 Study Site Wentloo_ii 31

3.3.12 Study Site Wentloo_iii 33

3.3.13 Study Site 3SBR 34


3.3.15 Study Site 56LBR 36

4. Area G – Junction 33 of M4 37

4.1 Site Description 37




4.2 Data Review and Analysis 39

4.2.1 Information Available 39

4.2.2 Analysis Approach 39

4.3 Flood Risk from Main Rivers and Ordinary Watercourses 39


4.4 Flood Risk from Sewers, Culverted Watercourses and Groundwater 40




4.5 Flood Risk from Surface Water Runoff 41

4.6 Summary of Flood Risk and Management 43


4.6.2 Study Sites 46LGRM 43

5. Area H – Cardiff West Central, Taff & Ely 45

5.1 Site Description 45




5.2 Data Review and Analysis 48

5.2.1 Information Available 48

5.2.2 Analysis Approach 48

5.3 Flood Risk from Main Rivers and Ordinary Watercourses 49

5.3.1 Flood Risk from Rivers Taff and Ely 49

5.3.2 Estimation of the Variation in Cardiff Bay levels during flood events 50

5.3.3 Final Flood Estimation on Taff and Ely based on the Variation in Cardiff Bay levels 51


5.4 Flood Risk from Sewers, Culverted Watercourses and Groundwater 53




5.5 Flood Risk from Surface Water Runoff 54

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5.6.1 Site 8LGR, Site 75SBR and Site 111LGR 56

5.6.2 Site 32LGR 56

5.6.3 Site 16LBR 57

5.6.4 Site 52LBR 58

5.6.5 Site 36LBR 58

5.6.6 Sites 31LBEM, 62LBR, 77LBR, 78LBRM and 83LBRM. 59

5.6.7 Site 70LBEM 60

5.6.8 Site 61LBRM 61

5.6.9 Site 96LGED 61

5.6.10 Site 101LBEM 61

5.6.11 Site 17LGR 62

6. Area I – Cardiff Bay to Pengam Green 63

6.1 Site Description and Flood Risks other than by Tidal Inundation 63



6.2 Flood Risk from Tidal Sources 66

6.2.1 Overtopping and breach from River Rhymney and the Coastal Frontage 67

6.2.2 Direct Inundation from Cardiff Bay and Cardiff Docks 69


6.3.1 Site 71LBC 73

6.3.2 Site 58SBR 74

6.3.3 Site 20LBU 74

6.3.4 Site 19LBU 75

6.3.5 Site 18LBM 77


6.3.7 Site 57LBR 80


6.3.9 Site 101LBEM 83

6.3.10 Site 94LBR 83

7. Conclusions 86

7.1 Site Summaries 86

8. Appendices 90

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List of Tables

Table 1-1 - Flood Risk for Sub areas of Cardiff 3

Table 3-1 - Area A Summary of Study Sites 15

Table 3-2 - Locations of Breaches for different time horizons and by event probabilities 16

Table 3-3 - Replication of Table 4 Hazard to People Classification using Hazard Rating 19 Table 4-1 - Area G Candidate Sites Summary 37

Table 5-1 - Area H – Summary of Study Sites 47

Table 5-2 - Flood event scenarios modelled to derive variations in Cardiff Bay levels 51

Table 6-1 - Area I – Summary of Study Sites 66 Table 6-2 – Extreme Tide Levels Severn Estuary at Cardiff Docks 67

Table 6-3 – fluvial flows assessed to occur coincident with extreme tidal flood events – Area I 67

Table 6-4 5 Levels resulting in Cardiff Bay for current and future extreme tidal events 72

Table 7-1 - Summary of Sub Area A – Wentloog Tidal Development Sites 86 Table 7-2 Summary of Area G – Junction 33, M4 87

Table 7-3 - Summary of Sub Area H – West Central Taff/Ely, Development Sites 87

Table 7-4 - Summary of Sub Area I - Bay Area to Pengam Green, Development Sites 89

Table 8-1 Scenarios modelled in 1D-2D ISIS TUFLOW model of Cardiff Bay and Roath Basin - current and

future extreme tidal events cxxii

Table 8-2 Levels resulting in Cardiff Bay for current and future extreme tidal events cxxiii

List of Figures

Figure 1-1 - Reports comprising the Cardiff SFCA* 2 Figure 3-1 – Area A Study Site Locations 14

Figure 3-2 – Hazard Score for 0.5% (1 in 200) annual chance event 2085 for access routes and study sites.20

Figure 3-3 - Site 3SBR location relative to other candidate sites and railway line 34

Figure 4-1 – Area G Study Site Locations 38 Figure 5-1 – Area H Study Site Locations 46

Figure 5-2 - ISIS Bay Model - 30 October 2000 flood event 51

Figure 5-3 - Area H Map of Surface Water Runoff Potential Screening Score 55 Figure 6-1 - Area I - Study Site Locations 64

Figure 6-2 - Extent of 2D tidal inundation model - Area I 68

Figure 6-3 Potential overland flow from Roath Basin into Cardiff Bay during extreme tide events 70

Appendices

Appendix A - SFCA Area Overview Map

A.1 Cardiff SFCA Area Overview Map – Drawing Number 5097656/RCF/002

Appendix B – Area Specific Maps

B.1 Drawing Numbers

Appendix C - Flood Depth and Area Tables

Appendix D – 2D Tidal Inundation Modelling.

D.1 Modelling approaches common to both Areas A & I

D.2 Limitations of the modelling results

D.3 Use of model results for sites not included in the SFCA Reports

Appendix E – Area A – 2D Tidal Inundation Modelling Assumptions and Results.

E.1 Model Set-up

E.2 Key Hydraulic Assumptions

5097656-70-DG017-3.docx v

E.3 Modelling Scenarios and Results

E.4 Future Improvements

Appendix F – Area I (Pengam Green to Cardiff Bay) 2D Tidal inundation modelling

F.1 Model Set-Up

F.2 Key Hydraulic Assumptions

F.3 Modelling Scenarios and Results

F.4 Future modeI refinements for site specific studies

Appendix G – Assessment of risk of tidal inundation via Cardiff Docks to Cardiff Bay.

G.1 Model Set-Up

G.2 Key Hydraulic Assumptions

G.3 Modelling Scenarios and Results

5097656-70-DG017-3.docx vi

Glossary of Terms

Term Meaning / Definition

AStSWF Map of Areas Susceptibly to Surface Water Flooding produced by the Environment Agency Wales

CFMP Catchment Flood Management Plans prepared by the Environment Agency Wales

DAMs Development Advice Maps (published by Welsh Assembly Government within TAN15)

DEFRA Department for Environment, Food and Rural Affairs (UK Government)

EAW Environment Agency Wales

EWL Extreme Water Level (tide level)

FCA Flood Consequence Assessment

FEH Flood Estimation Handbook method (for hydrological assessment)

FMSW Flood Map of Surface Water

HEC-RAS Hydrologic Engineering Center - River Analysis System (modelling software developed by US Army Corps of Engineers)

LDP Local Development Plan

LiDAR Light Detection and Ranging data (ground level information)

LPA Local Planning Authority

NFCDD National Flood and Coastal Defence Database

NGR National Grid Reference

PPW Planning Policy Wales (Welsh Assembly Government‟s Planning Policy)

QMED Medium Annual Flood

STAM Solutions for Total Asset Management database (Dwr Cymru Welsh Water records of reported sewer flood incidents)

SuDS Sustainable Drainage Systems

SWL Still Water Level (Tide level without Wave – effectively interchangeable with the EWL term)

TAN Technical Advice Note

TAN15 PPW Technical Advice Note 15: Development and Flood Risk

UKCIP United Kingdom Climate Impacts Programme

WAG Welsh Assembly Government

Zones A, B & C Flood Zones defined on Welsh Assembly Government DAMs

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1. Introduction This report represents an update of sections of the Cardiff Phase 2 SFCA of 2009, considering

an extended lifetime of development to 2110, compared to the period to 2060 previously

considered. As described in Section 1.3.1, Cardiff has been divided into Sub Areas for the

purpose of the SFCA.

The fluvially dominated flood risk to Area H and the tidally dominated risks to Area A and Area I

have been re-assessed. The assessments of all other sources of flood risk remain largely

unchanged and are as described in the SFCA Phase 2 Report (2009). The previous assessment

of surface water flood risk have been cross checked against the Environment Agency‟s Flood

Map for Surface Water and the Areas Susceptible to Surface Water Flooding map, which have

become available for planning purposes since completion of the 2009 SFCA Report. This

current report, therefore, represents an addendum to the SFCA Phase 2 (2009). The relevant

sections of that report are repeated below. The main section numbers are consistent with the

earlier report for ease of cross reference, if required.

1.1 Background

As part of the planning process, under the Compulsory Purchase Act 2004, planning authorities

in Wales are required to produce a Local Development Plan (LDP).

The Welsh Assembly Government‟s Planning Policy Wales (PPW) provides the planning policy

framework under which the Local Planning Authorities (LPAs) must prepare their Local

Development Plans (LDPs). The LDP has to show that it is encouraging patterns of development

that are economically, socially and environmentally sustainable. As part of this, PPW highlights

the need to reduce flood risk by avoidance of development within high risk areas, as opposed to

the previous approach of flood defence and mitigation of the consequences of flooding. The

guidance requires LPAs to consider the catchment as a whole and take a strategic approach to

flood risk.

PPW is supplemented by a number of Technical Advice Notes (TANs). Of key interest is TAN15:

Development and Flood Risk which provides guidance to LPAs regarding the assessment of

flood risk when they are preparing their LDPs. The advice note seeks to guide planning decisions

so that new development is directed away from areas that are considered to be at high risk of

flooding. Where development has to be considered within a high risk area, TAN15 outlines

justification tests in order to guide decisions regarding whether a specific development may

proceed.

Atkins Limited (Atkins) was appointed by Cardiff Council in January 2009 to undertake a

Strategic Flood Consequence Assessment (SFCA) for the City. The SFCA will form part of the

evidence base for the LDP and facilitates a risk based approach to development control in

Cardiff with respect to flood risk. This will provide clarity to both the Council and to potential

developers.

1.2 Purpose of the SFCA

The SFCA will provide an assessment of the flood risk for the study sites that are being

considered for development and provide guidance regarding the management of this risk. It

encompasses a broad level assessment and does not remove the requirement for more detailed

site-specific assessments which will follow the adoption of the LDP. Site-specific FCAs will need

to be undertaken by developers at the planning application stage, as required under TAN15.

The overarching aim of the SFCA is to provide sufficient data and information to enable the LPA

to apply a sequential approach to the allocating of sites, promoting flood Zones A and B before

Zone C (of the Welsh Assembly Government‟s Development Advice Maps – DAMs).

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The SFCA will assist Cardiff Council to:

Prepare appropriate policies for the management of flood risk within the LDP;

Ensure that flood risk is taken into account when considering options and in preparation of

strategic land-use policies;

Identify the level of detail required for site-specific FCAs in particular locations, and enable

the Council to determine the acceptability of flood risk in relation to emergency planning

capability;

Identify potential for producing local standing advice; and

Identify issues relating to managing surface water and use for sustainable drainage.

1.3 Approach to the SFCA

The SFCA considers both fluvial and tidal influences on flood risk within areas of the City.

Consideration is also given to other sources of flood risk; there are a number of ordinary

watercourses in the vicinity as well as public sewer networks.

Since the extent and suitability of the information available to inform the SFCA was not fully

known at the commencement of the study, the SFCA has been undertaken in two phases.

A number of reports comprise the overall Cardiff SFCA. How each of these reports relates to the

overall process is shown in Figure 1-1. The phasing of the SFCA and the definition of sub areas

A to I are described in the sections below.

Figure 1-1 - Reports comprising the Cardiff SFCA*

* PHASE 2 Part 1 Update revised November 2011 to include consideration of a flood path through Cardiff Docks to

Cardiff Bay following a query raised by the Environment Agency.

5097656-70-DG017-3.docx 3

1.3.1 Phase 1

The first phase of this project involved scoping out the requirements to be considered within the

SFCA. This included defining the areas within Cardiff where the flood consequences needed to

be determined and the extent and nature of the analytical assessments required to define the

risks.

It was felt that the recommendations indicated in the Environment Agency Wales‟ guidance note

to local authorities for SFCAs were too detailed for a city-wide assessment, and to assess all the

sites in detail against the TAN15 guidance would be prohibitively expensive.

A broad-level assessment using existing information was proposed for the study. The City was

divided into nine areas, existing information was gathered and reviewed, and an initial

assessment of the flood risk for each area was made, as described in the Phase 1 Report. The

initial Phase 1 flood risk by area is summarised as shown in the table below. The geographical

location of each area within Cardiff authority boundary is shown on Drawing 5097656/RCF/002,

Appendix A.

Table 1-1 - Flood Risk for Sub areas of Cardiff

SUB AREA TYPICAL RISK SCORE

Area A - Wentloog Tidal High

Area B - Rhymney North Pontprennau Varies Low to High

Area C - NE Cardiff - Roath Brook & Nant Glandulais High

Area D - Briwnant & Tongwynlais Low

Area E - Taff at Morganstown High

Area F - Cardiff West Low

Area G - Area of Junction 33, M4 Motorway Medium

Area H – Taff/Ely 2d modelled area High

Area I - Bay Area to Pengam Green High

Tasks required to fully define the risks for each area were identified and the cost of analysis

determined. The core activities undertaken in the SFCA assessment, which are dependent on

the initial risk score, are detailed in the Phase 1 report1.

1.3.2 Phase 2

In order to achieve best value from the budget available, the Phase 2 assessment was first

targeted on four sub-areas then considered to be of greatest strategic importance to Cardiff‟s

future development. Areas A, G, H and I were progressed first as they included a significant

number of proposed LDP allocations for housing and employment uses. The report on this first

Phase 2 assessment was delivered in November 2009 and is referred to hereafter as Phase 2

Part 1. In order to address areas of uncertainty in the level of flood risk to some areas identified

by the Phase 2 study, Cardiff Council commissioned a further 2D tidal inundation modelling

studies of Area A and Area I. This was reported in an Addendum Report to the SFCA, delivered

in March 2010. These reports considered a development lifetime of 50years to 2060.

This current report documents work undertaken to update the development lifetime to consider

the period up to 2110 for Areas A, H & I in accordance with the current Environment Agency

1 Report on Phase 1 (Scoping Study) and Proposed Methodology for Offer of Services for Phase 2 - the Assessment, Atkins Report

Ref.: 5076243.56.DG.032, April 2009.

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recommendation. It represents an update of Section 5 of the SFCA Phase 2 Part 1(2009) Report

and of the 2D Modelling Addendum of March 2010 (see Figure 1-1). The effects of new extreme

sea level estimates (see Section 2.3) have also been incorporated into the assessment.

Section 4, covering Area G – Area at Junction 33 of the M4 Motorway, has been updated to

include the sites submitted for the 2010 list of Candidate Sites.

The main section numbers of the SFCA Phase 2 Part 1 (2009) have been retained for this

current report for ease of cross reference.

Section 2 details the study areas and methodologies used in the assessment.

Each of Sections 3 through to Section 6 is specific to a certain area covering the main tasks

undertaken and the relevant findings.

It is worth noting, at this stage, that post-submission of the Phase 1 report, Cardiff Council

requested that some existing Allocations and Policy Areas be included in the SFCA in addition to

the Candidate Sites listed in Phase 1. For the purposes of analysis the site areas were treated

the same, and within the Phase 2 Part 1 (2009) and 2D Modelling Addendum reports (2010)

were referred to as “study sites”.

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2. The Study Area & Assessment

Methodologies

2.1 TAN15 Constraints

TAN15 has different requirements for sites depending on the type of development proposed and

the zones into which they fall. Zones are defined as follows:

Zone A – Little or no risk of fluvial or tidal flooding;

Zone B – Areas known to have been flooded in the past evidenced by sedimentary deposits;

Zone C1 – Areas at risk of flooding from fluvial or tidal sources (based on EAW extreme

flood outline for 0.1% annual chance event), but currently developed and served by

significant infrastructure, including flood defences; and

Zone C2 – Areas at risk of flooding from fluvial or tidal sources (based on EAW extreme

flood outline for 0.1% annual chance event) and without flood defence infrastructure.

Section 6 of TAN15 states that “new development should be directed away from Zone C and

towards suitable land in Zone A”. Where development is proposed within Zone C it must meet

the justification tests outlined within the Section. Where consideration of a development site can

be justified, the development must then meet the acceptability criteria detailed in TAN15 Section

7 and Appendix 1.

Development within Zone C1 or C2 must be flood free during a 1% (1 in 100) annual chance

fluvial event (including climate change) and the 0.5% (1 in 200) chance tidal event and have

acceptable consequences of flooding in the extreme 0.1% (1 in 1000) annual chance event as

defined by TAN15 A1.15. A development must not increase flooding elsewhere. In addition,

flood defences must be shown to be structurally adequate under extreme overtopping conditions,

and an emergency flood plan, including flood warning system and identified evacuation routes,

must be in place.

Emergency services and highly vulnerable development (as defined by TAN15 Section 5 and

including residential development) should not be considered within Zone C2.

All development, including that within Zone A (which has few constraints to development), must

give due consideration to the management of surface water; guidance is provided in TAN15

Section 8. Any development will result in changes to the natural hydrology of the catchment as a

result of increased runoff from impermeable ground and built-up areas. TAN15 states that

Sustainable Drainage Systems (SuDS) should be employed to manage surface water runoff

wherever possible, and if the use of conventional drainage systems is proposed, developers

must give good reasons why SuDS cannot be implemented. This approach is further reinforced

through recent legislation, the Flood and Water Management Act, 2010.

2.2 Development Sites

This current report considers the flood risk to candidate sites submitted to the Council for the

Draft LDP in 2010. Much of the analysis and assessment work for the SFCA had already been

completed based on the previous 2008 list of candidate sites. However, many of the sites in the

2010 list are the same or similar to those in the 2008 list.

The flood risks associated with development of each site are described in the site summaries in

Sections 3.3, 5.6 and 6.3. The assessments of the risks from sources which have not been

updated from the 2009 report are italicised in these site summaries. This indicates the text is

repeated from the earlier SFCA 2009 report.

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2.2.1 Site Areas

A review of the majority of the various sites identified for development was undertaken within the

Phase 1 assessment. This was undertaken for candidate site list as at 2008. The initial risk

categorisation of the individual sites led to the sub area risk categorisations shown within Table

1-1 above.

The three areas considered in this report as shown on Drawing Number 5076243/RCF/001 in

Appendix A, are as follows:

Area A – Wentloog Tidal

Area H – Taff/Ely 2d modelled area

Area I – Bay Area to Pengam Green

A detailed description of each area, and the development proposed there, is included in the area-

specific sections later in this report.

There is some overlap between Area I and Area H due to the potential for parts of each area to

be affected by overland flow paths from the other. Flooding mechanisms have been considered

for the overlap and study sites discussed within the most appropriate report chapter. Area

boundaries do not wholly correspond to those initially proposed within Phase 1 of the SFCA.

2.2.2 Lifetime of Development

TAN15 states that “the Assembly has a duty, under Section 121 of the Government of Wales Act,

to promote sustainable development in the exercise of its functions”, and part of this includes

“making provision for future changes in flood risk.” The guidance comments on the mounting

evidence that climate change will result in increased flood risk. Hence, flood risk must be

considered over the anticipated lifetime of each development.

Current Environment Agency Wales (EAW) policy is to accept a development lifetime of 75 years

for industrial/ commercial developments and 100 years for residential development. It previously

accepted a 50 year development lifetime, which was the basis of the analysis underlying the

Phase 2 Part 1 SFCA (2009).

The period of the development lifetime considered is important in areas where tidal influences

have an effect on the flood risk. This is because sea level is predicted to rise over time. The

longer the development lifetime considered, the greater will be the sea level rise. The allowance

made for future changes in fluvial flows is not, in practice, dependant on the period considered.

The latest Defra guidance adopted by the Welsh Assembly Government in 20072 for fluvial

flooding is to apply a 20% increase to peak river flows in order to evaluate the effects of climate

change. This is the same whether a 50 year or a 100year lifetime is considered. So for areas

affected by fluvial flooding only, the period of the development lifetime is less important than for

tidally affected areas.

In this SFCA, the peaks at the 1% (1 in 100) annual chance event in all watercourses have been

increased by 20% to account for the effect of climate change over the lifetime of the

development. No allowance for climate change was applied to a 0.1% (1 in 1000) annual chance

event, recognising the high uncertainty already inherent in estimating the flows for such extreme

events.

2 Welsh Assembly Government, July 2007, Flood and Coastal Defence Appraisal Guidance, FCDPAG3 Economic Appraisal,

Supplementary Note to Operating Authorities – Climate Change Impacts.

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2.2.3 Climate Change

The Environment Agency‟s guidance3 advises that allowances for climate change should be

included when assessing compliance with the threshold frequencies identified in the table within

A1.14 of TAN15. However, it does not expect an allowance to be included, when considering the

indicative values set out in the table within A1.15 of TAN15.

Unless development for emergency services is to be considered, which must be flood free over

the lifetime of the development, then consideration of climate change to a 0.1% (1 in 1000)

annual chance flood event is not required. Since no sites proposed for emergency services have

been assessed as part of this SFCA, forecast sea level rise has only been applied to 0.5% (1 in

200) annual chance extreme tide levels.

2.3 General Climate and Hydrology

2.3.1 Hydrological Situation

Cardiff falls within the Ely, Taff and Rhymney River Catchments. Two Catchment Flood

Management Plans (CFMP) cover the Cardiff area, the Taff and Ely CFMP and the Eastern

Valleys CFMP, which includes the River Rhymney.

The Taff Ely CFMP states that the catchment covers an area of approximately 696km² and

includes the catchments of the River Taff, River Ely, River Rhondda, River Cynon and the River

Clun. The Rhondda and Cynon are tributaries of the Taff, while the Clun is a tributary of the Ely.

The catchment generally drains in a southerly direction with both the Rivers Taff and Ely

discharging into Cardiff Bay, and subsequently into the Severn Estuary.

The headwaters of the Taff are located in the north of the catchment in the Brecon Beacons.

Moving southwards the land elevations fall. Whilst the river has a steep gradient in the upper

reaches (1:60 on the Taff Fechan upstream of Merthyr Tydfil) this gradient flattens (to

approximately 1:500) as it flows through Cardiff.

The Ely valley, running in a south easterly direction adjacent to the Taff catchment, is generally

less varied than the Taff. The steep upper reaches are characterised by deep narrow valleys

that open out into meandering lowland topography on the approach to Cardiff.

Current tidal influence on flood risk from the Rivers Taff and Ely is limited by the Cardiff Barrage.

However, future flood risk from the Taff and Ely will be affected by predicted increases in sea

level and the resulting effect on levels in Cardiff Bay during fluvial flood events (see section

5.3.2)

The River Rhymney drains to the Severn Estuary in the east of the city. A confluence of a

tributary, the Roath Brook, with the main Rhymney river channel is located at some 980m

upstream from the coast. The Rhymney has a catchment area of 223km² to downstream of this

confluence, with the Roath Brook contributing 20km² of this area. Tidal influence on river levels

is high in the lower reaches of the Rhymney and the Roath Brook. The tidal limit (the maximum

upstream location where the tide influences the river flows) is considered to extend to

Llanrumney.

The average annual rainfall for the Taff Ely CFMP catchment is 1,850mm and for the Rhymney

catchment is 1,380mm, compared to the average for Wales of 1,310mm. The steep upper slopes

of the catchments and above average annual rainfall result in rapid responses to rainfall events.

Downstream through Cardiff the rivers are relatively less responsive.

Further detail regarding the hydrological situation within each sub area is included in the relevant

report chapter.

1. 3 Environment Agency Operational Instruction 229_10, Flood Risk Management: TAN15 related Planning Applications

(Wales only), Oct 2010.

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2.3.2 Extreme Sea Levels

The results of a research project providing new extreme tide levels estimates were published by

Environment Agency/ Defra on the 21st February 20114. The research project provided an up-

to-date scientifically robust national evidence base and practical guidance on appropriate design

sea levels and uncertainty data around England, Wales and Scotland.

The estimated extreme tide levels and associated confidence intervals have been adopted by

Environment Agency Wales for assessment of flood risk for development control purposes, the

estimates provided being within the 95% confidence limit.

Updated tidal inundation modelling has therefore been undertaken for Areas A & I for the SFCA.

The results of the modelling and the amended assessments of tidal flood risks to the Wentloog

Levels and Cardiff Bay to Pengam Green areas are reported in section 3 and 6 below.

The extreme tidal events tested in the models are those with annual chance of exceedance of

0.5% (1 in 200) and 0.1% (1 in 1000). These are used for assessment of the study sites against

the guidance described in TAN15.

2.4 Sources of Flooding

The main risks of flooding to the study areas are:

the Rivers Ely and Taff,

from direct tidal inundation along the coast,

tidal flooding via the lower reach of the River Rhymney in the east of the City.

River Taff

The Taff catchment has experienced widespread flooding on a number of occasions over the last

60 years. These include the floods of 1946/47 (due to snowmelt), December 1960, December

1979, December 1992 and October 1998, all of which were from heavy rainfall. The most

notable flooding to Cardiff occurred in 1960 and 1979, where thousands of properties were

affected. This resulted in the Government spending a considerable sum of money on flood

defences.

River Ely

In contrast, the Ely catchment has only suffered from two significant events in the last few

decades. These were a result of heavy rainfall falling on a wet catchment in March 1998 and in

October 2000. The 1998 event only affected a small number of properties in the Ely Bridge area

of Cardiff, St Fagans, Peterston Super Ely, Brynsadler and Talbot Green. In October 2000 few

reports of flooding were reported and these were in St Fagans only.

Cardiff Bay Tidal

Tidal flood risk is considerably lower in Cardiff since the construction of the Cardiff Bay Barrage

in 1999. The barrage significantly reduces the risk of tidal flooding. However, the risk of flooding

in extreme events remains and is considered within Section 5.3.2 and Section 6.2.2 of this

SFCA.

The effects of extreme tidal events to flooding to the Cardiff Docks and the associated potential

flow routes into Cardiff Bay have been considered in Section 6.2.2

The defences to the east of the barrage are potentially susceptible to tidal overtopping. The

Environment Agency flood map indicates that the areas of Pengam Moors and Tremorfa are at

risk. Tidal flooding into these areas via the River Rhymney is discussed in Section 6.

4 Environment Agency, February 2011, Joint Defra & Environment Agency Research Project: SC060064. Coastal flood boundary

conditions for UK mainland and islands

5097656-70-DG017-3.docx 9

Tidal Severn Estuary

The Wentloog Levels are partially protected by flood defence embankments along the coast.

Improvements have been made to the defences over the years but floodplain remains

susceptible to flooding due to wave and tidal sources. Storm surge levels, tidal currents and the

wind wave climate along the Severn Estuary shoreline are significant. The existing salt marsh

has previously provided extra protection to the shoreline but is generally eroding. Overtopping or

breach of defences could result in significant flooding to the low lying area behind. Further

discussion of this risk is included in Section 3.2 and Appendix E.

Other flood risks

Other flood risks have been identified within the area, including:

Ordinary watercourses;

Culverted watercourses;

Sewers; and

Surface water runoff.

Groundwater flooding is not considered a strategically significant source of flooding in the study

areas, as evidenced in the CFMPs covering the Cardiff authority area. This source has not been

the subject of analysis for this SFCA. However, there will still be a need for future site specific

FCAs to consider the local conditions, including potential groundwater issues and any control

measures necessary.

The risks associated with each source of flooding are discussed on an area-specific basis.

2.5 Assessment Methodologies

Following review of available data for each area (see area-specific chapters), assessment of the

various flood risks for each area were made, as detailed below.

2.5.1 Flood Risk from Tidal Sources

As a coastal city, the risk of flooding to Cardiff from the sea has always been significant and the

effects of climate change are expected to make the situation worse. The 2007 report on climate

change published by the IPCC emphasises the certainty of accelerated climate change, and the

previously accepted guidance for sea-level rise of 5mm/ year for Wales has now been revised.

Current DEFRA guidance states that anticipated annual sea level rises are:

3.5mm/ year until 2025,

8.0mm/ year between 2025 and 2055,

11.5mm/ year between 2055 and 2085,

14.5mm/ year beyond 2085.

These values have been used within this SFCA. The allowances for future sea level rise,

provided by Defra and adopted by WAG, used information from the UKCIP scenarios, published

in 2002. Revised estimates from UKCIP were produced in 2009, although the Defra guidance

has not been revised.

As noted in Section 2.3 above, construction of the Cardiff Bay Barrage has considerably reduced

the flood risk to the city centre and areas in the Taff and Ely catchment. However, the risk to the

Wentloog Levels (Area A) and areas to the South and East of Cardiff (Area I) from direct tidal

flooding remains high. The flooding mechanisms for these two areas are entirely different and

hence have been analysed separately.

5097656-70-DG017-3.docx 10

Section 3 and Appendix E detail analysis undertaken in the determination of flood risk to the

Wentloog Levels. The assessment of potential tidal inundation was based on the Severn Estuary

Flood Risk Management Strategy (SEFRMS) previously undertaken for the Environment Agency

by Atkins. Two dimensional (2D) inundation modelling has been undertaken, incorporating the

risk of overtopping and breach of the defences to the area from the River Rhymney around the

Severn Estuary frontage to the River Ebbw. The methodology is described in more detail in

Appendix E.

For Area I, the SEFRMS analysis was extended to include an assessment of overtopping of tidal

waters that may propagate up the lower reach of the River Rhymney. This approach is

described in Appendix F.

The impact of tide level and future sea level rise on Cardiff Bay, and the resulting flood

predictions on the River Taff and River Ely is discussed in detail in Section 5.3.1. Although the

flood risk from these rivers is defined as fluvially dominated (with the 1% (100) and 0.1% (1 in

1000) annual chance events being considered), the risk arises from combinations of fluvial and

tidal effects.

Testing as part of this study has confirmed that in the river channels affecting study sites in Area

H, extreme fluvial events result in higher water levels in the Rivers Taff and Ely compared to

extreme tidal events of comparable probability.

By contrast extreme tidal effects in the Cardiff Bay area are dominant over extreme fluvial

events, i.e. result in higher flood levels. The tidal risk to study sites around Cardiff Bay is

discussed in Section 6. This includes the risk of tidal inundation into the Bay area via the Cardiff

Docks.

2.5.2 Flood Risk from Main Rivers

Fluvial flooding has been considered for the Rivers Taff and Ely (see Section 5.3.1). Tidal

influence is considered to be dominant over the fluvial effects on the River Rhymney at its

downstream end. The risks associated with this river are discussed within the tidal inundation

modelling for Area I and Area A described respectively in Appendix E and Appendix F.

Consideration of the potential impacts of climate change when assessing the consequences of

flooding have been made in line with the current DEFRA guidance and for fluvial flooding a 20%

increase to peak river flows has been included in order to reflect the influence of climate change.

Due to the high uncertainty inherent in estimating the peaks for the 0.1% (1 in 1000) annual

chance event, an allowance for climate change is not generally added to such extreme events.

2.5.3 Flood Risk from Ordinary Watercourses

No hydraulic models exist for the ordinary watercourses in the study areas and it was determined

within Phase 1 of this study that undertaking detailed modelling work as part of this study would

not be a cost effective use of resources given the anticipated level of risk. Therefore, a simple

assessment of the flood risk areas for the watercourses identified has been undertaken, with the

exception of the reen system in Area A.

