Braintree District Council Strategic Flood Risk Assessment Appendix A Braintree Supplementary Report

Report July 2008

Prepared for:

Photographs of Townsford Mill, Halstead and the River Stour at Pentlow Mill courtesy of Christopher Strickland

Braintree District Council D111532 Strategic Flood Risk Assessment

Revision Schedule

Mid Essex Strategic Flood Risk Assessment July 2008

Rev Date Details Prepared by Reviewed by Approved by

01 August 07 Draft SFRA for Nick Bosanko Liz Williams Jon Robinson comments Flood Risk Specialist Senior Consultant Associate

02 January 08 Final Draft Eleanor Cole Liz Williams SFRA Graduate Hydrologist Senior Consultant

03 April 08 Final SFRA Eleanor Cole Liz Williams Assistant Hydrologist Senior Consultant

04 May 08 Final SFRA Eleanor Cole Liz Williams Assistant Hydrologist Senior Consultant

05 July 08 Final SFRA Eleanor Cole Jon Robinson Assistant Hydrologist Associate Director

Scott Wilson 6-8 Greencoat Place, This document has been prepared in accordance with the scope of Scott Wilson's appointment with its client and is subject to the terms of that appointment. It is addressed London, to and for the sole use and reliance of Scott Wilson's client. Scott Wilson accepts no liability for any use of this document other than by its client and only for the purposes, SW1P 1PL stated in the document, for which it was prepared and provided. No person other than the client may copy (in whole or in part) use or rely on the contents of this document, without the prior written permission of the Company Secretary of Scott Wilson Ltd. Any advice, opinions, or recommendations within this document should be read and relied upon only in the context of the document as a whole. The contents of this document are not to be Tel: 020 7798 5200 construed as providing legal, business or tax advice or opinion. Fax: 020 7798 5001

© Scott Wilson Group PLC 2006 www.scottwilson.com Braintree District Council D111532 Strategic Flood Risk Assessment

Table of Contents

1 Non-Technical Summary...... 1 1.1 SFRA Background...... 1 1.2 SFRA Planning Objectives ...... 1 1.3 SFRA Report Layout ...... 2 1.4 Braintree District Council Considerations...... 2 1.5 Way Forward ...... 4 1.6 A Living Document ...... 5 2 Introduction and Background...... 6 2.2 Scope and Objectives ...... 6 2.3 Report Structure ...... 7 2.4 Braintree District Council Area ...... 7 3 Sources of Flooding in Braintree ...... 10 3.1 Introduction...... 10 3.2 Fluvial ...... 12 3.3 Overland Flow ...... 15 3.4 Arterial Drainage Network ...... 17 3.5 Groundwater...... 19 4 Level 1 Assessment ...... 20 4.1 Level 1 SFRA – Study Area, Flood Source Review and Data Review...... 20 4.2 Level 1 Mapping Methodology ...... 21 5 Level 2 Assessment ...... 23 5.1 Overview ...... 23 5.2 Fluvial ...... 23 5.3 Overland Flow ...... 24 5.4 Arterial Drainage Network ...... 25 5.5 Strategic Spatial Flood Source Summary...... 27 6 Bramston Sports Centre...... 31 6.1 Introduction...... 31 6.2 Site and Surrounding Area ...... 31 6.3 Regulatory Position ...... 33 6.4 Flood Status and Sources ...... 36 6.5 Assessment of Flood Risk...... 38 6.6 Potential Development Considerations ...... 39 Braintree District Council D111532 Strategic Flood Risk Assessment

6.7 Flood Mitigation and Management ...... 41 6.8 Bramston Sports Centre Conclusions ...... 44 7 Water Discharges...... 46 7.2 Effect of Development in Braintree...... 46 7.3 Mitigation ...... 47 8 SuDS Specific to Braintree...... 48 8.1 Introduction...... 48 8.2 Geology ...... 48 8.3 SuDS Recommendations ...... 52 9 Policy Specific Requirements to Braintree District ...... 54 9.2 Flood Risk ...... 54 9.3 Sustainable Drainage Systems (SuDS) ...... 55 9.4 Flood Mitigation ...... 56 9.5 Water Environment...... 57 10 Recommendations ...... 59 11 References ...... 60 12 Blackwater Modelling Report by Atkins ...... 61 Braintree Figures Appendix

Braintree District Council D111532 Strategic Flood Risk Assessment

List Figures Figure Number Description Area A1 Braintree District Location Map A2 District Wide Fluvial Flood Risk Map A3-1 Flood Zones 2007 Land East of Stebbing A3-2 Flood Zones 2107 Land East of Stebbing A4-1 Flood Zones 2007 Land West of Rayne A4-2 Flood Zones 2107 Land West of Rayne A5-1 Flood Zones 2007 Land East of Rayne A5-2 Flood Zones 2107 Land East of Rayne A6-1 Flood Zones 2007 Land South West of Great Notley A6-2 Flood Zones 2107 Land South West of Great Notley A7-1 Flood Zones 2007 Land East of Great Notley A7-2 Flood Zones 2107 Land East of Great Notley A8-1 Flood Zones 2007 Land South West of Braintree (Off London Road) A8-2 Flood Zones 2107 Land South West of Braintree (Off London Road) A9-1 Flood Zones 2007 Land North of Springwood Industrial Area, Braintree A9-2 Flood Zones 2107 Land North of Springwood Industrial Area, Braintree A10-1 Flood Zones 2007 Land at Dorewards Hall, Bocking A10-2 Flood Zones 2107 Land at Dorewards Hall, Bocking A11-1 Flood Zones 2007 Land North East of Braintree A11-2 Flood Zones 2107 Land North East of Braintree A12-1 Flood Zones 2007 Land Adjacent to Galleys Corner, Braintree A12-2 Flood Zones 2107 Land Adjacent to Galleys Corner, Braintree A13-1 Flood Zones 2007 Land South West of Silver End A13-2 Flood Zones 2107 Land South West of Silver End A14-1 Flood Zones 2007 Land East of Panfield Lane A14-2 Flood Zones 2107 Land East of Panfield Lane A15-1 Flood Zones 2007 Former Crittalls Factory, Silver End A15-2 Flood Zones 2107 Former Crittalls Factory, Silver End A16-1 Flood Zones 2007 Land South West of Witham A16-2 Flood Zones 2107 Land South West of Witham A17-1 Flood Zones 2007 Land North East of Witham A17-2 Flood Zones 2107 Land North East of Witham A18-1 Flood Zones 2007 Land East of Coggeshall A18-2 Flood Zones 2107 Land East of Coggeshall A19-1 Flood Zones 2007 Land West of Marks Tey A19-2 Flood Zones 2107 Land West of Marks Tey A20-1 Flood Zones 2007 Land North of Ferring A20-2 Flood Zones 2107 Land North of Ferring A21-1 Flood Zones 2007 Land West of Coggeshall Road, Kelvedon A21-2 Flood Zones 2107 Land West of Coggeshall Road, Kelvedon A22-1 Flood Zones 2007 Land West of B1023, Ferring A22-2 Flood Zones 2107 Land West of B1023, Ferring Braintree District Council D111532 Strategic Flood Risk Assessment

A23-1 Flood Zones 2007 Land between A12 and B1024, Ferring A23-2 Flood Zones 2107 Land between A12 and B1024, Ferring A24-1 Flood Zones 2007 Land North East of Halstead A24-2 Flood Zones 2107 Land North East of Halstead A25-1 Flood Zones 2007 Land East of Millennium Way A25-2 Flood Zones 2107 Land East of Millennium Way A26-1 Flood Zones 2007 Land North West of Earls Colne A26-2 Flood Zones 2107 Land North West of Earls Colne A27-1 Flood Zones 2007 Land Off Mount Hill, Halstead A27-2 Flood Zones 2107 Land Off Mount Hill, Halstead A28-1 Flood Zones 2007 Land South of Tye Green, Cressing A28-2 Flood Zones 2107 Land South of Tye Green, Cressing A29-1 Flood Zones 2007 Bramston Sports Centre A29-2 Flood Zones 2107 Bramston Sports Centre A30-1 Flood Zones 2007 Rifle Hill, Braintree A30-2 Flood Zones 2107 Rifle Hill, Braintree A31-1 Flood Zones 2007 Mill Hill, Southern Side of Braintree (Adjacent to Bypass) A31-2 Flood Zones 2107 Mill Hill, Southern Side of Braintree (Adjacent to Bypass) A32-1 Flood Zones 2007 Riverside Pool Site, Braintree A32-2 Flood Zones 2107 Riverside Pool Site, Braintree A33-1 Flood Zones 2007 Land East of High Street, Halstead A33-2 Flood Zones 2107 Land East of High Street, Halstead A34-1 Flood Zones 2007 Maltings Lane, Witham A34-2 Flood Zones 2107 Maltings Lane, Witham A35-1 Flood Zones 2007 Gimsons, Witham (Next to Recreation Ground) A35-2 Flood Zones 2107 Gimsons, Witham (Next to Recreation Ground) A36-1 Flood Zones 2007 Constance Close, Witham A36-2 Flood Zones 2107 Constance Close, Witham

A37 20 Year Depth 2007 Upstream of Braintree A38 20 Year Depth 2007 Braintree A39 20 Year Depth 2007 Braintree to Kelvedon A40 20 Year Depth 2007 Witham to Maldon A41 20 Year Depth 2107 Upstream of Braintree A42 20 Year Depth 2107 Braintree A43 20 Year Depth 2107 Braintree to Kelvedon A44 20 Year Depth 2107 Witham to Maldon A45 100 Year Depth 2107 Upstream of Braintree A46 100 Year Depth 2107 Braintree A47 100 Year Depth 2107 Braintree to Kelvedon A48 100 Year Depth 2107 Witham to Maldon A49 1000 Year Depth 2107 Upstream of Braintree A50 1000 Year Depth 2107 Braintree A51 1000 Year Depth 2107 Braintree to Kelvedon A52 1000 Year Depth 2107 Witham to Maldon Braintree District Council D111532 Strategic Flood Risk Assessment

A53 20 Year Hazard 2007 Upstream of Braintree A54 20 Year Hazard 2007 Braintree A55 20 Year Hazard 2007 Braintree to Kelvedon A56 20 Year Hazard 2007 Witham to Maldon A57 20 Year Hazard 2107 Upstream of Braintree A58 20 Year Hazard 2107 Braintree A59 20 Year Hazard 2107 Braintree to Kelvedon A60 20 Year Hazard 2107 Witham to Maldon A61 100 Year Hazard 2107 Upstream of Braintree A62 100 Year Hazard 2107 Braintree A63 100 Year Hazard 2107 Braintree to Kelvedon A64 100 Year Hazard 2107 Witham to Maldon A65 1000 Year Hazard 2107 Upstream of Braintree A66 1000 Year Hazard 2107 Braintree A67 1000 Year Hazard 2107 Braintree to Kelvedon A68 1000 Year Hazard 2107 Witham to Maldon A69 100 Year Hazard 2107 Bramston Sports Centre A70 1000 Year Hazard 2107 Bramston Sports Centre A71 Emergency Planning Map A72 A1 Flood Zones ALL FLUVIAL

Braintree District Council D111532 Strategic Flood Risk Assessment

List of Tables

TABLE 3-1SUMMARY OF POTENTIAL FLOOD SOURCES ...... 11 TABLE 3-2 TYPICAL INFILTRATION CO-EFFICIENTS FOR DIFFERENT SOIL TYPES...... 16 TABLE 5-1 SUMMARY OF POSTCODE AREAS AFFECTED BY FLOODING FROM SEWERS IN THE PAST ...... 26 TABLE 5-2 SUMMARY OF FLOOD RISK AT EACH POTENTIAL DEVELOPMENT ALLOCATION (FOR LOCATIONS SEE FIGURES APPENDIX) ...... 29 TABLE 6-1 VULNERABILITY OF PREDICTED LAND USAGES WITHIN THE BRAMSTON SPORTS CENTRE AREA. FOR MORE INFORMATION SEE TABLE D.2: FLOOD RISK VULNERABILITY CLASSIFICATION AND D.3: FLOOD RISK VULNERABILITY AND FLOOD ZONE ‘COMPATIBILITY’ IN PPS25 ...... 35 TABLE 6-2 SUMMARY OF POTENTIAL FLOOD SOURCES AT BRAMSTON SPORTS CENTRE ...... 36 TABLE 8-1 MAJOR SOLID GEOLOGICAL DEPOSITS FOUND THROUGHOUT THE DISTRICT ...... 50 TABLE 8-2 MAJOR DRIFT AND GEOLOGICAL DEPOSITS FOUND THROUGHOUT THE DISTRICT ...... 51

Braintree District Council D111532 Strategic Flood Risk Assessment

1 Non-Technical Summary

1.1 SFRA Background

1.1.1 Scott Wilson Ltd was commissioned by the Mid Essex Area Liaison group (MEAL) to undertake a ‘Stage 2’ Strategic Flood Risk Assessment (SFRA) of Mid Essex. An Inception Report, completed by Scott Wilson in November 2006, preceded this ‘Stage 2’ SFRA. The Inception Report located and identified available data and information that would be useful for completion of the SFRA. In addition the report outlined the extents of the study areas, the modelling approach and highlighted various specific flood risk issues within the Mid Essex area that should be covered within the main SFRA report.

1.1.2 MEAL incorporates the local councils of Chelmsford Borough Council, Colchester Borough Council, Braintree District Council and Maldon District Council.

1.1.3 This project was carried out in collaboration with the Environment Agency’s Anglian Region, and a draft of the full report was submitted to the Agency for their comments and observations. Mutually acceptable amendments have been incorporated into the final SFRA report.

1.2 SFRA Planning Objectives

1.2.1 The primary objective of the study was to enable the four participating local authorities to undertake Sequential Testing inline with Government’s flood risk and development policy document - Planning Policy Statement (PPS) 25: Development and Flood Risk - to inform the development of their emerging Local Development Framework (LDF) documents.

1.2.2 PPS25 requires local planning authorities to review flood risk across their districts, steering all development towards areas of lowest risk. Development is only permissible in areas at risk of flooding in exceptional circumstances where it can be demonstrated that there are no reasonably available sites in areas of lower risk, and the benefits of development outweigh the risks from flooding. Such development is required to include mitigation/management measures to minimise risk to life and property should flooding occur.

