Waterfront Quarter,...
Transcript of Waterfront Quarter,...
Strategic Sites Waterfront Quarter, Huddersfield Level 2 Flood Risk Assessment
April 2008
This report takes into account the
particular instructions and requirements
of our client.
It is not intended for and should not be
relied upon by any third party and no
responsibility is undertaken to any third
party
Ove Arup & Partners Ltd
New Oxford House, 30 Barkers Pool,
Sheffield S1 2HB
Tel +44 (0)114 272 8247 Fax +44 (0)114 275 9553 www.arup.com Job number 119046
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Ove Arup & Partners LtdIssue 2 April 2008
Document Verification
Page 1 of 1
Job number Job title Waterfront Quarter, Huddersfield
119046
File reference Document title Level 2 Flood Risk Assessment
Document ref
Revision Date Filename
Description First draft
Prepared by Checked by Approved by
Name Richard Wren Ian Drabble Matthew Lovell
Draft 1 19/03/08
Signature
Filename HWQ 3.1 Level 2 Flood Risk Assessment.doc
Description Initial comments from the EA incorporated into the text including notes on requirement for access to the river bank for maintenance
Prepared by Checked by Approved by
Name Richard Wren Ian Drabble Matthew Lovell
Issue 02/04/08
Signature
Filename
Description
Prepared by Checked by Approved by
Name
Signature
Filename
Description
Prepared by Checked by Approved by
Name
Signature
Issue Document Verification with Document �
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Ove Arup & Partners LtdIssue 2 April 2008
Contents
Page
1 Introduction 1
1.1 Site location and characteristics 1
1.2 The proposed development 1
1.3 Previous work 1
1.4 This commission 1
2 Risk of flooding on this development 3
2.1 Fluvial Flooding from the River Colne 3
2.2 Pluvial flooding 4
2.3 Flooding from the Huddersfield Narrow Canal 4
2.4 Flooding from groundwater 5
2.5 Existing flood defences 5
3 Implications for the proposed development 6
3.1 Flooding from the River Colne 6
3.2 Flooding from the Huddersfield Narrow Canal 7
3.3 Flooding from groundwater sources 7
3.4 Other constraints 7
4 Surface water management 9
4.1 General planning considerations 9
4.2 Existing management of surface water 9
4.3 Proposed management of surface water 10
4.4 Ground conditions – suitability for infiltration techniques 11
4.5 Use of attenuation systems 11
5 Conclusions and recommendations 12
Appendices
Appendix A
Current development proposals by DLG Architects (February 2008)
Appendix B
Environment Agency Flood Map for Waterfront Quarter and surrounding areas
Appendix C
Excerpts from Holme & Colne Flood Mapping Study Summary Sheets
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 1 Ove Arup & Partners LtdIssue 2 April 2008
1 Introduction
1.1 Site location and characteristics
The Waterfront Quarter development is located approximately 0.5 km south of the centre of
Huddersfield at national grid reference SE 140160. The existing site has a total area of
approximately 30,395 m2 (3.04 Ha) and can be divided broadly into two long and narrow
strips of land.
The northern part of the site is occupied by Kirklees Metropolitan Council and comprises
mainly of offices. Located at an elevation of between 76 and 82 m AOD, this part of the site
is characterised by changes of level, retaining structures and steep slopes. Until the 1960s
this area was residential, with houses fronting onto Manchester Road and Dale Street.
The southern part of the site is occupied by Sellers Engineers Ltd, with a part of the site
sub-let to Acacia Timber Products. The area has historically been used for industrial
purposes and is currently used for manufacturing and storage, with a small office block
located in the south eastern corner of the site. This area is lower lying, at between 70 and
76m AOD, and is more gently graded than the northern part of the site.
The site is located on the northern bank of the River Colne. The Colne is classed as a
“main river”, which means that the Environment Agency (EA) has powers to maintain and
improve the watercourse under the Water Resources Act 1991. The Yorkshire Land Drainage
Byelaws 1980 will also apply.
The Huddersfield Narrow Canal, which was reinstated during 2000-2001, passes beneath
the site in a tunnel.
The site is predominantly covered by buildings and hardstanding. These impermeable
areas are positively drained by means of below ground gravity drainage systems.
1.2 The proposed development
A mixed use development is currently proposed, comprising of a major new development for
Huddersfield Technical College and seven other buildings, some for commercial use and
some for residential accommodation.