The Wentloog levels are drained by a system of drainage channels, known as reens. The reens

system‟s main purpose is to move water from the fields to discharge in the sea during wet

seasons, but they also store water during dry periods for irrigation purposes. Water levels are

controlled by a system of sluices and the complexity of the system makes it difficult to assess

accurately. Since risk of flooding from the reens is potentially significant, a broad screening

assessment of the system was undertaken. This approach is described in Section 3.4.1 of the

Phase 2 SFCA (2009). The results are referenced for the relevant sites in the site summaries in

Section 3 of this current report.

For Areas H and I, reference was made to the LiDAR contour data in order to determine potential

flood extents and flow paths.

5097656-70-DG017-3.docx 11

The Dock Feeder in Area I is a potential source of flooding. This watercourse runs from an off-

take from the River Taff at Blackweir to Bute Dock through sections of open canalised channels

and culverts. A full discussion of its route is included in Section 6.4 of SFCA Phase 2 Part1

(November 2009). Since no existing flood studies exist for the watercourse an assessment of

potential risks was based on review of LiDAR data. However, it is recommended that a detailed

study of the hydrology and hydraulic modelling of the Dock Feeder channel is undertaken as

part, or prior to, any FCA for sites potentially affected in order to provide a better understanding

of the risks.

2.5.4 Flood Risk from Sewers

Cardiff has both separate and combined foul and surface water drainage systems. As a result of

historic development, unknown connections, and the existence of additional private sewer

systems within developed areas, there is significant uncertainty as to the local drainage

catchments in some areas. Although not all incidents are reported, the best information available

for determining flood risk from the sewerage system is the Dwr Cymru Welsh Water (DCWW)

Solutions for Total Asset Management (STAM) database. The database provides details of

reported flooding, pollution and odour incidents and, where appropriate, details of consequent

action taken.

With DCWW‟s permission, interrogation of the STAM incident database has been made for all

areas. DG5 sewer flood incidents, reported Severe External Flooding (SEF) and Other Flooding

(OF) incidents were all considered. It was noted in Phase 1 or this study that caution must be

exercised in interpretation of the database, since the nature of the incidents will depend on a

combination of local conditions at the time of individual storm events (blocked drains, local

maintenance protocols) and not all indicate a longer-term flood risk. In order to give indication of

areas of potential longer term risk, database incidents were screened to include incidents relating

to hydraulic overload only; known blockage incidents were excluded. The location of hydraulic

overload incidents have been considered in relation to the study sites taking account of local

ground profile in order to determine those which might be at risk from sewer flooding in the

future. Discussion of conclusions drawn from the analysis is included in the area-specific

chapters.

2.5.5 Flood Risk from Culverted Watercourses

The majority of watercourses within the study area remain in open channel for much of their

length; no risk from long culverted watercourses has been identified. However, there will be risks

associated with short culvert sections under roadways. Potential blockage within culverts could

result in waters backing up and watercourses overtopping and flooding land upstream of

culverts.

Flood risk has also been noted for two culverts within Area H and I; see Sections 5.4.2 and 6.4.2

respectively.

A large number of culverts within Area A are associated with the Reen System. Inspection and

assessment of each of these is beyond the scope of this strategic level study. It is anticipated

that these will be subject of site specific FCAs and are likely to be manageable through local

mitigation measures on the study sites. A limited amount of information was made available by

Network Rail, supplemented by site visit to inform the hydraulic inundation model of Area A (see

Appendix E) No separate assessment has been undertaken for flood risk linked to individual

culverts within this area.

2.5.6 Flood Risk from Groundwater

The brief for this SFCA did not include the requirement to consider risk of flooding from

groundwater sources. It is anticipated that the risk of groundwater flooding is low, and this is

confirmed by the Taff and Ely CFMP which states that groundwater flooding is not considered to

be a significant issue within the catchment. However, this does not negate that fact that risks

5097656-70-DG017-3.docx 12

associated with groundwater ought to be investigated as part of any site-specific FCAs carried

out in the future.

2.5.7 Flood Risk from Surface Water

Surface water risk was considered as part of the earlier reports forming the Parts of the Cardiff

SFCA (see Figure 1-1). These were completed prior to the Environment Agency‟s release of a

new Flood Map for Surface Water (FMSW) in late 2010. The FMSW represents the mechanisms

that cause surface water flooding. The map is available for 30- and 200-year storms.

Guidance issued by the Environment Agency states that the map is not intended to be definitive.

The maps should be used to support local flood risk management where better data are lacking.

The methods used to assess risk from surface water are described in section 2.5.7 of SFCA

Phase Report, November 2009.

In preparing this current report, the FMSW has been referred to. Where these maps indicate a

risk of surface water flooding which is different to that previously identified, this is noted. The

methods used previously and thus supplemented by the information in the FMSW are considered

to provide a level of assessment appropriate for this strategic level study.

5097656-70-DG017-3.docx 13

3. Area A – Wentloog Levels This section updates the assessment of flood risk for sites in the Wentloog Levels. It is based on

updated 2D tidal inundation modelling and supersedes the 2D Modelling Addendum Report of

March 2010 (see Figure 1-1 and section 1.3). The updated modelling takes account of new

estimates of extreme sea levels, released by the Environment Agency in February 2011 and

considers development lifetime up to 2110.

The assessments of all sources of flood risk, other than tidal, remain unchanged from those

reported in the SFCA Phase 2 Part1, November 2009. These assessments are, for

completeness and ease of reference, repeated under the summary of flood risk in Section 3.3,

together with the updated assessment of tidal flood risk. The text repeated from the earlier

Phase 2 report is italicised.

3.1 Site Description and Flood Risks other than by Tidal Inundation

Full details of the extent and description of Area A and the assessment methodologies used for

sources of flood risk other than tidal are given in the SFCA Phase 2 Part1 Report (2009).

As described in Section 1.3, the earlier SFCA 2D Model Addendum Report (March 2010)

considered the candidate sites to the Draft LDP which was withdrawn in 2010. The Candidate

Sites Submitted in 2010 for the current Draft LDP have been considered for this update.

3.1.1 General Site Description

Area A covers the area of the Wentloog levels that lie within the Cardiff Council Local Authority

Boundary. This extends from the Rhymney River in the West to St Mellons Business Park in the

East. The Wentloog Levels comprise an area of historically reclaimed land. As part of the

reclamation and subsequent management, an extensive network of ditches (reens) exists to

drain water out to sea. Land use in the area includes wide swathes of agricultural land, with a

large number of smaller conurbations to the North. Strategic infrastructure includes the main

London to south Wales railway line.

3.1.2 Proposed Development

A wide range of development types have been proposed for the Wentloog levels comprising

developments for employment, business, industry, housing and mixed use. The majority of the

sites in this area fall within the Environment Agency‟s extreme flood outline (0.1% (1 in 1000)

annual chance flood event) and Zone C1 of the TAN15 DAM. Although the area is defended

from tidal inundation, the sites were categorised as High Risk within Phase 1 of this SFCA due to

the potential for the defences to be either overtopped or breached.

The geographical extent of the study sites are shown on Figure 3-1 and site details are

summarised in Table 3-1 below.

5097656-70-DG017-3.docx 14

Figure 3-1 – Area A Study Site Locations

© Crown Copyright. Cardiff County Council, 100023376 (2011)

5097656-70-DG017-3.docx 15

Table 3-1 - Area A Summary of Study Sites

Site ID Name/ Location Existing Land Use

Development Category Area (ha)

76LGW South of Wentloog Avenue Previously Undeveloped Land

Waste Management Facility

60.0

22LGRM Trowbridge Mawr Previously Undeveloped Land

Residential/Industrial and Ancillary Community

35.0

21LGR Areas 9 - 12 St. Mellons Previously Undeveloped Land

Residential and Ancillary Community

18.8

23LGR Land bet. Crickhowell Rd and Willowbrook Rd

Previously Undeveloped Land

Residential and Ancillary Community

5.6

24LGRM Land East of Cypress Drive, St. Mellons

Previously Undeveloped Land

Mixed Use 9.5

74LGEMW Wentloog South of the Railway Line Previously Undeveloped Land

Employment and Waste 8.3

2LGRM Land at St. Mellons Business Park Previously Undeveloped Land

Mixed Use 3.9

1LGRM Land at Wentloog Levels Previously Undeveloped Land

Mixed Use 28.0

106LGEM Land at St Mellons/ Wentloog Previously Undeveloped Land

Mixed Use 71.0

GTchPk Action Area Previously Undeveloped Land

Green Tech Park 14.70

Wentloo_ii Action Area Previously Undeveloped Land

Business, Industry and Warehousing

15.60

Wentloo_iii Action Area Previously Undeveloped Land

Business, Industry and Warehousing

15.40

3SBR Pill Du Farm, Hendre Lane, Trowbridge, Cardiff

Yard and Scrubland

Residential 0.5

55LGR Trowbridge Road, Cardiff Greenfield Residential 2.6

56LBR St John‟s College Site, Greenway Road, Trowbridge, Cardiff

Brownfield Residential 2.51

3.2 Flood Risk from Tidal Sources

The Wentloog Levels floodplain is considered to be susceptible to flooding due to wave and tidal

effects. The defences are generally higher than extreme still water sea levels (SWL). The wave

climate, however, is such that during extreme storms, the quantity of water overtopping the

defences may be sufficient to cause damage to sections of the defences and potentially cause

breaches. The chances of extreme sea levels and high waves occurring together have been

considered. For each of the combinations, the resulting likelihood of breach of the coastal

defences has been assessed.

The risk of breaches has been assessed based on methodologies underlying the Environment

Agency‟s Severn Estuary Flood Risk Management Strategy (SEFRMS). The approach to the

modelling is described in Appendix E. Essentially the approach considers the joint probability of

wave and extreme tide levels, the probability of breach of the coastal defences and the

consequences of such breach events.

5097656-70-DG017-3.docx 16

Different combinations of extreme tide level and wave height, each with the same overall

probability were tested for scenarios at each time horizon. The modelling results indicate that

various combinations could result in breach at a number of locations along the defences over a

one hundred year development lifetime. These locations are referenced5 as listed below and are

marked on drawings 5097656/RCF-120 to RCF-122 in Appendix B.

Table 3-2 - Locations of Breaches for different time horizons and by event probabilities

Year Event chance Location References

2010 0.5% (1 in 200) SR1-RREB & SR1-2

0.1% (1 in 1000) As above plus SR1-0

2085 0.5% (1 in 200) As above plus SR1-3 & SR1-6

2110 0.5% (1 in 200) As above plus SR1-1 & SR1-REWB

SR1-RREB River Rhymney East Bank (downstream of rail bridge)

SR1-0 Little Wharf

SR1-1 Rhymney Great Wharf

SR1-2 Sluice Farm

SR1-3 Peterstone Great Wharf

SR1-6 Lighthouse Park

SR1-REWB River Ebbw West Bank

Currently two locations are considered susceptible to breach with a 0.5% (1 in 200) annual

chance in any year. For 0.1% (1 in 1000) annual chance events the number of sections of

defence currently considered susceptible to breach increases to three.

The weak points in the defences at Sluice Farm and on the east bank of the River Rhymney are

predicted to be damaged, but not breach, by events of a current 2% (1 in 50) annual chance of

exceedance. The Rhymney defence is predicted to be breached by tidal event with a 1% (1 in

100) annual chance, whilst the Sluice Farm defence will be breached by 0.5% (1 in 200) chance

events.

The numbers of susceptible sections of defence are predicted to increase into the future due to

the effect of sea level rise. The locations considered to be susceptible to breach over different

time horizon are summarised in the following table. It is considered likely that by 2085, four

locations will be susceptible to breach during 0.5% (1 in 200) chance events.

The results of the modelling are presented as flood boundaries on Drawings numbered

50976565/RCF/120 to 122 in Appendix B.

The consequences for the study sites indicated by the results of the modelling are described in

the following section.

5 Note that the referencing of the breach locations has been changed since the March 2010 SFCA 2D modelling addendum to be

consistent with the referencing used in the Environment Agency‟s SEFRMS project.

5097656-70-DG017-3.docx 17

3.3 Summary of Flood Risk Management

The flood risks to each study site in this area are described in the site summaries below. The

drawings in Appendix B which show the results of the 2D tidal inundation modelling for Area A

should be referred to in reading these site summaries.

Area wide impacts

By the year 2085, as a result of climate change, it is predicted that the majority of the Wentloog

levels within the Cardiff Authority area, especially south of the main rail line, are likely to be

inundated by 0.5% (1 in 200) tidal events as a result of breach of the coastal defences. All this

land therefore fails TAN15 criteria for the threshold of flooding (A1.14 of TAN15). Site specific

measures alone are unlikely to reduce the risk to acceptable levels for new development in line

with TAN15.

A strategic approach to flood alleviation to the Wentloog Levels as a whole is required if future

development is to proceed in this area.

Consideration is being given to improvements in flood risk management measures in the

Wentloog levels as part of the Environment Agency‟s Severn Estuary Flood Risk Management

Strategy (SEFRMS).

The SEFRMS consultation document for South East Wales6 contains a number of proposals to

manage flood risk in the Wentloog Levels. However, these aspirations cannot be guaranteed.

Funding and a commitment to progress the proposals would be required to mitigate the area

wide risk from 0.5% (1 in 200) chance events.

The proposed measures include:

improvements within 5 years to the weak point in the defences at Sluice Farm to bring the

general standard of defence to the Wentloog levels up to 0.5% (1 in 200) chance of

exceedance in any year.

In order to counteract the effects of climate change and keep the overall level of flood risk at

current day levels, further measures are proposed :

“Between 2060 and 2100” it is “proposed” to undertake “strengthening of the defences by

increasing the height of the embankments to keep pace with climate change”.

The objective is to “ensure that the chance of tidal flooding remains at 1 in 200 or less in any

year”.

However, as there is no guarantee that the costs of providing improvements to the defences can

be secured, for the purpose of the SFCA, it is assumed that the current defences will be

maintained at existing levels, and that the probability of overtopping and breach will increase into

the future due the effects of predicted sea level rise.

As part of the suite of mitigation options to be considered for each site, it is recommended that

flood management protocols should be developed for all study sites identified as being at risk of

flooding. This would need to take account of the risk to the site and the access and egress

routes.

3.3.0 Key Access Routes across the Wentloog Levels

For those sites situated south of the railway line, maintenance of access during extreme tidal

flood events poses a significant problem. This is less of an issue for those sites north of the

railway. The modelling indicates that key access routes across the Wentloog levels will be

affected by flooding within a 75 year development lifetime to 2085.

6 Managing Flood Risk on the Severn Estuary, South East Wales, Consultation Document, Environment Agency January 2011

5097656-70-DG017-3.docx 18

If development of any given site is to proceed in line with TAN15, it will need to be demonstrated

that escape/ evacuation routes remain operational under all conditions, as stated in A1.12

TAN15. This is a matter requiring expert assessment from the emergency services and local

authority emergency planners.

It is likely that this would include a more detailed assessment of the flood risk, considering the

actual site use as well as the depths and velocities of flood water.

An indication of the likelihood that access will remain operational can be gained by calculating a

hazard rating for locations based on the 2D tidal modelling of 0.5% annual chance events for

2085. This method is based on DEFRA research FD2320/TR2 and is recognised in England in

PPS25, (Planning Policy Statement 25: Development and Flood Risk: England). PPS25 adopts

a risk based approach. The hazard rating combines velocity, depths and a debris factor to

assess the combined hazard against conditions which are tolerable for emergency services to

function.

The PPS25 document references separate research7 which provides the guidance

recommended for development control and planning purposes. Although, this method has no

official endorsement through planning guidance in Wales, it can be considered to provide

evidence as to whether or not access will be operational for emergency services in the event of a

flood event.

The resulting hazard matrix from Defra Research Paper produced in PPS25, Table 4 is

replicated below as Table 3-3.

Figure 3-2 presents the hazard scores across the Cardiff Wentloog Levels area based on depths

and velocities which the modelling indicates can be expected during 0.5% (1 in 200) chance

events in 2085. This corresponds to the 75year development lifetime which is relevant to non

residential developments.

Based on this information and referencing the hazard matrix (Table 3-3) the following is

concluded with respect to the risk to access routes:

By 2085, along the majority of the flooded sections of main access roads, notably Wentloog

Avenue, the hazard score is classified at the highest rating of “Danger for All”. This indicates that

these sections of road would not be safe for emergency services to operate. Some lengths of

Wentloog Avenue are indicated to have a “Very low hazard” score, where the road is flood free

due to its elevation above the surrounding land. However, without mitigation, it cannot be

considered that access to sites which rely on this route (notably Site 76LGW) will remain

operational, and so fail to comply with TAN15 A1.12.

It is noted that the hazard rating is sensitive to the debris factor (DF) assumed (it is taken as 1.0

for all depths above 0.2m in the current analysis). Further more detailed assessment based on

local factors are required, This would include more detailed survey and consideration of the

actual likelihood of debris being a significant factor.

The potential to mitigate the risk to the main access routes may be achieved through raising key

sections – i.e. reduce the hazard score to at least “Danger for most” or below. Although subject

to confirmation, such measures might include raising of sections of low lying road. These would

need to be achieved without diverting flows and so increasing flood risk elsewhere.

A strategic approach to managing flood risk to the Wentloog levels in line with the proposals

included in the Environment Agency‟s Severn Estuary Flood Risk Management Strategy, would

reduce the risk to access across the area to all sites to acceptable levels.

7 Supplementary note on flood hazard ratings and thresholds for development planning and control purpose: Suresh Surendran and

Geoff Gibbs (Environment Agency),Steven Wade and Helen Udale-Clarke (HR Wallingford): May 2008:

5097656-70-DG017-3.docx 19

Table 3-3 - Replication of Table 4 Hazard to People Classification using Hazard Rating

5097656-70-DG017-3.docx 20

Figure 3-2 – Hazard Score for 0.5% (1 in 200) annual chance event 2085 for access routes and study sites.


/5097656-70-DG017-3.docx 21

3.3.1 Study Site 76LGW

This study site covers an area of some 60 ha. The proposed use is solely waste management.

If this includes waste disposal, this is classified under TAN15 as highly vulnerable development.

The site lies in TAN15 Zone C1. If waste disposal is proposed for the site, such highly vulnerable

development will need to be justified in line with Section 7 of TAN15.

The majority of the area of this previously undeveloped land lies south of Wentloog Avenue. The

site abuts or lies in close proximity to the coastal frontage over a length of some 2km. The south

west extremity of the site lies within 360m of one of the sections of the coastal defence assessed

to be at risk of breaching.

Tidal inundation

The results of the 2D tidal inundation modelling are summarised below.

Current situation 2010

The model results indicate that the site is flood free for a current 0.5% (1 in 200) annual

chance tidal event.

Large areas of the site would be inundated during a current 0.1% (1 in 1000) annual chance

event. Those areas remaining flood free would become islands surrounded by inundated

areas.

Depths of flooding would exceed 0.6m over some areas during this event, particularly in the

northern and south western parts of the site. Some localised areas around the existing

drainage channels could be expected to flood to depths exceeding 1.0m.

The corresponding velocities during a 0.1% (1 in 1000) chance event would be below

0.3m/s over the majority of the site. However, velocities would exceed 0.45m/s in the

southern part of the site and along existing drainage channels.

Considering climate change and sea level rise allowances to 2085:

The majority of the site area is within the 0.5% (1 in 200) annual chance tidal inundation

flood boundary. Islands of flood free ground would be isolated from existing access/egress

routes to Wentloog Avenue by the surrounding flooded areas.

Newton Road which runs from Wentloog Avenue through the site towards the coast is

predicted to be flooded in a 0.5% (1 in 200) annual chance event. Access along Newton

Road from the east would not meet TAN15 indicative acceptance criteria.

A potential access/ egress route to the site from the north over the rail line via Wentloog

Avenue is flooded along most of its length in a 0.5% (1 in 200) annual chance event.

Considering climate change and sea level rise allowances to 2110

The proposed development use of this site is for non residential development. Climate change

to 2110 would need to be considered if residential use were to be proposed.

Mitigation of the risk

It is possible that mitigation of the flood risks for at least part of the site over a 75 year period to

2085 could be achievable for non residential uses. The mitigation measures will need to be

explored as part as a site specific FCA. Options which may be considered include:

Options to manage the risk of flooding should consider a flood management plan within the

site operation plan, which would include a flood warning system and emergency evacuation

procedures.

Within the site it may be possible to raise ground levels to connect the flood-free portions of

the site. A site specific FCA would need to demonstrate that changes in ground and

highway levels would not adversely impact on flood risk elsewhere.

/5097656-70-DG017-3.docx 22

Ground raising to connect flood free area and provide flood free access across the site

(during a 0.5% chance event). Compensatory ground lowering would be required in other

appropriate areas, ensuring a level for level compensation;

Reducing the probability of breach through reinforcement of the coastal defences;

It is possible that raising some lengths of Wentloog Avenue to maintain safe access during a

0.5% (1 in 200) chance event could be explored for the benefit of the western Wentloog

area as a whole.

It is reiterated that TAN15 requires that mitigation measures must not adversely impact on flood

risk elsewhere.

Fluvial and other sources of flood risk

The flooding risk considered for sources other than tidal inundation are based on the broad scale

assessments as described in the Cardiff SFCA, November 2009.

Fluvial flood risk for the site is not considered to be significant, and no sewer or groundwater

issues have been identified for the site. Assessment of surface water indicates that there no

significant sources of runoff or flow paths in the area so risk from this source is also low.

Reference has been made to the Environment Agency surface water flood map (FMWS) does

not suggest any significant surface water management problem.

Site summary

By 2085, large areas of the site fail A1.14 of TAN15 as they are inundated by a 0.5% (1 in 200)

annual chance tidal event.

The safe access and egress routes to and from the site during this event would not be available

by 2085 if the Severn Estuary defences were to remain at their existing heights and conditions.

Without a strategic approach to future flood risk management, in line with the proposals included

in the Environment Agency‟s Severn Estuary Flood Risk Management Strategy, justification of

the site under TAN15 would be problematical.

Mitigation of the risk of breach through reinforcement of the sea defences may reduce the risk of

tidal flooding to an acceptable level to achieve TAN15 compliance for development of this site. A

site specific FCA would be required to fully define the risk and the extent of any possible

mitigation measures.

Waste disposal is categorised as “highly vulnerable” within TAN15 and the planning authority

would need to justify such a use in the flood plain as described in section 7 of TAN15. Without

further details of the proposed site use under the general waste management description, the

proposed site use is assumed to be highly vulnerable. It is recommended that the site use will

need further discussion with the Environment Agency to determine if the proposed use is treated

as highly or less vulnerable development under the terms defined in TAN15.

3.3.2 Study Site 22LGRM

This previously undeveloped land is known as Trowbridge Mawr. The proposed land use has

been changed since the original SFCA to include industrial uses as well as residential.

Tidal inundation

For the 2D tidal inundation modelling, the rail line is assumed to provide some protection from

tidal inundation waters for those areas to its north. This assumption would need to be confirmed

through structural assessments and ground investigation as part of any development proposals

at this site. The 2D tidal inundation modelling results, which assume that the railway provides an

effective barrier to tidal inundation, are described below.

/5097656-70-DG017-3.docx 23

Current situation

Undeveloped land at Trowbridge Mawr is estimated to be flood free for a current 0.5% (1 in

200) annual chance tidal event

Currently the site is indicated to be almost entirely flood free for 0.1% (1 in 1000) annual

chance flood events. A small area at the southern extent of the site would be inundated to

depths and velocities less than 0.6m and 0.15m/s, respectively.


The topography of the site is such that flood extents during 0.5% (1 in 200) annual chance

events for 2085 and 2110 are not significantly different in relation to the overall site area.

Approximately two-thirds of the site would be flooded by a 0.5% (1 in 200) annual chance

event and so fails A1.14 of TAN15. The north eastern part of the site would remain flood

free.

Flood free access/ egress would be possible from the north.

Fluvial flood risk

For a fluvial event, based on volumetric filling of the flood plain, analysis indicates there is

adequate storage capacity to ensure that a 0.1% (1 in 1000) annual chance rainfall event would

not impact the site. Local conditions will need to be assessed to determine measures required to

ensure drainage to the reen system remains sufficient to convey any flood flows. A number of

culverts under the rail line to the south will need to be assessed for condition, conveyance

capacity and risk of blockage.

Other sources of flood risk

As noted in Section 3.5.3 of the SFCA, the site has a history of high groundwater levels causing

ponding on the fields to depths of up to 0.3m. This source will need to be investigated further as

part of any site specific FCA. It is considered likely that the risk from this source could be

managed. Use of filter drains and other ground water management methods, along with options

to raise the elevation of developed areas of the site and make a feature of ponds, should be

considered within the site specific FCA.

Site summary

For the reasons described in Appendix D, the 2D modelling takes account of the characteristics

of the overland flow paths from the coastal defences to the study sites. The results provide a

representation of the distribution of flood flows and depths through the Wentloog Levels.

Our analysis indicates that water is conveyed overland from the coast to the study site via the

culverts under the railway embankment. By 2085, a 0.5% (1 in 200) tidal event would flood the

majority of the site. This area of the site fails to comply with A1.14 of TAN15 for all development

types, based on the frequency of flooding.

Although the majority of the site has been assigned a medium surface water risk score, it is

considered likely that surface water runoff can be accommodated in the existing drainage

system. The use of SuDS in the design of any development should be adopted to ensure that

runoff from the site does not adversely affect flood risk elsewhere.

Without mitigation of the tidal flood risk, the majority of the site fails to comply with TAN15

guidance. Access routes to the site which remain operational under all conditions should be

achievable.

Mitigation of the flood risks for at least part of the site over a 100 year period to 2110 should be

achievable and will need to be addressed by any site specific FCA. It is unlikely that the whole

site area will meet TAN5 acceptability criteria. The following options could be considered to

mitigate the flood risk to parts of the site and so increase the proportion of the site that meets the

TAN15 acceptability criteria:

/5097656-70-DG017-3.docx 24

Ground raising to reduce flooding to site areas. This would require compensatory ground

lowering in other appropriate areas, ensuring a level for level compensation;

Reducing the probability of breach through reinforcement of the coastal defences (likely to

require a multi agency strategic approach);

Provision of groundwater drainage measures; and

Provision of detailed flood management plan including flood warning system and emergency

evacuation procedures.

It is reiterated that TAN15 requires that mitigation measures must not adversely impact on flood

risk elsewhere.

Without a strategic approach to future flood risk management of the Wentloog Levels, in line with

the proposals included in the Environment Agency‟s Severn Estuary Flood Risk Management

Strategy, it is likely that approximately one third of the site area will comply with the indicative

acceptance criteria in TAN15.

3.3.3 Study Site 21LGR

This study site is known as Areas 9-12, St Mellons and is situated north of the rail line. The site

is proposed for housing and ancillary services.

Tidal inundation

Current Situation 2010

Currently flood free for current 0.1% (1 in 1000) annual chance events.

Considering climate change to 2110

2D tidal inundation modelling results indicates that the site is flood free for all events tested

including the future 0.5% (1 in 200) annual chance events through to 2110.


The flood risk from surface water and fluvial flooding is not anticipated to be significant. The

sites are situated at the bottom of sloping land which may contribute some runoff from the

surrounding developed areas although surface water runoff is likely to be managed in these

areas already.

The potential risk from groundwater should be clarified through a site specific FCA. As described

in Section 3.3.2, the adjacent Site 22 has a history of groundwater flooding. However, it is

considered likely that the risk from this source can be mitigated.

Site summary

No significant problems are anticipated for the development of this site with respect to TAN15

indicative guidance.

/5097656-70-DG017-3.docx 25


This site is understood to have extant planning permission. Any future issuing or renewal of

planning consent will need to reflect the management of the consequences of flooding in line

with current planning policy guidance.

This land between Crickhowell Road and Willowbrook Drive is proposed for residential and

ancillary services. Although the southern extent of the site lies within TAN15 Zone C1, our

assessment based on 2D modelling indicates that this land is situated outside the current 0.1%

(1 in 1000) annual chance tidal event flood extent. It is also predicted to remain flood free during

0.5% (1 in 200) annual chance events over a development lifetime of 100 years.

The site slopes from some 13mAOD in the north to 7.5mAOD in the south. The local topography

does not suggest any significant residual risk from the stream running through the site.

It is anticipated that housing development may be directed away from low end of the site where

runoff from the steep slope may cause local transient ponding. It may be possible to mitigate this

risk through the drainage design for any proposed development. Mitigation of any residual flood

risks for the site as a whole should be achievable. The site has a medium risk score for surface

water runoff.

Site summary

It is considered unlikely that there would be any problems with developing the site in line with

TAN15 indicative acceptability criteria, providing a site specific FCA gives adequate

consideration to the management of surface water runoff.


This site, at the East of Cypress Drive, lies in of TAN15 Zone C1.


The 2D tidal inundation modelling results indicate that site would be flood free for the current

0.1% (1 in 1000) annual chance tidal inundation event.


The modelling indicates that it is likely that the site would remain flood free during 0.5% (1 in

200) annual chance tidal to 2085.


During a 0.5% (1 in 200) annual chance event in 2110, a significant proportion of the site would

be subject to flooding in low lying areas in the east and the south of the site.

Mitigation

It is anticipated that the risk to parts of the flooded areas could potentially be mitigated by raising

ground levels and so increase the proportion of the site which might be developable in line with

TAN15. Some land would need to be used to provide compensatory flood storage for any areas

raised out of the flood plain. Therefore, even with mitigation, it is unlikely that all the site will

comply with the TAN15 indicative acceptance criteria.

A site specific FCA would be necessary to confirm that the mitigation would not create adverse

flooding impacts elsewhere.

Flood free access/ egress would be possible via Cypress Drive from the north.

Other sources

A reen runs around the eastern boundary of the site. An hydraulic assessment of this channel

during fluvial flood events will need to be undertaken as part of any site specific FCA. Similarly,

the site’s low elevation compared with the land to the west might make it susceptible to surface

/5097656-70-DG017-3.docx 26

runoff issues from existing residential areas and highways and this risk should be investigated

further within the FCA.

There are a number of flooding incidents to the north of this site recorded on DCWW’s STAM

database over a number of years (see SFCA Section 3.5.1). Whilst the potential risk to the site is

considered low, it is unclear if reported incidents indicate any under capacity in the sewerage

network which may need addressing. A site specific FCA should confirm the level of the risk and

impacts on the development of the site.

Site summary

It is likely that considering a 100 year period to 2110, mitigation of flood risk to the majority of the

site area should be achievable in line with the TAN15 indicative guidance for residential

development of this site.

3.3.6 Study Site 74LGEMW

Land at Wentloog south of the rail line has been identified for employment and waste

management, the latter of these being classed as “highly vulnerable development” within TAN15.

The site lies in TAN15 Zone C1.

Tidal inundation

The following is a summary of the 2D tidal inundation modelling results.

Current situation

The site would remain flood free during a 0.5% (1 in 200) annual chance tidal event.

The majority of the site would be flooded during a 0.1% (1 in 1000) annual chance tidal

event. Flood depths would generally be below 0.6m although some localised areas around

existing drainage channels would subjected to depths up to 1.0m. Flow velocities would be

less than 0.15m/s over much of the flooded area with velocities up to 0.3m/s in the south

eastern corner of the site.

An access/ egress route could be established to the west via Wentloog Avenue.


The site would be completely inundated during a 0.5% (1 in 200) annual chance tidal event

and without mitigation would fail A1.14 of TAN15.