1.2.3 The Strategic Flood Risk Assessment is the first step in this process, assisting in the development of the LDF’s by identifying flood risk areas and outlining the principles for sustainable development policies, informing strategic land allocations and integrating flood risk management into the spatial planning of the area. The SFRA thereby forms an essential reference tool providing the building blocks for future strategic planning.

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1.3 SFRA Report Layout

1.3.1 The main background and methodology information, including guidance on using the figures and potential measures for residual risk management are discussed in the Main SFRA report. For each participating authority a suitable appendix has been compiled to include background information on that area with regard to flood risk. Additional information requested as part of the SFRA Brief, in relation to each respective local authority and associated flood risk mapping, is also included.

1.3.2 There are four separate appendices for each authority. This report, Appendix A reflects the flood risk issues for Braintree District Council. Other Appendices reflect the flood risk issues in relation to each local authority.

1.4 Braintree District Council Considerations Background

1.4.1 The District of Braintree is bordered by Colchester to the east, Maldon to the south, and Chelmsford to the west. Braintree District Council is the only Local Authority within the MEAL Group that does not have a coastal boundary or areas at risk from tidal flooding. As such, tidal flooding has not been considered in this report.

1.4.2 The River Stour forms a large section of both the northern and eastern boundary of the District. A number of other large watercourses are situated within the District, including the River Brain, which flows through the towns of Braintree and Witham. The rivers Blackwater, Stour and Colne are also major rivers situated within the district.

1.4.3 The main settlement within the District of Braintree is Braintree, whilst Witham and Halstead represent the remaining key centres of population. Objectives

1.4.4 The Braintree District Council Appendix within the Mid Essex SFRA has been undertaken to meet the following key objectives:

1.4.5 The purpose of this report is to:

• Provide information and guidance to enable the Local Authority to apply the Sequential Test within their district; • Identification of flood risk to proposed allocation areas in both greenfield and brownfield sites; • Assess flood risk, on a more detailed level, to the potential redevelopment site at Bramston Sports Centre; and

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• Discuss the surface water drainage, maintenance and Sustainable Drainage Systems (SuDS) requirements for new developments, with special reference to critical drainage areas, such as Bocking. The Sequential Test

1.4.6 The process of the Sequential Test outlined in PPS25 aims to steer vulnerable development to areas of lowest flood risk. The SFRA aims to facilitate this process by identifying the variation in flood risk across the District, allowing an area-wide comparison of future development sites with respect to flood risk considerations.

1.4.7 The District of Braintree has been delineated into the flood zones outlined in PPS25 as Flood Zone 1, low probability, Flood Zone 2, medium probability and Flood Zone 3a, high probability. In addition, Flood Zone 3b, functional floodplain, has also been mapped. Table D.1 of PPS25 provides information on which developments might be considered appropriate in each flood zone, subject to the application of the Sequential Test and the Exception Test with a site-specific Flood Risk Assessment demonstrating safety.

1.4.8 In accordance with PPS12, Braintree District Council is required to produce a Core Strategy document. The Core Strategy must be in accordance with the policies of PPS25. Braintree District Council are in the process of producing their Preferred Options within their Core Strategy. The Sequential Test can then be performed when the preferred areas for development are identified. This test will identify the flood risks and development vulnerability of the preferred sites for development in order to assess the suitability of each development location. Where possible, more vulnerable developments should be steered to areas of lower flood risk. It is estimated that the Preferred Options Stage of the Core Strategy will be completed by summer 2008. The Exception Test

1.4.9 Where it can be demonstrated by the Local Planning Authority that the Sequential Test is passed, it will also be necessary in some circumstances for the Council to demonstrate that all three elements of the Exception Test are satisfied. Flood Sources

1.4.10 The dominant fluvial systems within the borough are the River Blackwater, the River Brain and the River Colne. These river systems flow within close proximity to a number of settlements (Figures A1 and A2). The River Brain flows through the towns of Braintree and Witham. The River Blackwater flows through Braintree, Coggeshall, Kelvedon and to the south of Witham. The confluence of the Rivers Brain and Blackwater is situated to the south east of Witham town centre. The River Colne flows through Great Yeldham, Sible Hedingham and Halstead. Due to the close proximity of these settlements to fluvial systems, they may be at risk from fluvial flooding.

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1.4.11 Areas of ‘functional floodplain’ (i.e. Flood Zone 3b) have been identified. These areas are generally open space areas that flood relatively frequently, and are not defended.

1.4.12 Fluvial flood extents have been remodelled as part of this study. These have been provided by Black and Veatch for the River Colne, and Atkins for the River Blackwater and the River Brain (see Section 12 – Blackwater Modelling Report by Atkins). These extents have been mapped to show the 1 in 1000 annual fluvial probability (Flood Zone 2), the 1 in 100 annual fluvial probability (Flood Zone 3a), and the 1 in 20 annual fluvial probability (Flood Zone 3b, or Functional Floodplain). Modelling has been carried out to show the present day Flood Zone extents (year 2007) and the future Flood Zone extents under 100 years of climate change (year 2107).

1.4.13 Surface water and groundwater records have been investigated to assess the risk of flooding originating from these sources. Anglian Water has supplied surface water and sewer flooding records for the District.

1.4.14 There are no land use or development restrictions within Flood Zone 1, Low Probability (i.e. all remaining areas of the Borough). These areas include large regions classified as Greenfield, proposed as broad locations for growth. It is important to note that surface run-off from development within these areas, if not carefully designed and managed, may exacerbate existing flooding and/or drainage problems further downstream. Therefore in accordance with PPS25 all developments should seek to incorporate Sustainable Drainage Systems (SuDS).

1.4.15 This SFRA seeks to use this data to identify the levels of flood risk across the District from all relevant sources of flooding. This will provide Braintree District Council with an improved understanding of flood risk issues to assist in the completion of their Sequential Test. This report also provides further information for sites such as the Bramston Sports Centre in relation to a specific flood risk to inform Exception Tests that may be required for future development in this area.

1.5 Way Forward

1.5.1 The risk of flooding posed to properties within the District arises from a number of different sources including river flooding, sewer and surface water flooding.

1.5.2 The main settlement in the District, Braintree, is situated within proximity to a main river and associated floodplain. Northern areas of Braintree town are within Flood Zones 2 and 3. Therefore some of the proposed site-specific allocations in the northern region of the town centre are at risk of flooding.

1.5.3 A spatial planning solution to flood risk management should be sought wherever possible. It is necessary for the local authority to consider, through the PPS25 Sequential Test, how to steer vulnerable development away from areas affected by flooding. This should also take into consideration other relevant strategies and studies

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in the area seeking to reduce flooding to those areas already at risk within the District. Specific planning recommendations have been provided at the end of this report.

1.5.4 Where other planning considerations must guide the allocation of sites to ensure the Sequential Test has been satisfied, further studies may be carried out to assist the local authority and developers to meet the Exception Test.

1.5.5 Engagement with the Emergency Planning Team and ‘Blue Light Services’ is imperative to minimise the risk to life posed by flooding within the Borough. It is recommended that the Council review their adopted flood risk response plan in light of the findings and recommendations of the SFRA.

1.6 A Living Document

1.6.1 The Mid Essex SFRA has been completed in accordance with PPS25 and the current guidance outlined in the Development and Flood Risk: A Practice Guide Companion to PPS25 ‘Living Draft’ (Feb 2007).

1.6.2 The SFRA has been developed by building heavily upon existing knowledge with respect to flood risk within the District. Ongoing modelling and potential improved flood defence and infrastructure may significantly improve current knowledge of flood risk within the District over time, and may alter predicted flood extents within the area. This may therefore influence future development control decisions within these areas.

1.6.3 In summary, it is imperative that the SFRA is adopted as a ‘living’ document and is reviewed regularly in light of emerging policy directives and an improving understanding of flood risk within the Borough.

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2 Introduction and Background

2.1.1 Scott Wilson has been commissioned by the Mid Essex Area Liaison (MEAL) group to undertake a Strategic Flood Risk Assessment (SFRA) on behalf of the local authorities of Braintree District Council, Chelmsford Borough Council, Colchester Borough Council and Maldon District Council.

2.1.2 The SFRA identifies flood risk issues relevant to both existing and proposed developments within the area of Mid Essex and the individual Local Authorities, allowing a direct input into the strategic planning of the Mid Essex region through local development frameworks. The SFRA process also aids local authorities to meet the requirements of Planning Policy Statement 25 (PPS25) ‘Development and Flood Risk’ published in December 2006.

2.1.3 In addition to the main report, Scott Wilson was commissioned to produce four reports to address the flood risk concerns specific to each local authority. This is the report for Braintree District Council. The scope and objectives for the Braintree area are addressed in the following section.

2.2 Scope and Objectives

2.2.1 This report has been undertaken for the Local Authority of Braintree District Council.

2.2.2 The purpose of this report is to:

• Provide information and guidance to enable the Local Authority to apply the Sequential Test within their district; • Identification of flood risk to proposed allocation areas in both greenfield and brownfield sites; • Assess flood risk, on a more detailed level, to the potential redevelopment site at Bramston Sports Centre; and • Discuss the surface water drainage, maintenance and Sustainable Drainage Systems (SuDS) requirements for new developments, with special reference to areas critical drainage areas, such as Bocking.

2.2.3 Additional and more general aspects of the study were not defined for the particular scope of this report, details of which can be found in designated sections of the main SFRA, relating to flood map applications, the Sequential Test, residual risk management and policy reviews.

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2.3 Report Structure

2.3.1 The report is structured to provide an assessment of flood risk based on the best available data. This data has been used to produce a Level 1 assessment of sites allocated in the Local Plan.

2.3.2 The Level 2 aspect of this report summarises the information pertaining to individual development allocations and the key flood sources. It also discusses, in more detail, the flood risk posed to the Bramston Sports Centre and makes recommendations for more detailed investigations.

2.3.3 Subsequently the report provides information pertaining to the Sequential Test and makes recommendations as to how it should be undertaken. This is followed by the identification of residual risk in an attempt to highlight the implications of inappropriate mitigation.

2.3.4 The report provides information and recommendations pertaining to surface water drainage for new developments. It also discusses some of the key considerations during the appraisal of Sustainable Drainage Systems (SuDS).

2.3.5 This report is comprised of one volume, which forms an appendix of the main SFRA report: Appendix A – Braintree District Council. The Main Report (Mid Essex Strategic Flood Risk Assessment, Scott Wilson) should however be referred to for Flood Mapping and Application and Methodology information.

2.4 Braintree District Council Area Physical Geography

2.4.1 The Braintree District is located to the west of Colchester Borough Council and to the north of Maldon District Council and Chelmsford Borough Council. Figure A1 is a location map of Braintree District Council; this also illustrates the key watercourses within the district.

2.4.2 The district of Braintree covers an area of almost 61,200ha and lies approximately 50 metres above sea level. The District is largely rural in land use with a general downward trend in topography towards the south-east and the sea. Braintree District Council is the only Local Authority within the MEAL Group that does not have any coastal zones.

2.4.3 The River Stour forms a large section of both the northern and eastern boundary of the District (Figures A1 and A2). A number of other large watercourses can be found within the District, including the River Brain, which flows through the towns of Braintree and Witham.

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2.4.4 The rivers Blackwater, Stour and Colne are major rivers and are situated within the district. The River Blackwater runs from its source near Saffron Walden in a southeasterly direction, where it flows through Braintree, Coggeshall, Kelvedon and Witham. The confluence of the River Brain with the River Blackwater can be found to the south east of Witham town centre. Subsequently the River Blackwater flows into the neighbouring district of Maldon before discharging into the Blackwater Estuary. The River Blackwater catchment as a whole is approximately 313 km2.

2.4.5 The River Colne flows through various settlements including Great Yeldham, Sible Hedingham, and Halstead, where it meets with the Bourne Brook and subsequently flows through White Colne before crossing into the neighbouring borough of Colchester. The River Colne has a catchment area of 250km2 to its tidal limit. There are numerous mill structures along the length of the River Colne.

2.4.6 The River Stour is the largest of the river systems located within the district. The watercourse has a catchment area of 862km2. However the majority of this catchment is located within the neighbouring district of Tendring. The River Stour is largely characterised by a rural landscape, much of which is designated as an Area of Outstanding Natural Beauty. Within the district of Braintree the watercourses flow through two small and isolated industrial areas and the small village of Bures St. Mary, which is located upon the border between Braintree and Tendring District Council.

2.4.7 Some of the additional watercourses within the District of Braintree are:

• River Pant • Pods Brook • River Ter • Belchamp Brook • Cambridge Brook • Roman River • Robins Brook Human Geography/Demographics

2.4.8 The Braintree District had a population of approximately 137,800 in mid-2005. The population of Braintree, the main town, consists of approximately 43,000 people, whilst the population of Halstead and Witham, key population centres, is 11,000 and 23,000, respectively.

2.4.9 In April 2007 Braintree District Council released the Braintree District 2025 Issues and Options Document, which forms part of the Local Development Framework (LDF) process. The document looks at the overall vision of the future for the development of the district, with various strategic options. At this early stage in the LDF process a

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number of sites have been identified for potential development following public consultation on the Core Strategy Issues and Options Document.

2.4.10 There are currently 26 potential development allocations identified in the Issues and Options stage of the Local Development Framework process that have been allocated following public consultation. It is not anticipated that all of these sites will be developed under the current planning regime. A further 7 urban capacity sites exist, which were identified under the LDF and have, as yet, not undergone development. These 33 potential development sites are referred to collectively throughout this report as potential development allocations.

2.4.11 Two of the potential development allocations consist of major settlement sites, which overlap into neighbouring Local Authorities. The first of which is located on land east of Stebbing, the majority of which is proposed within the District of Uttlesford. The second new settlement is proposed on land west of Marks Tey. The majority of the proposed settlement is located within the neighbouring borough of Colchester. Both of these potential development allocations are associated with greenfield land and constitute mixed use schemes. Up to 10,000 dwellings are proposed for both of these settlements. Leisure, employment and community facilities are also anticipated.

2.4.12 The majority of the remaining potential development allocations are outlined for residential purposes, many of which are associated with greenfield land. The number of dwellings proposed per site range from 100 to approximately 2,000. A number of potential brownfield redevelopment sites are also proposed for residential or mixed- use purposes.

2.4.13 The urban capacity sites are by definition all located within existing Town Development Boundaries, some of which have already undergone development within part of the allocation boundary. These sites are characterised by both green and brownfield development opportunities. Various development types are proposed, but the main emphasis is skewed towards residential development.