A major component of the masterplan for this development is the reinstatement of the
Huddersfield Narrow Canal at surface level. The existing tunnel structure will be opened up
and lock 3E will move downstream within the site. This will allow the canal to return broadly
to the original vertical alignment as it passes through the site, and will allow users of the site
to enjoy the proximity of the canal as well as the riverside.
The overall scheme architect is DLG Architects of Leeds. A plan showing the current
development layout is included in Appendix A.
1.3 Previous work
In January 2006 Arup issued a Preliminary Drainage Report. Reference to the Environment
Agency Flood Mapping available at that time stated that the site was located predominantly
in flood zone 1, i.e. the risk of flooding from the river is less than 1 in 1000 (or 0.1%) in any
year. It was noted that, given the statutory requirements in place at that time, no further
action was required.
1.4 This commission
Planning Policy Statement (PPS) 25, “Development & Flood Risk”, was published in
December 2006. PPS25 changes the framework within which flood risk is considered when
developing, or redeveloping, a site. It states that:
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 2 Ove Arup & Partners LtdIssue 2 April 2008
“Planning applications for development proposals of 1 hectare or greater in Flood Zone 1
and all proposals for new development located in Flood Zones 2 and 3 (see Table D.1,
Annex D) should be accompanied by a FRA. This should identify and assess the risks of
all forms of flooding to and from the development and demonstrate how these flood risks
will be managed, taking climate change into account. For major developments in Flood
Zone 1, the FRA should identify opportunities to reduce the probability and consequences
of flooding.”
Furthermore, the EA have procured new flood modelling studies for the River Colne
catchment, which have seen the boundaries of the flood zones redrawn. Inspection of the
current EA flood map for the area around the site (see Appendix B) shows that the EA
consider parts of this site adjacent to the river to be at risk of flooding.
Arup have been commissioned by Strategic Sites to provide a Level 2 Flood Risk
Assessment (FRA) report for the Waterfront Quarter development. This is a scoping study,
which aims to confirm all the sources of flooding that may affect the Waterfront Quarter
development site. In this document we will:
• Identify the pathways taken by floodwater in and around the site.
• Assess the vulnerability of the receptors, based on consideration of the site layout, land
uses and materials.
• Determine whether sufficient quantitative information is already available to complete an
assessment that is appropriate to the scale and nature of the risk.
• Take into account the impact of climate change as described in Annex B of PPS 25.
A Level 2 FRA will not involve any detailed hydrological or hydraulic modelling of the site,
but if necessary it will propose in outline the scope of any further investigation that is
required to complete the flood risk assessment process.
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 3 Ove Arup & Partners LtdIssue 2 April 2008
2 Risk of flooding on this development
2.1 Fluvial Flooding from the River Colne
The site is adjacent to the River Colne. The Colne is classed as a “main river” which means
that the Environment Agency (EA) have powers to maintain and improve the watercourse under
the Water Resources Act 1991. Responsibility for the river and river bank lies with the riparian
owner.
We made an enquiry to the EA in January 2008, requesting information relating to flooding
and flood risk at the development site. The response to this enquiry was received on 25
February 2008 and included the following information:
• Excerpt from the EA Flood Map - “Flood Zone Data For England” – showing the site
and surrounding area (see Appendix B)
• Extracts from the ISIS Hydraulic Model Summary Sheets produced as part of the “Colne
and Holme Flood Mapping Study” (see Appendix C).
The EA Flood Map defines areas as being within one of the following flood “zones”. (Note –
different criteria apply for areas subject to coastal flooding; these have been ignored in this
case.)
Zone 1 – Low Probability – This zone comprises land assessed as having a less than 1 in
1000 annual probability of river or sea flooding in any year (<0.1% Annual Exceedence
Probability, or AEP).
Zone 2 – Medium Probability – This zone comprises land assessed as having between a 1
in 100 and 1 in 1000 annual probability of river flooding in any year (1% – 0.1% AEP).
Zone 3 – High Probability – This zone comprises land assessed as having a 1 in 100 or
greater annual probability of river flooding in any year (>1% AEP).
Planning Policy Statement (PPS) 25, “Development and Flood Risk” provides guidance on
the types of development that are permitted in each of these flood zones. In order to
develop a site located within zones 2 or 3 for higher risk uses as defined in PPS25, it is
necessary to undertake the “sequential” and “exception” tests.