Access/ egress routes to the west of the site via Wentloog Avenue would be completely

inundated by 0.5% (1 in 200) event by 2085.

Other sources of flooding

No other significant flood risks have been identified for the site.

Site summary

Mitigation of the risk of tidal breach through reinforcement of the coastal defences would likely

achieve TAN15 compliance at this site. As stated previously, this would most likely be

undertaken as part of a broader effort to facilitate development of the Wentloog Levels,

consistent with the proposals described in the Environment Agency„s Severn Estuary Flood Risk

Management Strategy.

Without mitigation the site fails TAN15 for all development types.

It is possible that without strategic reinforcement of the coastal defences that TAN15 criteria may

be met for some small areas of the site through raising of ground levels. This would require

lowering of levels elsewhere to compensate for loss of flood storage. However, the inundation of

access routes would remain a problem (as described in section 3.3.0). Further site specific

assessment would be required to consider the viability of managing the risk to the access route

/5097656-70-DG017-3.docx 27

through a flood management plan, including flooding warning and emergency evacuation

procedures.

3.3.7 Study Site 2LGRM (formerly Site 77)

Site 2 runs approximately north to south around the eastern extent of the existing St Mellons

Business Park. The entire site lies within TAN15 Zone C1. The southern end of the site is low

lying (below 4.8mAOD) compared with the general ground elevation in the Wentloog Levels.

Ground elevations in the northern area of the site are above 6.5mAOD.

Tidal inundation

The rail line has been assumed to provide an effective defence against tidal inundation for areas

to its north. This would need to be confirmed by site specific FCA.

The following is a summary of the 2D tidal modelling results.

Current situation

The site is predicted to be flood free during a current 0.1% (1 in 1000) annual chance tidal

event.


Approximately 25% of the site would be flooded during a 0.5% (1 in 200) annual chance

tidal event. The northern part of the site would remain flood free.

In the event of a flood, the existing access via Heol Las would be cut off. A new access/

egress route to the flood free portion of the site would need to be provided from the north or

west.



tidal event.

A new access/ egress route to the flood free portion of the site would need to be established

from the north or west.

Fluvial flood risk

A drainage channel to the reen system forms the eastern boundary of the site. The broad scale

assessment of rainfall volumes in this study suggests the potential for runoff during a 0.1% (1 in

1000) annual chance event will exceed the storage capacity of the reens. Flooding of low lying

areas of the site can be expected. Existing information is insufficient to confirm compliance or

otherwise with TAN15, although it is anticipated that the reens could be managed to facilitate

development within the northern part of the site. Detailed hydrological and hydraulic assessment

of the drainage in this area will be required as part of a site specific FCA.


A high groundwater table, identified as a potential residual risk in Study Site 22 at Trowbridge

Mawr may also affect this low lying site. This will also need to be addressed as part of any site

specific FCA. There are no significant sources of runoff adjacent to the site.

The risk associated with recorded incidents of sewer flooding, including Pascal Close, as

described in Section 3.5.1 (of the SFCA), will also need to be addressed by any site specific

FCA.

Site summary

The risk posed by the existing drainage channel through the area will need to be understood in

more detail. Options for mitigation of this risk will need to be explored as part of a site specific

FCA. However, it is considered likely that this risk can be mitigated.

/5097656-70-DG017-3.docx 28

The tidal modelling indicates that approximately 60% of the site would meet TAN15 indicative

acceptability criteria for commercial development. A smaller proportion, approximately 50%, of

the site would meet the criteria for residential development which must consider a 100 year

development lifetime.

Access and egress routes are likely to be available via the flood free northern part of the site.

It is likely that if a strategic approach to future flood risk management of the Wentloog Levels, in

line with the proposals included in the Environment Agency‟s Severn Estuary Flood Risk

Management Strategy, then the majority of the site would comply with the indicative acceptance

criteria in TAN15 for all development types.


This site is being considered for residential, commercial and industrial development.

The entire extent of the site lies within TAN15 Zone C1. The land is low lying compared with the

rest of the Wentloog Levels, and is bounded in the north by the railway line.

Tidal inundation


Current situation

The majority of the site is predicted to remain flood free during a current 0.5% (1 in 200)

annual chance event.

During a 0.1% (1 in 1000) annual chance event, the entire site would be flooded entirely.

Flood depths over the majority of the site would remain below 0.6m with depths reaching up

to 1.0m along the south eastern boundary of the site. Flow velocities over the south eastern

half of the site would exceed 0.15m/s but would remain below 0.3m/s.


The modelling predicts flooding of the entire site during a 0.5% (1 in 200) annual chance

event in 2085, which fails TAN15 A1.14 for all development types based on existing

Environment Agency guidance.

It would be difficult to provide flood free access/ egress to the site as modelling results show

inundation of Heol Las along routes from the north and south.


As for 2085, the model results show complete inundation of the site and access/ egress

routes via Heol Las.

Fluvial flood risk

The site is likely to be flood free during a 1% (1 in 100) chance fluvial event (for a 24 hr storm)

based on simple volumetric filling of the floodplain.

However, the hydraulic capacity and risk associated with the Green Lane Reen which runs

parallel to Heol Las at the eastern boundary of the site will need to be assessed by any site

specific FCA.


A high groundwater table, identified as a potential residual risk for Study Site 22 at Trowbridge

Mawr, may also affect these low lying sites. This will also need to be addressed as part of any

site specific FCA.

The risk associated with the recorded incidents of sewer flooding as described in Section 3.5.1

(of the SFCA) will also need to be addressed by any site specific FCA.

/5097656-70-DG017-3.docx 29

Site summary

The site is predicted to be completely inundated by a 0.5% (1 in 200) annual chance tidal event

considering both 75 and 100 year periods.

Without a strategic option to maintain the current standard of service of the Severn Estuary

defences into the future, it is unlikely that this risk could be mitigated to provide compliance with

TAN15 indicative acceptability criteria for any of the development types proposed.

It is likely that a strategic approach to future flood risk management of the Wentloog Levels, in

line with the proposals included in the Environment Agency‟s Severn Estuary Flood Risk

Management Strategy, would result in the majority of the site complying with the indicative

acceptance criteria in TAN15 for all development types.

3.3.9 Study Site 106LGEM

Industrial and retail uses have been proposed for this site.

Site 106 lies within TAN15 Zone C1. The site is composed of two parts, one north and one south

of the railway embankment. Both parts of the site are low lying compared with the rest of the

Wentloog Levels.

Tidal inundation

It should be noted that the rail line has been assumed to provide an effective defence against

tidal inundation for areas to its north. This would need to be confirmed by a site specific FCA.


Current Situation

The great majority of the site is currently flood free for the 0.5% (1 in 200) annual chance

tidal event. Some limited localised flooding is indicated for around drainage channels in the

very south of the site.

During a 0.1% (1 in 1000) annual chance event, the majority of the site south of the rail line

would be flooded. Approximately 10% of the part of the site north of the rail line would be

inundated. Flood depths remain below 0.6m, and flow velocities would be less than 0.15m/s

over the majority of the flooded areas.


The site south of the rail line would be completely inundated during a 0.5% (1 in 200) annual

chance tidal event.

During the same event, approximately two-thirds of the site area north of the rail line would

be flooded.

In the event of a flood, an access/ egress route to the northern part of the site that meets the

indicative acceptance criteria detailed in TAN15 would be available via Fortran Road. .

Fluvial flood risk

The entire area of this site area is likely to be flood free during a 1% (1 in 100) chance fluvial

event (for a 24 hr storm) based on simple volumetric filling of the floodplain.

However, the hydraulic capacity and risk associated with the reen running along the western

boundary of the northern part of the site will need to be assessed by any site specific FCA.


A high groundwater table, identified as a potential residual risk in Study Site 22 at Trowbridge

Mawr may also affect this low lying site. This will also need to be addressed as part of any site

specific FCA.

The risk associated with the recorded incidents of sewer flooding as described in Section 3.5.1

(of the SFCA) will also need to be addressed by any site specific FCA.

/5097656-70-DG017-3.docx 30

Site summary

A large portion of the site north of the rail line and all of the site area south of the rail line are

predicted to be inundated by a 0.5% (1 in 200) annual chance tidal event by 2085. Without

mitigation, these areas fail A1.14 of TAN15.

The remaining portion of the site will be flood free during an event of this probability and comply

with TAN15 acceptability criteria and Environment Agency guidance for non residential

development.

It may be possible that mitigation measures could result in more of the site being flood free over

the development lifetime. A site specific FCA will be necessary to investigate the flood risk

posed by the reen draining along the western boundary of the site and to suggest measures for

mitigating this risk.

It is recommended that site specific FCAs consider the viability of the following mitigation

options:

Ground raising to reduce flooding to site areas with compensatory ground lowering in other

areas;

Reducing the probability of breach through reinforcement of the coastal defences;

Culvert replacement/improvement;

Provision of groundwater drainage measures; and

Provision of detailed flood management plan including flood warning system and emergency


To achieve any significant increase in the area complying with TAN15, a strategic scheme to

reinforce the Severn Estuary defences is likely to be required consistent with proposals

contained in the Environment Agency‟s Severn Estuary Flood Risk Management Strategy.

3.3.10 Study Site GTechPk

The Green Technology Park site is bounded to the north by the main London to Swansea rail line

and to the south by Wentloog Avenue. It lies within TAN15 Zone C1 and is almost 1km from the

coastal frontage.

The proposed use for this site is employment.

Tidal inundation

The 2D tidal inundation modelling results are described below.

Current situation

The site is flood free for a 0.5% (1 in 200) annual chance tidal flood event.

Modelling indicates that a 0.1% (1 in 1000) annual chance event would flood 70% of the

site‟s area, with maximum depths of flooding being less than 0.6m over most of this area.

Some deeper flooding is associated with existing drainage channels. Flood flow velocities

would generally remain below 0.15m/s.

An existing access route from Trowbridge Road over the rail line could provide access to the

northern, flood free portion of the site.

The main access/ egress route via Wentloog Avenue is flood free for the great majority of its

length during a 0.1% (1 in 1000) annual chance event. However, high velocities and depths

exceeding TAN15 indicative criteria affect parts of this route. It is considered likely that

mitigation of this risk on these short sections of road could be achievable through the raising

of road levels and alterations to culvert arrangements under the road. The feasibility of

such measures would need to be explored as part of any site specific FCA.

/5097656-70-DG017-3.docx 31


The majority of the site area is flooded by a 0.5% (1 in 200) annual chance tidal event. Only

a small area at the northern edge of the site would remain flood free.

By 2085, as indicated in Section 3.3.0, the main off site access/egress route via Wenloog

Avenue will not be passable during 0.5%(1 in 200) events. Reliance on this route alone

would fail TAN15 A1.12.

An access/ egress route for this site could be possible north via the existing rail line

overpass to Trowbridge Road. This would allow access to the site under all flood conditions

and so comply with TAN15 A1.12. The viability of this route would need to be confirmed as

part of a site specific FCA.

Fluvial flood risk

The site is not considered to be at risk from fluvial flooding.

Other sources

A site specific FCA will need to give adequate consideration to the management of surface water

runoff, although the risk from this source is considered to be low.

Site summary

Our analysis indicates that the site is impacted by the combined effects of overland flows from

the breach locations southwest and east of the site (Referenced SR1-0 and SR1-2 on Drawing

5097656/ RCF/ 120), Appendix B.

The site can be broadly divided into northern and southern areas based on compliance with or

failure to meet TAN15 indicative acceptability criteria.

The small area towards the northern edge of the site lies outside the 0.5% (1 in 200) tidal flood

extent over the 75 year period to 2085. Development of this part of the site should be achievable

in compliance with TAN15. This assumes that safe access and egress route can be

demonstrated to be maintained in the event of a flood.

However, the great majority of the site will be within the 0.5% (1 in 200) annual chance tidal flood

boundary by 2085 and so without mitigation would fail A1.14 of TAN15. It may be possible to

achieve compliance for at least part of this area through ground raising and compensatory

lowering in other parts. This would need to be achieved without affecting local overland flow

paths to avoid increasing flood risk elsewhere.

The strategic reinforcement of the Severn Estuary defences proposed as part of the Environment

Agency‟s Severn Estuary Flood Risk Management Strategy would provide mitigation of the tidal

risk and would increase developable area. This would require a strategic option to defend the

existing properties in the Wentloog Levels in general.

3.3.11 Study Site Wentloo_ii

This study site has been identified for potential commercial and industrial development and lies

south of the rail line and east of the proposed Green Technology Park to the west of study site

Wentloo_iii. The site is located within TAN15 Zone C2. The site boundary is 800m from its

closest point to the location of the coastal frontage assessed as at risk from breach.

Tidal inundation


Current situation

The site is predicted to be flood free during a 0.5% (1 in 200) annual chance tidal event.

A small localised area of the site would be flooded during a 0.1% (1 in 1000) annual chance

event. Depths and velocities would remain within TAN15 indicative acceptance criteria

/5097656-70-DG017-3.docx 32



exceeding TAN15 indicative criteria affect short sections of this route. It is considered likely

that mitigation of this risk on these short sections of road could be achievable through the

raising of road levels and alterations to culvert arrangements under the road. The feasibility

of such measures would need to be explored as part of any site specific FCA.


Approximately two-thirds of the site would be flooded during a 0.5% (1 in 200) annual

chance tidal event. The flood free areas would occur as isolated islands scattered across

the site.

As described for Site 74, existing access/ egress route to the west of the site via Wentloog

Avenue will be completely inundated by a 0.5% (1 in 200).

A potential emergency access/ egress route to flood free land may be available via an

existing rail overpass to Trowbridge Road, located north of the Site GTchPk.. This may

require crossing third party land.


Flood risk to the site from other sources is considered to be low.

Parts of the site are indicated to have a high risk score from as a result of the surface water

runoff screening. Although the area is fairly flat, it is heavily developed for industrial use and has

large areas of hard-standing which can generate significant runoff. However, it is considered

that the management of surface runoff through SuDS should be achievable for any new

development.

Site summary

The majority of the site fails to comply with TAN15 guidance. The main access and egress

routes to those parts of the site which remain flood free for 0.5% (1 in 200) chance events to

2085 would be flooded. It is possible that alternative access may be possible via an existing rail

overpass to Trowbridge Road (Figure 3-2). The viability of this route will need confirming. It is

likely that without mitigation the route to this overpass would not remain operational under all

flood conditions and so fail A1.12 of TAN15. Access and egress in line with TAN15 may prove

problematic.

If the access problems can be overcome TAN15 criteria may be met for parts of the site

considering the following measures to mitigate the risk.

Ground level raising with lowering of levels elsewhere to compensate for loss of flood

storage , may achieve some increase in site areas flood for a 0.5%(1 in 200) event by 2085.

All options to manage the risk of flooding should consider a flood management plan within

the site operation plan, which would include a flood warning system and emergency


A new crossing of the rail line could be considered or formalised access to the location of

the existing rail overpass to Trowbridge Road. This would provide emergency access and

egress in the event of a flood event.

A strategic approach to future flood risk management of the Wentloog Levels, in line with the

proposals included in the Environment Agency‟s Severn Estuary Flood Risk Management

Strategy, would result in the majority of the site complying with the indicative acceptance criteria

in TAN15 for all development types.

/5097656-70-DG017-3.docx 33

3.3.12 Study Site Wentloo_iii

This study site is identified for potential commercial and industrial development. It lies south of

the rail line and east of the Wentloo_ii study site and is within TAN15 Zone C1.

Tidal inundation


Current situation

The site is predicted to be flood free during a 0.5% (1 in 200) annual chance tidal event.


event. Flood depths and velocities would remain below 0.6m and 0.15m/s.



exceeding TAN15 indicative criteria affect short sections of this route. It is considered likely

that mitigation of this risk on these short sections of road could be achievable through the

raising of road levels and alterations to culvert arrangements under the road. The feasibility

of such measures would need to be explored as part of any site specific FCA.


The majority of the site would be flooded during a 0.5% (1 in 200) annual chance tidal event.

Without mitigation this fails TAN15 A1.14.

As described in 3.3.0, it is unlikely that off site access and egress via the main routes via

Wentloog Avenue would remain operation under all flood conditions as required

under A1.12 of TAN15.


Flood risk to the site from other sources is considered to be low.

Parts of the site are indicated to have a high risk score from as a result of the surface water

runoff screening. Although the area is fairly flat, it is heavily developed for industrial use and has

large areas of hard-standing which can generate significant runoff. However, it is considered

that the management of surface runoff through SuDS should be achievable for any new

development.

Site summary

With the exception of small islands of flood free areas, the great majority of the site is flooded by

a 0.5% (1 in 200) annual chance event by 2085 and so fails A1.14 of TAN15.

The remaining proportion of the site area which may be developable in line with TAN15 may be

increased through mitigation works. Such works may include raising parts of the site with

compensatory lowering of others. This viability of such measures would need further site specific

assessment. It is likely that the majority of the site would not meet TAN15 indicative acceptability

criteria following mitigation. Access in line with TAN15 A1.12, which requires access and egress

routes for a site remain operational under all conditions, will prove problematic.

Offsite access and egress cannot be demonstrated to be operational under all flood conditions

by 2085. The use of Wentloog Avenue for access/ egress during a flood would be very unlikely,

even for emergency services based on the hazard rating as described in 3.3.0. Alternative

access may be possible via the existing rail overpass to Trowbridge Road, west of the site. This

will require mitigation of the access over approximate 160m between Site Wentloo_ii and Site

GtchPk. This mitigation would most likely require a raised flood free path parallel to the rail line.

Overall, development of even parts of this site in line with TAN15 is likely to be problematic due

to the tidal flood risk.

/5097656-70-DG017-3.docx 34

A strategic approach to future flood risk management of the Wentloog Levels, in line with the

proposals included in the Environment Agency‟s Severn Estuary Flood Risk Management

Strategy, would result in the majority of the site complying with the indicative acceptance criteria

in TAN15 for all development types.

3.3.13 Study Site 3SBR

This study site at Pill Du Farm, Hendre Lane, which is identified for potential residential

development, has not previously been assessed as part of any earlier SFCA.

It lies between the Pill Du Reen to the north and the main railway line to the south in the central

part of the Area A. This site is within TAN15 Zone C1.

Figure 3-3 - Site 3SBR location relative to other candidate sites and railway line

Tidal inundation


Current situation

The site is flood free for 0.1% (1 in 1000) annual chance tidal breach events.


The site is flood free for 0.5% (1 in 200) annual chance tidal breach events and is within TAN15

indicative acceptability criteria and Environment Agency guidance for non residential

developments.


By 2110, the modelling indicates that tidal breach events of 0.5% (1 in 200) annual chance would

result in inundation of almost the entire site area and the access routes. The site therefore fails

to meet TAN15 A1.14 for residential development.

Depths of flooding are likely to exceed 0.6m on the access road to the site during a 0.5% (1 in

200) annual chance event.

/5097656-70-DG017-3.docx 35

Mitigation of the tidal risk is likely to require works off site. Raising parts of the site may be

possible. However, without further site specific assessment, it is unclear if compensatory

storage could be achieved within the site boundary.

Other Sources

The site‟s location at the bottom of gentle south sloping land may suggest some susceptibility to

surface runoff from surrounding areas. The surface water flood maps for the area also indicate

this potential for the area surrounding the site. However, the majority of the lies above the area

indicated on the Environment Agency Map of Area Susceptible to Surface Water Flooding

(AStSWF). It is likely that surface water runoff from the north is managed in that developed

areas. The location of the reen immediately north of the site would also serve to intercept runoff

from the north. Management of surface water runoff onto the site will need to be considered as

part of any site specific study, but it is considered likely that this risk could be mitigated. The

flood risk from surface water and fluvial flooding is not anticipated to be significant.

The potential risk from groundwater should be clarified through a site specific FCA. As described

in Section 3.3.2, the adjacent Site 22 has a history of groundwater flooding. However, it is

considered likely that the risk from this source can be mitigated

Site summary

Whilst it is likely that development of the site for non-residential purposes could be achieved, the

proposed development of the site for residential development would require mitigation of the tidal

flood risk over a development lifetime to 2110.

It is likely that mitigation of the risk to the site could be achieved through raising parts of the site.

However, it is unclear if there is sufficient area available on the site to provide sufficient

compensate for the resulting loss of flood storage. Access to the site may

If a strategic approach to future flood risk management of the Wentloog Levels were to be

promoted, in line with the proposals included in the Environment Agency‟s Severn Estuary Flood

Risk Management Strategy, it is likely that the majority of the site would comply with the

indicative acceptance criteria in TAN15 for all development types.


Trowbridge Road Cardiff, proposed for residential use, lies in TAN15 zone C1. It is located

between Site 21LGR to its east (shown on Figure 3.4, above) and Site 22LGRM to its west.

Trowbridge Road forms the western boundary of the site.

Tidal inundation

Current situation

The modelling indicates that the site lies outside the current 0.1% (1 in 1000) annual chance

flood outline.


The site is not indicated to be affected by tidal breach events of 0.5%(1 in 200) annual chance by

2110.

Other Sources

The site is low lying relative to surrounding areas to its north and northwest. Approximately 30%

of the site is indicated to be potentially at risk of surface water flooding on the Environment

Agency Map of Areas Susceptible to Surface Water Flooding (AStSWF). The ground contours in

the area suggest potential overland flow paths from the developed areas via open space to the

north of the site. It is likely that surface water on Trowbridge Road on the west boundary of the

site will bypass the site.

Mitigation of the risk is likely through the site layout and vertical alignments of the distributor

roads to ensure that overland flows do no impact on existing or new development.

/5097656-70-DG017-3.docx 36

Based on available information, we consider that it is likely that this risk can be managed within

the design of the development layout.

Surface runoff from the development site will need to be managed using appropriate sustainable

drainage systems (SuDS) to restrict runoff as required by section 8.5 of TAN15. If this is not

possible due to existing hydrological regime then attenuation systems will be required to reduce

the surface water discharge to at least Greenfield runoff.

Consideration will need to be given to any existing flow paths onto or away from the site

Site summary

The risk of tidal flooding to the site is considered very low.

Further site specific study will be required to define the risk of surface water flooding. It is

considered likely that the risk to this site can be managed in line with TAN15.

3.3.15 Study Site 56LBR

This site at St John‟s College, Greenway Road, proposed for residential use, lies in TAN15 zone

A, defined as little or no risk of flooding.

Site summary

It is considered likely that this site can be developed in compliance with TAN15. It is unlikely that

a site specific FCA would be required by the Environment Agency.

/5097656-70-DG017-3.docx 37

4. Area G – Junction 33 of M4 The sites in this area were assessed under the previous Phase 2 Report (November 2009). The

assessments presented in this chapter are based substantially on analysis undertaken at that

time. However, the Environment Agency has since released a new Flood Map for Surface

Water (FMSW). The FMSW represents the mechanisms that cause surface water flooding.

Where these maps indicate a risk of surface water flooding which is different to that previously

identified, this is noted. The site references have been changed to those in the 2010 list of

Candidate Sites for the LDP. Where the text previously reported in the 2009 report is repeated

here, it is italicised for clarity.

4.1 Site Description


Area G is mostly located to the North of the M4, at the north western edge of Cardiff, and to the

south of the village of Creigiau. The area is largely rural with a few main roads.

The area is fairly hilly, but the majority of slopes are not steep. There are two disused railway

lines in the area. The area is bounded to the west by the Nant Coslech River and the A4119

Llantrisant Road approximately bisects the area.


There are two candidate sites identified in the area. They cover areas of 37ha and 89ha

respectively and are on currently undeveloped land.

Site 102LGR borders the south of the existing settlement of Creigiau . Site 46LGRM lies

between the A4119 and the M4 and is the larger of the two sites. The south east of this site

borders the existing development of the services area at the M4, Junction 33. Both of the

candidate sites are located in rural areas away from any significant existing development.

The sites are shown on Figure 4.1 and site details summarised in Table 4-1 below.

Table 4-1 - Area G Candidate Sites Summary

Site ID Name/ Location Existing Land

Use Development Category/ Policy Type

Area(ha)

102 LGR Land South of Creigiau Agricultural Residential and Ancillary Community

37.64

46LGRM Junction 33 Agricultural Mixed Used Development

82.96

/5097656-70-DG017-3.docx 38

Figure 4-1 – Area G Study Site Locations

NANT GLASWG NANT HENSTAFF

NANT COSLECH

/5097656-70-DG017-3.docx 39


Rainfall from the upland hillsides enter a number of ordinary watercourses which have formed

around the village of Creigiau.

The most significant of these are the Nant Glaswg in the east, Nant Coslech in the west and the

Nant Henstaff which runs through Site 102GLR and skirts the western boundary of site 46LGRM.

The latter two watercourses combine before joining the River Ely. Nant y Glaswg joins the Nant

Dowlais before joining the River Ely further downstream.

All of these watercourses are culverted beneath roads and paths in the area, including the M4

motorway, at several locations.

The Environment Agency Flood Risk map shows that the area around the confluence of the Nant

Dowlais and Nant y Glaswg is at risk of flooding. This is not directly within any of the proposed

development sites.

The TAN15 Development Advice Map shows part of the Nant y Glaswg as being in TAN15 Zone

C2 and part of the Nant Henstaff is in TAN15 Zone B.

4.2 Data Review and Analysis

4.2.1 Information Available

A review of the available data has included the following:

TAN15 Development Advice Map;

EA Extreme Flood Outline Map;

DCWW STAM (Solution for Total Asset Management) Database; and

Light Detection and Ranging (LiDAR) data.

Environment Agency Flood Map of Surface Water (FMSW)

4.2.2 Analysis Approach

In Phase 1 of the SFCA all the sites in Area G were categorised as Medium risk due to potential

issues with culverted watercourses and known history of sewer flooding.

The sites fall outside of EA Flood Zone 1, and are in TAN15 Flood Zone A, so a strategic

assessment is considered appropriate at this stage to provide insight into the risks associated

with a development and guidance for the management of these risks.

The strategic assessment included the following:

Check for recorded sewer flooding.

Review of likely overland flow paths using LiDAR data.

Assessment of surface water runoff risk.

Site visit.

The site visit confirmed the initial assessment of medium risk for some parts of the sites, but also

showed that some parts of the sites could be re-assessed to Low risk. No sites were deemed to

be High risk.

4.3 Flood Risk from Main Rivers and Ordinary Watercourses


The EA flood maps do not show flooding for ordinary watercourses. However, some partial

modelling of the Nant Henstaff and the Nant y Glaswg has been carried out by the Waterman

/5097656-70-DG017-3.docx 40

Group for the International Business Park Environmental Statement. This shows the depths of

flooding that could occur in some parts of these watercourses.

Undertaking detailed modelling work to include the remaining sections of watercourse as part of

this study was not considered cost effective given the anticipated level of risk. Therefore, a

simple assessment of the flood risk areas was made with reference to the LiDAR contour data

and an assumed maximum rise in water levels within the watercourses of 3m. This was

considered a conservative estimate and such a rise would not be expected except in extreme

rainfall events concentrated on the watercourse’s catchment.

The combined analysis of existing data and estimated water level rises in the three watercourses

showed the following:

A rise of levels in the Nant Henstaff would affect small parts of the western end of Site 46LGRM.

Also the low central part of site 102LGR could become flooded.

Likewise, a rise in levels in the Nant y Glaswg could result in flooding to the eastern parts of site

46LGRM .

The areas affected are shown on Drawing Number 5097656/RCF/020/B, Appendix B.

4.4 Flood Risk from Sewers, Culverted Watercourses and

Groundwater


Candidate Site 102LGR gives cause for concern in this area.

The STAM database records that the wet well of the Ffordd Dinefwr Sewage Pumping Station

(SPS) regularly becomes surcharged during storm events, spilling via an outfall pipe to the

watercourse nearby. This indicates that the pumping station would be unable to accommodate

additional flows from the proposed development in the area, so alternative drainage

arrangements should be explored.

Near to this pumping station, are three surface water outfalls into the local watercourse which

serve the existing housing development. The head of the watercourse is within Candidate Site

102LGR and it was observed to be in poor condition at the points of discharge, with water

backing up, ponding and causing very wet ground conditions.

Flood reports are limited in this area, with only three isolated incidents reported. A repeat of one

of these, at Cardiff Road, could present a risk to Site 102LGR, as the topography would cause

overland flooding from Cardiff Road into the eastern end of the site.


There are 11 culverts that have been indentified within the area the blockage or undercapacity of

which could potentially result in flood risk to proposed development sites. Key culverts and

references used below are shown on Drawing Number 5097656/RCF/020/B.

A full investigation into the hydraulic capacity of the culverts and any regular maintenance

operations has not been undertaken. However, for the majority of the culverts identified it is not

unreasonable to except that due to their limited size blockage may occur during severe rainfall

events and a delay in clearing the blockage may result due to inaccessibility.

The Nant Henstaff is culverted at 4 locations in Area G: at Cardiff Road (H1), beneath the old

railway within Site 102LGR (H2), at the southern edge of Site 102LGR under the A4119 (H3) and

under the M4 at the edge of site 46LGRM (H4).

If culvert H2 beneath the old railway line becomes blocked, water in the Nant Henstaff could

back up and flood part of the eastern end of Site 102LGR, running alongside the railway

embankment. If culvert H3 beneath the A4119 became blocked, rather than flow across the

/5097656-70-DG017-3.docx 41

road, water could flow to the low point at the centre of Site 102LGR potentially resulting in

flooding to part of the site.

There is a risk of flooding to a small part of sites 46LGRM if culvert H4 becomes blocked causing

water to back up upstream.

The Nant y Glaswg is culverted at 3 locations potentially affecting site 46LGRM:

G2 - under the A4119 at the north eastern end of site 46LGRM,

G3 - under Heol Saint y Nyll

G4 under the M4 just to the east of Cardiff West motorway services.

Culvert G2 beneath the A4119 is fairly small. If it culvert blocks, floodwaters flowing across the

road from this location could affect a small area of 46LGRM before rejoining the watercourse.

Access to the culvert under Heol Saint y Nyll (G3) was not achieved during a site visit and its

size is not known. If it were to block a small strip of site46LGRM may become flooded. Water

overtopping the culvert would flow onto the road and rejoin the watercourse upstream of culvert

G4.

The M4 culvert (G4) is very large and unlikely to block. It is contained within a well defined

valley, so water would not flow far from the current route and no flood risk to study sites has

been identified from this culvert.


Susceptibility to groundwater flooding was not included as a part of this assessment. Risks

associated with groundwater should be investigated as part of site-specific FCAs.

4.5 Flood Risk from Surface Water Runoff

Surface water runoff was analysed using a number of different methods which sought to

complement each other and give a comprehensive overview of the flood risks. Following the

overview assessment of surface water runoff potential, included in Figure 4.2 below, LiDAR and

OS Mastermap data was reviewed to determine other potential flood risks and likely overland

flow directions resulting from any surface water runoff in the area. A site visit was undertaken to

confirm conclusions of the desk study.

In updating the assessments of these sites for 2010 list of candidate sites, for this current report,

the FMSW has been referred to. Where these maps indicate a risk of surface water flooding

which is different to that previously identified, this is noted in the site summaries below.

A significant number of sites are adjacent to already built-up residential areas and it is

anticipated that runoff from surrounding areas is already controlled by local surface water and

highway drainage systems. However the site areas are mainly rural undeveloped land and

without mitigation any development will result in increase runoff from the site itself. The flood risk

may be considered minimal providing an assessment of increased runoff rates from proposed

development is made and managed to avoid adverse impacts to both the new development and

existing properties in the area.