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3 Sources of Flooding in Braintree

3.1 Introduction Sources

3.1.1 The main SFRA report covers the whole of the Mid Essex area. This appendix of the report provides a summary of the flooding sources for the district of Braintree. Information regarding the different mechanisms of flooding is contained in the main SFRA.

3.1.2 This document assesses all flooding sources to the District.

3.1.3 Fluvial sources of flood risk have been assessed based upon Flood Zone coverage, as provided by the Environment Agency and recent modelling carried out as part of this study. It is understood that the information provided represents the most up to date information currently available. The information consists of flood outlines produced under the National Flood Zone project and more recent detailed mapping under the Strategic Flood Risk Mapping Framework.

3.1.4 The Environment Agency has indicated that flood risk in Witham could be mitigated in the future by a flood alleviation scheme, which, if undertaken, would consist of flood defence walls. This is currently in the concept stage and the Environment Agency is in the process of starting a viability study. As this project is in the very early stages of examination it may prove unfeasible and thus is not investigated as part of this SFRA.

3.1.5 The extent of existing flood defences within the district is limited. Information provided by the Environment Agency indicates that formal flood defences protect areas adjacent to the River Colne. More specifically this relates to the settlements of Halstead and White Colne. The latter protects only a small area, downstream of Colneford Hill. No further flood defences are understood to currently exist within the district.

3.1.6 PPS25 identifies the circumstances where a Flood Risk Assessment (FRA) must be submitted with a planning application for a proposed development, this is also discussed in the main SFRA Report. The specific scope of the assessment should address fluvial flooding from watercourses with the most up-to-date flood zone maps available from the Environment Agency. For smaller more localised streams where flood zone maps have not been produced, the appropriate method for assessing flood risk should be confirmed with the Environment Agency.

3.1.7 Figure A2 shows the areas at risk from fluvial flooding which have been classified as Flood Zones 2 and 3.

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3.1.8 There have been several occasions documented where flooding has occurred in various areas throughout the district, with no discernable correlation to river levels. The rural areas surrounding many of the settlements are characterised by relatively shallow London Clay deposits. Heavy rainfall can cause saturation of the soils or exceed the infiltration capacity and cause large quantities of surface water runoff to merge from surrounding fields. This mechanism often results in a rapid response to rainfall and in a local sense, can be more sudden compared to the impact of river flow.

3.1.9 Flood risk from overland flow can be widespread and is known to affect many areas within the district including Bocking, Witham and Coggeshall. This is often related to the location of the settlement on more convenient flat land at the base of the surrounding hills. The settlement areas may also exacerbate flood risk, attributed to surface water from highways or large areas of hard standing. Historic Flooding

3.1.10 The most recent significant flooding events in this area were the fluvial flooding events in October 2000 and October 2001, from various rivers throughout the district. In 2001 41 houses and 22 businesses were flooded in Halstead. It is understood that the flood defences were constructed within Halstead in response to this incident. A further 21 properties were flooded in Yeldham, 58 in Bocking, 153 in Kelvedon and 55 in Witham.

3.1.11 There have been various flood events in the past including 1979, where heavy rain coupled with snow melt on a frozen catchment caused extensive flooding in the River Stour and the River Colne. Summary of Flood Sources

3.1.12 Table 3-1 summarises the potential flood sources, pathways and the possibility of relevance to the district. Where relevant each of the sources is discussed in further detail.

Table 3-1Summary of potential flood sources Consider Flood Type Source Pathway further Blackwater, Floodplain ponding / Fluvial Yes Colne, Stour etc conveyance Tidal None None No Greenfield Flow paths merging Overland flow Yes runoff from high ground Arterial Drainage Urban runoff Surcharged sewers Yes Network Groundwater None None No

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3.2 Fluvial Sources

3.2.1 The main fluvial flooding source in the district of Braintree is associated with the River Brain, the River Colne and the River Stour. The River Stour forms part of the north and east boundary of the district. The River Pant is a major tributary of the River Blackwater, which drains in a south-easterly direction and forms the River Blackwater just north of Braintree town centre.

3.2.2 Pods Brook forms the River Brain to the south of Braintree town centre, which subsequently flows through Witham before discharging into the River Blackwater. The town of Braintree is therefore bounded to both the north and south by a large watercourse. However, fluvial flooding to the south of the town is limited as the capacity of Pods Brook is sufficient for the 1 in 100 year flows with restricted floodplains, whilst to the north (i.e. the River Blackwater) the floodplains are more widespread because of the more limited channel capacity.

3.2.3 The headwaters of the larger watercourses and some of the small tributaries are associated with chalk catchments, whilst others are predominately clay or a combination of the two. The geology of the catchment can impose a significant control upon the lag of the hydrological response to rainfall. Clay deposits generally cause the river system to respond quickly to rainfall. Chalk areas respond more slowly to rainfall as they allow groundwater storage within the fissures (joints) in the chalk. Nevertheless, ignoring the dissimilarities of the timing of the hydrological response, all of the watercourses within the district will exert a level of flood risk on the surrounding area.

3.2.4 Hydraulic structures such as weirs, mill structures, bridges and culverts may elevate water level and hence exacerbate flood risk in certain areas. Structures can promote blockage from flood debris, which may reduce the capacity of the watercourse locally. Furthermore, a reduction in capacity is often associated with structures themselves.

3.2.5 The ownership of the mill structures along the watercourses in the district varies, some are owned by the Environment Agency, others by Braintree District Council or by private mill owners. The Blackwater Flood Risk Study (Phase 5 Report) undertaken by Atkins involved an assessment of the impact of water levels if the gates of mill structures remained closed during a flood event. Flood levels were significantly increased upstream of the structures at four of the six structures investigated.

3.2.6 Quantification of the impact of structural failures (i.e. breaches) and overtopping of defences does not form part of the scope of this report. Furthermore, the mechanical or structural failure of localised defence barriers or mechanisms such as demountable flood boards, pumps designed to drain individual properties for example has not been considered in this report as it is not appropriate for a strategic level study. Hydraulic

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structures such as weirs, mill structures, bridges and culverts are typically taken into account during the detailed hydraulic modelling undertaken under the Strategic Flood Risk Mapping Framework studies, where these have been undertaken. The results of which where available (i.e. mapping) have been used for the flood zone mapping production.

3.2.7 As discussed in Section 3.1, a flood alleviation scheme in Witham is currently under investigation. This scheme is however in the very early stages of research and may not be viable. For this reason the scheme is not investigated as part of this SFRA. It is also understood that this summer Essex County Council are constructing a flood defence scheme to protect the village of Sible Hedingham to the 1 in 100 year flood event. It is believed to consist of two flood storage areas north of the village. However, according to the flood risk maps provided by the Environment Agency, none of the potential development allocations will benefit from these schemes. Consequently, no assessment has been made with respect to these defences.

3.2.8 All of the potential development allocations located within areas of floodplain are associated with the River Blackwater and its tributaries. The Environment Agency has not identified any future flood alleviation schemes currently under investigation upon these watercourses. Consequently, there has been no assessment with regards to flood alleviation schemes with respect to the impact upon potential development allocations within the floodplain.

3.2.9 The Blackwater Flood Risk Study (Phase 5 Report) outlines a number of flood defence measures that were identified to alleviate the existing flood risk associated with the River Blackwater and its tributaries. Due to the proximity of some of the development allocations to the River Blackwater and its tributaries, it is considered prudent to summarise alleviation measures that were identified as part of the study, which may or may not undergo further investigation in the future. It is understood at present that only one of these potential schemes identified has undergone further investigation at this stage. The potential alleviation measures identified are summarised below:

• Bocking – construction of a flood storage embankment upstream of the town • Coggeshall – construction of a flood storage embankment upstream of the town • Kelvedon – impounding flows behind the railway embankment upstream of the settlement • Kelvedon – construction of a flood wall upon right bank of the River Blackwater • Witham – construction of a flood storage embankment upstream of the town. • Witham - construction of flood walls upon both banks of the River Brain through a large proportion of its course through the settlement area. This is believed to constitute the alleviation works discussed above that is currently under further investigation.

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Pathways

3.2.10 The main pathway of fluvial flooding is as a result of high flow exceeding channel capacity and riverbank heights. Subsequently, floodwater will pond in the immediate fluvial floodplains or be conveyed within the larger floodplain area. The frequency and / or magnitude of fluvial flooding can be exacerbated under a range of scenarios, such as blocked culverts or failed flood defences. Receptors

3.2.11 The Environment Agency’s flood maps show the predicted extent of fluvial flooding in the district during the estimated 1 in 100 year (1%) and the 1 in 1000-year (0.1%) flood event in the absence of defences. In the upper reaches the flooding is generally confined to strips of land adjacent to the watercourses and villages directly adjacent to the watercourses such as Little Waltham are at high risk. However, flood risk is more widespread in the upper reaches of the Stour, near the district boundary.

3.2.12 Flooding generally becomes more widespread with increasing distance downstream, as floodplains naturally become wider. A number of towns and villages are at risk from fluvial flooding within the district. These include Halstead, Yeldham, Bocking, Kelvedon, Witham, Wakes Colne and Great Yeldham. On a smaller scale many individual dwellings, industrial premises and sewerage works for example, also form receptors to fluvial flood risk.

3.2.13 The capacity of Pods Brook is sufficient to contain the estimated 1 in 100 year (1%) and the 1 in 1000-year (0.1%) flood event throughout the majority of its course through Braintree. Therefore receptors associated with this watercourse are less significant. Functional Floodplain

3.2.14 PPS25 stipulates that functional floodplain is defined as any land that:

• would flood with an annual probability of 1 in 20 (5 per cent) or greater in any year, or at another probability to be agreed between the LPA and the Environment Agency (EA), or: • is designed to flood in an extreme (0.1 per cent) flood, or at another probability to be agreed between the LPA and the EA.

3.2.15 The functional floodplain is classed as Flood Zone 3b and in accordance with PPS25 residential development in this area would not be permitted.

3.2.16 The delineation of Flood Zone 3b (Functional Floodplain) has been undertaken using revised 1 in 20 year outlines from the River Blackwater and River Brain modelling as part of this study provided by Atkins, and the River Colne modelling provided by Black and Veatch. An outline has been provided for the River Blackwater, River Brain and

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River Colne, these are mapped as Flood Zone 3b in the Figure AX-1 series depicting current PPS25 flood zones for 2007.

3.2.17 To make an allowance for climate change for Flood Zone 3b, the 1 in 20 year event plus a 20% increase in flows was mapped. The models were re-run to determine these outlines which are mapped in the Figure AX-2 series which depict the PPS25 flood zones inclusive of climate change (in accordance with PPS25, 100 years of climate change have been mapped appropriate for residential development considerations).

3.3 Overland Flow

3.3.1 A patchwork of fields surrounds many of the key settlements within the district. The infiltration rate, capacity of the soil to allow rainfall to soak into the ground within each field, coupled with the volume or intensity of the rainfall and topography, will define the volume and rate of overland flow (i.e. runoff) generated from that particular field. The overland flow would naturally route towards the nearest watercourse, stream or ditch. The route taken by the overland flow is defined by the local topography.

3.3.2 Drainage routes are likely to have been disturbed significantly by urbanisation. This mechanism will result in urban areas receiving overland flow from surrounding hill slopes, which is a common occurrence worldwide.

3.3.3 The mechanism described above often forms one of the first hydrological responses to rainfall within a particular catchment. Therefore, overland flow from surrounding hill slopes can be observed before the river itself has responded to the rainfall and can potentially result in an early phase of flooding.

3.3.4 The hydrological response of particular catchments throughout the country varies significantly. The geological conditions of the catchment form a key factor in determining the hydrological response. The upper parts of the catchments within the Braintree District are generally defined by Chalk, whilst the geology of the lower catchment areas are characterised by London Clay. The two opposing geological conditions will result in significant differences in the catchment hydrological response to rainfall.

3.3.5 Catchments characterised by London Clay deposits will permit little infiltration and result in large quantities of overland flow. However, Chalk will promote infiltration and help sustain river water levels via a significant base flow input, (see Table 3-2 for more information).

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Table 3-2 Typical Infiltration co-efficients for different soil types

Soil Type Typical Infiltration Co-efficient (m/hr)

Chalk 0.001 - 100

Sandy clay loam 0.001 – 0.1

Clay <0.0001

*Taken from Table 4.7 of the CIRIA C697 SuDS Manual

3.3.6 During this study, reports of flooding identified from overland flow have been assessed. Many of these are concentrated in Bocking. Such occurrences have also been identified throughout many villages within the district. London Clay characterises the surrounding geological conditions at each of these settlements.

3.3.7 Braintree District Council has been involved in a number of small-scale flood alleviation projects. The flood events generally consist of flooding from overland flow, ditches and / or minor watercourses. Maintenance and clearing of ditches forms a large part of the work undertaken to assist in mitigating the potential consequences of overland flow.

3.3.8 A more sizeable flood alleviation project was undertaken by the Braintree District Council, towards the south-west of Witham. A minor watercourse that feeds into the River Brain has a history of flooding adjacent to Brook Walk; Howbridge Road has become impassable on several occasions. Flooding in the past has almost penetrated a number of nearby houses. A small reed-bed was constructed to form an off line storage area during the times of flood. It is understood that the scheme has been successful and has reduced the occurrence of flooding to these areas.

3.3.9 A number of other small schemes were identified in Great Yeldham, Coggeshall, Bocking (Church Street) and Silver End (Magdalene Crescent); however further information was not available. Under severe weather conditions, these small-scale projects are unlikely to prevent the occurrence of wide spread flooding, but may help to reduce the magnitude in localised areas. Therefore, it is not considered that these schemes will offer a significant improvement to development opportunities.

3.3.10 The small-scale projects described in this section differ from those discussed in Section 3.2, because the former are typically associated with locations / features too small to be considered under the Environment Agency flood maps.

3.3.11 None of the projects discussed in this section directly correspond with any of the potential development allocations proposed by Braintree District Council. However, the

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work undertaken at Bocking (Church Street) and Silver End (Magdalene Crescent) is within the proximity of two individual development allocations.