An initial review of the flood map shows that, broadly speaking, two small areas of the
Waterfront Quarter site are defined as being within flood zones 2 and 3.
• The area around the Sellers office buildings at the south eastern corner of the site.
• A strip along the bank of the river along the length of the site.
The information in the extracts from the hydraulic model has been considered in conjunction
with the topographical survey data for the site produced by Powers & Tiltman on behalf of
DLG Architects, to confirm the extents of zones 2 and 3 within the development area.
There are a number of control features in the river channel (e.g. weirs, bridges) and hence
there are a number of changes in water level along the length of the river. As a result we
have considered flood water levels at each of the modelled sections of the river.
2.1.1 Extent of flood zone 3
In Table 1 we have presented the modelled flood water level and the lowest surveyed
existing bank top level (as shown on the topographical survey). This relates to the area
near the cross section and may not be exactly at the cross section, thus allowing
consideration of low spots along the bank that may occur between section locations. These
levels do not take account of any walls that may be providing additional protection. The 1%
AEP water level data has been taken from the Environment Agency’s Colne & Holme Flood
Mapping Study ISIS Hydraulic Model (see Appendix C). Please note that the chainages
quoted are taken from the ISIS model – refer to Appendix C.
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 4 Ove Arup & Partners LtdIssue 2 April 2008
It can be seen that along the length of the river as it passes the Waterfront Quarter site, the
existing bank top is always higher than the 1% AEP water level. Therefore, the extent of
flood zone 3 should not extend beyond the top of the river bank into the development.
Chainage (m) Section description 1% AEP water
level (m AOD)
Lowest existing bank
top level (m AOD)
07369 Upstream face of Chapel
Hill highway bridge
70.434 70.48
07441 Immediately upstream of
lower weir
70.697 71.84
07468 Midway between weirs 70.633 72.48
07513 Immediately upstream of
upper weir
72.071 72.39
07630 72.003 74.04
07741 Upstream extent of
development
73.103 73.68
Table 1 – 1% AEP water levels (defining extent of flood zone 3)
2.1.2 Extent of flood zone 2
Comparison of the 0.1% AEP water levels given in the Flood Mapping Study with the
existing surveyed levels leads to the conclusion that the extent of flood zone 2 is broadly as
shown in the EA Flood Map (Appendix B). Development in this area should not include any
of the types defined in PPS25 as “highly vulnerable” unless the sequential and exception
tests have been applied.
2.2 Pluvial flooding
The site surface water drainage systems will be designed to current standards as defined in
BS EN 752 and other best practice guidance. The following minimum design standards will
be applied.
• No surcharge in pipework during the 1 in 2 year (50% AEP) event.
• No surface flooding (in pipework, chambers and balancing systems) during the 1 in 30
year (3.3% AEP) event.
• No flooding to buildings during the 1 in 100 year (1% AEP) event.
The development will, therefore, be protected from pluvial flooding up to and including a 1%
AEP event on the development site itself.
2.3 Flooding from the Huddersfield Narrow Canal
Upstream of the existing lock 3E at the western end of the development, the Huddersfield
Narrow Canal has a water level of approximately 73.25m AOD. The canal continues at this
level for some distance before the next change in level at lock 4E, upstream of Longroyd
Bridge. We note from the flood map (Appendix B) that in the area upstream of the
development site, flood zone 3 extends over and beyond the canal. We are not in
possession of surveyed level data to verify the extent of zone 3 beyond the limits of the
development. However, the profiles given on the Hydraulic Model Summary Sheets for
cross sections at chainage 07879 and 07946 (see Appendix C) indicate that the 1% AEP
water level crosses the towpath and would inundate the canal. This could generate a flow
path for floodwater along the canal and subsequently into the development.
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 5 Ove Arup & Partners LtdIssue 2 April 2008
It is noted that at the upstream (western) end of the development site, at chainage 07741,
the Hydraulic Model Summary Sheets indicate that the 1% AEP water level has fallen
significantly and is contained within the banks of the river.
We are not aware of any historical occurrences of flooding within the Waterfront Quarter site
as a result of water using the canal as a flood path. Currently, the canal passes in tunnel
beneath the development and this would provide an opportunity for any flood waters
entering the canal from the river to pass safely beneath the site with minimal impact to the
site.