/5097656-70-DG017-3.docx 42

Figure 4-2 – Area G Surface Water Runoff Potential

5097656-70-DG017-3.docx 43

4.6 Summary of Flood Risk and Management


The land located to the South of the village of Creigiau lies adjacent to the Ffordd Dinefwr estate.

The site is currently rolling grassland with some minor water courses crossing it.

There is a risk of flooding from surface water runoff from the north, particularly around the centre

of the site as most flows would gravitate to this area, although the local watercourses are likely to

convey any surface flow away from the site. The western half of the site slopes away to higher

ground from its centre and therefore is not at risk from surface water flooding. The eastern tip of

the site has a small risk of some flooding from surface water runoff from the road.

A risk of flooding to the site from sewers has been identified. There are reports of the Ffordd

Dinefwr sewage pumping station surcharging regularly and spilling to the watercourse at the

centre of the site; this could affect the proposed development. A manhole in the road at the

eastern end of the site has also previously been surcharged. If this was repeated flow would be

likely to gravitate on to the site.

Three culverts have been identified on the site (H2, H3, and U2); there is a risk of some localised

flooding to small areas of the site if any of these were to become blocked. Due to the

topography it is likely the worst affected area would be the centre of the site as flows would

gravitate here and along the edge of the railway embankment as this would enable flows to pond

there. This correlates with the area indicated to be at risk on the FMSW.

The site specific FCAs will need to give adequate consideration to the management of sewer

overflows and flooding adjacent to the watercourses. Options to mitigate flood risk should also be

investigated within the site specific FCA, including:

Avoiding development within the central part of the site;

Designating areas deemed to be at risk of surface water flooding as storage/ attenuation

ponds; and

Improving the drainage channels and culverts on the site to be able to convey flows and

reduce the risk of blockage.

Overall, a moderate level of flood risk has been identified for this site. However successful risk

mitigation is anticipated to be possible and no significant difficulties are expected in the

development of these sites with respect to flood risk.

4.6.2 Study Sites 46LGRM

The area of Site 46LGRM is currently rolling grassland and woodland.

Throughout the majority of the area the risk of surface water flooding is considered low as there

are no significant areas where water would runoff onto the site without being intercepted by

watercourses. Parts of the sites are located at a lower elevation than the adjacent motorway

embankments. If the motorway drainage infrastructure was poorly maintained then runoff onto

these sites could result. However, the risk of this being the case is considered low.

Analysis indicates there is a significant risk of some areas of the site flooding from watercourses.

In the northwest of the site, an area upstream of a pond on a tributary stream of the Nant

Henstaff could flood. If the culverts on the Nant y Glaswg (G2, G3) became blocked, areas of

the site in the shallow valleys of the watercourse would flood, affecting small parts of the north

east and east of the site. A site-specific FCA will be required in order to identify options to

confirm and manage the risk of flooding from these watercourses.

However, the areas at risk of flooding from watercourses make up a very small proportion of the

total site and it is thought that the primary means of mitigating against flood risk is to avoid

development in those areas affected. In addition other measures such as flood defences, culvert

improvements and site landscaping could be investigated to reduce the risk.

5097656-70-DG017-3.docx 44

It is likely that as indicated on the FMSW that surface water flood risk is likely to be limited to the

areas immediately adjacent to the streams and watercourses which currently drain the site

Development of the site is anticipated to be possible within TAN15 guidance.

5097656-70-DG017-3.docx 45

5. Area H – Cardiff West Central, Taff &

Ely

5.1 Site Description


Area H has been defined by the extents of the flood plains of the River Taff and the River Ely

which reach upstream from Cardiff Bay to Llandaff Weir (NGR ST 153784) and the A4232 bridge

crossing (NGR ST 111762), respectively. The area covers significant residential areas of Cardiff

including Grangetown, Riverside, Canton and Ely. The town centre is located to the east of the

River Taff, and to the southwest are the industrial areas of Leckwith and Penarth Moors.

Land between the two rivers is mostly flat and lies at a level of less than 10m AOD. South and

west of the River Ely the ground rises and is less developed with significant areas of open fields

and woodland.

The rivers both flow southeast into Cardiff Bay. The freshwater bay was created by the

construction of Cardiff Bay Barrage, which was completed in 2001. The tidal effects on the rivers

are restricted by the water levels in the bay. The Bay level is controlled by sluice gates in the

barrage and typically the level is maintained at 4.5mAOD.


A significant number of development sites originally considered for the SFCA within the Taff/Ely

area are no longer candidate sites for the current Draft LDP. However, the current candidate

sites include some extensive new residential areas proposed on undeveloped land in Ely to the

west of the area. These sites are on land south of the river and border existing residential

developments. Another cluster of sites proposed for mixed uses are located in the Ely Bridge

area, a notable example is the large site of the former Arjo Wiggins Paper Mill.

Several large sites have been identified for development in the existing industrial/ commercial

areas of Leckwith and Grangetown north of the River Ely and generally south of the railway

which runs southeast to Penarth.

The proposed development sites are shown on Figure 5-1 and details of the sites are

summarised in Table 5-1 below.

5097656-70-DG017-3.docx 46

Figure 5-1 – Area H Study Site Locations

(EAW 0.1% or 1 in 1000 annual chance flood outline also shown in blue)


5097656-70-DG017-3.docx 47

Table 5-1 - Area H – Summary of Study Sites

(Details extracted from the Candidate Sites Register, 2010. Cardiff Council)


Development Category

Area (ha)

8LGR West of Clos y Cwarra, Michaelston-super-Ely, Cardiff

Grazing for horses Residential 1.6

16LBR Ely Bridge, Western Business Centre, Cowbridge Road East

Unused Residential/

commercial/ retail/ community uses

22.3

17LGR Ely Bridge Farm, Dyfrig Road, Ely.

Unused Previously used agriculturally

Residential 0.7

31LBEM Land at Brindley Road, Leckwith, Cardiff

Nil use Employment uses 8.0

32LGR Land at Michaelston Road, Cardiff

Combination of pasture and partly wooded. Former quarrying and tipping site

Residential & Strategic Open

Space 9.8

36LBR Former Lansdowne Hospital Site, Sanatorium Road, Canton, Cardiff, CF11 8PL

Redundant hospital buildings and ancillary offices

Residential 1.5

52LBR Paper Mill Road, Cardiff , CF11 8PH

Vacant warehouse premises

Residential 0.8

61LBRM Penarth Road Car Park Site (adjacent to Cardiff Central Railway Station) of which includes Riverside Sidings/Semaphore House

S.A. Brain & Co Ltd land: Brewery

Railway land and station car park

3.0

62LBR Land adjacent to Clive Lane Operational railway land

Railway land 2.9

70LBEM Central Square Bus station / highway

Commercial/ Public Transport

Interchange 1.5

75SBR Land East of Great House Farm, Michaelston Road

Curtilage of Great House Farm House

Residential 0.2

77LBR Ferry Road, Grangetown, Cardiff

Part of site in operational use for Wales and West Utilities, due to cease.

Residential 11.7

78LBRM Wholesale Fruit Centre Cardiff

Former Wholesale Fruit Centre, Retail Warehouse and small industrial estate.

Residential/ commercial/ employment

7.0

5097656-70-DG017-3.docx 48


Development Category

Area (ha)

83LBRM Former J R Freeman Factory, Penarth Road, Cardiff

Commercial unit – former J R Freeman cigar factory

Mixed use – Employment/

Retail/ Residential 3.8

96LGED Land adjacent to Llandaff Campus to river bank and rear of campus

Greenfield Educational 0.72

111LGR Land north west of Great House Farm, Michaelston Road

Nil Residential 0.5


The Taff and Ely area is relatively flat and is drained by the two main rivers. The River Taff has a

catchment area of 510 km2 upstream of Cardiff Bay, and the River Ely has a catchment area of

163 km2. A number of ordinary watercourses are present in the area, many of which are partly

culverted under the densely developed areas of the city.

Environment Agency Flood Risk and TAN15 Development Advice Maps show that a significant

area is at risk of flooding. However, a number of flood defences have been constructed along the

River Ely to provide protection from fluvial events, and the Cardiff Bay Barrage has provided

significant enhanced protection from tidal flooding. As a result the majority of the city centre area

is classified as TAN15 Zone C1 (see definition in Section 2.1).

5.2 Data Review and Analysis

5.2.1 Information Available

A review of the available data has included the following:

TAN15 Development Advice Map;

EA Extreme Flood Outline Map;

DCWW STAM (Solutions for Total Asset Management) database;

DCWW Cardiff West and Cardiff Central InfoWorks sewer model;

Environment Agency 2-dimensional Ely/ Taff model (developed post-Phase 1 as part of the

Ely TUFLOW model update, July 2009);

Light Detection and Ranging (LiDAR) data.

5.2.2 Analysis Approach

The majority of the candidate sites identified within Area H fall within the low-lying areas adjacent

to the main rivers.

Of the sites assessed in Phase 1 of the SFCA (see Section 2.2 and Section 1.3.1), 56 fell within

EA Flood Zone 2 and were categorised as High Risk. A more detailed level of assessment was

proposed to determine the flood risks for these sites compared to those identified as low or

medium risk during Phase 1.

2D hydraulic model of the combined lower Ely & Taff Rivers

The detailed assessment made use of the 1S-2D ISIS-TUFLOW Ely/ Taff model which had

previously been completed by Atkins for the Environment Agency, as detailed in Section 5.3.1.

The existing EA model deals largely with current-day flood scenarios; therefore, additional model

5097656-70-DG017-3.docx 49

runs were required to map future flood event scenarios to reflect the effects of climate change

and sea level rises.

The model has been further developed for the SFCA to improve the representation of the

variation in levels in the impounded Cardiff Bay and the effect of this on flood risk upstream on

the two rivers.

Sites to the west, in the Ely area, were considered to have a lower risk of flooding. Only one of

the four sites considered within Phase 1, Site 111 (Site Ref. No.72 in SFCA Phase 1 Report),

was specified as Medium Risk due to its proximity to the River Ely and uncertainty with respect to

watercourses and culverts in the area. The other three sites were defined as Low Risk. A

strategic high level assessment was considered sufficient to understand the risks associated with

development in this area and provide initial guidance for the management of the identified risks.

The strategic assessment overlapped with the detailed TUFLOW model and assessment

techniques were applied to all the sites on the perimeter of the area shown to be at risk of

flooding from the Rivers Taff and Ely.

The strategic high level assessment included the following:

Check for recorded sewer flooding;

Review of likely overland flow paths using LiDAR data;

Assessment of Surface Water Runoff Risk; and

Site visit.

5.3 Flood Risk from Main Rivers and Ordinary Watercourses

5.3.1 Flood Risk from Rivers Taff and Ely

Existing Information

Atkins Limited has previously completed on behalf of the Environment Agency8, a combined one

and two dimensional (1D/2D) model of the rivers Taff and Ely using TUFLOW9 modelling

software. The extents of the Lower Ely ISIS-TUFLOW model correspond closely with the

upstream limits of Area H for both the River Taff and the River Ely. Downstream, the model

terminates at the confluences of the rivers with Cardiff Bay. The operation of the barrage was

not included in the model.

Hydrological assessment of the 1% (1 in 100) annual chance event including climate change,

and the current 0.1% (1 in 1000) annual chance event for both the rivers was made as part of the

TUFLOW modelling project. Given the proximity of the two river catchments the simultaneous

occurrence of the extreme events on the catchments is not considered overly conservative.

At the downstream end of both watercourses, the Cardiff Bay Barrage maintains water levels at a

typical level of 4.5m AOD. However, the level within the Bay fluctuates at every tide due to

periods when flows from the Rivers Taff and Ely cannot discharge into the sea, i.e. when tides in

the Bristol Channel exceed a level of 4.5m AOD. Extreme tidal events result in longer tide-

locked periods and when combined with greater flows in the rivers, this can mean that water

levels in the Bay significantly exceed the typical control level.

Although analysis undertaken by HR Wallingford determined that there is no direct correlation

between the occurrence of extreme fluvial events and extreme tidal events10

, various

combinations of tide levels and river flows will result in increased bay levels. Data from this joint

8 River Ely and Tributaries Model Update and Hazard Mapping, Atkins Job Number 5075684, final report reference:

5075684/71/DG/004, July 2009. 9 Area H Update used TUFLOW version 2009-07-AF-iSP, BMT WBM Hydraulics, Australia

10

A new joint probability appraisal of flood risk, P.G. Samuels and N. Burt, published in the Proceedings of the Institution of Civil Engineers Water & Maritime Engineering Journal 154 Issue 2, June 2002.

5097656-70-DG017-3.docx 50

probability report was used to provide the downstream boundary levels (i.e. the levels

representing the levels in Cardiff Bay) for the events previously tested in the Environment

Agency TUFLOW model.

The downstream boundary within the Environment Agency ISIS-TUFLOW model was a fixed

water level derived from this joint probability analysis.

5.3.2 Estimation of the Variation in Cardiff Bay levels during flood events

The effects of future sea level rises on level in Cardiff Bay were considered within the HR

Wallingford analysis.

However, in consultation with the Environment Agency, it was determined that extrapolation of

some of the key assumptions in the paper, without access to the underlying analysis, may lead to

overly conservative estimates of future levels in Cardiff Bay. This would have the knock on effect

of potentially significant over estimation of future flood levels in the lower reaches of the rivers

Ely and Taff.

In addition, the use of a fixed level in the ISIS-TUFLOW hydraulic model may also lead to over

estimation of flood levels, since it effectively assumes a permanent tide lock over the period of

the flood in the rivers. Whilst this conservative approach may be appropriate for flood mapping

areas currently at risk of flooding, it is considered overly conservative in estimating future flood

risk over the 100 year development lifetime.

Model of Cardiff Bay and Sluice Gates

To estimate the variation in the level in Cardiff Bay for design flows in the rivers and coincident

tide levels in the Severn Estuary, a 1D hydraulic model of Cardiff Bay was built using ISIS v3.3

software. This model incorporates the Bay and the operation of the 5 sluice gates in the

Barrage. Fluvial inflows to the model where derived from the outflows at the bottom of the EA

ISIS-TUFLOW model referred to above.

These flood flows were run against a tide curve representing the Mean High Water Spring

(MHWS) tide level downstream of the barrage.

Design event data which had been used to calibrate a previous model of the Bay prepared to

develop the control logic of the gate operation was supplied by the Cardiff Barrage Manager,

together with recorded flood event data from 30 October 2000.

The model was adjusted and considered to give good agreement with this verification and

calibration data. The model under predicts the peak Bay level recorded during the October 2000

event by 120mm.

Figure 5-2 shows the change in Bay level predicted by the model with the actual recorded Bay

levels recorded during the October 2000 event. As well as the difference at the peak, the two

Bay levels lines on the graph demonstrate the characteristic rise and fall in the Bay level with

fluvial inflows and coincident tide levels in the Severn Estuary.

5097656-70-DG017-3.docx 51

Figure 5-2 - ISIS Bay Model - 30 October 2000 flood event

Modelled Bay level compared to recorded Bay level. for recorded fluvial inflows and tide level data

5.9

6.23

0

50

100

150

200

250

300

350

400

450

500

3.25

3.5

3.75

4

4.25

4.5

4.75

5

5.25

5.5

5.75

6

6.25

6.5

6.75

Flo

w/ c

um

ec

Wa

te

r le

ve

l/ m

AO

D

Time hrs

Recorded Sea Level

Recorded Bay Level (enraf)

Model Bay Level Result

Ely flow

Taff flow

The variation in Bay level was tested in the model for the scenarios as summarised in the

following table.

Table 5-2 - Flood event scenarios modelled to derive variations in Cardiff Bay levels

Fluvial Event

(Annual

chance)

Year Peak River

Taff Inflow

(cumec.)

Peak River Ely

Inflow

(cumec.)

MHWS Tide

level

(mAOD)

Resulting

Model Output

Peak Bay

Level (mAOD)

1% (1 in 100) 2010 463 271 6.1 6.550

0.1% (1 in 1000) 2010 1023 262 6.1 6.79

1% (1 in 100) 2085 831 310 6.74 7.25

1% (1 in 100) 2110 808 330 7.1 7.62

5.3.3 Final Flood Estimation on Taff and Ely based on the Variation in Cardiff Bay levels

The Iterative approach to running the two hydraulic models

The two models (the ISIS Bay Model and the Ely Taff TUFLWOW model) could theoretically be

combined. However, to minimise model development time the two models have been

maintained as separate entities.

However, the outflows from the TUFLOW model, which provide the inflows to the Bay model, will

be affected by the downstream boundary applied. The flat downstream boundary in the

TUFLOW model may result in the flows into the Bay being underestimated. To overcome this

problem the following strategy was adopted, based around iterative runs of the two models.

5097656-70-DG017-3.docx 52

Two iterations of the model runs were carried out for each scenario listed in Table 5-2:

1st Run of ISIS Bay model - The inflows to the Bay model for each scenario were derived

from the Ely Taff ISIS-TUFLOW model which had been run with a flat downstream boundary

(i.e. representing a constant water level in the Bay). These inflows were run against a

MHWS spring tide cycle (with climate change applied for the future scenarios).

1st iteration – runs of the TUFLOW model were undertaken using the varying downstream

boundaries derived from the 1st runs of the scenarios in the Bay model.

2nd

Run of ISIS Bay model - The outflows from the TUFLOW model each scenario were then

used as inflows to the Bay model for a second set of runs of each scenario.

2nd

iteration – run of the Ely –Taff Model. The resulting variations in the Bay level from the

1st iterations were smoothed and used as the varying downstream boundary for the final runs

of each scenario in the Ely Taff ISIS TUFLOW Model.

Results

It is the results of this second iteration of the Ely Taff ISIS-TUFLOW model using time varying

downstream boundaries in the Bay that are presented in Drawing No. 5097656/RCF/320 to 322,

Appendix B., which show the flood risk to the candidate sites.

The model results provide a good understanding of the flood depths and velocities likely to be

encountered and enable full assessment of the sites in line with TAN15 guidelines.

Drawing Number 5076243/RCF/320 shows the maximum flood extents for the1% (1 in 100)

annual chance event currently and including climate change allowances to 2085 and 2110.

Appendix 1.14 of TAN15 requires that development sites remain flood free for the 1% event over

the lifetime of the development.

For the 0.1% (1 in 1000) annual chance event indicative limits on the maximum depth and

velocity of flooding are given in Appendix A1.15 of TAN15 (see Section 2.1). Drawing numbers

5076243/RCF/321 and 322 show, respectively, the maximum flood depths and corresponding

velocities predicted by the Ely/ Taff ISIS-TUFLOW model for the this event for the current day as

required by current Environment Agency guidance.

Please note that the results from this model should not be used to assess flood risk to areas

other than the candidate sites listed in Table 5.1. Checks have been undertaken and

amendments made to the digital terrain model in the area of the sites to reflect physical features

which impact on overland flow paths. Similar assessments would need to be undertaken before

the model for the detailed assessment of the other sites.

Main Flood Mechanisms indicated by the modelling relative to the candidate sites.

The results indicate:

1% (1 in 100) annual chance event - current day

Flooding occurs initially from overtopping from the River Taff at Blackweir into Pontcanna Fields

and into Bute Park on the Left Bank. Flooding from the River Ely occurs at Ely Bridge flowing

down Lansdowne Road into the Victoria Park area.

1% (1 in 100) annual chance event - 2085

Simulations indicate that by 2085 a 1% (1 in 100) annual chance event would extend south from

Pontcanna via Llandaff and into the roads of Canton. Bute Park on the left Bank of the Taff

would be completely inundated. Direct overtopping of the right bank of the Taff into the Riverside

area would occur upstream of the Castle Street Bridge. Direct overtopping from the River Ely

would extend over the southern half of Site 16LBR.

The overland flows from the two rivers would not meet under this scenario.

5097656-70-DG017-3.docx 53

1% (1 in 100) annual chance event - 2110

By 2110 the increased levels in Cardiff Bay due to sea level rise in the Severn Estuary would

result in flooding extending further across some areas of Cardiff. Notable areas are Riverside on

the right bank of the Taff and increased extents around Victoria Park. Flooding from the Ely

would extend onto Site 36LBR. Some overtopping of the left bank of the Taff south of the rail

station is also indicated for this scenario, although the nearest candidate site (Site 61LBRM)

would not be inundated.

0.1% (1 in 1000) annual chance event – current day

This event results in the most extensive flooding of the scenarios tested with overland flows from

the Taff and Ely meeting and interacting.

Simulations suggest that the flood extents include the whole of Pontcanna and Riverside, with

overland flows extending below the main railway line into Grangetown and parts of Leckwith

Moors.

Table C2 in Appendix C details the depths and areas of flooding for each study site during this

event.

Discussion of the individual sites is included in Section 5.7 below.


A number of ordinary watercourses flow into the main rivers. Of particular note is the Nant y Plac

on the western edge of Site 111LGR. This is a relatively minor watercourse and no hydraulic

model exists to accurately determine flood extents. However, the watercourse is located within a

narrow valley and the adjacent site area slopes steeply up away from both the Nant y Plac and

the River Ely to the northwest. If the Nant y Plac was to overtop its banks flow is predicted to be

contained within the narrow valley and quickly enter the main river; flooding would only affect the

very edge of the site, if at all.

At a strategic level, no other proposed development sites have been identified as at risk from

ordinary watercourses, although this will need to be confirmed within site-specific FCAs.

5.4 Flood Risk from Sewers, Culverted Watercourses and

Groundwater


The DCWW STAM database records one hundred and twelve DG5 sewer flooding incidents in

Area H.

Approximately 50% of these incidents occurred on 4th July 2001. The storm was assessed to

have an annual exceedance probability of 1.6% (1 in 60).

The majority of the reported incidents occurred in St Mary Street, Royal Arcade, Morgan Arcade

and Wyndham Arcade. No further developments are proposed in the vicinity of the retail

premises affected, but flooding was also reported at isolated properties at the southern end of

Clive Street. The nearest candidate sites are the proposed residential developments at Clive

Street and Ferry Road (sites 62 and 77). The potential adverse impact of these developments

on existing sewer flood risk should be investigated further as part of site specific assessments.

Flood incidents were reported on consecutive days at six properties on Station Terrace and six

properties in Redcliff Avenue. These are close to the extensive housing developments proposed

at the former Arjo Wiggins Paper Mill and adjacent sites (16, 52 and 36). Further investigations

are recommended when considering development in this area. Other reported incidents are

mostly located along the southwest border of Area H in Ely, and along the railway line heading

northwest.

5097656-70-DG017-3.docx 54

A number of isolated incidents are recorded, but these are either very old or not near proposed

development sites and are not considered to pose significant risks to the proposed

developments.


The Rivers Taff and Ely remain in open channel along their entire reach. A number of minor

watercourses flow into them at various points along their length and some of these are culverted

under existing residential and commercial properties, and under roadways. Of particular note are

the culverts under the A4232 which link drainage channels from Study Site 31LBEM to the River

Ely. The potential for flooding due to culvert blockage should be considered if development at the

site is to proceed.

No other culverted watercourses have been identified affecting proposed development sites.

However, should culverts be discovered, it is important to note that Environment Agency policy

generally does not allow development over existing culverts because inaccessibility can

exacerbate problems should they develop in the future.


Susceptibility to groundwater flooding was not included as part of this assessment. The Taff and

Ely CFMP states that groundwater flooding is not considered to be a significant issue within the

catchment.

It is noted that a large groundwater control scheme was introduced as part of the Cardiff Barrage

scheme. There is a groundwater control system built into the Millennium Stadium. There are

other similar schemes dotted throughout the low lying areas, designed to keep the groundwater

levels low.

It is not perceived that groundwater flooding would be a significant issue for the study sites.

However, risks associated with groundwater should be investigated as part of site-specific FCAs.

It is recommended that assessments should be made of additional control measures which may

be required for specific sites.

5.5 Flood Risk from Surface Water Runoff

Surface water runoff was analysed using a number of different methods which sought to

complement each other and give a comprehensive overview of the flood risks.

Initially a broad overview of risks was provided using a GIS technique; explanation of the

technique is included in Section 2.5.7. Figure 5-3 below shows the results of the surface water

risk screening analysis.

In addition to the above screening assessment a qualitative assessment of the likely flow paths

of surface water runoff was undertaken. This was based on a review of the contours derived

from LiDAR and using OS Mastermap to assess ground features which might serve to confirm

conclusions of the desk study; particular attention was given to those sites to the West of Area H

and those that were not considered at risk from flooding from the River Taff or River Ely.

5097656-70-DG017-3.docx 55

Figure 5-3 - Area H Map of Surface Water Runoff Potential Screening Score

110011LLBBEEMM

5097656-70-DG017-3.docx 56

A significant number of sites fall within built-up residential areas and it would normally be

anticipated that runoff within such areas is already controlled by local surface water and highway

drainage systems. Unfortunately, a comprehensive database of surface water flood incident does

not yet exist and, due to the historic piece-meal development of the drainage system in parts of

the city, there is a lack of understanding of the operation of surface water drainage systems and

the associated risk of flooding. In the absence of a more detailed investigation into the

management of surface water, assessment of the risk has been made on the basis of site

observations and standard assumptions.

Walkover surveys of the sites in Michaelston-super-Ely confirm the GIS assessment that minimal

risk of flooding from surface water runoff is anticipated for the majority of these sites providing

that surface water drainage facilities in existing upslope developed areas are of sufficient

capacity. The one exception to this is Site 32LGR, a full discussion of this site is included in

Section 5.6 below.


5.6.1 Site 8LGR, Site 75SBR and Site 111LGR

These sites at Great House Farm lie outside the defined TAN15 DAM Zone C2 and hydraulic

modelling indicates the sites are beyond the 0.1% event boundary. An ordinary watercourse

follows the boundary of Site 8 but is generally down slope from the site. Although the risk from

this source will need to be confirmed as part of a site specific study, it is considered likely that it

could be mitigated.

Flood risk from fluvial sources is considered to be low for these sites and would meet TAN15

guidelines for development.

The three proposed sites border an existing housing development situated on higher ground.

The drainage provision for the existing housing will need further investigation and any new

development in the area will need to consider the risk of surface water runoff and potential


Additionally, the runoff from the site itself will need to be managed to ensure no increased risk of

flooding elsewhere. TAN15, Section 8, requires that sustainable drainage systems (SuDS) are

considered for all new development. “Developers need to give good reason why SuDS could not

be implemented. If a conventional drainage system does not improve the status quo, then this

can be a valid reason for refusal of planning”.

Development of the site for housing in line with TAN15 requirements is likely. A site specific

assessment of the management of surface water for any development is recommended.

5.6.2 Site 32LGR

This site is located to the east of Michaelston Road, opposite sites 75 and 111. Although initially

no flood risk was identified from the main watercourses, sewers or surface water runoff , the site

visit brought to light a number of issues requiring further investigation prior to development of the

site.

A spring and small stream within the site will need managing in order to facilitate site

development. Within the southern end of the site a densely overgrown area was fenced off and

carried a sign warning of the risk of Weils‟ Disease. From remote visual inspection it appeared

that the structure was probably an abandoned sewage filter or similar. Further investigation will

be required as part of a site-specific FCA for this site in order to determine the exact purpose of

the structure and the level of flood risk posed by any connected underground pipe network.

Developers will need to satisfy themselves and the planning authority that there are no flood

risks from abandoned pipe work in the area.

Confirmation that ground contamination issues can be adequately addressed will also be

required prior to development of this site; however, since this is not a TAN15 issue it would not

need to be directly addressed in the site-specific FCA.

5097656-70-DG017-3.docx 57

Development of the majority of the site in line with TAN15 requirements is likely to be possible

subject to confirmation by a site specific assessment.

5.6.3 Site 16LBR

The site is understood to have planning permission, subject to the completion of a legal

agreement. Any change in the extant permission will need to reflect the management of the

consequences of flooding in line with current planning policy guidance.

The TUFLOW modelling results show that the northern end of the site is predicted to be flooded

by a current 1% (1 in 100) annual chance event (without climate change). The flood-free area

reduces significantly by 2085, with the majority of the site being inundated by a 1% (1 in 100)

chance fluvial event (approximately 60% by area inundated). The extent of inundation is

indicated to be marginally greater by 2110 as shown on Drawing Number 5076243/RCF/320.

Approximately 30% of the 22ha site is considered to remain flood free over the development

lifetime under current conditions. The majority of the site, therefore, would fail A1.14 of TAN15

without mitigation of the risk.

Modelling of the 0.1% (1 in 1000) annual chance event indicates that depths over the majority of

the site will be below 0.6m. However, over approximately 1.0 ha at the north and 1.8ha at the

south eastern end of the site flood depths will be over 1.0m. Flood depths will also exceed 1.0m

on the land to the south of the site and on land to the north east towards Victoria Park. The

potential access/ egress routes for any proposed development would need to take account of

this flood risk to the surrounding areas.

Over the majority of the site water velocities during a 0.1% chance event would be within the

indicative acceptance criteria given in A1.15 of TAN15. However, velocities would exceed those

acceptable limits at the northern tip of the site and over the entire portion of the site area on the

right (west) bank of the River Ely.

A number of flood risk mitigation options may exist for this site that could facilitate more

extensive development. A detailed FCA would be required to confirm the viability of options, but

the following potential solutions could be considered:

1) Provision of detailed flood management plan including flood warning system and emergency

evacuation procedures;

2) Ground raising within the central section of the site with lowered „pond‟ sections provided so

overall flood water storage volumes are not altered, and impacts on adjacent land is

minimised;

3) Ground raising for the whole site and construction of drainage channels to divert floodwater

flow volumes towards currently undeveloped land to the south of the site;

4) Provision of flood defences along the banks of the River Ely.

From initial consideration of the options it is anticipated that a flood management plan would be

unlikely to be sufficient to mitigate the risks as a stand-alone option but would need to be

combined with civil engineering works. Ground raising is likely to be most viable, although

compensatory storage volume would need to be provided (most likely within the site). TAN15

requires that development proposals should not cause flooding elsewhere.

The option to provide improved flood defences to the River Ely may be difficult to achieve without

increasing flood risk elsewhere. It is also worth noting that flooding to the south east of the site

results from overland flows from the River Taff merging with flooding from the River Ely. The

provision of a defence barrier along the River Ely may result in the restriction of River Taff flood

flow paths and increase flood risk to other sites.

Detailed two-dimensional modelling would be required to assess the impacts on adjacent land of

any of the proposed mitigation options that involve physical changes.

5097656-70-DG017-3.docx 58

Should development of any part of the site be proposed, the detailed FCA would also be required

to demonstrate acceptable access/ egress routes are available for emergency services during a

flood event. Accident and Emergency facilities in Cardiff are located to the North of the River

Taff in the Heath area, access to which would potentially require crossing flooded areas.

The FCA should also address the potential flood risk from sewers, as identified in Section 5.4.1

above. The cause of reported flood incidents should be determined and if necessary, measures

to manage the future risk identified before development in the vicinity proceeds.

5.6.4 Site 52LBR

This site lies immediately to the east of Site 16LBR, Papermill Road being located between two

railway lines.

The site is currently considered to be flood free for a 1% (1 in 100) annual chance event. Taking

climate change to 2085 into account, an event of this probability will result in inundation over

approximately 30% of the 7ha site area. The extent of this event by 2110 will be little changed.