3.3.12 Flooding from overland flow is considered to be of a smaller scale in relation to inundation and consequences than flooding associated with fluvial sources. Pathways

3.3.13 Again, pathways of overland flow will be defined by local topography. Natural or unnatural features may influence the route that floodwater will take. In urban areas roads can form common pathways helping to dictate the area affected by flooding. Receptors

3.3.14 Flooding from overland flow (or any other source for that matter) does not discriminate upon the particular receptor. However, flooding from overland flow is more likely to occur where large hill slopes or areas of hard standing are situated nearby to a particular site. On a site specific scale the risks from this flood source should be identified within a flood risk assessment.

3.4 Arterial Drainage Network Sources

3.4.1 Localised flooding can occur as a result of severe storms, which are localised in extent and duration. The intensity of the rainfall in urban areas can create runoff volumes that temporarily exceed the urbanised sewer and natural drainage capacities, creating ‘flash’ flooding, referred to in this document as surface water flooding.

3.4.2 Flooding may occur when the rainfall intensity exceeds the capacity of the surface water sewer system. This is likely to become a more common occurrence in the future, due to climate change and an increase in the number and intensity of convective storms. It is now fairly widely accepted that one of the main effects of climate change in the South East will be a higher intensity rainfall and more frequent winter storms, which will increase the risk of flooding from all sources of flooding.

3.4.3 As part of this study a request was made to Anglian Water for information regarding existing surface water and sewer flood risk in the area. A series of postcodes were provided for general areas in which flooding from this source had been recorded in the past.

3.4.4 Comparison with the recorded events and allocation sites will help to identify the risk local sewers may have upon particular sites. However, it should be noted that all sewers represent a degree of flood risk through restricted capacities for transporting large volumes of water. Therefore irrespective of their flooding history, an assessment from this source should be made as part of a site specific FRA.

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3.4.5 Braintree has commissioned a Water Cycle Study to investigate the occurrence of flooding from this source. In addition to investigating the current risk of sewer flooding, the study will investigate the potential increase in sewer flooding as a result of new development and the increased pressures of climate change on this system.

3.4.6 Flooding generated by intense rainfall events leading to overland flow and ponding should also be investigated in a site specific FRA. The local topography should be taken into account. Where steep urbanised areas exist local to the proposed development site, or the site it situated within a depression, the potential for overland flow to be routed to the site of the proposed development and result in localised flooding should be investigated. Anglian Water does not hold information relating to this form of flooding. Pathways

3.4.7 If the capacity of sewers is inadequate or a blockage were to occur, manholes are likely to become surcharged, forcing floodwater to spill out of the manholes and flood the surrounding areas.

3.4.8 The extent of the flood will be defined by the surrounding topography and volume of floodwater involved. For example, if a manhole surcharges in a low-lying area/depression, surrounded by higher ground, flooding may be relatively deep but affect an isolated and relatively small area. Whilst if it were to occur near the top of a hill, floodwater may follow the gradient of the hill into areas of lower ground. This could potentially exacerbate flooding elsewhere rather than cause flooding near the responsible sewer.

3.4.9 Braintree District Council has been unable to provide details pertaining to specific drainage networks such as highways drains, drainage ditches or non-main river watercourse that may pose flood risk within the Borough. Receptors

3.4.10 Anglian Water have provided a list of incidents of flooding that have been reported and recorded for general households and businesses to date. These are discussed in Section 5.4. Policy and Mitigation

3.4.11 The goals of SuDS, (see Chapter 9 in the Main Report and Chapter 8 in this report), can be achieved by utilising a management plan incorporating a chain of techniques, (as outlined in Interim Code of Practice for Sustainable Drainage Systems 2004), where each component adds to the performance of the whole system. These are prevention measures, source control, site control and regional control strategies.

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3.5 Groundwater

3.5.1 During the study there has been no indication that groundwater flooding forms a significant risk within the district. Approximately 70% of the district is underlain by London Clay, which includes the location of all of the main settlements.

3.5.2 The presence of London Clay throughout the majority of the district will form an impenetrable barrier to any groundwater at depth, limiting the risk from this source. The presence of groundwater and any incidents of groundwater flooding should be fully investigated in a site-specific FRA.

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4 Level 1 Assessment

4.1 Level 1 SFRA – Study Area, Flood Source Review and Data Review

4.1.1 The objective of the Level 1 SFRA is intended to collate and review available information on flood risk for the study area. This document forms both a Level 1 and Level 2 report, as the study brief was based on PPG25 and did not recommend the 2 staged approach as outlined in PPS25.

4.1.2 The Level 1 SFRA addresses Objective 1 of Section 1.2 and forms part of the evidence base (Objective 2) for the study area.

4.1.3 A level 1 SFRA is designed to be sufficiently detailed in order to allow the application of the Sequential Test on the basis of Table D1 of PPS25 and to also identify whether application of the Exception Test is expected to be necessary. Information from this stage can also be used to assess how any environmental objectives relating to flooding, as defined in the sustainability appraisal, may be affected by any additional proposed developments.

4.1.4 This stage in the SFRA is primarily a desk-based study, which should use existing information for a number of sources, outlined below.

• Environment Agency Flood Map. • Regional Flood Risk Appraisal (RFRA) (including all the sources of data referred to in the guidance provided on their preparation). • National Flood and Coastal Defence Database (NFCDD) and National Flood Risk Assessment (NaFRA). • Any available expert advice from the Environment Agency. This may be in the form of reports containing the results of detailed modelling and flood mapping studies or historic flood events. • Consultation with other flood risk professionals including: water companies, highways authorities, local authorities (in their role as statutory drainage (operating authority)), navigation authorities and informed local sources. • Maps of geology and soil. These allow the potential for the implementation of source control and infiltration techniques, groundwater and overland flood risk to be investigated and assessed.

4.1.5 This information as a whole should be sufficient to allow application of the Sequential Test and subsequently inform the Sustainability Appraisal and any succeeding plan policies.

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4.1.6 In the event of the Level 1 SFRA demonstrating the potential need of the application the Exception Test, either due to current levels of flood risk or due to an increase in flood risk resulting from climate change, further data collection and/or analysis will need to be carried out, this should be investigated and incorporated into the Level 2 SFRA. Figures A3-A36 are split into two series of maps, Figure AX-1 and Figures AX- 2. The AX-1 series show the current flood zones in accordance with PPS25 for 2007.

4.1.7 PPS25 states an appropriate allowance should be included for climate change over the lifetime of the development, considered for residential development to be 100 years. Therefore the Figure AX-2 series show the flood zones with 100 years of climate change for 2107.

4.1.8 Braintree District has been delineated in the Figures A3-1 to A36-2 into the flood zones outlined in PPS25 as Flood Zone 1, low probability, Flood Zone 2, medium probability and Flood Zone 3a, high probability. In addition, Flood Zone 3b, functional floodplain and the flood outlines with an inclusion of climate change, has also been mapped. Table D.1 of PPS25 provides information on which developments might be considered to be appropriate in each flood zone, subject to the application of the Sequential Test and either the Exception Test or a site-specific Flood Risk Assessment demonstrating safety.

4.1.9 In accordance with PPS25 Braintree District will complete a Sequential Test process for their spatial strategy and each of their proposed strategic locations. This will identify the flood risks and development vulnerability in order to assess the suitability of each development location, and where possible steer more vulnerable developments to areas of lower flood risk.

4.2 Level 1 Mapping Methodology Fluvial 2007 and 2107 outlines

4.2.1 The delineation of Flood Zones 3a and 3b has been undertaken using outlines from the recent River Colne modelling undertaken as part of this study provided by Black and Veatch. The River Blackwater and River Brain modelling has also been revised as part of this study by Atkins. Flood outlines have been provided for the Rivers Colne, Brain and Blackwater, these are mapped as Flood Zone 2, 3a and 3b in the Figure AX- 1 series depicting the current PPS25 flood zones for 2007.

4.2.2 To make an allowance for climate change for Flood Zones 2, 3a and 3b, the 1 in 1000 and 1 in 20 year events plus a 20% increase in flows were modelled as part of this study. The 1 in 100 year plus climate change was supplied by the Environment Agency from previous modelling. The models for the Rivers Colne and, Brain and Blackwater were re-run by Black and Veatch and Atkins respectively to determine these outlines, which are mapped in the Figure AX-2 series which depict the PPS25 flood zones inclusive of climate change (in accordance with PPS25, 100 years of

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climate change have been mapped appropriate for residential development considerations).

.

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5 Level 2 Assessment

5.1 Overview

5.1.1 The objective of the Level 2 SFRA is to reduce the level of uncertainty regarding flood sources for any development sites that cannot be located in Flood Zone 1 after application of the Sequential Test.

5.1.2 The majority of the potential development allocations are located entirely within Flood Zone 1. Only seven of the allocation areas in the District (including Bramston Sports Centre) include small areas of Flood Zone 2 and 3. Through the Sequential Test, Braintree District Council should seek reasonably alternative sites that lie completely within Flood Zone 1.

5.1.3 Therefore it is unlikely that any of the Braintree allocations will require the Exception Test. As a result the objective of the Level 2 SFRA with respect to the Braintree District is to summarise the key sources of flood risk (i.e. fluvial, overland flow and the arterial drainage network) at each of these allocation sites by bringing together all of the information collated. If development within these allocation areas is later proposed through planning applications in the Flood Zones 2 and 3, further detail may be required as part of the site specific flood risk assessments.

5.1.4 In the first instance the individual sources of flooding are summarised, following which an assessment is made based upon individual site allocations from each of these sources.

5.1.5 The second objective consists of a more detailed assessment of flood risk to the potential redevelopment site at Bramston Sports Centre, Witham.

5.1.6 The assessment has been made at a strategic level and is intended only to inform how the potential development allocations may be at risk from each source. This should therefore form a ‘stepping-stone’ for site-specific FRA’s, considering the recommendations discussed throughout in this report and the main SFRA report.

5.2 Fluvial

5.2.1 In total there are 34 potential development allocations within the Braintree District, identified in the Issues and Options stage of the LDF and preceding the Local Plan. Seven of these potential allocations are partially located within an area classified as floodplain, including the Bramston Sports Centre. All of these potential allocations are identified and discussed within Section 4. Braintree will need to complete the Sequential Test on the proposed Issues and Options allocations applying the principals of PPS25 and the sequential approach to flood risk and development.

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5.2.2 Some of the potential allocation sites are located adjacent to the floodplain. Redefinition of the adjacent floodplain based upon detailed site topography and adjacent design flood levels may improve the accuracy of the floodplain boundary and potentially affect the development opportunities at individual sites. Hazard Mapping

5.2.3 To provide a greater level of detail on the fluvial flood risks, an assessment has been made on the hazard associated with the River Blackwater, River Brain and River Colne. The hazard mapping was based on the outputs from the River Blackwater and River Brain modelling (by Atkins) and River Colne modelling (by Black and Veatch) for the 1 in 20, 1 in 100 and 1 in 1000 year plus climate change outlines. The modelling is 1-dimensional and does not have an associated velocity output; therefore the hazard has been classified as a function of depth, assuming zero velocity.

5.2.4 The Hazard categories have been mapped using the “FD2320/TR2 – Flood Risk Assessment Guidance for New Development” depths and associated hazard with an assumed zero velocity as shown in Table 13.1 of that document. This is explained in more detail in Section 6 of the main SFRA.

5.2.5 An estimation of velocity to refine the hazard classification for a site could be made on a site-specific basis in relation to distance from the river, local topography, flow paths etc.

5.3 Overland Flow

5.3.1 As identified in Section 3.3 London Clay underlies all of the main settlements and the surrounding areas. This will promote large quantities of overland flow, which can contribute to or result in flooding.

5.3.2 Bocking in particular has been affected from this source in the past. Numerous other villages throughout the district have also been affected from overland flow. However, no further detailed information has been provided that could identify more specific or additional areas at risk from overland flow.

5.3.3 It is considered that any settlements surrounded by hill slopes will be at significant risk. This will be exacerbated if the hill slopes are characterised by London Clay, or similarly impermeable geological conditions.

5.3.4 Several small-scale flood alleviation schemes were identified in Section 3.3. It is not considered that these will offer significant benefit to any adjacent developments proposals.

5.3.5 It would be prudent to recommend that where possible any hard surfaces such as parking areas are designed with semi-permeable surfacing such as block pave to facilitate increased infiltration and reduce where possible overland flow. The impact of

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overland flow on future development sites could be mitigated through the suggested use of SuDS and attenuation of future runoff flows to greenfield rates, the specific details of which should be investigated during the preparation of site-specific FRAs.

5.4 Arterial Drainage Network

5.4.1 This section discusses the results of the assessment made with regard to the potential development allocations and the information that has been provided by Anglian Water.

5.4.2 There were 13 historic flood events associated with sewer flooding within the district that were within 500 metres of a particular site allocation. These are summarised in Table 5-1.

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Table 5-1 Summary of postcode areas affected by flooding from sewers in the past

Postcode Closest site (ref.) Development type Proximity of site

CM7 5JN 8 Mixed use 250m W

CM7 9AX 9 Residential 500m SW

CM8 1BW UC5 Mixed use 50m SE

CM8 1XJ UC7 Residential 100m S

CO6 1EJ 17 Mixed use 400m S

CO6 1SX 16 Residential 0m W

CO6 2RX 24 Residential 250m E

CO9 1HA 25 Residential 250m W

CO9 1HD 25 Residential 300m W

CO9 2AP UC4 Mixed use 50m E

CO9 2JA UC4 Mixed use 75m E

CO9 2JZ 25 Residential 100m N

CO9 2LR 25 Residential 0m W

5.4.3 The above information (Table 5-1) is a summary of the sites situated within close proximity to and affected by sewer flooding. A site specific FRA should investigate the pathway of the floodwater from the source (sewer in this situation) to the receptor (site). For example, a site situated some distance away from the source, but down gradient of that source, may be more susceptible to this form of flooding than a site situated within a short distance from the source but located at a similar or higher topographic level.

5.4.4 The above table (Table 5-1) is only a summary of the areas affected by sewer flooding. There may be additional sewer systems that could lead to sewer flooding. In addition, as mentioned earlier, this form of flooding is likely to increase in the future due to the effects of climate change.

5.4.5 Several sites are identified as being flooded from this source on more than one occasion, which suggests that the local sewers flood relatively frequently. A more thorough investigation should be undertaken under these circumstances to examine existing capacity and the potential to improve this and reduce the occurrence of

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flooding in these locations. It is understood Braintree District Council will be completing a water cycle strategy. This document should therefore look at these specific areas in relation to capacity and related flooding events.

