Broxbourne Borough Council Level 2 Strategic Flood Risk ......Mapping of groundwater flood risk has...

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Broxbourne Borough Council Level 2 Strategic Flood Risk Assessment Final April 2017 Broxbourne Borough Council Bishops' College Churchgate Cheshunt WALTHAM CROSS EN8 9XQ

Transcript of Broxbourne Borough Council Level 2 Strategic Flood Risk ......Mapping of groundwater flood risk has...

Page 1: Broxbourne Borough Council Level 2 Strategic Flood Risk ......Mapping of groundwater flood risk has been based on the Areas Susceptible to Groundwater (AStGWF) dataset. The AStGWF

Broxbourne Borough Council Level 2 Strategic Flood Risk Assessment

Final

April 2017

Broxbourne Borough Council

Bishops' College

Churchgate

Cheshunt

WALTHAM CROSS

EN8 9XQ

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JBA Project Manager Joanne Chillingworth The Library St Philip’s Courtyard Church End COLESHILL B46 3AD

Revision History

Revision Ref / Date Issued Amendments Issued to

Draft v1.0 / April 2017 Issue of draft Kim Harding, Broxbourne Borough Council

Final v2.0 / April 2017 Client comments actioned

Kim Harding, Broxbourne Borough Council

Final v3.0/ April 2017 Additional 2D modelling appendix added

Kim Harding, Broxbourne Borough Council

Contract This report describes work commissioned by Broxbourne Borough Council. The Council’s representative for the contract was Kim Harding.

Prepared by .................................................. Freyja Scarborough MSc BSc

Assistant Analyst

Reviewed by ................................................. Joanne Chillingworth BSc MSc MCIWEM C.WEM

Chartered Senior Analyst

Purpose This document has been prepared as a Final Report for Broxbourne Borough Council. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared.

JBA Consulting has no liability regarding the use of this report except to Broxbourne Borough Council.

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Acknowledgements We would like to acknowledge the assistance of Broxbourne Borough Council.

Copyright © Jeremy Benn Associates Limited 2017

Carbon Footprint A printed copy of the main text in this document will result in a carbon footprint of 49g if 100% post-consumer recycled paper is used and 63g if primary-source paper is used. These figures assume the report is printed in black and white on A4 paper and in duplex.

JBA is aiming to reduce its per capita carbon emissions.

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Contents 1 Introduction ............................................................................................................ 1

2 Methodology .......................................................................................................... 1

2.1 Flood Zones ....................................................................................................................... 1 2.2 Climate change .................................................................................................................. 1 2.3 Surface Water .................................................................................................................... 1 2.4 Groundwater ...................................................................................................................... 2 2.5 River networks ................................................................................................................... 2 2.6 Sewer and reservoir flooding ............................................................................................. 2 2.7 Usage of 2D hydraulic modelling ........................................................................................ 2 2.8 Consistency with EA Flood Zones ...................................................................................... 3 2.9 Note on SuDS Suitability .................................................................................................... 3

3 Detailed Site Summary Tables .............................................................................. 4

Appendices ......................................................................................................................... 6

A Appendix A – Site Summary Tables and Mapping ................................................ 6

B Appendix B – JFLOW Technical Summary ........................................................... 2

List of Tables Table 2-1: Summary of SuDS Categories .......................................................................... 4

Table 3-1: List of Detailed Summary Tables ...................................................................... 4

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1 Introduction The Level 2 SFRA assesses the level of flood risk associated with sites identified through the Council’s strategic land availability assessment exercise and shortlisted for their development potential.

A site was shortlisted for Level 2 assessment if it met the following criteria:

• The site is within Flood Zone 2 and/or 3; and/or

• An ordinary watercourse runs through or adjacent to the site;

• And it may be suitable for development

This Level 2 SFRA assessment of sites helps to determine variations in flood risk across the proposed site allocations, identifying site-specific FRA requirements and helping guide local policies to provide sustainable developments, as well as managing and reducing flood risk to existing communities.

2 Methodology

2.1 Flood Zones

The data used to prepare the fluvial mapping for this study is based on Flood Zones and the results from hydraulic models either provided by the Environment Agency or prepared for the purposes of this Strategic Flood Risk Assessment (SFRA). Hydraulic models used include:

• River Lee 'M02'

• River Lee and Tributaries 'M05_06'

• River Lee and Tributaries 'M07_08'

2.1.1 Flood Zones 2 and 3a

Flood Zones 2 and 3a have been taken from the Environment Agency’s Flood Zone 2 and 3, as shown on their long term flood risk information website.