The whole site will be flooded by a 0.1% chance event. Depths of flooding will be less than 0.6m

over approximately 55% of the total site area. Depths will exceed 0.6m but less than 1.0m over

35% of the area. Depths exceed 1.0m over 10% of the site area. Corresponding velocities will

be less than 0.15m/s over the majority of the site.

Access south along Papermill Road via the rail underpass to Sanatorium Road will also be

flooded during a 1% chance event by 2085. During a current 0.1% (1 in 1000) annual chance

fluvial event, the maximum depths on the road would be less than 0.6m on the road immediately

adjacent to the site, but would exceed 0.6m at the rail underpass onto Sanatorium Road. The

velocities during this event are indicated to be very high on the approach to the rail underpass, in

excess of 0.45m/s. There are currently no alternative access routes. The site‟s location between

two railway means that options are very limited.

Under existing conditions, without mitigation, safe access to the site cannot be demonstrated to

be maintained in the event of flooding over the 75 year development lifetime recommended by

the Environment Agency for commercial developments. The flood risk to the site itself complies

with the TAN15 indicative criteria for retail and commercial development over approximately 55%

of its area.

Since flooding of the site and access routes originates from both the south east and northwest of

the site, hard engineering solutions are likely to be challenging and costly. These might include

improved defences to the River Ely, but for the reasons given above in relation to Site 16LBR,

the feasibility of this option will need extensive further appraisal. Options within the site layout

itself would not mitigate the flood risk associated with the access.

Any options to mitigate the risk would include a flood management plan.

Future development of the site would need to demonstrate that the consequences of flooding to

the site would not be exacerbated compared to the existing site use. Based on the existing flood

risk and mitigation options, significant re-development of this site in line with TAN15 would be

extremely challenging.

5.6.5 Site 36LBR

The site is considered to be flood free during a 1% (1 in 100) annual chance event including

climate change allowance up to and including 2085. By 2110 a 1% (1 in 100) chance event

would inundate part of the west end of the site.

The entire site would be inundated by a current 0.1% (1 in 1000) chance fluvial event, with

depths exceeding 0.6m over approximately 50% of the site which fails TanAN15 A1.15 for

residential development. Depths exceed 1.0m over approximately 30% of the total site area,

which fails TAN15 for all development types. The corresponding velocities would be less than

0.15m/s over the whole site.

5097656-70-DG017-3.docx 59

The existing road access to the east of the site will be flooded by a 1% (1 in 100) chance event

by 2085. Flood free access will not be maintained to the site and so fails TAN15. In addition, a

current 0.1% (1 in 1000) chance event would result in velocities exceeding 0.45m/s on the main

access routes east, crossing the railway lines via Sanatorium Road and Leckwith Road and

south via Lawrenny Avenue. These fail TAN15 criteria for safe access and egress.

Although the area of existing housing immediately to the south would remain flood free during a

1% (1 in 100) chance event to 2110, access to the wider area east of the rail lines would be

inundated by the 1% chance event by 2085.

The site, therefore, fails TAN15 criteria. If development is to proceed, measures to mitigate the

risk will be required to comply with TAN15 guidance. Such mitigation measures could include

engineering works to improve the flood defence to the River Ely. However, such mitigation is

unlikely to be straight forward and may prove problematic. Mitigation of the risk through a flood

management plan for non residential development may be less problematic than mitigation to

standards required for residential development.

Any FCA for this site will also need to address the potential flood risk from sewers, as identified

in Section 5.4.1 above. The cause of reported flood incidents should be determined and if

necessary, measures to manage the future risk identified before development in the vicinity

proceeds.

5.6.6 Sites 31LBEM, 62LBR, 77LBR, 78LBRM and 83LBRM.

Three of these sites; Site 31LBEM, Site 78LBRM and Site 83LBRM are understood to have

planning permission subject to completion of legal agreements. Any future issuing or renewal of

planning consents will need to reflect the management of the consequences of flooding in line

with current planning policy guidance

The five sites within the Leckwith Industrial Estate and Penarth Moors areas are shown to remain

flood free for a 1% (1 in 100) annual chance event over a development lifetime to 2110.

However, during a more extreme 0.1% (1 in 1000) annual chance event flooding is expected to

affect either parts of the sites themselves or the potential access/ egress routes for the sites.

The velocity of flow would be expected to remain below 0.15m/s over all five sites during a 0.1%

chance flood event.

Site 31LBEM - This site is flood free for 0.1% chance event and flood free access for this site via

Brindley Road and Hadfield Road is considered to be maintained during this event. A risk of

flooding to part of this site from culverts under the A4232 has been identified as noted in Section

5.4.2. Culverts under this road link drainage channels from the site to the River Ely. Although

the extent of flooding from culvert blockage may be considered to be low, the potential frequency

of flooding means that this risk should be addressed by potential developers.

It is considered likely that development of this site can be achieved in line with TAN15.

Site 77LBR – Approximately 50% of the total area of this site located east of the railway line at

Ferry Road will be inundated by a 0.1% (1 in 1000) chance fluvial flood event. Maximum depths

and corresponding velocities over the flooded area would remain within TAN15 criteria for all

development types. The existing access to the site to its northeast, the Ferry Road roundabout,

will be flooded to depths over 1.0m during this extreme event. Works to the roundabout or to

establish alternative routes to mitigate the flood risk to the access route may be feasible. These

would need to be developed as part of a flood management plan for any proposed development.

Few difficulties would be anticipated should a developer propose development of the southern

end of the site providing it is demonstrated within the FCA that the risk of sewer flooding (raised

by incidents reported in nearby Clive Street for the 4th July 2001 event) can be adequately

managed. Suitable access/ egress routes will also need to be identified.

Site 62LBR – The model results indicate that this site to the west and parallel with Clive Street

would remain flood-free during a 0.1% (1 in 1000) chance flood event. However, the potential

5097656-70-DG017-3.docx 60

access routes to east and south will be flooded to depths exceeding 1.0m. Depths of flooding

exceed 1.0m on the roundabout to Ferry Road at the south end of this site. Land to the west will

be flooded to depths varying from in excess of 0.6m to greater than 1.0m. Access to the north

via Penarth Road may be viable, although this route is flooded to a depth of greater than 0.6m.

Management of access/ egress for emergency services will, therefore, also need addressing.

The flood risk to the site itself is low. However, measures to mitigate the risk to access routes

and provide safe access during a 0.1% (1 in 1000) chance event may prove challenging. A flood

management plan as part of any site specific FCA for any proposed development of this site

would be advisable.

Site 78LBRM - A mixed use development is proposed at site 78 on Hadfield/ Penarth Road. With

the current ground profile of the site, approximately 60% of its area would be flood free for all

events modelled. The majority of the eastern part of the site is flooded to depths greater than

0.6m, but less than 1.0m. Flood depths would exceed 1.0m on only 3% of the overall site area.

Without mitigation, the inundated part of the site would be suitable only for industrial uses.

Access from all parts of the site to Hadfield Road would meet TAN15 criteria for depths and

velocity of flooding under current conditions.

Mitigation of the risk to at least part of the affected area of the site could be achieved through

raising part of the site, with compensatory lowering of other parts. This would allow other

development types to be considered i.e. commercial & retail uses, in compliance with TAN15

guidance.

A site-specific FCA would be required and careful consideration of changes to overland flows

and any potential adverse impacts on flooding elsewhere. It is likely that at least part of this site

can developed in line with TAN15 guidance.

Site 83LBRM –under existing conditions, the entire site would be inundated during a 0.1% (1 in

1000) chance fluvial event. However, for a little over 60% of the site area depths would not

exceed 0.6m during such an extreme event. Development of this part of the site for all

development types is likely to be achieved in line with TAN15 guidance. The depths over the

remaining 36% of the site area would be below 1.0m. Under existing conditions this part of the

site is likely to be suitable only for industrial uses, based on TAN15 acceptability criteria. Ground

re-profiling may increase the area available for uses other than industrial. Care will be required to

ensure any resulting changes to overland flow paths do not adversely affect flooding elsewhere.

On the main access/egress route for the site, via Penarth Road, maximum flood depths are

mostly below 0.6m. There are some localised areas of depths in excess of 0.6m but below 1.0m

on this road. It is considered likely that with careful planning and design of any proposed

development, that the risk to safe access can be managed safely.

There are no reported incidents indicating a problem with sewer flooding in the area of this site.

5.6.7 Site 70LBEM

The site is expected to remain flood free during a 1% (1 in 100) chance fluvial event over a

development lifetime to 2110.

A cluster of reported sewer flooding incidents is located to the east of this site. Further liaison

with Dwr Cymru Welsh Water is recommended. This should ensure that the source of these

incidents is understood and that any potential overland flow paths towards the site are identified

to allow mitigation. It is considered likely that the risk from this source can be mitigated. The

magnitude of any works required would need to be determined from the further investigation.

Overall the flood risk to this site is considered to be low (noting the recommended further

investigation of the sewer flood risk) and it is likely that the risk can be managed in line with

TAN15.

5097656-70-DG017-3.docx 61

5.6.8 Site 61LBRM

Site 61LBRM is defined as within TAN15 DAM Zone C1 (protected).

Modelling shows that this site is not at risk from the 1% (1 in 100) annual chance event flood

within the next 100 years.

Although areas to the south and west of the site, towards the Taff, are estimated to flood in a 1%

chance event by 2110, alternative flood free access and egress routes would be available.

The site is also considered flood free for a current 0.1% (1 in 1000) annual chance fluvial event.

From consideration of the fluvial flooding on the Rivers Taff and Ely, it is concluded that the

development of this site is likely to meet TAN15 requirements.

The town centre, north of the rail line has already been identified as being at risk from sewer

flooding. Further investigation of the underlying reasons and any implications being for

development of the site should be undertaken as part of the design of the development.

Overall, the flood risk to the site is considered to be low and development within TAN15

guidance is considered to be likely.

The EAW‟s Flood Map of Surface Water (FMSW) indicates that Dumballs Road may be at risk

from surface water flooding. This risk would need to be considered in more detail as part of a

site specific assessment. It is considered that management of this risk is likely to be achievable.

The detailed site specific assessment for the site would need to demonstrate that the developer

has considered all flood risk issues associated with developing the site, including the provision of

acceptable access/ egress routes for emergency services, and the use of SuDS.

5.6.9 Site 96LGED

This site in Llandaff lies between an existing college campus and the right bank of the River Taff.

It lies in TAN15 DAM Zone C1 (protected) and benefits from defences from the River Taff..

The 2D model results indicate that the site would remain flood free for a 1% (1 in 100) chance

flood event over the lifetime to 2110. During a 0.1% (1 in 1000) chance event, currently, the

model results indicate that there would be some overtopping from the river defences at the

upstream (west) end of the site. However, maximum flood depths and corresponding velocities

would be well within TAN15 A1.15 criteria.

Surface water management will need to be given careful consideration. The EA Flood Map of

Surface Water (FMSW) indicates flooding only for the more severe of the rainfall events tested (1

in 200 annual chance) and this is indicated to be “deep” over a very localised area.

The nearest reported sewer flooding incidents are remote from the existing college or the

proposed development site. The capacity of the sewer system to accommodate any new

development would need to be explored with the sewerage undertaker as part of the

development design.

It is considered likely that, subject to a more detailed site specific FCA being undertaken, the

flood risk associated with development of this site can be managed in line with TAN15 guidance.

5.6.10 Site 101LBEM

Site 101LBEM lies partially within the TAN15 DAM Zone C1 (protected) and partially in Zone B.

Modelling shows that this site is not at risk from the 1% (1 in 100) annual chance event flood

within the next 100 years.

The site is also considered flood free for a current 0.1% (1 in 1000) annual chance fluvial event.

From consideration of the fluvial flooding on the Rivers Taff and Ely, it is concluded that the sites

will meet TAN15 requirements and development of the area may be possible.

5097656-70-DG017-3.docx 62

A detailed site specific assessment for the site would need to demonstrate that the developer

has considered all flood risk issues associated with developing the site, including the provision of

acceptable access/ egress routes for emergency services, and the use of SuDS.

5.6.11 Site 17LGR

Site 17 lies adjacent to the defended right bank of the River Ely. The site is flood free over the

majority of its area during a 1% (1 in 100) annual chance flood event over the development

lifetime to 2110.

However, by 2085 the access route to the site via Dyfrig Road would be completely inundated by

a 1% (1 in 100) chance event as a result of overland flow from the River Ely upstream of the site.

The site itself will be completely inundated by a current 0.1% (1 in 100) annual chance flood

event as a result overland flow from the Ely via Dyfrig Road.

Although depths of flooding would be within TAN15 criteria over almost the entire site area, the

velocities over parts of the site would exceed TAN15 acceptability criteria. The velocities on the

site near the current site access to Dyfrig Road would be particularly high.

Velocities on Dyfrig Road, the main access/ egress route to the site would exceed 0.45m/s over

approximately 200m from the entrance to Site 17.

The site, therefore, fails TAN15 criteria. If development is to proceed, measures to mitigate the

risk will be required to comply with TAN15 guidance. Such mitigation measures could include

engineering works to improve the flood defence to the River Ely. However, such mitigation is

unlikely to be straight forward and may prove problematic.

5097656-70-DG017-3.docx 63

6. Area I – Cardiff Bay to Pengam Green This section updates the assessment of flood risk for sites in the Pengam Green to Cardiff Bay

Area. It is based on updated 2D tidal inundation modelling and supersedes the 2D Modelling

Addendum Report of March 2010 (see Figure 1-1 and section 1.3). The updated modelling takes

account of new estimates of extreme sea levels, released by the Environment Agency in

February 2011 and considers development lifetime up to 2110. Following comments from the

Environment Agency, the potential for tidal flood water to enter Cardiff Bay via the Docks has

been investigated and assessed. This is described in Section 6.2.2.

As described in Section 2.5.7, surface water flood maps have been released by the Environment

Agency since completion of assessments of surface water flooding reported in the SFCA Phase

2 Part 1, November 2009. As part of the current assessment reference has been made to the

surface water flood maps to check the previous assessments for those candidate sites which

were the subjects of the November 2009 report. Otherwise, the assessments of all sources of

flood risk, other than tidal, remain unchanged for those sites which were previously assessed.

For completeness and ease of reference, these earlier assessments are repeated under the

summary of flood risk in Section 6.3, together with the updated assessment of tidal flood risk.

The text repeated from the earlier Phase 2 report is italicised.

Four of the current list of candidate sites (site references: 20LBR, 71LBC, 73LBRM and 58SBR)

have not been previously assessed; Phase two assessments in line with the methodologies

described in the SFCA November 2009 and the 2D tidal modelling update has been undertaken

for these sites.

6.1 Site Description and Flood Risks other than by Tidal Inundation

Full details of the extent and description of Area I and the assessment methodologies used for

sources of flood risk other than tidal are given in the SFCA Phase 2 Part 1 Report (2009).

As described in Section 1.3, the earlier SFCA 2D Model Addendum Report (March 2010)

considered the candidate sites to the Draft LDP which was withdrawn in 2010. The Candidate

Sites Submitted in 2010 for the current Draft LDP have been considered for this update.


Area I is bounded to the east by the River Rhymney, to the southeast by the coast and to the

west by the flood plain of the River Taff and Cardiff Bay. The centre of the City of Cardiff lies to

the northwest of Area I. The area is approximately bisected into east and west portions by the

spur rail line to Cardiff Docks. To the east are the areas of Tremorfa and Pengam Green. To the

west of the rail line are the areas of Splott and Butetown. Along the coast the Cardiff Flats

comprise an area of historically reclaimed land. The Cardiff Barrage lies in the south of the area

with the lock gate entrance to the Docks, being to the east of the barrage. The existing land use

in the area includes industrial, residential and dock developments.

The topographical elevations in the area vary between 6mAOD to over 10mAOD.


The proposed development types comprise residential, mixed use and employment. The

individual sites cover areas of between 0.03ha to 24ha. The largest site at Pengam Green is

proposed for Employment and Industry and covers some 24ha.

The geographical extent of the study area is shown together with the individual site locations on

Figure 6-1 and site details are summarised in Table 6-1, below.

5097656-70-DG017-3.docx 64

Figure 6-1 - Area I - Study Site Locations

(EAW 0.1% or 1 in 1000 annual chance flood outline also shown in blue)

QQuueeeenn AAlleexxaannddrraa DDoocckk

RRooaatthh DDoocckk

RRooaatthh BBaassiinn

CCaarrddiiffff BBaarrrraaggee

EEnnttrraannccee ttoo DDoocckkss

5097656-70-DG017-3.docx 65

This page is intentionally blank

5097656-70-DG017-3.docx 66

Table 6-1 - Area I – Summary of Study Sites

Site ID Name/ Location Existing Land Use Development Category

Site Area (ha)

71LBC Wales Millennium Centre 'Phase 3' Land

Occasional outdoor events

Arts related activities

0.1

58SBR Moorland Road, Cardiff

Vacant buildings and adjacent yard

Residential 0.03

20LBU Queens Gate Car Park Vacant Unknown 1.6

19LBU Pengam Green Vacant Employment 24.1

18LBM Queen Alexandra Head Vacant

Mixed use including leisure

4.7

60LBR Porth Teigr (formerly known as Roath Basin South), Cardiff Bay, Cardiff

Part – land under construction;

Part – vacant awaiting development;

Part – Cardiff Bay

Mixed use residential, retail, commercial and leisure

16.6

57LBR Hannah Street, Bute Town, Cardiff

Vacant, cleared site Residential 0.1

73LBRM Cardiff Graving Docks / Landsea Gardens / Landsea Square / Havannah Street Car Park, Cardiff Bay

Surface car park, small vessel moorings, public waterside access, restaurant, public space

Mixed use 2.1

101LBEM Land off Dumballs Road, Cardiff

Mixed industrial use (B1, B2 & B8) and temporary car park

Mixed use 3.7

94LBR Colchester Avenue, Cardiff

Education Residential 2.0

6.2 Flood Risk from Tidal Sources

The most significant flood risk in the east of the area is due to extreme tide levels in the Severn

Estuary, the River Rhymney and the Roath Brook. Tidal events in the lower reaches of the River

Rhymney and Roath Brook have been confirmed to result in higher flood levels than comparable

fluvial events (tested using EAW 1D HEC RAS model11

).

In the west of the area the main sources of flood risk arise from potential increases in water

levels in Cardiff Bay due to combined fluvial and tidal influences in concert with the impoundment

of the Cardiff Bay Barrage. There is also a potential for tidal inundation through Cardiff Docks.

The level in Cardiff Bay has been tested for tidal and extreme fluvial events using the 1D ISIS

Bay model described in Section 5.3.2. For extreme tidal events this model has been developed

further to test the effect of flooding into the Bay from the Docks via the Roath Basin (see Section

6.2.2).

11

Environment Agency Wales, 2007, Lower Rhymney and Roath Brook model, v2, HEC-RAS

5097656-70-DG017-3.docx 67

Extreme Tide Levels and coincident fluvial flows into Cardiff Bay

It is noted that events described as tidal will have coincident fluvial component and vice versa.

For this current assessment the following combinations of events have been assumed in the

modelling:

Tidal events:

0.5% (1 in 200) annual chance tide level – coincides with QMED in Rivers

0.1% (1 in 1000) annual chance tidal level – coincides with QMED in Rivers

Fluvial events:

0.5% (1 in 200) annual chance fluvial flow – coincides with a Mean High Water Spring level

in Severn Estuary

0.1% (1 in 1000) annual chance fluvial flow – coincides with a Mean High Water Spring level

in Severn Estuary

The maximum extreme still water tide levels considered for this assessment are presented in the

table below.

Table 6-2 – Extreme Tide Levels Severn Estuary at Cardiff Docks

(source Environment Agency Wales, 2011)

Annual chance of exceedance

2010 2085 2110

0.5% (1 in 200) 8.4mAOD 9.1mAOD 9.4mAOD

0.1% (1 in 1000) 9.0mAOD *- *-

*Not assessed for the SFCA

The peak fluvial flows assumed to coincide with these extreme tides are presented in Table 6-3

below. These are referenced in the descriptions of the modelled flood levels in the following

sections.

Table 6-3 – fluvial flows assessed to occur coincident with extreme tidal flood events – Area I

QMED

(50% or 1 in 2 annual chance)

20%

(1 in 5 annual chance)

QMED inc climate change allowance

River Taff 356 m3/s 443 m

3/s 430 m

3/s

River Ely N/A12

78 m3/s 78 m

3/s

River Rhymney N/A12

112 m3/s 112 m

3/s

Roath Brook N/A12

12 m3/s 12 m

3/s

6.2.1 Overtopping and breach from River Rhymney and the Coastal Frontage

The Cardiff Flats floodplain is considered to be susceptible from flooding due to wave and tidal

sources. Storm surge levels, tidal currents and the wind wave climate along the shoreline are

significant.

12

In the absence of existing estimates of the QMED flows, we consider that use of the Q5 or 20% (1 in 5) annual chance flow represents a conservative approach. We have confirmed, based on analysis of ratios of ReFH flow estimates that the Q5 is greater than a QMED+20%. The Q5 has therefore also been used for the climate change scenarios and current day estimates.

5097656-70-DG017-3.docx 68

The flooding due to overtopping and breach has been assessed using the 2D inundation model

(see Appendix D and Appendix F for further details). The extent of the model is shown in the

following Figure.

The right bank of the River Rhymney in the lower reaches comprises a mixture of infrastructure

which would not all have been designed formally for a flood and coastal risk management

function. They are not all Environment Agency maintained assets and there may be private ad-

hoc maintenance.

The tidal risk is considered dominant over the fluvial influences for the lower reaches of the

Rhymney and Roath Brook.

The main source of flood risk to the eastern side of Area I is overtopping of the right bank of the

River Rhymney. Extreme tide levels are propagated up the channel of the Roath Brook. The

effects of sea level rise also increase the risk of breach to the coastal frontage due to the

combined effect of wave and still water tide levels (SWL).

Figure 6-2 - Extent of 2D tidal inundation model - Area I

Green line indicates the extent of the tidal boundaries assessed

Reproduced from Figure F.1, Appendix F

The risk of breach has been assessed based on methodologies underlying the Environment

Agency Severn Estuary Flood Risk Management Strategy (SEFRMS). The approach to the

modelling is described in Appendix F. The increase in level up the Roath Brook during extreme

5097656-70-DG017-3.docx 69

tide events has been tested by routing a fluvial flood event against a fixed tide level through an

Environment Agency 1D HEC RAS model of the Roath Brook and Lower Rhymney.13

The breach assessment essentially comprises consideration of the joint probability of wave and

extreme still water tide levels (SWL), the probability of breach and the consequences of breach

events.

Different combinations of SWL and wave height, corresponding to a series of joint probabilities

were tested for scenarios at each time horizon.

Results

The results of the 2D modelling of overtopping and breach from the coastal frontage, and the

River Rhymney and Roath Brook, are presented on Drawing Numbers 509765/RCF/420 to 422,

Appendix B.

Current situation

There is no breach indicated for a current 0.5% (1 in 200) event. The flooding indicated on

Drawing No. 5097656/RCF/420 results from overtopping from the River Rhymney and the Roath

Brook.

A single location at Tremorfa - labelled SR1-RRWB on Drawing No. 5097656/RCF/420 is

indicated to breach during a current 0.1% (1 in 1000) annual chance event.

Considering Climate Change to 2110

A single breach in the same location is indicated for a 0.5% (1 in 200) annual chance event in

2085 and 2110.

6.2.2 Direct Inundation from Cardiff Bay and Cardiff Docks

The sites in the west of the area will potentially be affected by elevated levels in the impounded

Cardiff Bay. The flood levels in the Bay are affected by a combination of fluvial and tidal

influences. A comparison has been made between the estimated flood levels in Cardiff Bay and

ground levels for the study sites.

The existing ground and defence levels are presented here for comparison with the results of the

modelling of flood levels in Cardiff Bay (presented below in this section). The LIDAR ground

levels around the Bay generally exceed 8.2mAOD. There are flood defence walls at two

locations of lower ground, at Windsor Esplanade and around the Graving Docks. The top of

defence level at Windsor Esplanade is 8.3mAOD. It is understood that a spot level survey

indicates that the top of the defences around the Graving Dock are approximately 8.0 to

8.1mAOD, although this requires further verification. It had previously been understood that this

defence was at least 8.2mAOD (as reported in the August 2010 issue of SFCA). Prior to

construction of the Graving Dock defence the hinterland area to the north, including Stuart

Street, Adelaide Street and the site of the Techniquest building, had been subject to flooding.

This area lies in a low lying corridor of land which stretches north towards central Cardiff. Any

water overtopping the Graving Docks defences will tend to flow north into this area.

Flow Path Through Cardiff Docks (Roath Basin)

Sites around Cardiff Docks, most notably Sites 20LBU, 60LBRM and 18 LBM, are potentially at

risk from elevated water levels in the docks. It is understood from the operators of the Docks,

Associated British Ports (ABP) that the current operating procedure requires opening the lock

gates to the Queen Alexandra Dock on tides in excess of 7.3mAOD. This effectively allows the

water in the docks to equal the level of the tide. Therefore, areas around the dock are at risk of

13


5097656-70-DG017-3.docx 70

flooding for the extreme tides assessed for the SFCA (the tide level of 7.3mOAD is exceeded for

all events tested).

Inspection of LIDAR ground levels between the Roath Basin and the impounded Bay indicate

that potential overland flow routes exist by which water overtopping the dockside can weir into

the impounded Cardiff Bay. Under normal operating conditions, sea water from the dock is

prevented from passing into the impounded Bay by a bund structure in the disused lock adjacent

to the Norwegian Church. LiDAR data indicates a top of bund elevation of 7.6mAOD. The land

either side of this bund rises up to 9.0mAOD. The potential flow paths into the Bay via the Docks

are indicated on Figure 6-3.

Figure 6-3 Potential overland flow from Roath Basin into Cardiff Bay during extreme tide events

The Area I 2D tidal inundation model was used to test the volume of tide water entering the

docks during extreme tidal events under two scenarios:

Assuming the lock gates to the Queen Alexandra Dock to be open allowing the full area of

the lock to convey tide water into the dock, i.e. approximating to the actual situation.

Assuming the lock gates to remain closed. Sea water can only enter the dock by weiring

over the lock gates and any low lying land either side of the lock. i.e. limiting (i) the

conveyance area for flow and (ii) the time of overtopping to a relatively short period either

side of high water.

The gate open scenario is considered to approximate more closely to the current operating

procedure for tide levels exceeding 7.3mAOD. However, the second scenario has been tested

to inform the potential flood mitigation benefits of closing off the flow path through the open lock.

The extent of flooding around the Docks and the overland flow paths into Cardiff Bay has been

assessed as described in Appendix G. Essentially, the 1D ISIS hydraulic model of the Bay

described in Section 5.3.2 has been developed further to include a link to a 2D model of the area

Location of bund separating Cardiff Bay from the Docks

5097656-70-DG017-3.docx 71

between the Roath Basin and the Bay14

. This models the overland flow which will develop from

the Roath Basin during extreme tidal events and the resultant effect on levels in Cardiff Bay.

In order to test compliance with TAN15 A1.14, a comparison was made between the 1% (1 in

100) annual chance fluvial events and 0.5% (1 in 200) annual chance tidal events to determine

whether fluvial or tidal events result in the higher levels. For assessment of TAN15 A1.15, the

0.1% (1 in 1000) annual chance fluvial and tidal events were compared.

It is industry practice to compare the 1% (1 in 100) annual chance fluvial events and 0.5% (1 in

200) annual chance tidal events when determining fluvial – tidal dominance for flood levels. i.e.

do extreme fluvial or tidal events result in the higher levels.

The effect of climate change on Bay levels due to increased fluvial flows and predicted sea level

rise has been considered in the assessment of the study sites.

The increase in the level in the Bay during extreme tidal events is predominantly due to the tide

locking of the incoming fluvial flows. To test the relative significance of the flow path into the Bay

from the Docks, sensitivity tests were undertaken using the model based on assumption of no

flow in via this route. i.e. assuming only fluvial flows into the Bay. These are presented in Table

6-4 5.

A separate sensitivity test of the Area I 2D only model (not linked to the 1D Cardiff Bay model)

was also run to estimate the levels likely to develop in the Docks if the gates were re-engineered

to allow their closure during extreme tidal events. This was tested for the most extreme level

only, the 0.5% (1 in 200) chance tidal event in 2110, a tide level of 9.4mAOD. The result is

presented in Table 6-4 5.

Tide Levels

The extreme tide levels in the Severn Estuary (on the seaward side of the Cardiff Barrage) used

in the assessments are presented in Table 6-2.

Results

The levels in Cardiff Bay for extreme tidal events predicted from the modelling are presented in

Table 6-4 5, the extent and depths of flooding are indicated in Drawings 5097656/RCF/420 and

421, Appendix B. The levels presented in the table include both the existing conditions and

assuming a mitigation scenario. This latter situation assumes the tidal inundation of the Docks is

mitigated through re-engineering of the lock gates to enable their closure during extreme tides.

The economic viability of such an option has not been assessed.

14

This is separate to the larger Area I 2D model used to assess direct coastal breach and inundation via the River Rhymney.

5097656-70-DG017-3.docx 72

Table 6-4 5 Levels resulting in Cardiff Bay for current and future extreme tidal events

Assumes coincident QMED flows in the River Taff and River Ely

Scenario Tide level (mAOD)

Water Levels (mAOD)

TIDAL EVENT YEAR Key model assumption *

Roath Basin Impounded Bay

0.1%

(1 in 1000) annual chance

2010 Existing condition - Dock Lock Gates

open

9.0 9.0 7.66

No flow from Roath Basin

9.0 N/A 7.58

0.5% (1 in 200) annual chance

2085 Existing condition - Dock Lock Gates

open

9.1 9.1 8.08


9.1 N/A 7.96


2110 Existing Condition Dock Lock Gates

open

9.4 9.4 8.29


9.4 N/A 8.11

Lock Gates to Docks assumed to be retained closed

(overtopped only) **

9.4 8.2 N/A

* Open Lock Gates is current situation

** 2D only model

Current situation

During extreme 0.1% (1 in 1000) annual chance fluvial events in both the Taff and Ely Rivers, the

Bay level is predicted to rise to a peak level of 6.79mAOD (see Table 5-2 under Area H). All

sites in Area I would be flood free from this source during such an event.

During the comparable 0.1% (1 in 1000) annual chance tidal event, the tide level in the Severn

Estuary is predicted to be 9.0mAOD. The model results indicate that the Bay level would rise to

a peak of 7.66mAOD, if this event were to coincide with a QMED (50% or 1 in 2 annual chance)

fluvial flood event.

This level would be contained within the impounded Bay by the defences around the Graving

Docks, at Windsor Esplanade and by the general ground levels around the eastern sides of the

Bay. Some localised flooding to the low lying area in front of the Millennium Centre building

would occur during the peak level in the Bay.

Localised areas in the eastern Bay and Roath Basin area would be flooded due to tidal

inundation via the docks. This will result in overtopping from the Roath Basin into Cardiff Bay

and flooding to land at the dock side. (see Drawing 5097656/RCF/421C, Appendix B)

5097656-70-DG017-3.docx 73

If the flow path from the Docks into the Bay were to be mitigated against, then the Bay level

would peak at less than 7.60mAOD as a result of the reduction in the volume of water entering

the Bay.


The peak Bay level is predicted to increase to 8.08mAOD considering a 0.5% (1 in 200) annual

chance tide of 9.1mAOD. This assumes a greater flow into the Bay as a result of climate

change.