5.4.6 If a particular area is subject to relatively frequent flooding from sewers, this should not prevent a particular site from undergoing development. Certain mitigation measures can be designed into the development to manage flood risk of this kind such as improved drainage and sewer systems (often replaced as part of new development) and raised floor levels.

5.4.7 In those worst affected areas Anglian Water upgrade the sewer networks in an attempt to alleviate flooding. However, this is a relatively lengthy process considering the large region that Anglian Water serve. Anglian Water could not provide details regarding the location of future alleviation schemes.

5.5 Strategic Spatial Flood Source Summary

5.5.1 Table 5-2 identifies the impact at each potential allocation site from a combination of the three key flood sources within the district (i.e. fluvial, historical and overland flow flood sources). It is intended to summarise the potential consequence of flooding from each source at each potential site allocation. A weighting factor has been applied to each source. This is intended to represent the different scale or magnitude of the associated flood risk, with the presence of fluvial flood risk forming the most heavily weighted factor. Weighting factors associated with each flooding source are outlined below (Table 5-2).

5.5.2 Information pertaining to fluvial flood risk is also deemed to be the most reliable source of information and forms the key consideration of the Sequential Test, and hence determines the appropriate location of development. It should be noted that some allocations are large, with only a small portion lying within Flood Zones 2 and 3. In such cases Flood Zone 1 could easily be utilised for the proposed development.

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Table 5-2 Weighted factors associated with each flooding source

STRATEGIC SPATIAL FLOOD SOURCE SCORING SYSTEM Value Classification Notes Fluvial Flood Sources Potential site allocation is Presence of the floodplain within the settlement buffer zone located within the Environment has been attributed the heaviest weighting due to the Agency floodplain implications on breach of defences or overtopping of the river 6 banks. In this report, the score relating the presence of the floodplain is anticipated to be the most crucial in assessment of flood risk. A small stream (or equivalent) flows through the site/adjacent 3 to the site boundary, which is not defined by an Environment Agency floodplain Sites characterised by an 2 isolated ditch network or a series of ponds for example 0 No water features exist Historical Flood Sources Development allocation is Historical flooding classification is based on the results identified as affected by summarised in Table 2 1 flooding in the past therefore may be at risk No flood incidents in the 0 development allocation area in the past Overland Flow Flood Sources Where potential site allocations are located within areas known 3 to have been affected by historical flooding from overland flow 2 Values assigned dependent on The underlying solid geology of the site helps to dictate the underlying solid geology (with volume of overland flow generated. A more detailed respect to clay (value of 2) and investigation may identify recent drift deposits that may 1 chalk (value of 1)) of the potentially influence the mechanisms that govern the individual site generation of overland flow.

5.5.3 The total maximum value for individual site allocations equates to a value of 10, which would suggest that the development allocation is potentially at significant flood risk. As discussed above sites located within the floodplain should be avoided. However, the

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settlement allocation areas are very large in many cases. Development within the area could be undertaken avoiding the flood affected areas and preferentially locating development in either Flood Zone 1 or 2, which would be acceptable under the Sequential Test. If residential development is located within the floodplain (i.e. floodplain value of 6), the Exception Test may be required depending on the proposed development, or where possible development should be guided away from the floodplain within the particular site. With the presence of a stream/ditch network within the development allocation, (a floodplain value of 2), appropriate steps should be taken to address the impact of this source.

5.5.4 There is a lack of arterial drainage network information, (or overland flow, for example). It is however considered unlikely that flood risk from this source will render a site un-developable. Arterial drainage networks should be considered and investigated as part of any site specific FRA, which should identify what mitigation measures may be necessary to protect against flooding from each source.

5.5.5 Table 5-2 summarises which sources of flooding are anticipated to affect each potential site allocation. It is intended that this will inform a site-specific FRA for the appropriate sites. The summary is by no means conclusive; it should however provide a useful guide of flood risk associated within individual sites. The methodology undertaken is considered to be very subjective, but is however based upon the key factors identified and used within this report to define flood risk from each individual source.

Table 5-2 Summary of flood risk at each potential development allocation (for locations see Figures Appendix)

Flood Sources

Site Fluvial Overland Flow Historical Total

1 – Land east of Stebbing 3 2 0 5 2 – Land west of Rayne 2 2 0 4 3 – Land east of Rayne 2 2 0 4 4 – Land south-west of Great Notley 1 2 0 3 5 – Land east of Great Notely 3 2 0 5 6 – Land south-west of Braintree 6 3 0 9 7 – Land north of Springwood Industrial 2 3 0 5 area 8 – Land at Dorewards Hall, Bocking 3 3 1 7 9 – Land north-east of Braintree 3 3 1 7 10 – Land adjacent to Galleys Corner, 2 3 0 5 Briantree 11 – Land south-west of Silver End 6 2 0 8

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12 – Land east of Panfield Lane 1 3 0 4 13 – Former Crittalls Factory, Silver End 1 2 0 3 14 – Land south-west of Witham 1 3 0 4 15 – Land north-east of Witham 2 3 0 5 16 – Land east of Coggeshall 3 3 1 7 17 – Land west of Marks Tey 6 2 1 9 18 – Land north of Feering 6 2 0 8 19 –Land west of Coggeshall Road, 1 2 0 3 Kelvedon 20 – Land west of B1023, Feering 6 2 0 8 21 – Land between A12 and B1024, 1 2 0 3 Feering 22 – Land north-east of Halstead 3 2 0 5 23 – Land east of Millennium Way 1 3 0 4 24 – Land north-west of Earls Colne 1 2 1 4 25 – Land off Mount Hill, Halstead 1 2 1 4 26 – Land south of Tye Green, Cressing 1 2 0 3 Bramston Sports Centre 6 3 0 9 UC1 – Rifle Hill, Braintree 1 3 0 4 UC2 – Mill Hill, Braintree 1 3 0 4 UC3 – Riverside Pool Site, Braintree 6 3 0 9 UC4 – Land east of High Street, 1 2 1 4 Halstead UC5 – Maltings Lane, Witham 3 3 1 7 UC6 – Gimsons, Witham 1 3 0 4 UC7 – Constance Close, Witham 1 3 1 5

5.5.6 The highlighted sites score higher than 5 with respect to associated flood risk and should therefore have detailed site specific flood risk assessments undertaken with particular reference to the related flood source specified i.e. fluvial, overland flow or drainage.

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6 Bramston Sports Centre

6.1 Introduction

6.1.1 The scope of the Braintree Appendix of the Mid Essex SFRA report included a more detailed assessment of flood risk associated with the redevelopment of Bramston Sports Centre (Figure A29-1 and A29-2). This is the only area identified within the scope of the Braintree SFRA for further and detailed flooding investigation.

6.1.2 This section is intended to identify the key considerations regarding flood risk at the site, which can then be taken forward in the production of a detailed FRA to be submitted with the planning application. This assessment addresses Objective 3 in Section 2.2 and provides additional detail to the Level 1 assessment and summary information identified above. Aims and objectives

6.1.3 This section forms a desk-based assessment of flood risk at the proposed Bramston Sports Centre. This will present supplementary information to the SFRA, as requested in the initial scope for the Braintree SFRA, to inform on the specific issues and suitability for development of the Bramston Sports Centre site. This will involve:

• Analysis of relevant information regarding flood risk in the vicinity of the site • Discussion of flood risks that are anticipated to affect the site • General recommendations regarding layout of the site with in accordance with PPS25 • Investigation of SuDS that are likely to be appropriate at the site

6.2 Site and Surrounding Area Location of site

6.2.1 The Bramston Sports Centre site is located near the centre of Witham and to the west of the River Brain, (for location of Witham within the District of Braintree see Figure A1). The approximate National Grid Reference for the centre of the site is TL 816 143, and the approximate postcode of the site is CM8 1BP. The immediate surrounding area is primarily residential. To the north west of the site is a railway line, further to the north west of which is predominantly arable land. Site Appearance

6.2.2 The site is approximately 15 hectares in size. Spinks Lane dissects the centre of the site. To the east of Spinks Lane a sports centre and a secondary school can be found.

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Associated car parking and playing fields are also located in this area. To the west of Spinks Lane lies an adult education centre, car park, a football pitch and pavilion. Development Proposals

6.2.3 The proposed Bramston Sports Centre forms part of the development proposals. The proposed development involves the retaining of the existing adult education centre and school, with the development of a large part of the site for residential purposes. The existing sports centre could either be demolished and replaced or refurbished. At the time of writing the proposals had not been finalised. Surrounding areas

6.2.4 To the south of the Bramston Sports Centre site is King Edward Way and Hatfield Road, to the west of the site is the railway line, to the north Blunts Hall Road, Stevens Road, Highfields Road, and Guithavon Road, and to the east lie Mill Lane and Bridge Street. The immediate surrounding area is primarily residential.

6.2.5 There are a number of vegetated regions within the built up area of Witham, including to the east and north of the Bramston Sports Centre site. Surrounding the built up residential area is agricultural land, the primary use of which is arable cropping. Geology/hydrogeology

6.2.6 A borehole log of the shallow geology at the site was ascertained from British Geological Survey (BGS). This was taken towards the northeast corner of the site and indicates that approximately 4 metres of alluvial deposits exist beneath the topsoil. This consists of clayey sand and gravels, which is underlain by firm to stiff clayey silt. Water was encountered approximately 3.5 metres below ground level.

6.2.7 The Environment Agency’s Ground Water Source Protection Zones map, available on the Agency’s website indicates that the Bramston Sports Centre site is not within a Protection Zone, and there are no Ground Water Source Protection Zones in the locality of the proposed development area. Ground Water Source Protection Zones indicate the risk of contamination from any activities that might cause pollution in the area and preventative pollution measures are set up in high-risk areas. The Agency’s classification of Source Protection Zones around potable groundwater abstraction points is designed to limit potential pollution activities.

6.2.8 The site is situated within a Catchment Abstraction Management Strategy area. These are areas, classified by the Environment Agency, where the removal of water is managed. This aims to control water abstraction during times of drought to ensure wildlife is not harmed.

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6.3 Regulatory Position PPS25 Sequential Test and Vulnerability Classification

6.3.1 In accordance with PPS25 Table D.1, developers and local authorities proposing to develop in Flood Zones 2 and 3 should seek opportunities to:

• Reduce flooding by considering the layout and the form of the development and the appropriate application of sustainable drainage techniques • Relocate existing development to land in zones with a lower probability of flooding, and • Create space for flooding to occur by restoring functional floodplains and flood flow pathways and by identifying, allocating and safeguarding open space for storage.

6.3.2 The Environment Agency floodplain map (Figure Ai) indicates that the eastern region of the site lies partially within Flood Zones 1, 2 and 3 of the River Colne, classified as low, medium and high probability respectively.

Figure Ai Title: Bramston Sports Centre Environment Agency Flood Zone Map

Flood Zone 3 - Flooding from rivers or sea without defences Flood Zone 2 - Extent of xtreme flood Flood defences Areas benefiting from flood defences

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6.3.3 Flood Zone 1 areas have a low probability of flooding from fluvial sources and have been assessed as having less than a 1 in 1000 annual probability of river or sea flooding in any year (<0.1%).

6.3.4 The Flood Zone 2 areas have a medium probability of flooding from fluvial sources and have been assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1%-0.1%).

6.3.5 The Flood Zone 3 areas with the highest probability of flooding from fluvial sources and have been assessed as having a 1% or greater annual probability of fluvial flooding (>1 in 100 year flood).

6.3.6 PPS25 separates Flood Zone 3 into 3a and 3b, Zone 3a is classified as an area of ‘High Probability’ flood risk and Zone 3b is classified as the ‘Functional Floodplain’. Each of these zones requires different planning responses depending on the vulnerability of the proposed development. The Functional Floodplain has not been differentiated in the Environment Agency maps for the River Brain local to the proposed development site (Figure Ai). Modelling produced for this SFRA by Atkins does however differentiate Flood Zone 3 into 3a and 3b (Figure A-29-1).

6.3.7 The inundated areas presented in the Environment Agency flood zone map (Figure Ai) do not take into account the effect of flood defences or climate change. Figures A29-1 and A29-2 show the effects of climate change on the modelled flood depths produced by Atkins for the River Brain modelling project undertaken as part of this SFRA. In accordance with PPS25 100 years of climate change has been modelled for residential development, commercial development need only consider 60 years of climate change.

6.3.8 As this site is located within a high-risk flood zone and it is proposed to situate vulnerable developments on the site, Braintree District Council should seek alternative sites to take forward in the LDF. Only if this is not possible, i.e. the Sequential Test is passed, should this site be considered for development. Even then, the development of the site should be subject to a strict policy to ensure the sequential zoning of the site. This site can only be developed if the Sequential Test is passed. Development Vulnerability

6.3.9 A sequential risk-based approach to determining the suitability of land for development in flood risk areas should be applied at all levels of the planning process, whilst taking the flood risk vulnerability of the proposed land usage into account. The vulnerability of some of the predicted usages is listed in the following table (Table 6-1), according to the ‘Flood Risk Vulnerability Classification’ Table D.2 in Annex D, PPS25.

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Table 6-1 Vulnerability of predicted land usages within the Bramston Sports Centre Area. For more information see Table D.2: Flood Risk Vulnerability Classification and D.3: Flood Risk Vulnerability and Flood Zone ‘Compatibility’ in PPS25

When Exception Proposed Development Vulnerability Classification Test Required

Retail* Less Vulnerable N/A

Office Use* Less Vulnerable N/A

Small Business Units* Less Vulnerable N/A

Restaurants, Cafes and Less Vulnerable N/A Hot Food Takeaways*

Drinking Establishments More Vulnerable Flood Zone 3a

Residential* More Vulnerable (basements are Flood Zone 3a classified as Highly Vulnerable) (Flood Zone 2)

Leisure* Less Vulnerable N/A

Public open space Water -compatible N/A

Public car parking Less Vulnerable/ Water N/A compatible

* indicates areas not compatible with Flood Zone 3b according to Annex D of PPS25

6.3.10 All classifications of development are subject to the Sequential Test and should be steered to the lowest Flood Risk zone first. Only if there are no alternative sites in Flood Zones 1 or 2 can development be ‘appropriate’ in Flood Zone 3 subject to the Exception Test.

6.3.11 For the Exception Test to be passed the development must fulfil the following requirements (according to Annex D of PPS25):

a) it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. If the DPD has reached the ‘submission’ stage – see Figure 4 of PPS12: Local Development Frameworks – the benefits of the development should contribute to the Core Strategy’s Sustainability Appraisal;

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b) the development should be on developable1 previously-developed land or, if it is not on previously developed land2, that there are no reasonable alternative sites on developable previously-developed land; and c) a FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible, will reduce flood risk overall.