2.4 Flooding from groundwater
We are not aware of any historical records of groundwater flooding on the Waterfront
Quarter site. Ground investigation activities in the site occupied by Kirklees MC, carried out
on behalf of the Huddersfield Technical College design team, found that the groundwater
level in cohesive deposits was between 1.0 – 2.5m below ground level. This implies that
groundwater may be encountered during activities involving deep excavations and design of
permanent cuttings may need to take account of the need to collect and discharge
groundwater.
The Geo-environmental Desk Study produced by Arup in December 2006 notes that the
sheet pile walls of the canal tunnel are likely to locally affect the groundwater regime. This
may be further exacerbated by raising the water level in the canal.
2.5 Existing flood defences
The Environment Agency does not maintain any flood defences in the area around the site.
The existing river bank levels provide protection in excess of the 1% AEP water levels.
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 6 Ove Arup & Partners LtdIssue 2 April 2008
3 Implications for the proposed development
3.1 Flooding from the River Colne
Building levels should be located above the water level occurring during the 1% annual
exceedence probability plus climate change (AEP+CC) event. For fluvial flooding, the
current guidance on modelling the allowance for climate change is given in Annex B of PPS
25. For a design horizon beyond 2025, PPS 25 recommends consideration of the effect of
an increase of 20% on the discharge in the watercourse. The modelled 1% AEP flow for the
River Colne adjacent to the development, taken from the Hydraulic Model Summary Sheets
provided by the EA, is 69.0 m3/s (see Appendix C). When an increase of 20% is applied this
equates to a discharge of 82.8 m3/s. The 0.1% AEP flow is 196.7 m
3/s.
The Hydraulic Model Summary Sheets give water levels for the 1% AEP and the 0.1% AEP
events. In order to estimate the water level during the 1% AEP+CC event, we have
interpolated between the water levels given for the 1% and 0.1% AEP events. Although
linear interpolation does not provide a precise assessment of the water level corresponding
to a particular discharge, this simple assumption provides the best estimate available in the
absence of modelled flows.
Chainage 1% AEP
water level
(m AOD)
0.1% AEP
water level
1% AEP+CC
flow (m3/s)
1% AEP+CC water
level
07369 70.434 72.821 82.8 70.691
07441 70.697 72.949 82.8 70.940
07468 70.633 72.954 82.8 70.884
07513 72.071 73.027 82.8 72.174
07630 72.003 73.071 82.8 72.118
07741 73.103 74.284 82.8 73.231
Table 2 – 1% AEP + climate change allowance water levels
The implications for the current proposed floor levels are as follows:
3.1.1 Basement car parking level beneath buildings L and M
The sections appropriate to this structure are at chainage 07369, 07441 and 07468. The
minimum proposed floor level is 71.000m AOD and the maximum 1% AEP+CC water level
is 70.940m AOD. A minimum freeboard of 60mm is therefore achieved above the 1%
AEP+CC water level.
The entrance to the basement car park should be designed so that rainwater runoff and
overland flood flows from external areas of the development do not flow into the basement.
3.1.2 Ground floor level in buildings A, B and C
The sections appropriate to this structure are at chainage 07513, 07630 and 07741. The
minimum proposed ground floor level is 73.350m AOD and the maximum 1% AEP+CC
water level is 73.231m AOD. A minimum freeboard of 119mm is therefore achieved above
the 1% AEP+CC water level.
3.1.3 Ground floor levels in all other buildings
All other buildings proposed on the Waterfront Quarter site have proposed ground floor
levels at or above 74.00m AOD. These buildings are, therefore, above the highest 1%
AEP+CC water level of 73.231m AOD.
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 7 Ove Arup & Partners LtdIssue 2 April 2008
3.2 Flooding from the Huddersfield Narrow Canal
As noted in section 2.3, the relative levels of the River Colne and the Huddersfield Narrow
Canal some distance upstream of the development site are such that a possible mechanism
exists for flood water to pass from the river into the canal during the 1% AEP event. This
mechanism could potentially result in a flood path along the canal and into the development.