A 1% (1 in 100) chance fluvial event would result in a peak water level of 7.62mAOD in the Bay

(see Table 5-2 under Area H), assuming a coincident Mean High Water Spring level (including

climate change) in the Severn Estuary of 7.1mAOD.

The 0.5% (1 in 200) annual chance tidal event level in the Severn Estuary is expected to rise to

9.4mAOD, based on current sea level rise predictions.

The modelling indicates that the Bay water level would rise to a peak of 8.3mAOD, assuming the

peak of the tide coincides with a QMED (including +20% climate change allowance) flood in the

Rivers Taff and Ely and with the lock gates to the docks open. Flooding to the area around the

docks and the area between the Roath Basin and the impounded Bay would result as indicated

on 5097656/RCF/421, Appendix B.

If the dock gates were re-engineered to ensure that they can be kept closed during extreme tide

events, then the reduced conveyance of sea water into the docks would result in reduced

flooding around the docks and less overtopping into the Bay. The model indicates that a peak

Bay level of 8.1mAOD would result.

The potential impacts of the predicted water levels on candidate sites are discussed in the

following section.


The results of the modelling of 0.5% (1 in 200) annual chance events showing the predicted

extent of flooding over the development lifetime are presented on Drawing 5097656/RCF/420 in

Appendix B. The depth and velocities estimated by the modelling for a current 0.1% (1 in 1000)

annual chance tidal event are shown on Drawings 5097656/RCF/421 to 422. These should be

referred to in reading the following site summaries.

Note: For the Docks area, only the Roath Basin has been modelled in 2D. Flow velocities are

available only for this part of the Docks. For the other areas of the Docks, the depths of flooding

have been estimated by contouring the extreme tide levels over the existing LIDAR ground

levels.

For all sites, surface runoff from developments will need to be managed using appropriate

sustainable drainage systems (SuDS) to restrict runoff as required by section 8.5 of TAN15.

6.3.1 Site 71LBC

Ground levels on this site are above 9.0mAOD, and generally over 9.5mAOD over most of the

site.

Fluvial and tidal sources of flooding

Current situation and considering climate change to 2110:

The site is not considered to be at risk from fluvial or tidal sources over a 100 year development

lifetime.

5097656-70-DG017-3.docx 74


Surface Water Flood Maps indicate some limited localised risk to part of the site for a 0.5% (1 in

200) rainfall event.

Site summary

It is considered likely that this site can be developed in line with TAN15. The management of

surface water within the site will be required. It is likely that the surface water risk to the site can

be managed within the design of the site layout and vertical alignments of any footpaths or roads

to ensure flow paths do not impact on any proposed or existing developments.

6.3.2 Site 58SBR


Current situation

The modelling indicates that this site is flood free for the fluvial and tidal events.


The modelling indicates that this site is flood free for the fluvial and tidal events assessed over

the development lifetime to 2110.

Although flood free to 2085, the access road to the north and areas west of the site lie within the

0.5% (1 in 200) annual chance flood outline by 2110 as indicated on Drawing 5097656/RCF/420.


The areas immediately north and west of the site are also indicated to be potentially at risk of

surface water flooding based on the EAW FMSW. The management of surface water within the

site will require consideration within the development design. It is considered likely that surface

water risk to the site can also be managed.

Consideration will need to be given to maintenance of access to the site under all conditions in

line with A1.12 of TAN15. The land south of the site is indicated to be flood free and there is the

potential to design emergency access via this route.

Site summary

It is considered likely that development of the site in line with TAN15 acceptability criteria can be

achieved, subject to confirmation through a site specific FCA.

6.3.3 Site 20LBU

This site lies partially in TAN15 Zone B, but with the majority within Zone A. The proposed use

for this site is unknown but assumed to include residential development.

The general site LIDAR levels are indicated to be at or above 9.0mAOD. Ground levels in the

southern half of the site are generally higher, reaching 9.6mAOD. There is a shallow depression

in the ground levels in the northern half of the site. The levels, bordering the Dockside at the

southern boundary are more typically 8.7mAOD.


Current situation

The site is flood free for tidally or fluvially dominated 0.1% (1 in 1000) annual chance flood

events.


The site is elevated above both the tidally dominated 0.5% (1 in 200) annual chance flood

levels and so is considered flood free.

5097656-70-DG017-3.docx 75

The site is therefore complies with TAN15 within a 75year development lifetime and is

considered suitable for industrial/ commercial developments.


The majority of the site lies below the 9.4mAOD tide level for a 0.5% (1 in 200) annual

chance event level by 2110.

However, the flood depths during this event would be less than 0.4m over the majority of the

site.

Without mitigation, large parts of the site fail to comply with TAN15 over the 100year

development lifetime required for assessment of residential developments.

Mitigation of the risk may be achievable for example, through the construction of raised defences

to the dockside of the site. The modelling indicates that if the lock gates to Queen Alexandra

docks are re-engineered to allow them to remain closed during extreme tide levels, then dock

water levels would remain below the site levels.

The risk to any proposed development could be also managed by limiting the use of the ground

floor to non residential uses with residential units restricted to above ground flood level. The

duration of any tidal flooding would be limited to a period of a few hours around high water. Flood

free refuge could be sought above ground level during this time.

The access to the site from the north is likely to remain flood free over a 100year development

lifetime.


The EAW Surface Water Flood maps indicate that the depression in the northern part of the site

is susceptible to surface water flooding. It is considered likely that this risk can be mitigated

through consideration of surface water in the design of the vertical alignments of roads and paths

as part of any future development.

The site is flood free and access routes to the site are likely to remain operational under all

conditions.

Site summary

The current risk to the site from fluvial or tidal sources is considered to be low. However, within

the 100year lifetime the site is at risk from a 0.5% (1 in 200) annual chance tidal flood event.

It is likely that site specific mitigation measures, such as defences to the dockside could allow

development to proceed in compliance with TAN15. The risk to any proposed development

could also include limiting any residential elements to above ground floor levels.

Changes to the lock gates to Cardiff Docks to allow closure during extreme tide events would

mitigate the risk to this site and improve flood risk to the docks and wider Roath Basin area.

The site design will need to take account of potential surface water risk and management of

surface water within the site.

It is considered likely that development of this site can be achieved in compliance with TAN15

guidance, subject to appropriate mitigation being demonstrated as part of a site specific FCA.

6.3.4 Site 19LBU

The Study Site at Pengam Green is proposed for employment and lies in TAN15 Zone C2.

Tidal Inundation

Current situation

The area around the site is affected by overland flow routes from the north and east due to

overtopping of the right bank of the River Rhymney.

5097656-70-DG017-3.docx 76

The majority of the site is flood free for a current 0.5% (1 in 200) annual chance event with

small localised areas being inundated in the narrow strip of land adjacent to the River

Rhymney. The inundated area approximates to less than 10% of the site area.

The model results indicate that during a 0.1% (1 in 1000) annual chance flood event, a

number of small localised areas comprising approximately 20% of the site will be flood free.

50% of the site will flood to depths greater than 1.0m. Flood depths would remain below

0.6m over approximately 40% of the site.

The velocities of flood water are not indicated to exceed 0.15m/s over the majority of the

site. Velocities exceeding 0.45m/s are indicated in parts of the thin strip of land which lies

adjacent to Rover Way near the River Rhymney at the north east boundary of the site.

The site therefore fails to meet TAN15 A1.15 acceptability criteria for commercial

development over approximately 60% of the site‟s area. Approximately 50% of the site

would comply with A1.15 for Industrial development, due to the slightly greater depths of

flooding considered acceptable for such development types.

Consider climate change and sea level rise allowances to 2085

Approximately 80% of the site‟s area would be inundated by a 0.5% (1 in 200) annual

chance tidal event. Without mitigation of this risk, these areas of the site would fail the

TAN15 criteria for threshold of flooding, A1.14 of TAN15.

It is considered likely that routes for off-site access and egress would be available to the

south and west of the site via Rover Way and Seawall Road during this event and so would

comply with the requirement of A1.12 of TAN15 that access remain operational under all

conditions. It is noted that existing access to the east via Rover Way would be completely

flooded during this event and it is unlikely that this route would remain operational.

Consider climate change and sea level rise allowances to 2110

The proposed site use is for employment which requires consideration of a 75 year development

lifetime. The longer 100 year lifetime is considered for residential developments. The modelling

indicates that the entire site would be inundated by a 0.5% (1 in 200) tidal event by 2110. If use

of any part of the site were to be considered for residential use, this would fail A1.14 of TAN15.


It is considered that the risk of flooding from other sources is unlikely to be significant compared

to the tidal flood risk, although they are not negligible.

Within the screening assessment the surrounding areas are indicated to have a high surface

runoff risk score. The site is generally at lower ground levels than the surrounding land. Runoff

risk from the surrounding areas would also need to be considered as part of the design for the

development of any part of this site. Although it is anticipated that surface water management

measures incorporated in the overall site design should be able to address the risk.

Two sewer flooding incidents at Greenbay Road and another at the Pengam Green Tesco are

close to the proposed sites. The causes are unknown. The area sewer provision should be

investigated as part of any proposed development to determine if any significant risk of sewer

flooding exists e.g. possibly related to under capacity issues or blockage.

Site summary

The results of the 2D modelling are considered to provide a good representation of the

distribution of flood flows and depths through Area I for a strategic broad scale assessment. (The

2D modelling is described in Appendix F).

During a current 0.5% (1 in 200) tidal event, the majority of the water flooding the area results

from overtopping of the right bank of the River Rhymney. During a current 0.1% (1 in 1000)

chance event and a future 0.5% (1 in 200) chance tidal event, breach of the coastal frontage is

predicted.

5097656-70-DG017-3.docx 77

Without mitigation, the site fails to comply with TAN15 over the majority of its area.

Access and egress routes are likely to remain available during a 0.5% (1 in 200) annual chance

tidal event to 2085.

Mitigation of the tidal risk to the site to meet TAN15 acceptability criteria may be problematic.

Increasing defence levels on the River Rhymney could be considered. This may be considered

as part of a flood alleviation strategy for the existing properties at risk from this source. This

would require a multi agency approach involving the Environment Agency and the local authority.

Such a scheme may reduce risk to the site sufficiently to meet the TAN15 acceptability criteria

and so facilitate development. However, the extent of such works is likely to be considerable.

The design of flood resilient buildings should also be considered. In addition, it should be noted

that the Environment Agency are not normally supportive of proposals which rely on flooding of

voids under buildings being used in mitigation of loss of flood plain storage.

Ground raising with compensatory lowering of other appropriate areas within the site may reduce

flooding to meet TAN15 criteria for at least part of the site area which is predicted to be

inundated. This measure alone would not allow development of the entire site, due to the need

to provide equivalent compensation for loss of flood plain storage. The increase in the proportion

of the site which may meet TAN15 would be subject to further more detailed assessment.

Interception of the overland flow from the north could also be considered to reduce flood risk to

the site. This would have the added benefit of potentially protecting existing properties which are

currently lie in the overland flow paths. However, consideration would have to be given to any

adverse impacts on flood risk elsewhere arising from changes in the overland flow routes.

Without mitigation, it is likely that only a small proportion of the site could be developed in line

with TAN15 guidance.

6.3.5 Site 18LBM

This site is proposed for mixed use development including leisure. It is almost entirely within

TAN15 Zone B or ZONE A. Site levels are generally above 9.0mAOD with a significant

proportion of the area above 10mAOD.

Current Situation

The site is elevated above a 0.1% (1 in 1000) annual chance tidal flood level. The current risk of

flooding is considered to be low.


The inundation modelling indicates that the site is elevated above potential flooding from the

Docks. A low lying strip of land crosses the site, including a ditch on land north of the existing

Harbour Authority and ABP Offices. The ground elevations in these areas are as low as

approximately 8.8mAOD. Inundation of these areas and north into the site would occur from the

lock entrance to the Queen Alexandra Dock, during the 0.5% (1 in 200) annual chance tide level

of 9.1mAOD. Some overtopping into the Bay is possible via this route. The period of

overtopping into these areas is likely to be short, limited to a few hours either side of high water.

The volumes entering the Bay via overland flow paths across the site would not be expected to

significantly affect the water level in Cardiff Bay.


The area of inundation of the site during a 0.5% (1 in 200) annual chance tide level of 9.4mAOD

will be slightly more extensive than the situation for 2085. The overland flow paths developing

across parts of the site are likely to be of short duration, of approximately 1 hour, around the

peak of the tide.

5097656-70-DG017-3.docx 78

It is considered that mitigation of the risk through the design of the site layout and ground levels

should be achievable without causing adverse impact elsewhere, with the potential to direct

water back to the open coast.

Access

LIDAR levels indicate that access bridges across the docks north of the site are likely to be

flooded for a 0.5% (1 in 200) chance tidal event by 2085. However, the depths of flooding are

likely to be less than 0.6m and will be of short duration. Similarly the new bridge access via Site

60LBRM from the Norwegian Church area would be flooded, albeit to predicted depths less than

0.6m and for a relatively short duration.





Flood free refuge would be available over a large area of the site itself during any periods when

of site access could not be maintained.

Other sources

There is no indication of a significant surface water risk to the site.

Site summary

Some areas of the site are at risk from extreme tidal events as indicated by the flood extents

indicated on Drawings RCF/420 to RCF421, Appendix B. It is likely that mitigation of the direct

inundation from the coast and lock gate area could be achieved by ground raising or raised

defences without adverse impacts elsewhere.

The access routes to the north via the Roath Basin and eastern Bay area are likely to flood,

albeit to depths less than 0.6m and for short duration.

More involved mitigation based around re-engineering of the lock gates to the Queen Alexandra

Dock such that the gates remain closed during extreme tidal events could restrict the flood

outline to the cross hatched area indicated on Drawing 5097656/RCF/420. This would mitigate

the risk to the site itself and the off-site access routes to the north.

It is considered likely that the site can be developed in compliance with TAN15. The scale of

mitigation options necessary will need further investigation and development.

6.3.6 Site 60LBRM

This majority of the site lies within TAN15 Zone B. Some parts of the site lie in Zone C1, but

these are a small proportion of the overall 16.6 hectare site.



with current planning policy guidance.

Current Situation

Whilst the majority of the site area is elevated above a 0.1% (1 in 1000) annual chance flood

level, flooding to some parts of the site can be expected during such an event. The flooding of

this site derives from two directions during this extreme tidal event;

i) due to the increase in the level in Cardiff Bay during tide lock,

ii) inundation of tide water from the Roath Basin dock.

5097656-70-DG017-3.docx 79

A small area of the lower lying part of the site bordering an old dock on Cardiff Bay will flood

during a 0.5% (1 in 200) tidal event. The flood depths in this low lying area will be less than 0.6m

during a 0.1% (1 in 1000) chance event (excluding the dock area itself).

Overtopping is also predicted from the Roath Basin onto the lower lying areas in the north central

part of the site during a 0.5% (1 in 200) chance tidal event. During a 0.1% (1 in 1000) chance

tidal event, the flood depths over the majority of the affected areas are likely to remain below

0.6m. Some localised areas will be affected by flooding over 0.6m. Flood depths greater than

1.0m will be restricted to the areas of the actual docks. The flow velocities across the site are

likely to be benign, the flooding being due to steady filling of both the Bay and the Docks. High

velocities can be expected near a new access bridge north of the site by the Norwegian Church.

This is adjacent to the main flow path for water overtopping the bund separating the Roath Basin

dock from Cardiff Bay (described in section 6.2.2 and indicated on Figure 6-3).

The flooding from Roath Basin reflects the flow path through Cardiff Docks under the existing

conditions whereby the lock gates must be opened during extreme tide levels (See Section 6.2.2

and Appendix G).

Although the current risk of flooding to the majority of the site is considered to be low, the lower

lying areas of the site which lie in the 0.5% (1 in 200) flood outline fail to comply with TAN15

guidance.


Although the modelling of levels in Cardiff Bay suggests that the majority of the site area is

above the 0.5% (1 in 200) annual chance level estimated in the Cardiff Bay (8.01mAOD)

overland flows will develop from Roath Basin dock around peak of high tide. Approximately 50%

of the site area will be affected during this event. The flooding will be of relatively short duration

of a few hours and the depths of flooding over the majority of the area will remain below 0.6m.

The velocities across the site are likely to be largely benign as the flooding derives from steady

filling of the docks and eventual overtopping of the dock sides.

Consider climate change to 2110

The flooding across the site during a 0.5% (1 in 200) chance tidal event will be slightly more

extensive than the case for 2085, as indicated on Drawing 5097656/RCF/420/B, Appendix B.

Access





LIDAR levels indicate that the approaches to the access bridges across the docks north of the

site are likely to be flooded for a 0.5% (1 in 200) chance tidal event by 2085. However, the

depths of flooding are likely to be less than 0.6m and will be of short duration. It is considered

likely that the access routes north of the site will remain operational for all but few hours near the

peak of the tide for the extreme flood events assessed. This would need to be confirmed by a

site specific study as part of any FCA for the site. It is noted that the new bridge access to Site

60LBRM from the Norwegian Church area would be flooded, albeit to predicted depths less than

0.6m and for a relatively short duration. However, the velocities at this bridge are likely to be

high, due to its proximity to the flow path over the bund which separates the Roath Basin dock

from the Bay. It is likely that during the period around the peak of the extreme tides, this access

route would not be operational.

Flood free refuge would be available over a large area of site 60LBRM during any periods when

off site access is not operational.

5097656-70-DG017-3.docx 80

Other sources

There is no indication of a significant surface water risk to the site. Evidence does not suggest a

risk of sewer flooding.

Site summary

All but the very lowest areas of the site, bordering the old dock on Cardiff Bay are elevated

above the highest flood levels predicted for Cardiff Bay during current day extreme tidal events.

The main source of flood risk to the site results from the tidal filling of the Roath Basin dock

during extreme tide events. The small proportion of the site will be affected by a current 0.1% (1

in 1000) annual chance event. The resulting flood depths will be below 0.6m over much of the

affected areas with some localised flooding above 0.6m. Depths greater than a 1.0m will be

restricted to the old docks within the site.

The current flood risk to the majority of the site area is considered to be low.

By 2085, relatively short duration shallow flooding is predicted for approximately 50% of the site‟s

area during a 0.5% (1 in 200) chance tidal event. Flood depths will be greater in those areas

bordering the docks.

By 2110, the depths of flooding will increase during the 0.5% (1 in 200) chance event. Flood

depths are predicted to exceed 1.0m around the dock sides. These depths will affect areas up to

60m from the dock side.

Flood free refuge will be available within the site for the duration of the flooding which is likely to

be of short duration of a few hours around the peak of the tide.

It is considered likely that the majority of the site can be developed in line with TAN15 guidance

for residential and mixed use development. The low lying areas of the site may be suitable for

boating, boatyards, marinas or other water related uses as recognised in TAN15.

Mitigation options to increase the area suitable for all development types might include raised

defences to the Roath Basin dock to reduce tidal flooding. Mitigation based around re-

engineering of the lock gates to the Queen Alexandra Dock to allow their closure during extreme

tidal events could restrict the flood outline to the cross hatched area indicated on Drawing

5097656/RCF/420. This would mitigate the risk to the site itself, the wider Roath Basin and

Cardiff Docks area including the off-site access routes to the north.

The level of the access bridges to the site will need to be confirmed to inform any assessment to

determine if escape/ evacuation routes remain operational under all conditions, as required by

A1.12 TAN15. This is a matter requiring expert assessment from the emergency services and

local authority emergency planners.

Flood free refuge would be available over a large area of the site itself during any periods when

such access/egress were not operational.

It is considered likely that the site can be developed in compliance with TAN15, The scale of

mitigation options will need further investigation and development.

6.3.7 Site 57LBR

This site at Hannah Street Church, Hannah Street, is proposed for residential development and

lies in TAN15 Zone C1. The surrounding area comprises a low lying corridor west of the Central

Cardiff to Cardiff Bay Rail line. Typical ground levels are between 7.4mAOD and 7.8mAOD.

The site benefits from defences to Cardiff Bay to the south and the River Taff to the west.

Tidal inundation

The 2D tidal inundation modelling results indicate that this site is not affected by extreme tidal

inundation events over the assessed 100 year period to 2110.

5097656-70-DG017-3.docx 81

Based on the 1D modelling, the peak water level in Cardiff Bay during a 0.5% (1 in 200) event is

estimated to reach a peak of 8.1mAOD by 2110. Defences to the Bay area prevent water

overtopping and flowing north and towards this site.

The site is considered to be flood free for a 100 year development lifetime.

The flood risks from other sources as assessed in the SFCA are repeated below.

Fluvial flood risk

River modelling of the River Taff described for Area H in Section 5.3 indicates that this site is

flood free during a current 0.1% (1 in 1000) annual chance fluvial event.

The site is also flood free for a 1% (1 in 100) annual chance event over a 100 year period to

2100. This takes account of the effect of 20% increase in river flow and the effect of sea level

rise on tide locking of flows into Cardiff Bay.

Other sources

The Surface Water Flood risk screening score indicates a potential risk between medium and

high. Inspection of the LiDAR levels indicates that ground levels in the area are generally low

lying compared to surrounding areas. However, Site 57LBR is elevated above its immediate

area. Overland flow paths developing as a result of highway and surface runoff are likely to

divert away from the site. The EA Map of Surface Water (FMSW) indicated that the access

roads north and south of the site may be susceptible to surface water flooding.

The risk of surface water flooding will need to be taken into account in the design of the

development. The areas further north of the site are indicated on the FMSW to be at risk from

widespread surface water flooding. The FMSW map indicates that access and egress for Site 57

should be maintained in the event of extreme rainfall events via routes to the south and east of

the site.

The study site is in close proximity to a large number of recorded sewer flooding incidents. The

site‟s elevation above its immediate area should prevent overland flows impacting the site. It is

recommended that the risk from this source is investigated further as part of the site

development.

The risk will need to be determined and any mitigation measures such as surface water

management plan will need to be developed as part of the development design.

Site summary

It is considered likely that the site will remain flood free for the flood events assessed over the

development lifetime to 2110. A site specific assessment will need to give adequate

consideration to the management of surface water runoff, although the risk from this source is

considered to be low. Subject to investigation and confirmation of the sewer flooding risk and

suitable mitigation measures, it is likely that the site could be developed in line with TAN15

guidance.

6.3.8 Site 73LBRM

This site is proposed for mixed use including residential development and lies in TAN15 Zone

C1.

The site is discussed in terms of eastern and western parcels of land, in relation to Havannah

Street which lies northwest of the St David‟s Hotel site. Ground levels on the westernmost parcel

are above 8.0mAOD over approximately 80% of its 0.7ha area. The parts of the site east of

Havannah Street comprise the Graving Docks area.

Anecdotal reports from the Manager of the Cardiff Barrage indicate that this area and the

Techniquest building were subject to flooding prior to impoundment of the Bay. In the current

post impoundment situation, the area between the Graving Docks is reported to contribute to the

5097656-70-DG017-3.docx 82

flood storage during elevated levels in Cardiff Bay. The hinterland to this site, Stuart Street and

north into Cardiff is protected by hard defences. It is understood that a spot level survey

indicates that the top of the defences around the Graving Dock are 8.1mAOD although this

requires further verification.

Tidal inundation

Current Situation

The current 0.1% (1 in 1000) annual chance tide level in the Severn Estuary is 9.0mAOD. If this

were to occur coincident with QMED (1 in 2 annual chance) flows in the Taff and Ely Rivers, the

model results indicate that levels in the Bay would rise to 7.66mAOD (compared to 6.79mAOD

for a fluvial dominated 0.1% (1 in 1000) chance event – see Table 5-2).

The areas of the site on the Bay side of the defences around the Graving Docks would flood.

The areas on the landward side of the defences would remain flood free. All of the site west of

Havannah Street would be flood free during a 0.1% (1 in 1000) annual chance tidal event.

Climate Change to 2085

By 2085, the peak level in Cardiff Bay during a 0.5% (1 in200) tidal event is predicted to rise to

just over 8.0mAOD. The defended areas of the site would be expected to remain flood free

during an event of this chance.

A small area of the site west of Havannah Street lies below 8mAOD. Taking a precautionary

approach, this area is highlighted as within the 0.5% (1 in 200) chance outline for 2085 on

Drawing 5097656/RCF/420. The flooding to this area is likely to be shallow and of short

duration. Verification of the level of the defence along Windsor Esplanade may show this area to

be defended.

Climate Change to 2110

Modelling results indicate that a 0.5% (1 in 200) annual chance event coincident with a QMED (1

in 2) flow in the rivers would result in the Bay level rising to 8.3mAOD. (This compares to an

estimated Bay level of 7.62mAOD during level fluvially dominated 1% (1 in 100) annual chance

event in the Rivers Taff and Ely - as shown in Table 5-2). This would result in overtopping of the

defences around the Graving Docks. The water would flood the areas of the Candidate Site

behind the defences and overland flows would develop into the low lying areas north of this site

as indicated by the arrows on drawing 5097656/RCF/420, Appendix B.

The majority of the site west of Havannah Street would remain flood free with some minor

overtopping in to the small lower lying area in the north part of this western portion of the site.

Although slightly deeper than this event in 2085, the flooding to this area is likely to be shallow

and of short duration. Verification of the level of the defence along Windsor Esplanade may

show this area to be defended.

Other sources

The Environment Agency Surface Water Flood Map does not indicate any significant risk from

this source.

Site summary

Those parts of the site on the landward side of the existing flood defences will remain flood free

during a current 0.1% (1 in 1000) chance tidal event. The site would remain flood free for all

fluvially dominated events over the development lifetime to 2110.

The lower lying areas of the Graving Docks, on the undefended side of the defences serve as

designated flood storage and would flood during a current 0.5% (1 in200) annual chance tidal

event. Further details of the proposed development uses included in the “mixed use” description

may allow certain development in this area for boating, boatyards, marinas or other water related

uses as recognised in TAN15. Further detailed assessment would be required to ensure the risk

can be managed and will be dependent on the specific land use proposed.

5097656-70-DG017-3.docx 83

The defence levels around the Graving Docks require verification, currently understood to be

between 8.0 and 8.1mAOD. By 2085 a 0.5% (1 in 200) chance tidal event could result in the

peak Bay level of 8.08mAOD just overtopping or coming within 0.02m of the top of the defence.

Subject to verification of the defence levels, the defended parts of the site would either suffer

flooding at the peak of the tide or just remain flood free to 2085.

By 2110, a 0.5% (1 in 200) chance event would overtop the defences resulting in flooding to the

site and the development of overland flow paths north into Stuart Street and Adelaide Street and

the low lying hinterland to the north as indicated by the arrows on Drawing Number

5097656/RCF/420/C, Appendix B . The period of overtopping would be of the order of ¾ hour.

Options to mitigate the future risk could include raising the levels of the existing defences to the

Graving Docks.

The section of the site west of Havannah Street lies on higher ground and all but a small part of

this area of the site would remain flood free over a development lifetime to 2110.

Up to 2085, it is considered likely that access would remain operational under all conditions to

the defended parts of the site and the high ground of the site area west of Havannah Street.

By 2110, it is likely that mitigation measures will be required to manage the risk to the site itself

and to the off-site access routes. Flood free refuge would be available in the higher ground in

the west of the site, west of Havannah Street.

Considering Site 73LBRM overall, it is considered likely that a large proportion of this site can be

developed in compliance with TAN15, subject to a more detailed site specific FCA being

undertaken.

6.3.9 Site 101LBEM

This site is not considered to be at risk from tidal inundation. The assessment of the flood risks

to the site are described in Section 5.6.10.

6.3.10 Site 94LBR

This site at Colchester Avenue lies within TAN15 Zone C2 (not benefitting from defence). It is

noted that the road at Colchester Avenue over which overland flows would need to pass to affect

the site is designated as being in TAN15 Zone C1 (benefitting from defence infrastructure).

The site is proposed for residential use which is considered highly vulnerable development as

defined in TAN15.



with current planning policy guidance

Tidal inundation

The site lies on the left (northern) west bank of the Roath Brook. This is outside the area for

which the 2D model was designed to assess the flood risk. The defence levels to the left bank of

the Roath Brook are based on the LIDAR levels within the 2d domain, rather than surveyed

levels. This will tend to underestimate the actual defence levels. A comparison has been made

between the LIDAR levels representing defences in the 2D model and surveyed levels used in

the EAW 1D ISIS model of the Roath Brook and Lower Rhymney15

. It is considered that the

LIDAR levels provide a reasonable estimate of the defences and do not result in overly

pessimistic estimates of flooding to the area which includes Site 94LBR.

The results of the 2D tidal inundation modelling for this site can be considered as indicative and

are likely to provide conservative estimates. Incorporating surveyed levels into the model may

15

Lower Rhymney and Roath Brook Flood Risk Mapping Study, ISIS v3, Environment Agency Wales, 2008

5097656-70-DG017-3.docx 84

reduce the risk slightly for the current situation. It is considered unlikely that the future scenarios

will be materially affected due to the extensive overtopping of the left bank.

The modelling indicates that the site is affected by water overtopping the left bank of the Roath

Brook during extreme tidal events.

At the time of writing, it is understood that the Environment Agency is developing a combined 1D

2D inundation model of the Roath Brook and its flood plain, including the area at Colchester

Avenue. It is likely that the results of the EAW modelling will represent an improvement in the

level of detail in this area compared to the inundation model developed for this SFCA.

Nevertheless, the current model is considered to provide a level of detail appropriate to the

strategic broad scale nature of the SFCA. Further detailed modelling will be required, to confirm

the viability of mitigation measures to mitigate the flood risk.

The results of the 2D modelling in relation to Site 94LBR are described below.


The model results indicate that the site is inundated by a current 0.5% (1 in 200) annual

chance tidal event over approximately 75% of the site area. Depths of flooding would be

shallow, estimated to be generally less than 0.1m.

The modelling indicates that the majority of the site would be flooded to depths of at least

1.0m during a current 0.1% (1 in 1000) annual chance event. A small portion in the east of

the site would remain flood free.

The corresponding velocities during a 0.1% (1 in 1000) chance event would be below

0.3m/s over the majority of the site and the access to Colchester Avenue.

Based on the modelling results, the site fails to comply with TAN15 A1.14 and A1.15


The extent of flooding to the site would not increase significantly for a 0.5% (1 in 200)

annual chance tidal inundation event, but depth would increase dramatically to over 1.0m.

The flooding would extend north of the site to properties on Sinclair Drive.

Recognising the limitations of the modelling, the hazard score (see Section 3.3.0 above for

definition) during 0.5% (1 in 200) annual chance event indicates that access via Colchester

Avenue would remain passable to emergency services only.


Depths of flooding to the site would increase further by 2110 during a 0.5% (1 in 200)

annual chance event.

The eastern portion of the site would remain flood free for this event.

Without mitigation, it is unlikely that an off-site access and egress route for the flood free

area could be maintained via Colchester Avenue under all conditions. Alternative

emergency access could potentially be available via land to the east of the site.

Based on the modelling results, the site fails to comply with TAN15 A1.14 and A1.15.

Mitigation of the risk

Any mitigation of the risk to the site would need to be achieved without increasing the flood risk

elsewhere.

The installation of a structure on the culvert outfall of the Roath Brook to the Rhymney could be

considered and may provide benefit in preventing tide levels backing up the Brook channel.

However, it is questionable if there is sufficient storage in the Roath Brook channel to contain the

fluvial volumes which would accumulate during tide lock periods.