6.3.12 The Exception Test should only be applied in partnership with the Local Planning Authority. For successful application it is important that the argument presented for justification through the Exception Test is in line with policies set out in Local Plans and Local Development Frameworks and supported by reference to other national policies such as development of brownfield sites.

6.3.13 The ‘Less Vulnerable’ classification of land use is considered appropriate within Zones 2 and 3a only where there is no suitable available land in a lower flood risk zone.

6.3.14 In accordance with PPS25 Flood Zone 3b, the functional floodplain, should be avoided for all development types. Water compatible land uses are appropriate in this zone as are essential infrastructure land uses where the Exception Test has been applied. Less, More and Highly Vulnerable land uses are not compatible in this zone.

6.4 Flood Status and Sources Sources of Flood Risk

6.4.1 PPS25 recommends that an FRA should consider all possible sources of flooding for a given site. Table 6-2 summarises the range of potential flood sources, pathways and the possibility of relevance to the site.

Table 6-2 Summary of potential flood sources at Bramston Sports Centre

Flood Type Source Pathway Consider further

River Brain Out of bank flow Yes Fluvial Unnamed stream to Out of bank flow Yes the north and south

Tidal None None No

Overland flow Greenfield runoff Runoff routed towards site Yes

Arterial Blockage / extreme Floodwater routed towards site Yes Drainage

1 Developable sites are defined in Planning Policy Statement 3 (PPS3) Housing as those sites which should be in a suitable location for housing development and there should be a reasonable prospect that the site is available for, and could be developed at the point envisaged. 2 Previously-developed land definition (commonly known as Brownfield Land). See Annex B of Planning Policy Statement 3 Housing

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Network event

Groundwater None None – clay present No

Fluvial

6.4.2 A small strip of the site is located within the Environment Agency floodplain (Figure A29-1). More specifically this relates to Flood Zone 3b. As discussed in the Functional Floodplain section of this report, PPS25 states that Functional Floodplain is attributed to the 1 in 20 year event. If any area of the site falls within Flood Zone 3b the site should be refused, under the guidance of the Sequential Test (assuming the site is not intended for water compatible uses or essential infrastructure that can not feasibly be located elsewhere). If this site is allocated, (in the situation where there are no other available sites), it would have to be made clear in associated policies that no development is proposed in areas classified as Flood Zone 3b, functional floodplain.

6.4.3 The land within the floodplain can also be attributed to Flood Zone 3 (i.e. High Probability; land which has been assessed to flood with an annual probability of 1 in 100 (>1%) or greater in any one year) and Flood Zone 2 (i.e. Medium Probability; land which has been assessed as having between a 1 in 100 and 1 in 1000 annual probability of fluvial flooding (1% - 0.1%)). The majority and remaining part of the site is located within Flood Zone 1 (i.e. Low Probability; land assessed as having a less than 1 in 1000 annual probability of flooding in any one year (<0.1%)). Figure A29-1 illustrates the present day flood zone map produced for the site including the updated modelling as part of this SFRA study.

6.4.4 Revised modelling has been undertaken for this watercourse by Atkins as part of this SFRA to ascertain flood zone extents within the River Brain in the vicinity of the site. This represents the best data available for design flood levels associated with this watercourse. Nevertheless, it would be prudent to obtain the adjacent flood levels from the Environment Agency and refine the sites Flood Zone Maps based upon detailed site topographical data.

6.4.5 No flood zone data was made available for the unnamed tributaries of the River Brain to the north and south of the site. Under these circumstances it can be assumed that the Flood Mapping Study of the River Brain has not incorporated these catchments into the mapping, due to the small size of each catchment. However, each stream exerts a degree of flood risk upon the site, especially considering the close proximity and large number of culverts that are evident within the site area (Figure A29-1). Therefore flooding from both of these sources should be quantified appropriately. A site specific FRA should investigate these tributaries and assess the flood risk associated with these watercourses.

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6.4.6 A site visit should be undertaken as part of the FRA in the first instance and discussions should be held with the Environment Agency to ascertain what they would deem an appropriate assessment of flood risk from these small streams. Overland Flow

6.4.7 During the investigative process of this report there has been no direct reference made to historic flooding from overland flow in the immediate vicinity of the site. However, a desk-based review suggests that the surrounding hills slope towards Witham and the site. Furthermore, the slopes are characterised by an impermeable solid geology (London Clay) and form a relatively large catchment area. Flooding from this source should therefore not be discounted and a full assessment made on site as part of a site specific Flood Risk Assessment. This should include an evaluation of flow paths to determine how / if these potential mechanisms affect the site. Arterial Drainage Network

6.4.8 One of the recorded sewer flooding incidents provided by Anglian Water was within the vicinity of the site. However, this was located upon the opposite bank of the River Brain and is therefore considered not to affect the site. However, as mentioned throughout this report, flood risk from nearby sewers should not be discounted on the basis that there are no recorded incidents.

6.4.9 The site is located upon a relatively low-lying area adjacent to the river. Surrounding areas, particularly from the west, fall towards the site. It is therefore considered that a range of pathways exist that could route floodwater towards the site if a sewer were to flood in the nearby area.

6.4.10 Typically, suitable mitigation measures can be incorporated into the development to protect it from flooding from what tends to be more manageable sources of flooding, such as overland flow and the drainage network. These are anticipated to be more manageable because under design rainfall conditions they typically result in less significant flooding. Some relevant mitigation measures are discussed below.

6.5 Assessment of Flood Risk

6.5.1 This section focuses on the modelling outputs from the River Brain (including recent revisions by Atkins). The 1 in 20, 1 in 100 and 1 in 1000 events have been modelled and mapped, both for the present day and the climate change scenarios. Fluvial

6.5.2 Figures A29-1 and A29-2 show the modelled flood extents for Flood Zones 2, 3a and 3b of the River Brain, produced by Atkins both for the current situation (2007) and for the climate change scenario (2107). These figures show the flood risk from the River Brain affects the eastern region of the site. In addition, Figures A69 and 70 show the

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hazard associated with the 1 in 100 and 1 in 1000 year flood event with the influence of climate change.

6.5.3 Within the present day scenario (Figure A29-1) the functional floodplain (Flood Zone 3b) inundates the area surrounding the River Brain. The inundation extent extends within the boundary of the Bramston Sports Centre and is located to the east of the school buildings. Flood Zone 3a inundates the area to the east of the school complex and the eastern area of the vegetated sports field. Flood Zone 2 inundates an area of the vegetated sports field to the north of the school buildings and towards the east of the proposed development area.

6.5.4 Within the climate change scenario (Figure A29-2) the extent of the functional floodplain (Flood Zone 3b) increases between the school complex and the River Brain. The extent of Flood Zones 2 and 3a increase slightly in the climate change scenario.

6.5.5 There are no areas of high hazard associated with this stretch of the watercourse (Figures 69 and 70). Towards the east of the proposed development site is an area classified as medium hazard, fringed by an area of low hazard. Due to the abundance of land classified as no hazard within the area of the site west of the River Brain, development should not be situated within areas with an associated hazard.

6.6 Potential Development Considerations Site Layout and Design

6.6.1 A sequential approach should be taken into consideration when determining the masterplanning of the Bramston Sports Centre development area. Using the flood zone maps, more vulnerable uses should be directed into lower vulnerability zones, i.e. Flood Zone 1.

6.6.2 Watercourse corridor protection zones are considered to form an essential aspect of sustainable development. The protection zones provide room for the safe conveyance and storage of floodwater, they help to maintain riparian habitat, allow for natural processes of erosion and depositing, recreational opportunities and access for future watercourse maintenance or improvement. A minimum of 9 metres setback/buffer zone of any permanent development should be provided from the top of the riverbank of any ‘Main River’, such as the River Brain. Either 3 or 5 metres from non-Main Rivers (the un-named watercourses to the north and the south of the site), may also be requested, dependent on the dimensions of the watercourse. In accordance with the Water Resources Act 1991 and the Land Drainage Byelaws, the prior written consent of the Agency is required for any proposed works or structures in, under, over or within 9 metres of a designated ‘main river’ or the associated flood defences.

6.6.3 Any works in, over, under or within 9 metres of a designated main river or flood defence requires formal written consent from the Environment Agency prior to the

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works commencing. This includes the construction of any buildings, culverts, bridges, footways and outfalls. In addition, any works that could affect the flow of an ordinary watercourse (i.e. not designated as a Main River) require consent from the Environment Agency prior to the commencement of works. This includes culverting, diverting, and can include outfalls and bridges depending on the likely affect to the flow of the watercourse.

6.6.4 In accordance with PPS25, development of highly vulnerable, more vulnerable or less vulnerable land uses would not be permitted in Flood Zone 3b. It is therefore suggested that this area be outlined for open space within the overall scheme for the site.

6.6.5 With respect to the Bramston Sports Centre, it must be demonstrated that the Sequential Test is has been passed before development is considered. Following this, the Exception Test will be required in addition to a Flood Risk Assessment demonstrating the safety of the development for the lifetime of the development, i.e. the Exception Test is passed. Assuming the Sequential Test and Exception Test are passed, the following suggestions can be made regarding the masterplanning of the area:

• No development should be located within Flood Zone 3b, the functional floodplain. This is situated along the eastern boundary of the site. Water compatible land uses can be located in this area, such as canoe clubs. It is suggested that this area retains its current land use as playing field or is used as recreational or landscaped areas.

• Highly vulnerable land uses (e.g. mobile homes) cannot be positioned within Flood Zone 3a, which is situated along the eastern boundary of the site, extending to the west of Flood Zone 3b. If more vulnerable land (e.g. residential dwellings) uses are to be situated within this zone the Exception Test will be required. Less vulnerable land uses (e.g. shops) are compatible with Flood Zone 3a. It is suggested that this area remain as vegetated playing field. If it is necessary to develop this area, it is suggested that non-residential land uses are situated in this location, e.g. leisure facilities. Residential development may be placed in this flood zone providing the Exception Test is applied.

• Highly vulnerable land uses (e.g. mobile homes) can be situated within Flood Zone 2, situated to the east of the site, extending to the west of Flood Zone 3a, providing the Exception Test is applied. All other land uses can be situated within this zone. Residential development may be situated in this area.

• All land uses can be situated within Flood Zone 1. As such a large proportion of the site is classified as Flood Zone 1, it is suggested that this area be developed in preference to areas of Flood Zone 2 and 3. The Sequential Test should guide development preferentially into land classified as Flood Zone 1. Where

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development is situated on Greenfield land within this area, runoff will be limited to the pre-development Greenfield runoff rate. Sustainable Drainage Systems (SuDS) should be utilised to achieve this (see Section 8). Floor Levels

6.6.6 Floor levels should be set to the 1 in 100 year scenario with the influence of climate change level. There should then be a 300mm freeboard allowance in addition to the above scenario.

6.6.7 Levels have been extracted from the modelled data from the River Brain modelling produced by Atkins and LiDAR data. The flood level located along the eastern boundary of the site, for the 1 in 100 year scenario with the influence of climate change, is 16.85m AOD. A 300mm freeboard should be added to this level, thus finished floor levels for any developments should be set above 17.15m AOD. A site specific flood risk assessment should however investigate this level using a ground topographic survey. Access

6.6.8 Safe access and egress can be gained via Spinks Lane, this road remains dry. From Spinks Lane, Blunts Hall Road to the north and Hatfield Road to the south remain dry and should be used to gain access/egress to/from the site during times of flood. Safe access/egress routes should not involve crossing the River Brain.

6.7 Flood Mitigation and Management

6.7.1 The assessment undertaken above is intended to draw attention to the main sources of flood risk posed to the site. This section will identify some of the options that exist that are intended to manage each of the flood sources identified. A discussion regarding some initial considerations is also included below regarding an appropriate surface water strategy. Fluvial Flood Risk

6.7.2 It is understood that the area proposed for residential occupancy is located adjacent to the River Brain. No detailed plans have been received. However, this could potentially encroach into the floodplain identified in Figure A29-1 and A29-2. Detailed flood mapping using a site topographical survey as discussed in Section 5.2 and Section 6 of this report should be undertaken first to confirm the floodplain extents.

6.7.3 The Sequential Test, as contained within PPS25, states that residential premises are regarded as ‘more vulnerable’ and should not be located within Flood Zone 3b (Functional Floodplain). The Exception Test is used for development in areas where the Sequential Test has been passed and the development is ‘exceptional’ i.e. no alternative options exist. The compatibility between Flood Zone 2 and residential

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premises is considered to be acceptable. More details of the Sequential Test are located within Section 3 of the main report.

6.7.4 The majority of the site is not located within the floodplain (i.e. located within Flood Zone 1). Through the sequential approach it is recommended that all proposed development is steered away from the floodplain to the east of the site and located entirely in Flood Zone 1 to the west of the site.

6.7.5 Avoiding development in the floodplain would be considered the most appropriate means of managing the fluvial flood risk. This can be achieved easily at the site by avoiding the strip of land along the east site boundary, which is shown to be within the floodplain.

6.7.6 The final development proposals including potential mitigation measures should be considered in a full FRA when more information is made available.

6.7.7 The Level 1 assessment for the site has been undertaken in Section 4, Figure A29-1 and A29-2. Overland Flow and Drainage

6.7.8 A site visit should be undertaken to allow an appropriate assessment of overland flow and drainage sources. This will also help identify what mitigation measures may be necessary. This could involve one or more of the following:

• Raise finished floor levels above the 1 in 100 year plus climate change fluvial event, with a 300mm freeboard allowance; • Provision of channels to control flow paths through the site and thus guide water away from the proposed developments and into surface water drainage systems; • Divert flow paths to prevent entry to the site, (it must be ensured and demonstrated that flow is not being moved to another area and thus increasing the flood risk elsewhere); • Use water resistance building materials to limit the potential damage caused from flood water inundation • Incorporate SuDS into the designs of any landscaped areas within the Masterplan area (see Section 8 for more information). Groundwater

6.7.9 Groundwater flooding can occur when groundwater levels rise above ground levels at the site of interest.

6.7.10 Due to the close proximity of the eastern area of the site with the River Brain there is a possibility that groundwater in this area may be in hydraulic conductivity with the River

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Brain. This should be fully investigated in a site investigation report and Flood Risk Assessment of the site.