Currently, the canal passes in tunnel beneath the site. However, a major component of the
masterplan for this development is the reinstatement of the Huddersfield Narrow Canal at
surface level. Lock 3E is to be relocated and the normal water level in the canal will
subsequently match that in the upstream reach, at approximately 73.25m AOD. Any
significant variations in the normal water level in the canal could, therefore, pose a risk of
flooding to the adjacent buildings and / or external areas and is thus undesirable.
As discussed in section 2.3, at the upstream (western) end of the development site the
hydraulic model summary sheet for chainage 07741 indicates that the 1% AEP water level
is contained within the banks of the river. The estimate in Table 2 above gives the 1%
AEP+CC water level at 73.231m AOD, which is below the normal water level in the canal
(approximately 73.25m AOD). This suggests that the provision of an overflow would allow
flood waters to pass back to the river, thus minimising the risk of flooding of the
development. This could be achieved by locally reducing ground levels between the canal
and the river, providing a preferential route for floodwaters. The details of these protection
measures, including the responsibilities for any maintenance, should be considered as part
of the design of the vertical realignment of the canal.
It is noted that the proposed minimum floor level for buildings A, B & C are 73.350m AOD
and 71.000m AOD for buildings L & M. The latter is below the normal water level in the
canal and the former is only marginally higher. Provided that the canal water level is
controlled by means of the protection measures described above to prevent the canal from
overtopping its banks, the risk of buildings being flooding from the canal will be minimised.
3.3 Flooding from groundwater sources
Shallow groundwater (at between 1.0 and 2.5m below existing ground level) has been
recorded on parts of this site. The basement/ground floor car parking levels beneath
buildings A, B & C and L & M could potentially be below the surrounding groundwater levels.
Further assessment is required to determine the groundwater levels in these locations and
to assess any potential implications of the increased water level in the canal. To establish
the groundwater regime in detail across the site, it is recommended that piezometers or
standpipes are installed within boreholes located across the site. These installations should
subsequently be monitored for changes in groundwater level over a representative period.
If possible, this monitoring should extend beyond such time as the canal is realigned to
monitor any changes in the groundwater level as a result of the realignment.
Depending on the results of these tests, it may be necessary to design the below ground
levels of affected buildings as waterproof structures and to provide appropriate drainage
systems.
3.4 Other constraints
The EA have stated that there should be no new buildings, structures (including gates, walls
and fences) or raised ground levels within 6 metres of the top of the river bank. This is to
allow access to the watercourse for maintenance and provide for overland flood flows.
Where existing buildings currently encroach on this “stand off” zone, but these buildings are
to be demolished and replaced, the EA would require that improved access is made
available.
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 8 Ove Arup & Partners LtdIssue 2 April 2008
An application for land drainage consent must be made to the EA for any development
taking place within 8 metres of the bank top. This process is subject to a statutory
consultation period of eight weeks.
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 9 Ove Arup & Partners LtdIssue 2 April 2008
4 Surface water management
4.1 General planning considerations
PPS 25 states that:
“Site layout and surface water drainage systems should cope with events that exceed the
design capacity of the system, so that excess water can be safely stored on or conveyed
from the site without adverse impacts.
“The surface water drainage arrangements for any development site should be such that
the volumes and peak flow rates of surface water leaving a developed site are no greater
than the rates prior to the proposed development, unless specific off-site arrangements
are made and result in the same net effect.
“For new development, it may be necessary to provide surface water storage and
infiltration to limit and reduce both the peak rate of discharge from the site and the total
volume discharged from the site.”
4.2 Existing management of surface water
The Baseline Drainage Report by Arup issued in March 2008 considers in detail the
implications of the management of surface water on the Waterfront Quarter site. It presents
the findings of a survey of the existing site drainage, carried out during December 2007,
which is used to determine the likely catchments and discharge capacities of the receiving
sewers or waterbodies. A summary of the conditions imposed by the various organisations
is given below.
4.2.1 British Waterways (BW)
Approximately 44% of the existing site discharges surface water to the canal. BW have
stated verbally that they would be prepared to accept unattenuated discharge from the
development to the canal. However, BW also expressed concerns about the capacity of the
overflow from the canal at Aspley Basin, approximately ½ mile downstream of the site. This
may place limits on the capacity of the canal to accept surface water and further discussions
are required to determine how this may impact on the permitted discharge from the site.
The Baseline Drainage Report assumed that the proposed discharge would not increase
beyond existing levels, which is in line with the second statement in section 4.1 above.