5097656-70-DG017-3.docx 85

Localised raising of ground levels on the site may mitigate the flood risk to some parts. However,

the displaced volumes are likely to be significant, given that the depths of water on the site are

indicated by the modelling to exceed 1.0m for a current 0.1% chance event and the future 0.5%

( 1 in 200) chance event. Compensatory storage may be available on the site, but this would

reduce the available development area.

The site lies in an area which is already heavily urbanised. Raising defences to the Roath Brook

would have benefits alleviating flood risk to the existing properties. It is possible that such

mitigation may reduce the risk sufficiently to facilitate development of Site 94LBR.

Strategic solutions for the area as a whole would need to be explored in partnership with the

Environment Agency. These may provide benefits to reducing risk to the site 94LBR sufficient

for TAN15 criteria to be met. Further detailed work would be required to confirm this.

Fluvial Flood Risk

The flood levels in the Roath Brook during extreme fluvial flood events have been checked

against the relevant tidal events (i.e. those required for assessment of A1.14 and A1.15 of

TAN15) with reference to an Environment Agency 1D HEC RAS model of the Roath Brook16

.

Fluvial flood levels are less than those for the corresponding tidal events. The flood risk in the

lower reach of the Roath Brook is therefore considered to be tidally dominated. It is likely that

mitigation of the tidal flood risk to Site 94LBR will also mitigate the fluvial flood risk.

Other sources of Flood Risk

Reference to the EAW‟s FMSW map indicates that shallow flooding may affect some small areas

of the site during a 1 in 30 year rainfall event. More extensive flooding across approximately

60% of the site is indicated for a 1 in 200 year rainfall event. These maps are indicative only.

The management of surface water risk to the site and the runoff from any proposed development

would need to be managed as part of the overall design of the site. It is likely that these risks

can be mitigated.

Site summary

Under existing conditions, the majority of the site fails TAN15 for all development types. A small

parcel of land (approximately 0.24ha) in the east of the site remains flood free over development

lifetime to 2110. Providing that an emergency access and egress route can be established,

development of this part of the site for residential use may be achievable in line with TAN15

guidance.

Any more significant development of this site will require further more detailed assessment of the

existing flood risk. This is likely to require more detailed modelling. This could be informed by a

2D inundation model of the area which we understand is under development by the Environment

Agency.

Mitigation of the risk to the site is likely to be problematic. A strategic solution to alleviate

flooding to the Colchester Avenue and Newport Road areas may reduce flood risk to the site

sufficiently to facilitate development in line with TAN15 guidance. Such a strategic solution is

likely to require a multi agency approach involving the Environment Agency, Cardiff Council and

Welsh Government.

16


5097656-70-DG017-3.docx 86

7. Conclusions

7.1 Site Summaries

The following tables summarise the situation assessed for each study site, based on the existing

information. The main flood risk for each site is highlighted together with its current planning

status and a cross reference to the relevant section above which describes the likelihood of any

new planning application achieving compliance with the current planning policy guidance in

TAN15. Each of the four sub areas are summarised in Table 7-1 to Table 7-4.

Table 7-1 - Summary of Sub Area A – Wentloog Tidal Development Sites

Site ID Name/ Location Main Flood Risk Reference

76LGW South of Wentloog Avenue Tidal S.3.3.1

p.21

22LGRM Trowbridge Mawr Tidal S.3.3.2

p.22

21LGR Areas 9 - 12 St. Mellons Surface Water S.3.3.3

P.24

23LGR Land between Crickhowell Rd and Willowbrook Rd

Surface Water S.

p.25

24LGRM Land East of Cypress Drive, St. Mellons

Tidal S.3.3.5

p.25

74LGEMW Wentloog South of the Railway Line Tidal S.3.3.6

p.26

2LGRM Land at St. Mellons Business Park Tidal S.3.3.7

p.27

1LGRM Land at Wentloog Levels Tidal S.3.3.8

p.28

106LGEM Land at St Mellons/ Wentloog Tidal S.3.3.9

p.29

GTchPk Action Area Tidal S.3.3.10

p. 30

Wentloo_ii Action Area Tidal S.3.3.11

p.31

Wentloo_iii Action Area Tidal S.3.3.12

p. 33

3SBR Pill Du Farm, Hendre Lane, Trowbridge

Tidal

Surface Water

S.3.3.13

p.34

55LGR Land at Trowbridge Road Surface Water S.3.3.14

p.35

56LBR Land at Former St John‟s College Negligible S.3.3.15

p.36

5097656-70-DG017-3.docx 87

Table 7-2 Summary of Area G – Junction 33, M4

Site ID Name/ Location Main Flood Risk

Reference

102 LGR Land South of Creigiau

Sewers including (SPS) Fluvial (minor watercourses/ culvert blockage)

S.4.6.1

p.43

46LGRM Junction 33

Fluvial (minor watercourses/ culvert blockage)

S.4.6.2

p. 43

Table 7-3 - Summary of Sub Area H – West Central Taff/Ely, Development Sites


Reference

8LGR West of Clos y Cwarra, Michaelston-super-Ely, Cardiff

Surface water Potential risk to part of site from ordinary watercourse

S.5.6.1

p. 56

16LBR Ely Bridge, Western Business Centre, Cowbridge Road East Fluvial

S.5.6.3

p.57

17LGR Ely Bridge Farm, Dyfrig Road, Ely. Fluvial

S.5.6.11

p.62

31LBEM Land at Brindley Road, Leckwith, Cardiff

Limited

Consider culvert blockage

S.5.6.6

p.59

32LGR Land at Michaelston Road, Cardiff Low risk

Possible Sewer

S.5.6.2

p.56


Fluvial S.5.6.5

p.58

52LBR Paper Mill Road, Cardiff , CF11 8PH Fluvial

S.5.6.4

p.58

61LBRM Penarth Road Car Park Site (adjacent to Cardiff Central Railway Station) - includes Riverside Sidings/ Semaphore House

Low Risk S.5.6.8

p.61

62LBR Land adjacent to Clive Lane Fluvial risk to access routes

S.5.6.6

p.59

70LBEM Central Square Low risk

Possible sewer

S.5.6.7

p.60

75SBR Land East of Great House Farm, Michaelston Road

Low risk

SW management

S.5.6.1

p.56

5097656-70-DG017-3.docx 88


Reference

77LBR Ferry Road, Grangetown, Cardiff Fluvial

Access during 0.1% chance event

S.5.6.6

p.59

78LBRM Wholesale Fruit Centre Cardiff Fluvial

0.1% chance only

S.5.6.6

p.59

83LBRM Former J R Freeman Factory, Penarth Road, Cardiff

Fluvial

0.1% chance only

S.5.6.6

p.59

96LGED Land adjacent to Llandaff Campus to river bank and rear of campus

Low Risk

Possible SW runoff

S.5.6.9

p.61

101LBEM Land off Dumballs Road, Cardiff Negligible S.5.6.10

p.61

111LGR Land north west of Great House Farm, Michaelston Road

Low Risk

Management of on site drainage channels.

S.5.6.1

p.56

5097656-70-DG017-3.docx 89

Table 7-4 - Summary of Sub Area I - Bay Area to Pengam Green, Development Sites

Site ID Name/ Location Main Flood Risk Reference

71LBC Wales Millennium Centre 'Phase 3' Land

Surface Water S.6.3.1

p.73

58SBR Moorland Road, Cardiff Low risk

Tidal & Surface Water impacts to access routes

S.6.3.2

p.74

20LBU Queens Gate Car Park Surface Water S.6.3.3

p.74

19LBU Pengam Green Tidal S.6.3.4

p.75

18LBM Queen Alexandra Head Tidal S.6.3.5

p.77

60LBRM Porth Teigr (formerly known as Roath Basin South), Cardiff Bay, Cardiff

Tidal S.6.3.6

p. 78

57LBR Hannah Street, Bute Town, Cardiff Surface Water impacts to access routes

S.6.3.7

p.80

73LBRM Cardiff Graving Docks / Landsea Gardens / Landsea Square / Havannah Street car park, Cardiff Bay

Tidal S.6.3.8

p.81

101LBEM Land off Dumballs Road, Cardiff Negligible

See Area H

S.6.3.9

p.83

94LBR Colchester Avenue, Cardiff Tidal S.6.3.10

p.83

8. Appendices Appendix A - SFCA Area Overview Map

Appendix B – Area Specific Maps



Appendix E – Area A – 2D Tidal Inundation Modelling Assumptions and Results.

Appendix F – Area I (Pengam Green to Cardiff Bay) 2D Tidal inundation modelling

Appendix G – Assessment of risk of tidal inundation via Cardiff Docks to Cardiff Bay.

5097656-70-DG017-3.docx

Appendix A - SFCA Area Overview Map

A.1 Cardiff SFCA Area Overview Map – Drawing Number

5097656/RCF/002

5097656-70-DG017-4.docx

Appendix B – Area Specific Maps B.1 Drawing Numbers

5076243/RCF/120 – Area A - Wentloog Tidal. Flood Extents for 0.5% (1 in 200) Annual Chance Events – Current and with Climate Change to 2085 and 2110.

5076243/RCF/121 – Area A – Wentloog Tidal. 0.1% (1 in 1000) Annual Chance Flood event - 2010 Current Day. Predicted Maximum Flood Depths Contours

5076243/RCF/122 – Area A – Wentloog Tidal. 0.1% (1 in 1000) Annual Chance Flood event – 2010 Current Day. Predicted Velocity Contours

5076243/RCF/020/B – Area H – Cardiff West Central Taff & Ely. Flood Extents for 1% (1 in 100) Annual Chance Events – Current and with Climate Change to 2085 and 2110.

5076243/RCF/320 – Area H – Cardiff West Central Taff & Ely. Flood Extents for 1% (1 in 100) Annual Chance Events – Current and with Climate Change to 2085 and 2110.

5076243/RCF/321 – Area H - Cardiff West Central Taff & Ely. 0.1% (1 in 1000) Annual Chance Flood event – 2010 Current Day. Predicted Maximum Flood Depths Contours

5076243/RCF/322 – Area H - Cardiff West Central Taff & Ely. 0.1% (1 in 1000) Annual Chance Flood event – 2010 Current Day. Predicted Velocity Contours

5076243/RCF/420/B – Area I – Cardiff Bay to Pengam Green Flood Extents for 0.5% (1 in 200) Annual Chance Tidal Events – Current and with Climate Change to 2085 and 2110.

5076243/RCF/421/C – Area I – Cardiff Bay to Pengam Green. 0.1% (1 in 1000) Annual Chance Flood event - 2010 Current Day. Predicted Maximum Flood Depths Contours

5076243/RCF/422/B – Area I – Cardiff Bay to Pengam Green. 0.1% (1 in 1000) Annual Chance Flood event - 2010 Current Day. Predicted Velocity Contours.


Areas A & H

5097656-70-DG017-3.docx

Table C.1 Proportion of Study Sites in Area A affected by Indicative Depths of flooding for 0.1% annual chance tidal flood event 2010

Site ID Name/ Location Existing Land Use Development Category Site

Area (ha)

Flood free or

<0.6m

>0.6m, <1.0m

>1.0m

76LGW South of Wentloog Avenue Previously Undeveloped Land

Waste Management Facility 60.0 36.6 12.5 9.2

22LGRM Trowbridge Mawr Previously Undeveloped Land

Residential/Industrial and Ancillary Community

35.0 35.0 0.0 0.0

21LGR Areas 9 - 12 St. Mellons Previously Undeveloped Land

Residential and Ancillary Community 18.8 20.4 0.0 0.0

23LGR Land bet. Crickhowell Rd and Willowbrook Rd Previously Undeveloped Land

Residential and Ancillary Community 5.6 5.6 0.0 0.0

24LGRM Land East of Cypress Drive, St. Mellons Previously Undeveloped Land

Mixed Use 9.5 9.2 0.0 0.0

74LGEMW Wentloog South of the Railway Line Previously Undeveloped Land

Employment and Waste 8.3 7.7 0.5 0.1

2LGRM Land at St. Mellons Business Park Previously Undeveloped Land

Mixed Use 3.9 4.0 0.0 0.0

1LGRM Land at Wentloog Levels Previously Undeveloped Land

Mixed Use 28.0 25.6 2.6 0.0

106LGEM Land at St Mellons/ Wentloog Previously Undeveloped Land

Mixed Use 71.0 72.6 0.4 0.0

GTchPk Action Area Previously Undeveloped Land

Green Tech Park 14.7 12.6 1.4 0.6

Wentloo_ii Action Area Previously Undeveloped Land

Business, Industry and Warehousing 15.6 15.6 0.0 0.0

Wentloo_iii Action Area Previously Undeveloped Land

Business, Industry and Warehousing 15.4 15.3 0.1 0.0

3SBR Pill Du Farm, Hendre Lane, Trowbridge, Cardiff Yard and Scrubland Residential 0.5 0.5 0.0 0.0

55LGR Trowbridge Road, Cardiff Greenfield Residential 2.6 1.5 0.0 0.0

56LBR St John‟s College Site, Greenway Road, Trowbridge, Cardiff

Brownfield Residential 2.5 2.5 0.0 0.0

5097656-70-DG017-3.docx

Table C.2 Proportion of Study Sites in Area H affected by Indicative Maximum Depths of flooding for 0.1% annual chance fluvial flood event – 2010

Site ID Name/ Location Development Type

Total Site Area (ha)

Area of Site affected (ha)

Flood free or <0.6m

>0.6m, <1.0m >1.0m

8LGR West of Clos y Cwarra, Michaelston-super-Ely, Cardiff Residential 1.60 1.0 0.0 0.0

16LBR Ely Bridge, Western Business Centre, Cowbridge Road East

Residential/ Commercial/ Retail/ Community uses

22.30 0.8 2.1 2.6

17LGR Ely Bridge Farm, Dyfrig Road, Ely. Education & Mixed Use 0.70 1.0 0.0 0.0

31LBEM Land at Brindley Road, Leckwith, Cardiff Employment uses 8.00 1.0 0.1 0.1

32LGR Land at Michaelston Road, Cardiff Residential & Strategic Open Space.

9.80 1.0 0.0 0.0


Residential 1.60 0.4 0.5 0.5

52LBR Paper Mill Road , Cardiff , CF11 8PH Residential 0.80 0.6 0.3 0.1

61LBRM Penarth Road Car Park Site (adjacent Cardiff Central Railway Station) Riverside Sidings/ Semaphore House

Residential & Mixed-use 3.00 1.0 0.0 0.0

62LBR Land adjacent to Clive Lane Residential 2.50 1.0 0.0 0.0

70LBEM Central Square Commercial/ Public Transport Interchange

1.58 1.0 0.0 0.0

75SBR Land East of Great House Farm, Michaelston Road Residential 0.24 1.0 0.0 0.0

77LBR Ferry Road, Grangetown, Cardiff Residential 11.73 1.0 0.3 0.0

78LBRM Wholesale Fruit Centre Cardiff Residential/ Commercial/ Employment

7.00 0.7 2.1 0.2

83LBRM Former J R Freeman Factory, Penarth Road Mixed use – Employment/ Retail/ Residential

3.62 0.6 1.3 0.0

111LGR Land northwest of Great House Farm, Michaelston Road Residential 0.74 1.0 0.0 0.0

5097656-70-DG017-3.docx


D.1 Modelling approaches common to both Areas A & I

The 1D-2D hydraulic models have been constructed using ESTRY TUFLOW, which is an

industry standard hydraulic modelling software package. The models‟ outputs provide estimates

of both flood depths and flow velocities over the modelled extents.

The modelling of tidal overtopping and breach of the defences to the Severn Estuary is based on

a development of the approach and information gathered by the Environment Agency for the

Severn Estuary Flood Risk Management Strategy (SEFRMS).

Digital Elevation Model (DEM)

The Digital Elevation Models (DEM) of the study areas form a basis of the 2D hydraulic models.

The DEMs of the study areas have been generated using a combination of filtered LiDAR data

and NEXTMap data. The NEXTMap data has been used at places where the LiDAR data was

not available or the resolution was poor. The DEMs for both areas, A & I, were created by

extracting ground elevations on a computational grid having a cell size of 10m x 10m. The

chosen grid size of 10m provided a reasonable balance between level of topographic details

required to achieve accuracy appropriate for the nature of study, and reasonable model

runtimes.

The DEM is based on LiDAR available at November 2009.

D.2 Limitations of the modelling results

The 2D modelling provides a significant improvement in the level of confidence in the estimated

flood depths and velocities during extreme flood events and so informs the assessment of flood

consequences to the study sites. However, further detailed modelling will be required for site

specific studies, incorporating surveyed levels on the study sites themselves and key overland

flow routes. No testing of any proposed site layouts has been included as part of this strategic

modelling study.

D.3 Use of model results for sites not included in the SFCA Reports

The modelling for Area A and Area I has been developed specifically for the study sites listed in

the list of Study Sites in Section 3 and Section 4, respectively.

The results of the flood modelling should not be extrapolated to sites other than those specifically

listed in this report without due consideration being given to the assumptions inherent in the

model development. The confidence in the model results away from these sites will vary, being

affected by the assumptions made in developing the model.

5097656-70-DG017-3.docx

Appendix E – Area A – 2D Tidal Inundation

Modelling Assumptions and

Results.

E.1 Model Set-up

The model simulates the spreading out of tidal floodwaters onto and over the flood plain during

extreme tides which overtop and in places may breach the existing flood defences. It will also

estimate flood levels and velocities across the proposed development sites. It should be noted

that the model is not intended to predict flood levels upstream on the River Rhymney or River

Ebbw.

The Wentloog Levels area is considered at risk of tidal flooding resulting from overtopping from

rivers and overtopping and breach of the Severn Estuary defences. The methodology adopted to

predict tidal flood risks to the area and the key assumptions are set out below.

E.1.1 Spatial Extent of the Model

The Area A model covers the Wentloog Levels area. The western extent of the model is located

along the River Rhymney and the eastern extent is located along the River Ebbw. The Severn

Estuary shoreline forms the southern extent of the model.

E.1.2 Key Topographic Details

A site visit was undertaken prior to building the hydraulic model to assess key topographic

features which are likely to influence local flooding mechanisms and therefore need incorporating

into the model. Features such as the flood defences to the estuary, along the River Rhymney

and River Ebbw; the railway embankment and the culverts underneath it, are likely to affect

overland flow paths. The methods adopted to incorporate these features into the model and

modelling assumptions made are set out below.

Left Bank / Defence Levels on the Lower Rhymney

The levels along the left (eastern) bank of the River Rhymney would govern the volume of

floodwater overtopping from the river and potentially developing into overland flow paths towards

the study sites. These levels were extracted from the 1D HEC-RAS model of the River Rhymney

provided by the Environment Agency (EA Wales, Lower Rhymney and Roath Brook, v2 2007).

The bank levels were stamped onto the existing DEM, interpolating the levels between the

locations of the HEC RAS cross sections.

Since issue of the March 2010 SFCA 2D modelling addendum, an additional weakness in the left

bank defences was highlighted to the Severn Estuary Flood Risk Management Study

(SEFRMS17

) team by the Environment Agency‟s Area Flood Risk Manager. This improvement in

the available information has been incorporated into the SFCA hydraulic model for Area A. The

modelling of this newly identified location of potential breach in the defences is described in

section E.2.3, below.

Right Bank / Defence Levels on the River Ebbw

Limited topographic information on the bank levels along the River Ebbw is available and

therefore bank levels are based on the LiDAR data. These were compared with top of bank level

in an Environment Agency ISIS model of the lower River Ebbw. Retention of the LiDAR levels

17

Severn Estuary FRM Strategy, Baseline Flood Consequences Assessment, Working Draft Report, Environment Agency June 2009.

5097656-70-DG017-3.docx

are considered a conservative assumption as LiDAR will tend to underestimate levels on the top

of defence structures.

Coastal Defence Levels along the Shoreline

These levels are based on detailed topographic survey18

which was undertaken for the

Environment Agency‟s SEFRMS19

.

Railway Embankment and Culverts Underneath

The Railway embankment which runs across the Wentloog Levels area serves as an informal

defence to prevent tidal flooding spreading overland north of the railway line. Levels along the

railway embankment are based on the LiDAR data.

Several culverts through the railway embankment exist which could allow conveyance of

floodwaters to the north of railway line. A site visit was undertaken to assess the significance of

these culverts to convey flows across the railway line. Culverts which are likely to affect flood

risks to the proposed development sites are incorporated into the model. No detailed topographic

survey data is available for these culverts. The sizes of some of the culverts were available from

the Network Rail. For the culverts where no data is available, sizes have been assumed based

on on-site observations and the channel width derived from maps of the area. Inverts for all the

culverts are based on the LiDAR data. These culverts are modelled as one-dimensional (1D)

elements in the model.

E.2 Key Hydraulic Assumptions

The Wentloog Levels area is considered at risk of tidal flooding due to a combination of

overtopping and breach of the coastal defences. The volumes entering the flood plain as a result

of breach of a defence far exceed those resulting from overtopping.

E.2.1 Modelling Breach of Defences

The coastal defences along the Wentloog levels have been assessed for susceptibility to breach

as part of the SEFRMS.

The formation of a breach in the defences is dependent on the flow of water down the back of

the defence structure as a result of overtopping. Once the tolerable flow of water is exceeded,

erosion of the structure will commence leading to failure and potentially breach of the defence.

The susceptibility of a defence will depend on a number of interrelated factors including the

defence height, type of defences, slope etc.

It is assumed that the defences will be maintained in their current condition and will be

maintained into the future. No raising of defences in the future is assumed. This results in an

increased probability of overtopping and breach of the defences if sea levels rise as predicted.

E.2.2 Joint Probability Still Water Tide Level and Deep Water Wave Height

The water overtopping a defence will result from a combination of the tide level (still water level

or SWL) and the coincident wave heights. As part of the SEFRMS the joint probability of a series

of tide levels and wave heights were assessed. For events of a given probability only a limited

number of combinations of tide level and wave will result in breach of the defence. As the tide

level (SWL) increases the coincident wave height decreases.

The joint probability assessment has been updated for the SFCA to reflect the new extreme tide

level estimates released by the Environment Agency in February 2011 (see Section 2.3.2).

18

Topographic Survey of flood defences (Infomap Ltd, May 2009) for SEFRMS 19

Severn Estuary FRM Strategy, Baseline Flood Consequences Assessment, Working Draft Report, Environment Agency June 2009.

5097656-70-DG017-3.docx

E.2.3 Breach Locations and related model assumptions

Based on the detailed breach analysis methodology undertaken as a part of the SEFRMS study,

seven locations in the Wentloog levels are considered to be at risk of breach over the 100 year

period to 2110.

These locations are referenced20

as listed below and are marked on drawings 5097656/RCF-120

to RCF-122 in Appendix B.

SR1-RREB River Rhymney East Bank (downstream of rail bridge)

SR1-0 Little Wharf

SR1-1 Rhymney Great Wharf

SR1-2 Sluice Farm

SR1-3 Peterstone Great Wharf

SR1-6 Lighthouse Park

SR1-REWB River Ebbw West Bank

The numbers of locations considered susceptible to breach increases over the time horizons

considered, due to the effect of predicted sea level rise, as follows:

Table E.1 - Locations of Breaches for different time horizons and by event probabilities

Year Event chance Location References

2010 0.5% (1 in 200) SR1-RREB & SR1-2

0.1% (1 in 1000) As above plus SR1-0

2085 0.5% (1 in 200) As above plus SR1-3 & SR1-6

2110 0.5% (1 in 200) As above plus SR1-1 & SR1-REWB

The boundary conditions adopted to represent the breaches in the 2D model are those

developed for the SEFRMS. The main parameters defining the breach in the model are:

A period of 1 hour was set to represent the time for the full breach to form from the time of

its commencement.

All the these breaches are assumed to develop to 50m in width once initiated.

The toe levels to which the defences reduce at each breach location are:

SR1-RREB - 7.5mAOD

SR1-0 - 6.0mAOD

SR1-1, - 5.7mAOD

SR1-2 - 7.0mAOD

SR1-3 - 7.0mAOD

SR1-6 - 7.0mAOD

SR1-REWB 7.3mAOD

20

Note that the referencing of the breach locations has been changed since the March 2010 SFCA 2D modelling addendum to be consistent with the referencing used in the Environment Agency‟s SEFRMS project.

5097656-70-DG017-3.docx

These levels are based on the higher of the average toe levels (base of the defence

embankment) on the landward or seaward side of the defences (or the river side in case of the

river defences).

Table E.1 includes combinations of Still Water tide Levels (SWL) and Deep Water Wave Height

(HO) which have been assessed to cause breach at each of the locations. The Table also

includes trigger levels (level at which breach is initiated) for different modelling scenarios. The

trigger level reflects the minimum SWL that is assessed to cause breach in combination with a

given HO.

E.2.4 Boundary Conditions

A spatially varying tidal boundary condition (tide levels versus time) along the shoreline has been

used to model overtopping and breach of the Severn Estuary defences during extreme tidal

events. The boundary condition has also been extended up the channels of the River Rhymney

and the River Ebbw to model overtopping of river banks.

River Ebbw

Along the River Ebbw, the tidal boundary condition extends up to A4042 roundabout (Docks

Way). Only limited topographic levels are available along the River Ebbw upstream of this

location. Inspection of the LiDAR data indicates that upstream of the A4042 roundabout area,

defences exist to the east of residential development at Beech Grove and Lighthouse Road.

Levels along these defences are higher than the tide levels. It should be noted that LiDAR data is

likely to underestimate the defence height and therefore in absence of accurate defence levels,

tidal boundary conditions were not extended upstream of A4042 roundabout.

It should be noted that the model is not designed to give detailed accurate results in the area of

the River Ebbw upstream of the A4042 roundabout. Any additional flood risk to the study sites

from this potential source is considered unlikely to be significant, given that the nearest site is

over 5½ km from the Ebbw. Caution should be exercised in interpreting the model results to

assess sites further east than the study sites listed in Table 3-1.

River Rhymney

Along the River Rhymney, the tidal boundary condition is extended to a point approximately 0.75

km north of the A48 roundabout. The defence levels upstream of this location are high enough

to prevent overtopping during the extreme tidal events tested and therefore, the tidal boundary

conditions are not extended beyond this location.

During extreme tidal events, the levels along the River Rhymney channel are likely to be higher

than the tide levels at its mouth due to the contribution of fluvial flows in the river channel. An

Environment Agency one-dimensional (1D) HEC-RAS model has been used to estimate

increases in level up the river channel during extreme tidal events. A 20% (1 in 5) annual

chance fluvial flow was routed against the extreme tide levels. 21

The resulting increase in water level up to the Newport Road bridge is less than 150mm for all

tide levels tested. The impact on flooding to the Wentloog levels is not significant. The elevated

water levels in the river channels are nonetheless incorporated into the model for completeness.

This is achieved by means of three level change points along the route of the Rhymney, the tide

levels being linearly interpolated along the model boundary.

These elevated water levels in the river channels have been used to simulate overtopping of the

defences.

21

In the absence of existing estimates of the QMED flow on the Rhymney or Roath Brook, we consider that use of the Q5 or 20% (1 in 5) annual chance flow represents a conservative approach. We have confirmed, based on analysis of ratios of ReFH flow estimates that the Q5 is greater than a QMED+20%. The Q5 has therefore also been used for the climate change scenario in 2060.

5097656-70-DG017-3.docx

Change in Tide level along the Severn Estuary

The predicted tide level (SWLs) for an extreme event of a given annual chance will increase

along the Severn Estuary from the mouth of the Rhymney to the River Ebbw. This is

incorporated into the hydraulic model by interpolation of the tide levels.

Table E.2 shows tide levels used for different modelling scenarios and the variation in tide levels

between the River Rhymney and the River Ebbw.

E.3 Modelling Scenarios and Results

The latest development of the 2D inundation model has been run for different scenarios as listed

in Table E.2. The spread of floodwater as represented by the model have been analysed. The

predicted maximum flood depths and corresponding velocities for each scenario are plotted in

the drawings presented in Appendix B.

The consequences for each of the study sites are described in Section 3.3 of the main report.

An overview of the tidal flood risk to the Wentloog Levels, specifically that part within the Cardiff

Authority area, is given below.

0.5% (1 in 200) annual chance events, 2010

For the existing situation, base year 2010, flooding during 0.5% (1 in 200) chance tidal events

results from some overtopping from the Severn Estuary at Sluice Farm and the left (east) bank of

the River Rhymney. Currently the most significant source of tidal flooding is the predicted breach

of the Severn Estuary flood defence at Sluice Farm (SR1-2). (This section of the defence

currently has a 2% (1 in 50) annual chance of breach in any year based on the SEFRM

analysis). The flooding extends immediately northwest of the breach site, but does not extend

quite as far as the nearest study site (Site 74GEMW). The overland flow path does, however,

extend sufficiently north to cover the south eastern boundary of Site 1LGRM. The breach of the

defence on the River Rhymney downstream of the rail bridge is less significant.

The main access routes into the general Wenloog area would remain flood free for this event

currently.


The two events modelled, each with a 0.1% (1 in 1000) annual chance of exceedance, represent

the highest tide level and the lowest tide level at which breach will occur. The lower tide (SWL)

is coincident with a higher deep water wave height. The results of the modelling for these events

are presented on Drawing 5097656/RCF/121, Appendix B.

The breaches at Little Wharf and Sluice Farm result in wide spread flooding across the area;

although, within the Cardiff Authority area this is largely restricted to the seaward side of the

main rail line. The higher tide level of 9.2mAOD results in fewer breaches of the Severn Estuary

defences, but causes increased overtopping from the River Rhymney. As a result more water

spreads east along the route of the rail line and Lamby Way, but is not significant for the study

sites assessed.

Depths of flooding remain below 0.6m over the majority of the Cardiff Authority area. However,

depths do exceed 1.0m in a number of locations. This occurs in areas generally local to the

routes of the drainage channels or reens.

Of particular note is the roundabout on Lamby Way in the east of the area where depths are

predicted to exceed 1.0m with corresponding velocities greater than 0.45m/s. This is the main

access route west to the Rhymney crossing into Cardiff.

Depths and velocities are also indicated to be high at two locations of overland flows crossing

Mardy Road. This affects the main crossing north out of the Wentloog levels over the rail line.

5097656-70-DG017-3.docx

Depths and velocities are predictably high local to the breach locations at Little Wharf (SR1-0)

and Sluice Farm (SR1-2). Site 76LGW is affected most. The impacts on all study sites are

described in relation to TAN15 A1.15 acceptability criteria in Section 3.3 of the main report.

It is also noted that flow velocities are high, in excess of 0.45m/s, at a localised area along the

rail line in the far west of the area. This does not affect the study sites directly, but may be worthy

of further investigation given this rail line‟s strategic importance to south Wales.


This scenario, the shorter of the two development lifetimes considered, is used to assess

commercial sites against A1.14 of TAN15, i.e. the threshold of flooding.

By 2085 the Severn Estuary Defence at Peterstone Great Wharf is indicated to be susceptible to

breach during tidal events of 0.5% (1 in 200) annual chance, in addition to the locations at Sluice

Farm, Little Wharf and the left bank of the River Rhymney. The result is widespread flooding

extending over the Wentloog Levels area including the majority of study sites and the access

egress routes.


The extent of the flooding during events of this annual chance is not significantly greater than for

2085, although sites at the edge of the area flooded in 2085 will be completely inundated, Site

3SBr, Site 24LGRM and 105LGEM.

There are two additional locations subject to breach during such events, but these are located a

considerable distance to the east in the Newport Authority area. Any impact on the study sites is

masked by the effects of the breaches more local to the study sites, indicated on the Drawings in

Appendix B.