6.7.11 Typically, suitable mitigation measures can be incorporated into the design of the development to protect it from sources of flooding from overland flow and the drainage network. These are anticipated to be more manageable because under design rainfall conditions they typically result in less significant flooding. Some relevant mitigation measures are discussed below. However, prior approval from the Environment Agency must be granted. Post-redevelopment Drainage

6.7.12 The redevelopment should not exacerbate flood risk to the site or third parties. Increased impermeable coverage following redevelopment can generate additional surface water runoff discharged from the site. The existing receptor of surface water runoff should be investigated. It is anticipated that that the site currently either discharges into a sewer, or discharges straight into the River Brain, or a combination of the two. Discussions should be held with the Environment Agency and potentially Anglian Water to agree a surface water strategy. Greenfield runoff rates should be maintained with runoff restricted to the pre development rates as part of the proposed drainage strategy.

6.7.13 A borehole log of the shallow geology at the site was ascertained from British Geological Survey (BGS). This was taken towards the northeast corner of the site and indicates that below the topsoil approximately 4 metres of alluvial deposits exist. This consists of clayey sand and gravels, which is underlain by firm to stiff clayey silt. Water was encountered approximately 3.5 metres below ground level.

6.7.14 The drainage strategy may necessitate the use of infiltration, attenuation and / or water harvesting techniques.

6.7.15 Whilst the presence of a relatively shallow water table indicates that soakaways may not be practical, the use of infiltration should certainly be considered as a key technique of surface water management. This could include permeable paving, swales and infiltration basins for example.

6.7.16 The upper alluvial deposits (i.e. clayey sand and gravels) located beneath the site are anticipated to transfer groundwater into the adjacent River Brain. The nature of this mechanism is generally a slow process, introducing a lag between infiltration into the ground and seepage into the adjacent watercourse. Assuming that the distance and / or geological conditions are sufficient the lag will allow surface water to seep into the River Brain once the peak flows conditions have receded.

6.7.17 A more detailed assessment of the geology beneath the site should be undertaken to ensure that these methods are suitable. This may necessitate drilling a range of boreholes, percolation testing and a contaminative study. If for any particular reason

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infiltration is found to be unacceptable, other techniques should be considered such as storage. Section 8 discusses the role that SuDS can play as part of sustainable development and identifies what should be considered for the successful implementation of these techniques. Additional Constraints

6.7.18 Watercourse corridor protection zones must be considered when planning the eastern part of the site. A buffer zone of 9 metres is generally stipulated from the adjacent top bank of Main Rivers, such as the River Brain. Either 3 or 5 metres from the small tributary to the north and south would also be requested, dependent upon the dimensions of the watercourse. No development is typically allowed within this buffer zone. Land Drainage Consent would be required for the construction of any proposed surface water outfalls into any adjacent watercourses such as the River Brain.

6.7.19 An acceptable access and egress route to and from the site should, in general, remain dry during a 1 in 1000 year flood event. With respect to flooding from the River Brain this is not considered to be problematic as Spinks Lane is not located within the floodplain.

6.8 Bramston Sports Centre Conclusions

6.8.1 Braintree District Council has singled out the Bramston Sports Centre for detailed flood risk investigation and Section 6 in this report has addressed this.

6.8.2 A strip of land to the east of the site is located within the floodplain.

6.8.3 A full FRA should be undertaken and submitted with the planning application, focused upon the assessment of flooding from local watercourses, overland flow and the arterial drainage network. It should also contain an investigation of the proposed drainage strategy.

6.8.4 The Masterplan for this area should be applied in accordance with the sequential approach, where possible placing more vulnerable development in areas of lower flood risk. In particular development within functional floodplain 3b should be avoided. Most development types would not be permitted in this area in accordance with PPS25. Details with respect to the compatibility of proposed development is identified within Section 3.4 of the Main SFRA Report, which incorporates advice stipulated within PPS25.

6.8.5 As this site is located within a high-risk flood zone and it is proposed to situate vulnerable developments on the site, Braintree District Council should seek alternative sites to take forward in the LDF. Only if this is not possible, i.e. the Sequential Test is passed, should this site be considered for development. Even then, the development

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of the site should be subject to a strict policy to ensure the sequential zoning of the site. This site can only be developed if the Sequential Test is passed.

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7 Water Discharges

7.1.1 Undeveloped areas generally rely on natural drainage to absorb surface water or to allow flow across the ground. This natural overland flow provides the baseflow to local watercourses as well as having environmental and ecological benefits to local ecosystems.

7.1.2 Developments generally reduce the permeability of the site area, effectively reducing the amount of water infiltrating into the ground and disrupting the natural overland flow processes. Conventional surface water drainage uses underground piped systems that are designed to remove water from the site as quickly as possible. These systems can disrupt natural flows within the catchment, in particular the natural recharge of groundwater and provision of baseflow to watercourses. In addition, these systems can be surcharged causing localised flooding to people and property.

7.1.3 PPS25 outlines that new developments should, where reasonably practicable, mimic the existing situation in terms of surface water runoff. Consequently, developments on existing greenfield areas are typically required to attenuate runoff to greenfield rates. On previously developed land, the Environment Agency typically requires some reduction in runoff rates when compared to the existing situation.

7.1.4 PPS25 outlines that one of the impacts of climate change is a storm of higher intensity and of more frequent occurrence. As a result, flooding events resulting from overland flow sources or from surcharged drainage systems are likely to become more prevalent. In light of this, management of surface water discharge will become an important consideration in the majority of planning decisions.

7.1.5 An inevitability associated with development is the discharge of foul water. This is typically discharged into foul sewers that are often connected to the surface water drainage system. As a consequence, surface water flooding can become polluted with effluent within the foul water system, creating an additional hazard to a population from waterborne disease.

7.2 Effect of Development in Braintree Surface Water

7.2.1 The majority of the developments areas are located on Greenfield sites and are large in size. These developments could potentially have a cumulative effect on the amount of surface and foul water discharged into the existing systems.

7.2.2 The few proposed development areas within built up settlements are located on previously developed land. As a result, it is likely that existing surface water from these locations is discharged into the local river system, with relatively little

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attenuation. Continuing development in such areas is likely to exacerbate these problems, however, this is likely to have a less significant impact than development of areas currently located on greenfield land.

7.2.3 Development in these areas, particularly those that are located on predominantly greenfield areas will require significant efforts in order to attenuate surface water runoff. Those areas with a high percentage of brownfield land have an opportunity to improve the existing situation. Foul Water

7.2.4 Many of the proposed developments within Braintree have the potential to significantly increase the amount of water discharged into the foul sewer system. Logically, there is likely to be a greater increase in foul water discharge as a result of developments on previously greenfield land or areas of lower intensity in terms of population. For example, replacing a car parking area with high intensity dwellings or offices is likely to increase the volume of water discharged into the foul drainage system.

7.2.5 Some of the key development areas within Braintree are likely to result in a significant increase in volume of water discharged into the foul sewer system. Many of the proposed developments on currently Greenfield land all have the potential to significantly increase foul water discharge. Therefore these areas should consider separate drainage and sewer systems with increased capacities to meet the existing and future development demands.

7.3 Mitigation

• Mitigating surface water drainage typically involves the implementation of Sustainable Drainage Systems (SuDS), as detailed in Section 8 below.

7.3.1 Generally, the most favourable systems are infiltration techniques that allow water to enter the ground through infiltration and seepage. Such techniques mimic a natural, greenfield system and encourage a more natural groundwater recharge. However, such techniques are not always applicable to every site. Therefore, the application of SuDS should be assessed on a site-by-site basis.

7.3.2 Mitigating foul water typically involves techniques that seek to reduce water usage or that utilise on site recycling methods. Such measures include, for example, reducing toilet cistern volumes, recycling some wastewater to flush toilets and water collection for landscaping irrigation.

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8 SuDS Specific to Braintree

8.1 Introduction

8.1.1 This section is intended to address the final objective (i.e. Objective 4) as outlined in Section 1.2 in a more general sense, whilst the more specific details pertaining to Bocking are discussed throughout this report.

8.2 Geology

8.2.1 The solid geology of the Braintree District can be separated into three main types:

• The northern section of the District is underlain by the Upper Chalk Formation. • A thin section through the centre of the District extending from Sudbury in the east heading southwest to Castle and Sible Hedingham in the centre of the District and Finchingfield in the east is underlain by the Lower London Tertiaries (Lambeth Group and Thanet Sands). • The southern area of the District is underlain by the London Clay Formation. Occasional pockets of Crag are located overlying the Chalk within the northern area of the site and one large pocket to the south of Wethersfield.

8.2.2 Drift deposits overlying the solid geology consist mainly of the Lowestoft Formation, which comprises Glaciofluvial Deposits, Till and Glaciolacustrine Deposits. In areas around large river channels outcrops of Kesgrave sands and gravels, River Terrace Deposits, Head Deposits alluvium are located and in some areas the underlying solid formation has been exposed.

8.2.3 The following tables (Table 8-1 & Table 8-2) highlight the main solid and drift geology deposits with an indication of potential corresponding appropriate SuDS. Specific geological conditions at individual sites should be ascertained from geological maps and where relevant, during more detailed stages of design, verified as part of a site investigation.

8.2.4 It is important to note that various geological formations have variable permeability, which can mean that the particular formation may consist of differing geological deposits. For example, the alluvial deposits of a particular river may consist of widespread sands and gravels, characterised by a high permeability. Whilst more isolated pockets may exist where the alluvial deposits consist of clay and silts, with a low permeability.

8.2.5 Furthermore, the Upper Chalk Formation located throughout the northern section of the District could potentially form an ideal receptor of surface water generated on an overlying site. However, in many circumstances the solid geology is not exposed at or

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near ground level and is overlain by drift deposits, which can be in excess of 200 metres. Under these circumstances the presence of the drift deposits could negate the practicality of infiltration techniques.

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Table 8-1 Major solid geological deposits found throughout the district

Drift Deposit Permeability General Characteristics Locations Potential appropriate SuDS techniques

Found within river valleys and tributaries of Infiltration and combined infiltration/attenuation systems and Variably Generally clay with some gravel Alluvium the River Glem, Stour, Colne, Pant, Brain attenuation systems e.g. permeable surfaces, sub surface infiltration, Permeable sand and silt and Ter and Bourne Brook and Pods Brook basins and ponds, swales and filter strips i.e. a combined system

Found within river valleys and tributaries of Infiltration and combined infiltration/attenuation systems and River Terrace Variably Sandy gravel, clayey in places the River Glem, Stour, Colne, Pant, Brain attenuation systems e.g. permeable surfaces, sub surface infiltration, Deposits Permeable local veneer of clayey silt and Ter and Bourne Brook and Pods Brook basins and ponds, swales and filter strips i.e. a combined system

Generally Found within river valleys and tributaries of Kesgrave permeable Sands and sandy gravels with less Infiltration systems e.g. permeable surfaces, sub surface infiltration, the River Glem, Stour, Colne, Pant, Brain Formation (dependant than 10% clay and silt swales and filter strips. and Ter and Bourne Brook and Pods Brook fines content)

The Lowestoft Formation consists of Glacial deposits comprising Till, Glaciofluvial Deposits and Glaciolacustrine Deposits.Till This is the main superficial deposit found deposit: sandy clay with chalk throughout the District. Thicknesses of the fragments (formerly known as deposit vary greatly. Within buried valleys it Attenuation systems e.g. basins and ponds, green roofs, tanks, Boulder Clay)Glaciolacustrine Lowestoft Variably is considered that the formation could rainwater harvesting etc are likely to be the most suitable techniques. Deposits: calcareous silt which is Formation Permeable potentially extend up to 300m in thickness Infiltration systems may be appropriate within more the deposits found finely interbedded with the and within certain areas of current river characterised by sands and gravels. Glaciofluvial Deposits which occur channels the deposits have been eroded below within and above the Till away to expose the soil geology deposits. The Glaciofluvial Deposits consist of gravelly clayey sand with interbeds of clayey sand and silt.

Found within river valleys and tributaries of Attenuation systems e.g. basins and ponds, green roofs, tanks, Head Deposits Impermeable Silty or sandy clay the River Glem, Stour, Colne, Pant, Brain rainwater harvesting etc and Ter and Bourne Brook and Pods Brook

Generally Limited isolated pockets associated with the Attenuation systems e.g. basins and ponds, green roofs, tanks, Peat Brown/ black organic rich clay impermeable upper reaches of water courses rainwater harvesting etc

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Table 8-2 Major drift and geological deposits found throughout the district

Solid Geology Permeability General Characteristics Locations Potential appropriate SuDS techniques

Outcrops are generally located in the Includes: Mainly fine grained buff to northern part of the District overlying the Infiltration and combined infiltration/attenuation systems and Variably brown, locally shelly, micaceous The Crags Chalk. A large outcrop is located to the attenuation systems e.g. permeable surfaces, sub surface infiltration, Permeable sands, with local rounded flint south of the band of The Lower London basins and ponds, swales and filter strips i.e. a combined system gravels Tertiaries to the south of Wethersfield.

Comprises the Woolwich and Reading Formation and Thanet The Lower London Tertiaries are located in The Lower Sand Formation. a band extending from Sudbury in the east Infiltration and combined infiltration/attenuation systems and Variably London heading southwest to Castle and Sible attenuation systems e.g. permeable surfaces, sub surface infiltration, Permeable The Woolwich and Reading Tertiaries Formation consists mottled clays Hedingham in the centre of the District and basins and ponds, swales and filter strips i.e. a combined system sands silts whilst the Thanet Finchingfield in the east Formation comprises silty sands.

Generally permeable (Heavily White and grey chalk, nodular and Infiltration and combined infiltration/attenuation systems and Chalk deposits are located within the Chalk weathered soft with flint seams in upper part attenuation systems e.g. permeable surfaces, sub surface infiltration, northern area of the District material can and localised marl bands basins and ponds, swales and filter strips i.e. a combined system be impermeable)

Clay, Orange brown becoming blue grey with depth, variably silty with London Clay thin sand and rare pebble beds. The London Clay Formation is located within Attenuation systems e.g. basins and ponds, green roofs, tanks, Impermeable Formation Some siltstone nodules and bands the southern area of the District rainwater harvesting etc and Selonite Crystals, occasional shell fragments.