4.2.2 Yorkshire Water (YW)
Approximately 32.5% of the existing site discharges surface water to the public combined
sewer system in Chapel Hill. Yorkshire Water have stated that they will accept a discharge
of surface water to the existing combined sewers, restricted to the level of runoff from the
existing/previous use of the site. If it can be demonstrated by means of investigation that
the existing or previous use of the site has (or had) a surface water connection to sewer,
then YW would accept a discharge with the same rate of flow and/or contributing area.
4.2.3 Environment Agency (EA)
Approximately 6% of the existing site discharges surface water to the River Colne. Where
an existing site can be shown to drain to a watercourse, the EA have stated that they will
accept discharge of surface water from the redeveloped site to the watercourse subject to
the discharge being reduced by a minimum of 20% from the existing rate.
The EA have also stated that areas of the site not currently draining to a watercourse may
be allowed to discharge to the watercourse subject to discharge being attenuated to the
equivalent greenfield runoff rate for a 1 in 1 year rainfall event, which is considered to be 5
litres per second per hectare. The attenuation facility should be designed to accommodate
the discharge from a 1 in 30 year rainfall event. The discharge associated with a 1 in 100
year rainfall event should be retained within the site, without causing flooding to buildings. It
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 10 Ove Arup & Partners LtdIssue 2 April 2008
is noted that these requirements correlate well with the proposed performance standards
outlined in section 2.2.
4.2.4 Landscaped and other “permeable” areas
The remainder of the site area, approximately 17.5%, is landscaped or derelict and
therefore considered not to contribute to the rainfall runoff for the purposes of these studies.
4.3 Proposed management of surface water
4.3.1 Anticipated conditions on discharge of surface water
The Baseline Drainage Report has presented evidence and calculations to determine the
existing peak rates of discharge of surface water from the development site. These
calculations have been used to determine the permissible peak rate of discharge, based on
the requirements of the various authorities. The results are presented in Table 3 below. For
more information on this, and the derivation of the assumed rainfall intensity, please refer to
the Baseline Drainage Report.
Authority Conditions of discharge Conditions on
additional flows
Estimated permissible
discharge
YW (sewers) Like-for-like
(assumed 40mm/hr)
Not accepted 110 l/s
BW (canal) Like-for like
(assumed 50mm/hr)
Not accepted 185 l/s
EA (river) 20% reduction on existing
(assumed 87.5mm/hr)
Greenfield runoff
(max 5 l/s/Ha)
36 l/s
Anticipated permissible surface water discharge from
proposed development
331 l/s
Table 3 – Restrictions on surface water discharge, imposed by maintaining authorities
4.3.2 Surface water discharge generated by the proposed development
The overall site area, calculated using the proposed development plan in Appendix A,
(excluding existing off-site highways) is 30,395 m2. The proportion of the site area that is
proposed to be landscaped (permeable) is estimated to be 10%. This gives a total
proposed impermeable area of 27,356 m2, which is acknowledged to be greater than the
existing scenario.
The standard to which drainage systems are required to perform has become more onerous
over time. In particular, the EA have stated that the site must be designed so that buildings
do not flood in a 1 in 100 year storm event. When a site is redeveloped it is therefore
inevitable that it will be required to convey surface water more effectively than the
development it replaces.
The predicted surface water run off for the total impermeable area has been calculated
using the Rational Method, with a rainfall intensity of 75mm/hr. We estimate that the peak
discharge from the site will be as follows:
Q = 27,356 x (1/3600) x 75
= 570 litres per second
It can be seen that the proposed development is predicted to generate a significantly greater
peak discharge of surface water than the limits that may be imposed by the various
authorities. Additional discharges from this site will be severely restricted. In order to
reduce the peak discharge to acceptable levels it will be necessary to provide some form of
on-site attenuation, or use sustainable drainage systems, or a combination of these
techniques. The volume of attenuation required can only be calculated once a detailed
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 11 Ove Arup & Partners LtdIssue 2 April 2008
hydraulic model of the proposed network has been built, and the magnitudes of the
allowable discharges have been agreed with the controlling authorities.
Refer to the Baseline Drainage Report for recommendations relating to the permissible peak
surface water discharge from individual plots and the suggested connection/outfall locations.