2.1.2 Flood Zone 3b

Flood Zone 3b has been identified as land which would flood with an annual probability of 1 in 20 years (5% AEP). It has been derived from the 20-year defended modelled flood extent, where detailed Environment Agency hydraulic models exist, and where no detailed models exist, this uses 2D hydraulic modelling (Jflow+) produced as part of this SFRA.

There may be some minor discrepancies between Flood Zone 3b and Flood Zone 3a in places as the Jflow+ modelling has not yet been used to update Environment Agency Flood Zones.

2.2 Climate change

The three climate change allowances for the ‘2080’s were modelled by re-running the Environment Agency's detailed models of the River Lee and Tributaries, and running the 2D modelling along the Turnford Brook, Theobalds Brook, Wormleybury Brook.

Watercourses covered by Environment Agency Flood Zones but not by the models detailed above, were modelled for climate change as part of this SFRA using Jflow+.

The mapping provides a strategic assessment of climate change risk – developers should undertake detailed modelling of climate change allowances as part of a site-specific FRA, following the guidance set out in the SFRA main report.

2.3 Surface Water

Mapping of surface water flood risk in Broxbourne borough has been taken from the updated Flood Map for Surface Water (uFMfSW) published online by the Environment Agency.

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Category Definition

High Flooding occurring as a result of rainfall with a greater than 1 in 30 chance in any given year (annual probability of flooding 3.3%)

Medium Flooding occurring as a result of rainfall of between 1 in 100 (1%) and 1 in 30 (3.3%) chance in any given year.

Low Flooding occurring as a result of rainfall of between 1 in 1,000 (0.1%) and 1 in 100 (1%) chance in any given year.

Very Low Flooding occurring as a result of rainfall with less than 1 in 1,000 (0.1%) chance in any given year.

The uFMfSW is derived primarily from identifying topographical flow paths of existing watercourses or dry valleys that contain some isolated ponding locations in low lying areas.

Although the uFMfSW offers improvement on previously available datasets, the results should not be used to understand flood risk for individual properties. The results should be used for high level assessments such as SFRAs for local authorities. If a particular site is indicated in the Environment Agency mapping to be at risk from surface water flooding, a more detailed assessment should be required to more accurately illustrate the flood risk at a site-specific scale. Such an assessment will use the uFMfSW in partnership with other sources of local flooding information to confirm the presence of a surface water risk at that particular location.

2.4 Groundwater

Mapping of groundwater flood risk has been based on the Areas Susceptible to Groundwater (AStGWF) dataset. The AStGWF dataset is a strategic-scale map showing groundwater flood areas on a 1km square grid. It shows the proportion of each 1km grid square, where geological and hydrogeological conditions indicate that groundwater might emerge. It does not show the likelihood of groundwater flooding occurring and does not take account of the chance of flooding from groundwater rebound. This dataset covers a large area of land, and only isolated locations within the overall susceptible area are actually likely to suffer the consequences of groundwater flooding.

The AStGWF data is indicative and should only be used in combination with other information, for example local data or historical data. It should not be used as sole evidence for any specific flood risk management, land use planning or other decisions at any scale. However, the data can help to identify areas for assessment at a local scale where finer resolution datasets exist.

2.5 River networks

Main Rivers are represented by the Environment Agency's Statutory Main River layer. Ordinary Watercourses are represented by the Environment Agency's Detailed River Network Layer.

2.6 Sewer and reservoir flooding

Due to licencing and confidentiality restrictions, sewer flooding data has not been represented on the mapping. Please see the main SFRA report for further information.

2.7 Usage of 2D hydraulic modelling

Where there are no Environment Agency detailed hydraulic models covering the site, 2D modelling was undertaken using JFlow to determine Flood Zone 3a, Flood Zone 3b and Flood Zone 2, as well as the effects of climate change, for a number of ordinary watercourses flowing through or adjacent to sites. Appendix B gives a technical summary of this modelling technique. Using this information combined, detailed site summary tables have been produced for the proposed site allocations. Each table sets out the following information:

• Site area

• Existing drainage features

• Proportion of the site in each Flood Zone

• Proportion of the site in each surface water scenario

• The presence of any flood defences

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• Whether the site is covered by a flood warning service

• Whether there are any access and egress issues for the site

• Climate change allowances for ‘2080s’ epoch

• The potential impacts of climate change in the future

• A broadscale assessment of suitable SuDS techniques and considerations

• NPPF and Exception Test guidance

• Advice on the preparation of site-specific FRAs and considerations for developers.