5097656-70-DG017-3.docx

Table E.2 - Sea level scenarios tested for Area A – considering the joint probability of surge tide level and wave height

YEAR

Tidal Events - Annual

Probability

Tide level variation

River Rhymney to River Ebbw

(mAOD)

Tide Level at which Breach starts at each location – mAOD to one decimal place

(coincident wave height required to cause breach)

Scenario Reference

SR1-RREB SR1-0 SR1-1 SR1-2 SR1-3 SR1-6 SR1-REWB

River Rhymney east bank

(No significant

wave climate)

Little Wharf Rumney Great Wharf

Sluice Farm Peterstone Great Wharf

Lighthouse Park

River Ebbw West Bank

2010

0.5% 8.6 to 8.9 8.2 No Breach No Breach No Breach No Breach No Breach No Breach SFCA_Scenario1_2011

8.2 to 8.5 8.2 No Breach No Breach 8.3

(0.81m wave)

No Breach No Breach No Breach SFCA_Scenario2_2011

0.1% 9.2 to 9.4 8.2 a

No Breach 8.8

(zero wave)


8.7 to 9.0 8.2 8.6

(0.7m wave)

No Breach 8.2

(0.9m wave)


2085

0.5% 9.2 to 9.5 8.2 9.1

(0.5m wave)

No Breach 9.1

(zero wave)


8.9 to 9.2 8.2 9.0

(0.9m wave)

No Breach 8.3

(0.9m wave)

8.8

(0.9m wave) No Breach No Breach SFCA_Scenario6_2011

2110

0.5% 9.6 to 9.9 8.2 9.6

(zero wave)

No Breach 9.1

(zero wave)

No Breach No Breach 9.8

(zero wave)

SFCA_Scenario7_2011

8.8 to 9.1 8.2 8.1

(1.3m wave)

8.5

(1.3m wave)

7.8

(1.3m wave)

8.4

(1.3m wave)

8.9

(1.3m wave)

No Breach SFCA_Scenario9_2011

5097656-70-DG017-3.docx

E.4 Future Improvements

Two-dimensional hydrodynamic modelling has been undertaken to assess the risk of

tidal flooding to the proposed development site in the Wentloog Levels. The modelling

assumptions and the level of topographic and hydraulic details incorporated within the

model are based on the current available information.

We recommend that further improvements in the understanding of some of the

information and data used in the modelling should be investigated. It is possible that

further refinement of the hydraulic model may be appropriate for further analysis as

part of site specific FCAs.

Some areas to be investigated further are identified below:

Topographic survey of defence levels along the left bank of the lower Rhymney

would be required to confirm the levels used in the current modelling.

No assessment of the structural integrity of defences along the left bank of the

River Rhymney has been undertaken.

A location considered susceptible to breach on the left bank of the Rhymney

immediately downstream of the railway bridge has been incorporated. The level

of detail applied in this study should be improved upon if site specific

assessments are required of sites located nearer the potential breach location in

the west of the area.

The current study used flow sets which were supplied with the model by the

Environment Agency, and which are considered reasonable in assessing the

“order of magnitude” of increase in levels up the river channel. Given the small

increase in river levels relative to the magnitude of overtopping of the defence

levels used in the current study, we would not anticipate that revision of the

hydrology will significantly affect the flood risk to the Study Sites.

Culverts present underneath the railway embankment are based on data

supplied by the Network Rail. For culverts where data was not available, sizes

have been assumed based on on-site observations and channel width. It is

considered likely that more accurate information will confirm culvert dimensions

are narrower than those assumed in the model. We consider that the approach

adopted for the current study is conservative and will tend to overestimate

conveyance of tidal flood flow north of the rail line through the Wentloog levels.

Modelling of proposed site layouts would be required to identify any potential

impacts on flood risk elsewhere and to inform the development of potential


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Appendix F – Area I (Pengam Green to

Cardiff Bay) 2D Tidal

inundation modelling

F.1 Model Set-Up

The study area of Pengam Green to Cardiff Bay is considered to be at risk of tidal

flooding resulting from overtopping both from the River Rhymney and over the

defences to the Severn Estuary. Breach of the Severn Estuary Defence is also a

potential source of flooding. The methodology of the 2D tidal inundation modelling

adopted to predict tidal flood risks to the study area and the key assumptions made

are set out below.

F.1.1 Spatial Extent of the Model

The Area I 2D model covers Pengam Green to Cardiff Bay area (excluding the

impounded Bay itself). The spatial extent of the model extends from the Cardiff Bay

area in the west to the River Rhymney in the east.

Figure F.1 - Extent of Area I Inundation Model

5097656-70-DG017-3.docx

The Severn Estuary shoreline forms the south eastern extent of the model. The Roath

Brook runs across the north of the study area in a west-easterly direction.

F.1.2 Key Topographic Details

Key topographic and hydraulic features specific to the study area include flood

defences along the Roath Brook and River Rhymney, the culvert at the confluence of

the Roath Brook and River Rhymney, the railway embankment and culverts/openings

across railway embankment which may affect propagation of overland flow paths. Site

visits were undertaken prior to building the hydraulic model to assess key topographic

features which are likely to influence local flooding mechanisms and therefore, need

incorporating into the model. The methods adopted to incorporate these features into

the model and modelling assumptions made are set out below.

Roath Brook - Old Course and New Cut

The Roath Brook bifurcates into two routes at the Sainsbury‟s store site off Colchester

Avenue (see Figure F.1).

Figure F.1 - Location of the Old Course and New Cut of the Roath Brook and confluence

with the River Rhymney

The first route to the confluence of the Roath Brook with the River Rhymney is

referred to as the New Cut. This follows a north easterly direction to the confluence

just north of the A4232 roundabout with the Newport Road. The second route, which

is referred to as the Old Course, follows a general easterly direction to discharge to

the River Rhymney just north of the Lamby Way/Rover Way roundabout. This old cut

off channel is understood to only receive highway and surface water drainage from a

housing development to the south of Newport Road.

5097656-70-DG017-3.docx

It is considered that overland flow paths from the Roath Brook could potentially affect

the proposed development sites and therefore, this watercourse is included in the

model.

Earlier versions of the model assumed that tide levels in the Rhymney would

propagate up both channels of the Roath Brook. However, information from the

Environment Agency suggests that the Old Course from Sainsbury‟s serves only as a

surface water drain. Any back flow up the Old Course is prevented by a flap vale on

the end of the culvert through which it discharges to the River Rhymney. By contrast,

since no such structure is present on the end of the culvert at the downstream end of

the New Cut, water is able to back flow up the New Cut of the Roath Brook when

flood levels are elevated in the River Rhymney.

For the tidal inundation model, tide level boundary has been incorporated into the

New Cut of the Roath Brook.

Right Bank / Defence Levels on the Roath Brook

The defence levels along the right bank of the Roath Brook would govern the volume

of floodwater overtopping the brook and subsequent development of overland flow

paths towards the study sites. These levels were extracted from the 1D ISIS22

model

of the Roath Brook provided by the Environment Agency. The bank / defence levels

were stamped onto the existing DEM.

Right Bank / Defence Levels on the River Rhymney

The defence levels along the right bank of the River Rhymney were provided by the

Environment Agency. These levels are based on topographic survey undertaken in

January 2009 by Total Surveys Limited for the Environment Agency Wales‟ Lower

Rhymney Embankments Standard of Service Assessment.

Defence Levels to the Severn Estuary

The frontage to the Severn Estuary from the mouth of the River Rhymney south to a

Travellers‟ Site on Rover Way (OSNGR 321781, 178578) is based on survey provided

by the Environment Agency. The levels south along the coast to the Cardiff Barrage

are based on LiDAR data.

Railway Embankment and Culverts Underneath

Levels along the railway embankment which runs across the study area are based on

LiDAR data. The Roath Brook Old Course is understood to be culverted under the rail

line. Inspection of the OS MasterMap data did not reveal any other culverts under the

railway embankment which could affect/influence the overland flow paths.

F.2 Key Hydraulic Assumptions

A review of the Environment Agency existing hydraulic models23

of the Lower

Rhymney and Roath Brook indicates that fluvial flood levels are lower than tidal flood

events of same chance of occurrence. i.e. over the lower reaches of the Rhymney

and Roath Brook tidal flood risk is considered to be dominant over fluvial.

Modelling Breach of Defences

The principles underlying the breach assessment for Area I are essentially the same

as those described for Area A in Appendix E.

22

Environment Agency Wales, ISIS model, Lower Rhymney and Roath Brook v3, 2008 23

Environment Agency Wales, HEC-RAS model, Lower Rhymney & Roath Brook, v2, 2007

5097656-70-DG017-3.docx

A joint probability assessment of extreme tide levels (EWL) and coincident waves

heights along the coastal defences to Area I has been updated for the SFCA to reflect

the new extreme tide level estimates released by the Environment Agency in

February 2011 (see Section 2.3.2).

The susceptibility of the defences to breach along the Severn Estuary and the lower

reach of the River Rhymney bordering Area I was assessed by Atkins Limited as part

of the Environment Agency‟s Severn Estuary Flood Risk management Strategy

(SEFRMS).

Since it is assumed that the defences will be maintained in their current condition into

the future the probability of overtopping and breach of the defences increases into the

future, if the sea levels rise as predicted.

The formation of a breach in the defences is dependent on the flow of water down the

back of the defence structure as a result of overtopping. Once the tolerable flow of

water is exceeded, erosion of the structure will commence leading to failure and

potentially breach of the defence. The susceptibility of a defence will depend on a

number of inter-related factors including the defence height, type of defences and

slope.

The water overtopping a defence will result from a combination of the tide level (SWL)

and the coincident wave heights. As part of the SEFRMS the joint probability of a

series of tide levels and wave heights were assessed. For events of a given

probability only a limited number of combinations of tide level and wave will result in

breach of the defence. As the tide level (SWL) increases the coincident wave height

decreases. It is assumed that the defences will be maintained in their current

condition and will be maintained into the future. No raising or reinforcement of

defences in the future is assumed. This assumption results in an increased

probability of overtopping and breach of the defences if sea levels rise as predicted.

Based on the detailed breach analysis methodology, a single location on the Severn

Estuary defences to Area I is considered to be at risk of breach over the 100 year

period to 2110.

This is located on the coastal frontage to the Severn Estuary at Rover Way, Pengam

Green, shown labelled as SR1_RRWB on Figure F.1 above and Drawings

5097656/RCF/420 to 422, Appendix B.

The breach assessment of a range of EWL and wave height combinations indicates

that this location has a current standard of service in excess of a 0.5% (1 in 200)

annual chance event. This location is currently at risk of breach during a more

extreme 0.1% (1 in 1000) annual chance event. The probability of breach will

increase into the future.

The boundary conditions adopted to represent the breach in the Area I 2D model are

those developed for the SEFRMS. The main parameters defining the breach in the

model are:

A period of 1 hour for the full breach to form following its commencement.

The breach is assumed to develop to 50m in width once initiated.

The defence reduces to a toe level of 7.65mAOD following breach.

5097656-70-DG017-3.docx

Boundary Conditions

A tidal boundary condition (tide levels versus time) along the Severn Estuary

shoreline has been used to model overtopping of tidal defences. The tidal boundary

condition has also been used along the Roath Brook and the River Rhymney to model

overtopping of defence and subsequent development of overland flow paths. Only

the New Cut course of the Roath Brook, downstream of the Sainsbury‟s store has

been incorporated into the model. The tidal boundary condition along the Roath Brook

extends to Roath Mill garden area, approximately 1.6 km from its confluence with the

River Rhymney. Upstream of this location, banks levels are such as to prevent any

overtopping during extreme tidal events.

Along the River Rhymney, tidal boundary condition extends up to A48 roundabout,

beyond which flood levels are unlikely to affect the proposed development sites.

During extreme tidal events, the levels along the River Rhymney and the Roath Brook

are likely to be higher than the tide levels in the Severn Estuary due to a contribution

of fluvial flows in the river channel.

The Environment Agency Wales‟ one-dimensional (1D) HEC-RAS model (Lower

Rhymney & Roath Brook, v2, HEC-RAS, 2007) has been used to estimate the

increase in water levels up the river channel by routing a 20% (1 in 5) annual chance

fluvial flows against extreme tides such as 0.1% (1 in 1000) and 0.5% (1 in 200)

annual chance events. In the absence of an estimate of QMED or 50% (1 in 2)24

annual chance flow, we consider that the use of 20% (1 in 5) annual chance flow

represents a conservative approach. The increase in water levels up the river

channels of the Roath Brook and River Rhymney are presented in Table F.1 - . These

elevated water levels in the river channels have been used to simulate overtopping of

the defences.

To incorporate the increases in water levels up the river channels in the event of tidal

events coincident with Q5 or 20% (1 in 5) annual chance fluvial flood flows, a number

of level change points have been used along the River Rhymney and the Roath

Brook. In total 2 change points have been used for the River Rhymney and 3 change

points for the Roath Brook. Tides were then linearly interpolated between these points

and input into the hydraulic model.

Table F.1 - shows tide levels which were used for the different modelling scenarios

and the increases in flood levels up the river channels due to coincident fluvial flows in

the River Rhymney and the Roath Brook

24

In the absence of existing estimates of the QMED flow on the Rhymney or Roath Brook, we consider that use of the Q5 or 20% (1 in 5) annual chance flow represents a conservative approach. We have confirmed, based on analysis of ratios of ReFH flow estimates that the Q5 is greater than a QMED+20%. The Q5 has therefore also been used for the climate change scenario in 2060.

5097656-70-DG017-3.docx

Table F.1 - Tide Levels and increases up the Channels of the River Rhymney and Roath

Brook for the tidal scenarios modelled

Year Tidal Event

Probability

Tide levels

at Coast

Water levels at Change Points (mAOD)

Model Run Case Ref. ID

(mAOD)

Down-stream

extent of the

Roath Brook

Just north of Newport

Road Round-about

Approx. 0.35km north of

Sainsbury’s

Near Waterloo Gardens

Near Roath

Mill Gardens

(Change Point 1)

(Change Point 2)

(Change Point 3)

(Change Point 4)

(Change Point 5)

RHYMNEY XS 1116

RHYMNEY XS2554

ROATH XS 569.40

ROATH XS

1181.40

ROATH XS

1637.50

2010

0.50%

8.6 8.65 8.72 8.67 8.78 8.82 AREA"I"_CASE10 (1 in 200)

0.10%

9.2 9.2 9.26 9.22 9.3 9.33 AREA"I"_CASE 11 Sensitivity 2 (1 in 1000)

2085

0.50%

9.2 9.3 9.35 9.31 9.39 9.42 AREA"I"_CASE 12 Sensitivity (1 in 200)

0.50% 8.9 8.95 9 8.96 9.04 9.07 AREA"I"_CASE13

(1 in 200)

2110 0.50%

9.6 9.66 9.71 9.67 9.75 9.78 AREA"I"_CASE14 (1 in 200)

1 – Note: levels were applied in the model to the nearest one decimal point.

In addition to testing the overtopping from the rivers and the coastal frontage, the

probability of breach at the coastal frontage has been incorporated into the modelling.

The tide levels at which breach is initiated for the scenarios tested are given in the

following table.

Table F.2 – Breaches Assumed in Scenarios Area I Tidal inundation

Year Tidal Event Probability

Model Run

Case Ref. ID

BREACH @ SR1-RRWB (Coastal Frontage Pengam

Green)

2010 0.5%

(1 in 200) AREA"I"_CASE10 No Breach

0.1%

(1 in 1000) AREA"I"_CASE 11 Sensitivity 2

Breach commences at 9.0mAOD tide

2085 0.5%

(1 in 200) AREA"I"_CASE 12 Sensitivity No Breach

0.5%

(1 in 200) AREA"I"_CASE13

Breach commence at 8.9mAOD tide

2110 0.5%

(1 in 200) AREA"I"_CASE14

Breach commence at 8.7mAOD tide

5097656-70-DG017-3.docx

F.3 Modelling Scenarios and Results

The model has been run for different scenarios as listed in Table F.1 - , above.

Predicted maximum flood depths and corresponding velocities for each scenario were

analysed to estimate worst case flood depths and corresponding velocities. The

results are presented on Drawings 5097656/RCF/420 to 422, Appendix.B.

Current Situation 2010

0.5% (1 in 200) Annual chance event 2010

Flooding during this event derives from overtopping from the River Rhymney and

Roath Brook.

The 2D model allows estimation of the overland flow paths from the locations of tidal

overtopping of right (west) bank of the River Rhymney, notably into the area of

allotment gardens upstream of the A4232 crossing of the Rhymney. Water from this

source flows west over Rover Way into Tremorfa and south, reaching the boundary of

Site 19LBU.

Overland flows from overtopping of the River Rhymney upstream of the main railway

line d combine with those overtopping the right bank of the Roath Brook to inundate

the Newport Road area. Flooding to this area extends up Colchester Avenue to

inundate across Site 94LBR. Flooding from both banks of the Roath Brook at the

Waterloo Gardens Area does not affect any of the study sites.

Flood water is also indicated to spread along the main railway line, extending

approximately 750m west of the Rover Way crossing over this rail line.

0.1% (1 in 1000) Annual chance event 2010

As well as overtopping, the breach of the coastal defence at SR1-RRWB (see Figure

F.1) close to the Study Site 19LBU contributes to extensive flooding across the

Tremorfa and Pengam Green area.

Greater overtopping volumes from the right bank of the River Rhymney and the Roath

Brook enter the flood plain by the same mechanisms as described above for the 0.5%

chance event. In addition direct overtopping from the River Rhymney occurs

immediately east of Site 19LBU.

The flood water from north of the main rail line flows south and merges with the

overland flows overtopping south of the railway line.

Flood water also derives from the breach of the coastal defence south of Site 19LBU,

merging with the flow paths from the north on this site.

Floodwater would also extend approximately 800m further west along the main

railway line compared to the 0.5% (1 in 200) annual chance event.

0.5% (1 in 200) Annual chance events, 2085

As a result of the higher tide levels, a greater volume of floodwater enters the

floodplain and the overland flow paths extend further south and west across the

Tremorfa area and Study Site 19LBU particularly. North of the Roath Brook, the flood

extents are greater than the 2010 base year, with significantly greater depths of water

on Study Site 94LBR and the surrounding area, including the access/egress routes for

the site. In addition to the overtopping from the Roath Brook and River Rhymney,

overtopping and breach of the coastal defences contributes significantly to the

volumes of flood water, causing extensive flooding across site 19LBU.

5097656-70-DG017-3.docx

The overland flow along the main rail line extends further west, with some flooding to

roads away from the immediate locality of the rail line, although this flow path does

not impact any of the Study Sites over this time horizon.

0.5% (1 in 200) Annual chance events, 2110

The greater volumes entering the flood plain from the River Rhymney, the Roath

Brook and the coastal frontage result in more extensive flooding than the situation in

2085. Overland flows from the Tremorfa area, east of the Dock rail line merge with

flows which develop south from the main Swansea London rail line. The modelling

indicates that large areas of Splott and Tremorfa will be flooded during a 0.5% (1 in

200) tidal event by 2110. It is noted that Study Site 58SBR in the Splott area would

remain flood free.

In the west of the area, the modelling indicates that the 0.5% (1 in 200) annual chance

tide level of 9.4mAOD would overtop the dock gates and the local area. The high

ground levels bordering this area serve to contain the extent of flooding. The limited

time of the overtopping and the limited flow conveyance area, results in the flood level

in the Roath Basin Docks being very much less than the tide level in the open coast of

the Severn Estuary. The flood level which develops in the Roath Basin Docks will

depend on the exiting dock water level prior to the extreme tide. A relatively small

area of Study Site 60LBRM would be affected by 2110.

In summary, the 2D model provides improved estimates of the local distribution of

flood water through the area assessed.

F.4 Future modeI refinements for site specific studies

We suggest that further investigation into some of the information used in the

modelling should be undertaken. It is possible that further refinement of the hydraulic

model may be appropriate for further analysis as part of site specific FCAs.

Some areas to be investigated further are identified below:

The Environment Agency are currently undertaking 2D modelling of the lower

Roath Brook which may provide a source of improved information.

Improved topographic survey of the defence levels along the left bank of the

Roath Brook would improve the level of confidence in the model results,

particularly for sites north of the Roath Brook. It is unlikely that this will change

the baseline condition for the future scenarios. It will be required to inform

mitigation options.

Investigation should be undertaken to confirm the arrangements of any structures

(such as flap valves) on the outfalls of the two routes of the Roath Brook at their

respective confluences with the River Rhymney. The current model assumes a

functioning flap valve on the Old Course.

No detailed assessment of the structural integrity of defences along the River

Rhymney has been undertaken. The current model assumes that defences

would not fail during extreme events. Site specific detailed study would need to

consider the possibilities of defence breach, building on the “Standard of Service

Assessment” undertaken in 2005 by Atkins on behalf of the Environment Agency.

The results of such a detailed assessment of the probability of breach of the

River Rhymney defences could have an impact not only for potential

development at Study Sites but also for the risk to existing properties in Area I.

More detailed hydrological assessment of the River Rhymney and Roath Brook

could be undertaken to confirm the increases in water levels up the river

5097656-70-DG017-3.docx

channels due to fluvial flows coincident with extreme tidal events. However, we

consider that in this current study, the use of the flow sets supplied by the

Environment Agency with the HEC RAS model, are likely to be reasonable in

assessing the “order of magnitude” of increase in levels up the river channel. We

would not expect this to have a significant impact on the volumes of water

entering the flood plain and the resulting overland flows to the Study Sites.

Modelling of proposed site layouts would be required to identify any potential

impacts on flood risk elsewhere and to inform the development of potential


Before undertaking on any further modelling as part of site specific studies, the

Environment Agency should be consulted for the results and limitations of the 1D - 2D

inundation model of the Roath Brook which is currently under development.

Appendix G – Assessment of risk of

tidal inundation via Cardiff

Docks to Cardiff Bay.

G.1 Model Set-Up

The potential for extreme tide levels to enter Cardiff Bay from the Docks via Roath

Basin Dock were tested by developing a 1D-2D ISIS TUFLOW hydraulic model.

The methodology of the 1D-2D tidal inundation modelling adopted to predict tidally

dominated flood risks to Cardiff Bay and the key assumptions made are set out below.

G.1.1 Spatial Extent of the Model

The 1D element of the model comprises the Bay model described in outline in Section

5.3.2 of the main report.

Overland flow from Roath Basin to Cardiff Bay

The 1D ISIS model of the Bay described in Section 5.5.5 of the SFCA was developed

and combined with a new section of 2D domain covering the Roath Basin and the

land bordering the eastern Bay. This 1D-2D ISIS TUFLOW model was used to test

the extent of overland flow from the Roath Basin into the Bay and the effect on peak

water levels in the Bay.

Channel and bund separating Roath Basin Dock from Cardiff Bay

Under normal operating conditions the water in Cardiff Docks is prevented from

entering Cardiff Bay by a bund located in an old lock channel adjacent to the

Norwegian Church (structure shown in Figure G.1 - and location shown on Figure

G.3 ).

This channel and bund were represented by adding a 1D link section of channel to the

1D ISIS model of Cardiff Bay (described in section 5.3.2). The bund itself is

represented as a spill unit. The dimensions of the link sections were estimated from

OS Mastermap25

data. The widths of the channel sections of the link are set at 25m

and the invert level is an assumed level of -1.7mAOD. The error in estimating the

depth of the channel and so the velocity losses calculated in the model are likely to be

small and are considered appropriate for a strategic level assessment. Overall each

section is 95m wide to include the flow in the out of channel area of the dockside

bordering the lock. The spill level is set at 7.6mAOD based on the LIDAR levels to the

top of the bund.

The resulting 1D ISIS model was linked to a 2D TUFLOW domain to represent the

area of overland flow around the Roath Basin and into Cardiff Bay. The 2D domain

was created based on LIDAR levels.

The 1D ISIS geometry is presented schematically on Figure G.2, below.

The extent of the 2D domain is indicated by the black & magenta outline on Figure

G.3.

25 © Crown Copyright. Cardiff County Council, 100023376 (2011)

Figure G.1 - Bund separating Roath Basin Dock from Cardiff Bay. View from Roath Basin

Figure G.2 - 1D ISIS Geometry

The overland flows spilling into the Bay (i.e. not via the 1D link channel) were

estimated by allowing the water reaching the edge of the 2D domain to spill into

infinity. The volumes spilling were estimated using “po lines” within the TUFLOW

model along the boundary of the 2D domain with the edge of the Bay. These are

shown as green lines on the Figure below.

Figure G.3 - Extent of area of 2D domain to test overland flow paths into Cardiff Bay.

Green lines are po lines in the TUFLOW model

The area of the Bay in the model is 1.64km2. Dividing the volume of water crossing

the po lines within the model by this area provides an estimate of the order of

increase in the Bay level. This additional level is added to the peak modelled water

level calculated within the 1D ISIS model to derive an estimated peak level in Cardiff

Bay.

Note that the flow over the bund separating the Roath Basin from the Bay is

accounted for directly in the 1D ISIS link (sections Dock_Sect to CH0002 shown in

schematic Figure G.1)

G.2 Key Hydraulic Assumptions

G.2.1 Operation of the lock gates to Queen Alexandra Dock

The operation of the lock gates to Cardiff Docks is based on information obtained

from the port operators, ABP Limited.

The lock to Queen Alexandra (QA) Dock connects Cardiff Docks to the Severn

Estuary. However, these lock gates cannot restrain tides higher than the level in the

QA Dock. To remain in the closed position a positive head is required, i.e. a higher

level in the QA Dock relative to the tide level in the Severn Estuary.

The normal water level in the dock is 6.71mAOD i.e. 13.01m above Admiralty Chart

Datum (mACD).

The level to the top of the lock gates is 7.7mAOD and the sill level to the lock

entrance is understood to be -6.7mAOD.

The impounded dock level can be increased up to a maximum of 7.3mAOD

(13.6mACD). Therefore, a positive head of water can be maintained inside the dock

for any height of tide up to 7.3mAOD and the lock gates can be retained in the closed

position.

However, for tide elevations above 7.3mAOD, the lock gates are opened and the level

in the dock equalises with that of the tide at the lock entrance. The gates can be

closed once the tide has dropped below 7.3mAOD.

This level compares with the range of 8.4mAOD to 9.4mAOD extreme tide levels

tested for the SFCA.

G.2.2 Boundary Conditions

The Area I 2D inundation model, described in Appendix F, was extended to the

eastern end of the Cardiff Barrage to include the entrance to the QA Dock.

This 2D model was run to test the effect of the lock gates being open during extreme

tide events. This confirmed that the order of magnitude of the conveyance available

through the locks is sufficient for the extreme tide levels to be achieved throughout the

docks to the Roath Basin (the dock immediately east of the impounded Cardiff Bay).

It was established that the level in the Roath Basin will vary, to a close approximation,

with the tide level in the Severn Estuary for tide levels above 7.3mAOD.

As a result, a head-time (HT) boundary of the extreme tide cycles was applied within

the Roath Basin in the 2D domain in the 1D-2D ISIS TUFLOW model.

A separate HT tidal boundary is also applied in the 1D ISIS model, the downstream

boundary (TF_DS_001) on the Severn Estuary side of the sluice gates representing

the Cardiff Bay Barrage.

The fluvial flows into the Bay from the rivers Taff and Ely are applied as flow-time

(QT) boundaries at the top of the respective reaches in the 1D model.

A QMED flow estimate for the Taff and a Q5 estimate for the Ely were assumed to

occur with each tide event tested.

Flow estimates derived from existing Environment Agency models were used. The

combined Ely-Taff 1D-2D ISIS TUFLOW model does not contain a QMED estimate.

A QMED estimate was taken from an earlier model developed for the Environment

Agency, the Lower Taff 1D ISIS Flood Risk Mapping update, November 2007. No

QMED estimate was available for the River Ely. As a surrogate, a 20% (1 in 5) annual

chance estimate (Q5) was taken from the Environment Agency, River Ely 1D ISIS

model, CFMP, 2008. For the future scenarios a 20% increase in fluvial flow was

applied to the QMED on the Taff. No increase in flow was applied to the River Ely Q5

estimate as this is considered to approximate the QMED +20%.

The model flows for the Ely represent an over estimate of a QMED flow. This was

considered reasonable given the Ely‟s relatively small contribution to overall flow into

the Bay at the peak of the tide. The Taff flow is assumed to occur at the same time as

the peak of the tide. The Ely peaks some 5hours after the peak on the Taff.

G.3 Modelling Scenarios and Results

This combined 1D-2D ISIS TUFLOW model was used to:

a) Determine the extent of flooding during extreme tide events

b) The depths and velocities in the overland flow paths from the Roath Basin to

Cardiff Bay.

c) Calculate the volumes of overland flow entering the Bay

Note: For areas of the Docks outside the 2D domain of the model, the tide levels were

contoured over the LIDAR and a depth grid calculated within a proprietary GIS

software package. No velocity profiles were therefore available for areas outside the

limits of the 2D domain representing the western Roath Basin.

G.3.1 Scenarios Modelled

The boundary conditions were applied for each of the scenarios as listed in Table 8.1.

Two sets of model runs were:

1. The existing situation with respect to the lock gates to Cardiff Queen Alexandra

Dock being open during extreme high tides.

2. Mitigation situation – assumes that water is prevented from entering Cardiff Bay

from the Roath Basin Dock.

Table 8-1 Scenarios modelled in 1D-2D ISIS TUFLOW model of Cardiff Bay and Roath

Basin - current and future extreme tidal events

Assumes coincident QMED flows in the River Taff and River Ely**

Scenario Tide level (mAOD)

Peak Fluvial flows

TIDAL EVENT

Annual Chance

YEAR River Taff

River Ely**

0.1% (1 in 1000) 2010 9.0 360 78

0.5% (1 in 200) 2085 9.1 429 78

0.5% (1 in 200) 2110 9.4 429 78

* Open Lock Gates is current situation

** Ely Q5 Flow used as existing QMED estimate not available

G.3.2 Results

Table 8-2 Levels resulting in Cardiff Bay for current and future extreme tidal events

Assumes coincident QMED flows in the River Taff and River Ely

Scenario Tide level

(mAOD)

TIDAL EVENT

(Annual Chance)

YEAR Assumption regarding flow path

from Roath Basin into

Cardiff Bay

Peak Model Bay

Level - excludes overland

flow

(mAOD)

Volume Spilling into Bay

via overland flows.***

(m3)

Derived increase in Bay level

(m)

Final Estimated Bay Level

m(AOD)

0.1%

(1 in 1000)

annual chance

2010

9.0 Existing condition - Dock Lock Gates open

7.66 3,123 0.002 7.66

9.0 No flow from Roath

Basin

7.58 N/A N/A 7.58


2085


8.06 32,108 0.02 8.08


Basin

7.96 N/A N/A 7.96


2110


8.22 111,069 0.07 8.29


Basin

8.11 N/A N/A 8.11

*** Flows extracted from po lines in TUFLOW 2D domain

The resulting extents and depths of flooding are presented in the Drawings in

Appendix B and the consequences for the Candidate Sites discussed in Section 6 of

the main Report.

Atkins Limited West Glamorgan House 12 Orchard Street Swansea SA1 5AD Telephone: +44(0)1792 641172 Fax: +44(0)1792 472019

Atkins Limited West Glamorgan House 12 Orchard Street Swansea SA1 5AD Telephone: +44(0)1792 641172 Fax: +44(0)1792 472019

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