4.4 Ground conditions – suitability for infiltration techniques
A ground investigation was carried out by Soil Mechanics on behalf of Huddersfield
Technical College (HTC) and WML Consulting during June 2007. This investigation
focussed on the proposed HTC plot, in the area currently occupied by KMC.
Four water inflow tests were performed in boreholes to examine the potential for use of
infiltration techniques. More detail relating to the analysis of the results is presented in the
Baseline Drainage Report. The analysis found that the permeability of the cohesive
deposits and underlying bedrock was approximately 4 x 10-6
m/s or less. For infiltration
techniques to be considered viable, the required minimum permeability is generally in the
region of 1 x 10-5
m/s.
Measurements taken in the cohesive deposits also found groundwater levels at between 1.0
and 2.5m below ground. This occurrence of shallow groundwater, when considered with the
relatively poor permeability values, indicates that infiltration is unlikely to be suitable within
the HTC site. This rules out the use of a number of sustainable drainage techniques.
Investigation of existing ground conditions throughout the remainder of the site has not yet
been undertaken. Further tests to identify the permeability should be carried out as part of
the wider ground investigation to establish if and where surface water infiltration is possible.
4.5 Use of attenuation systems
Given the likely increase in the total peak surface water discharge from the development,
the requirement to limit the peak discharge and the poor permeability of the underlying soils
and rock, it is likely that some form of attenuation will be required. This could be provided
on a plot-by-plot basis or by means of one or more attenuation tanks serving the wider
development.
Strategic Sites Waterfront Quarter, HuddersfieldLevel 2 Flood Risk Assessment
J:\119000\119046-00\0 ARUP\0-01 CIVIL\0-01-08 REPORTS\HWQ 3.0 - FLOOD RISK ASSESSMENT REPORT\HWQ 3.1 LEVEL 2 FLOOD RISK ASSESSMENT.DOC
Page 12 Ove Arup & Partners LtdIssue 2 April 2008
5 Conclusions and recommendations
The Environment Agency (EA) have provided us with an excerpt from mapping of “Flood
Zone Data For England” and extracts from the ISIS Hydraulic Model Summary Sheets
produced as part of the “Colne and Holme Flood Mapping Study”. This information has
allowed us to make an assessment of the risk of flooding to this development based on
actual surveyed levels on the site.
The Waterfront Quarter development site is located entirely within flood zones 1 and 2 as
defined by Planning Policy Statement (PPS) 25. Parts of the site adjacent to the river are
designated as flood zone 2 – that is, the risk of flooding from the river in any year is between
1 in 100 (1% annual exceedence probability, or AEP) and 1 in 1000 (0.1% AEP). These
areas can be developed for all uses except those defined in PPS 25 as “highly vulnerable”,
whereupon the sequential and exception tests must be considered. It is not currently
proposed to locate development types classed as “highly vulnerable” within these areas.
The remainder of the site is located in zone 1 and it is therefore development of any type is
permitted under the guidance given by PPS 25.
Water levels have been estimated for the 1% AEP plus climate change (+CC) flood event,
using data supplied by the EA. This report has recommended that floor levels in the
proposed development be located at or above these levels.
To minimise the risk of flood water entering the site along the canal during extreme events,
the provision of some form of protection should be incorporated as part of the design of the
vertical realignment of the canal. This could comprise of a flood gate and overflow from the
canal to the river at the upstream extent of the development, or other protection measures.
The basement car parking levels beneath buildings A, B & C and L & M could potentially be
below the groundwater level, particularly if groundwater is affected by the vertical
realignment of the canal. Further assessment is required to determine the groundwater
level in these locations and to assess any potential implications of the increased water level
in the canal. Depending on the results of these investigations, it may be necessary to
design the below ground levels of affected buildings as waterproof structures and to provide
appropriate drainage systems.
The surface water drainage systems on this development will be managed so as to not
increase the peak discharge of surface water to any of the existing receiving watercourses
or other drainage assets, when compared to the existing scenario. In some cases a
reduction in the peak discharge will be achieved. The initial ground investigation studies
have determined that the ground is not suitable for the use of infiltration techniques
commonly associated with sustainable drainage systems. Other techniques, including
below ground attenuation, will be required to achieve the discharge conditions.
The management of surface water on the site will be such that the buildings will not flood
during a 1% AEP (1 in 100 year) rainfall event.