• Mapping including Flood Zones, climate change, surface water, depth, velocity and hazard mapping

2.8 Consistency with EA Flood Zones

It is important to recognise that for the SFRA a number of different sets of data have been used to present the Flood Zones. The detailed site summary tables shown in Appendix A as part of the Level 2 assessment may differ to the Environment Agency Flood Zones and ‘Flood Map for Planning’ (Appendix B of Level 1 SFRA report). In general, where there is detailed model coverage, these should match the Environment Agency Flood Zones for Flood Zones 2 and 3 as the Environment Agency updates the Flood Maps with detailed modelling periodically. Flood Zone 3b uses the defended 20-year flood extent from these hydraulic models.

Where there are no detailed models present, 2D modelling has been undertaken to provide an indication of flood risk at the site for the purpose of the SFRA. This uses the same modelling technique as the Environment Agency Flood Zones, but the extents may differ as more up-to-date data has been used (for example model software version, ground terrain data etc), since those published on the Environment Agency’s website. This modelling was needed to provide the 20-year and climate change extents, as well as the depth, hazard and velocity information at the site. Flood Zone 3b in this case is classed as undefended, as the 2D modelling does not take into account channel survey or structures.

It is recommended that any sites covered by 2D modelling are investigated in further detail at Flood Risk Assessment stage, which may require detailed hydraulic modelling informed by channel topographic survey, to confirm flood risk at the site.

2.9 Note on SuDS Suitability

The hydraulic and geological characteristics of each proposed site allocations were assessed to determine the constraining factors for surface water management. This assessment is designed to inform the early-stage site planning process and is not intended to replace site-specific detailed drainage assessments.

The assessment is based on catchment characteristics and additional datasets such as the AStGWF map and Soil maps of England and Wales which allow for a basic assessment of the soil characteristics on a site by site basis. LIDAR data was used as a basis for determining the topography and average slope across each development site. Other datasets were used to determine other influencing factors on potential SuDS. These datasets include the following:

• Historic landfill sites

• Source Protection Zones

• Groundwater Vulnerability Zones

• Detailed River Network

• Environment Agency Flood Zones

• OS open data on Sites of Special Scientific Interest (SSSI)

This data was then collated to provide an indication of particular groups of SuDS systems which might be suitable at a site. SuDS techniques were categorised into five main groups, as shown in Table 2-1, and are included in each site summary table as part of the Level 2 assessment. This assessment should not be used as a definitive guide as to which SuDS would be suitable but used as an indicative guide of general suitability. Further site-specific investigation should be conducted to determine what SuDS techniques could be utilised on a particular development.

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Table 2-1: Summary of SuDS Categories

SuDS Type Technique

Source Controls Green Roof, Rainwater Harvesting, Pervious Pavements, Rain

Gardens

Infiltration Infiltration Trench, Infiltration Basin, Soakaway

Detention Pond, Wetland, Subsurface Storage, Shallow Wetland, Extended Detention Wetland, Pocket Wetland, Submerged Gravel Wetland,

Wetland Channel, Detention Basin

Filtration Surface Sand filter, Sub-Surface Sand Filter, Perimeter Sand Filter,

Bioretention, Filter Strip, Filter Trench

Conveyance Dry Swale, Underdrained Swale, Wet Swale

The suitability of each SuDS type for the proposed site allocations has been described in the summary tables. The assessment of suitability is broadscale and indicative only; more detailed assessments should be carried out during the site planning stage to confirm the feasibility of different types of SuDS. The LLFA should be consulted at an early stage to ensure SuDS are implemented and designed in response to site characteristics and policy factors.

3 Detailed Site Summary Tables As part of the Level 2 SFRA, detailed site summary tables have been produced for the proposed site allocations below:

Table 3-1: List of Detailed Summary Tables

Site Ref Settlement Flood Zone Coverage (%) OW within 100m

(Y / N) FZ3 FZ2

Brookfield Wormley 10% 12% Yes

Cheshunt Lakeside (Delamare Road Strategic Site)

Cheshunt 15% 98% Yes

Wolsey Hall, Windmill Lane Cheshunt 88% 96% Yes

Turnford Surfacing Site Hoddesdon 19% 94% Yes

Derwent Turnford Turnford 59% 66% Yes

Waltham Cross - Northern High Street (Land off Sturlas Way).

Waltham Cross 0% 4% No

Land East of Dinant Link Road Hoddesdon 6% 34% Yes

Gas Holder Site Broxbourne 2% 6% Yes

South of Church Lane Wormley

Broxbourne 0% 0.2% Yes

Theobald’s Station car park Waltham Cross 0% 0.1% Yes

Waltham Cross Trade Centre Waltham Cross 0% 29% No

Rush Meadow Cheshunt 1% 3% Yes

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Appendices

A Appendix A – Site Summary Tables and Mapping

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B Appendix B – JFLOW Technical Summary

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