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8.3 SuDS Recommendations

8.3.1 PPS25 indicates that Regional Planning Bodies and Local Authorities should promote the use of SuDS for the management of surface water runoff generated by development. Runoff rates from new developments should not increase following redevelopment, including an allowance for climate change.

8.3.2 PPS25 recognises that flood risk and other environmental damage that can be caused by traditional drainage systems can be managed through the use of SuDS. Flood risk can be reduced by systems that minimise the changes in the volume and rate of surface runoff from development sites. This is complementary to the control of development within the floodplain. Where possible SuDS should be incorporated into all new developments. However, if for instance infiltration techniques were proposed, this must first be assessed subject the appropriate geology and ground conditions to accommodate this technique.

8.3.3 In addition, drainage of rainwater from roofs and paved areas around buildings should comply with the 2002 Amendment of Building Regulations Part H (3). The requirements are as follows:

• Adequate provision shall be made for rainwater to be carried from the roof of the building. • Paved areas around the building shall be so constructed as to be adequately drained. • Rainwater from a system provided pursuant to sub-paragraphs (1) or (2) shall discharge to one of the following in order of priority: • An adequate soakaway or some other adequate infiltration system; or where that is not reasonably practicable; • A watercourse; or where that is not reasonably practicable a sewer.

8.3.4 SuDS seek to manage surface water as close to its source as possible, mimicking surface water flows arising from the site, prior to the proposed development. Typically this approach involves a move away from piped systems to softer engineering solutions inspired by natural drainage processes.

8.3.5 SuDS should be designed to take into account the surface run-off quantity, rates and also water quality ensuring their effective operation up to and including the 1 in 100 year design standard flood including an increase in peak rainfall of up to 30% to account from climate change.

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8.3.6 Wherever possible, a SuDS technique should seek to contribute to each of the three goals identified below with the favoured system contributing significantly to each objective. Where possible SuDS solutions for a site should seek to:

1. Reduce flood risk (to the site and neighbouring areas), 2. Reduce pollution, and, 3. Provide landscape and wildlife benefits.

8.3.7 These goals can be achieved by utilising a management plan incorporating a chain of techniques, (as outlined in Interim Code of Practice for Sustainable Drainage Systems 2004), where each component adds to the performance of the whole system:

• Prevention: good site design and upkeep to prevent runoff and pollution (e.g. limited paved areas, regular pavement sweeping) • Source control: runoff control at/near to source (e.g. rainwater harvesting, green roofs, pervious pavements) • Site control: water management from a multitude of catchments (e.g. route water from roofs, impermeable paved areas to one infiltration/holding site) • Regional control: integrate runoff manage from a number of sites (e.g. into a detention pond)

8.3.8 The application of SuDS is not limited to a single technique per site. Often a successful SuDS solution will utilise a combination of techniques, providing flood risk, pollution and landscape/wildlife benefits. In addition, SuDS can be employed on a strategic scale, for example with a number of sites contributing to large scale jointly funded and managed SuDS. It should be noted, each development site must offset its own increase in runoff and attenuation cannot be “traded” between developments.

8.3.9 In accordance with PPS25 the local authority and Environment Agency would require any greenfield development should retain the existing greenfield runoff rate post- development and should allow for climate change. SuDS should be included in new developments where possible to manage surface water.

8.3.10 PPS25 requires the use of SuDS as an opportunity of managing flood risk, improving water quality and increasing amenity and biodiversity. SuDS should be located in accordance with the restrictions set out in Policy and Practice for the Protection of Groundwater.

8.3.11 The ground conditions of the potential development allocation have not been assessed as part of this SFRA and would be subject to the results of ground investigations for the site determining their potential for SuDS. However, a preliminary investigation was undertaken for the Bramston Sports Centre (see Section 4.6)

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9 Policy Specific Requirements to Braintree District

9.1.1 PPS25 was issued in December 2006 and further reinforces the guidance issued in PPG25 on managing development and flood risk. PPS25 strives to reinforce the importance of a flood risk assessment and mitigation of flood risk, if any to a development. The flood risk should be considered as part of development proposals which is dependant on the scale and nature of the development.

9.1.2 PPS25 puts more emphasis on the Sequential Test and introduces the Exception Test in order to ensure that only exceptional developments are located in areas at risk of flooding. PPS25 also places a greater emphasis on the importance of managing surface water and its safe disposal in order to ensure flood risk is not increased elsewhere as a result of a development. This should be carefully considered when developing areas such as developments on greenfield sites, where inappropriate development techniques could exacerbate flood risk to third parties.

9.1.3 National and local policies have been reviewed against the local flood risk issues and objectives identified by the Environment Agency in the CFMP. From these policies the following catchment wide and specific area strategies have been developed under the headings Flood Risk, SuDS, Flood Mitigation and the Water Environment. Integration of these suggested policy considerations into LDF / LDD should ensure that the objectives and aspirations of the Environment Agency and national policy are met whilst strengthening the position of the LPA with regard to Flood Risk.

9.1.4 The natural environment and countryside will be conserved to protect the Borough’s diverse history, archaeology, geology, and biodiversity. Development should be directed away from sites of landscape and conservation importance and land at risk from flooding.

9.2 Flood Risk Catchment Wide Strategies

1. Ensure the Sequential Test is undertaken for all allocations to reduce the flood risk to the allocation and ensure that the vulnerability classification of the proposed development is appropriate to the flood zone classification; 2. Flood Risk Assessments should be undertaken for all resulting allocations within Flood Zones 2 and 3 to assess the risk of flooding to the development and identify options to mitigate the flood risk to the development, site users and surrounding area, and where floodplain storage is removed, the development should provide

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compensatory storage on a level for level basis to ensure that there is no loss in flood storage capacity.; 3. Flood Risk Assessments are required for all developments in Flood Zone 1 that are greater than 1.0ha. However, if a critical drainage problem has been identified on a development in Flood Zone 1 Flood Risk Assessments will then required for developments that are greater than 0.5ha or over 10 dwellings. 4. Flood Risk to development should be assessed for all forms of flooding; 5. If a critical drainage area is identified, then all developments that may have an impact on the local drainage should produce a FRA/drainage strategy, not just major developments. 6. The anticipated affects of climate change should be assessed and mitigated for, if necessary, for the proposed lifetime of the development; 7. Where there are no Flood Zones associated with a particular stream an appropriate assessment of flood risk should be made in accordance with advice from the Environment Agency; Area Specific Strategies

1. Consideration to flooding from overland flow should be given for developments occurring throughout the District, but with particular regard to Bocking, Braintree, Witham and Coggeshall.

9.2.1 Through integration of these suggestions, the emerging LDF is anticipated to comply with PPS25 and the aspirations and policies contained within the Braintree District Local Plan Review, more specifically policy RLP 66, RLP 67 and RLP 68. The LDF needs to ensure compliance with national policies and regional policies as set out in RSS14.

9.3 Sustainable Drainage Systems (SuDS) Catchment Wide Strategies

1. SuDS must be included in new developments as a way to manage surface water. 2. PPS25 requires the use of SuDS as an opportunity of managing flood risk, improving water quality and increasing amenity and biodiversity. 3. Flood risk assessments should be undertaken for developments in Flood Zone 1 that are greater than 1ha in size, to ensure that flood risk is not increased to other properties due to increased site runoff; 4. Runoff rates from new developments should not increase following redevelopment, including an allowance for climate change;

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5. Runoff rates should be restricted to greenfield runoff rates in areas known to have a history of sewer flooding; 6. SuDS should be located in accordance with the restrictions set out in Policy and Practice for the Protection of Groundwater. 7. Infiltration techniques may not be appropriate in areas affected by ground contamination, unless suitable mitigation is undertaken. Area Specific Strategies

1. Runoff rates should be restricted for both greenfield and brownfield developments in Bocking, Braintree, Witham and Coggeshall in particular, this is also likely to be appropriate within other settlements to ease surface water flooding and drainage capacity exceedence; 2. Infiltration techniques are unlikely to be appropriate where the site is underlain by London Clay, such as in Bocking, Braintree, Witham, Halstead, Coggeshall and Kelvedon. Attenuation techniques should be imposed in these circumstances.

9.3.1 Through integration of these suggestions, the emerging LDF is anticipated to comply with PPS25 and the aspirations and policies contained within the Braintree District Local Plan Review, more specifically policy RLP 69.

9.3.2 As defined within the 2002 Amendment of Building Regulations Part H (3) surface water should discharge into one of the following, in order of preference:

• An adequate soakaway or some other adequate infiltration system; or where that is not reasonably practicable; • A watercourse; or where that is not reasonably practicable a sewer.

9.3.3 If surface water is discharged into a watercourse and / or sewer, a practical restriction upon the rate of discharge should be imposed. This should be assessed on a site by site basis and agreed with the Environment Agency.

9.4 Flood Mitigation

9.4.1 Flood Mitigation Policies should address the following issues: Catchment Wide Strategies

1. Where an allocation borders an area benefiting from flood defence, opportunities should be sought for the maintenance of these flood defences to be partly funded by the development for its lifetime; 2. A minimum setback distance of 9m should be maintained along fluvial watercourses to ensure maintenance access for flood defences, adhering to the

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Environment Agency’s Land Drainage bylaw along main rivers. This 8m strip should be kept free of all permanent development and where possible should encourage leisure and public access routes. 3. Opportunities should be sought to deculvert rivers, where possible, to return them to a natural system, reducing back up of flows and under capacity where this does not exacerbate the flooding elsewhere; 4. River channel restoration should be undertaken where possible to return the river to its natural state and restore floodplain to reduce the impact of flooding downstream; 5. Emergency planning strategies should be in place in order to direct people to safety during times of flooding. Area Specific Strategies

9.4.2 Emergency planning should be taken into account to facilitate safe access and egress from areas identified in this SFRA to be prone to flooding. In addition, Figures A69 and A70 should be consulted to direct people to places of refuge during times of flooding. The location of any future emergency services or potential large scale refuge centres, such as hospitals, fire stations, community halls, should take into account the risk of flooding to ensure these centres remain operational during times of flooding.

9.4.3 Details of Emergency Accommodation and procedures are outlined in ‘Emergency Accommodation – Step by Step Management Guide’, provided by Braintree District council. This document outlines emergency accommodation procedures and lists the principal Rest Centres3 in the area as: Alec Hunter High School – Braintree, Notley High School and Sports Centre – Braintree, The Hedingham School – Sible Hedingham, Tabor High School and Sports Centre – Braintree, Honeywood School – Coggeshall, Ramsey School and Sports Centre – Halstead, Bramston School and Sports Centre – Witham, and Rickstones School, Witham.

9.4.4 The above mentioned document also outlines local radio stations and other information systems where information regarding severe weather warnings will be issued. In the District of Braintree information will be available by tuning into BBC Essex (95.3 and 103.5 Fm / 729, 765, 1530 MW), Essex Radio (96.3 and 102.6FM) and Breeze (1359MW and 1431 AM). In addition, information regarding severe weather warnings can be found on the meteorological office web page.

9.5 Water Environment

9.5.1 Water Environment Policies should address the following issues:

3 Building or part of a building designated by the Local Authority for the provision of temporary accommodation for Evacuees.

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Catchment Wide Strategy

1. Development should not have a detrimental impact on the water environment through changes to water chemistry or resource. 2. All developments should incorporate water efficiency measures, not just those in water stressed areas. The emerging East of England Plan advocated strict water efficiency levels which will have to be incorporated into new developments; 3. Any development should not be located within 9 metres of the riverbank to ensure access for maintenance but also to ensure a riparian corridor for improvement of the riverine environment. 4. River channel restoration should be undertaken where possible to return the river to its natural state and restore the floodplain to reduce the impact of flooding downstream, by allowing the river to flood naturally. In addition, there are multiple environmental benefits to restoring the river to a natural state including encouraging fauna and flora into the area and establishing a variety of ecosystems, as well as increasing the aesthetic value of the river system. . Area Specific Strategies

9.5.2 Policy RLP 86 reinforces the need for sensitivity near watercourses stating development would not be permitted that would harm the open character, nature conservation importance or recreational importance of the floodplains of the River Stour, Colne, Brain, Pent, Blackwater, Ter Valley and their tributaries and the Chelmer and Blackwater Navigation.

9.5.3 Where appropriate, development proposals adjoining the rivers will be required to incorporate riverside paths and open spaces.

9.5.4 Any proposals requiring the provision of a new bridge shall ensure a minimum of 2.3 metres headroom above normal water level to allow for river use and provide fauna passages suitable as wildlife corridors.

9.5.5 Integration of the strategies suggested above with the merging policies of the LDF will aid to strengthen the position of the Local Planning Authority and help to improve the way in which flood risk is assessed and managed throughout the District.

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10 Recommendations

10.1.1 The Strategic Flood Risk Assessment for Mid Essex has been completed in accordance with PPS25 and the current guidance outlined in the Development and Flood Risk: A Practice Guide Companion to PPS25 ‘Living Draft’ (Feb 2007). This report forms the specific appendix in relation to Braintree District Council.

10.1.2 Existing knowledge with respect to flood risk in the District has been utilised in order to produce this SFRA. Over time, knowledge about flood risk within the District will change, for example with proposals for flood alleviation measures. This may alter predicted flood extents within the Borough through improved defence (or alleviation measures), therefore influencing future development control decisions within these areas.

10.1.3 It is imperative that this SFRA is adopted as a ‘living’ document and is reviewed regularly in light of emerging policy directives and improved understanding of flood risk within the District.

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11 References

Atkins (2004), Blackwater Flood Risk Study, October 2004

DCLG (2006), Planning Policy Statement 25: Development and Flood Risk, The Stationary Office, London

DCLG (2006), Consultation - Planning Policy Statement: Planning and Climate Change - Supplement to Planning Policy Statement, The Stationary Office, London

Environment Agency (2006), Draft North Essex Catchment Flood Management Plan, April 2006

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12 Blackwater Modelling Report by Atkins

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Blackwater Modelling Report Flood Risk Mapping Study Design Model Runs

6 Design Model Runs

6.1 Design Conditions for Hydrological Models

This section describes the design conditions for the hydrological (Mike 11 RR) models. The design hydrology involves generating flood frequency curves for locations within the catchment to provide adequate representation of peak flows throughout the catchment for the appropriate range of return periods.

With respect to whole catchment modelling it is necessary to simulate different events in the model to produce a single flood extent for a given return period. Depending on the catchment characteristics the rainfall event that results in a 100yr flow at the top of the catchment can be very different from that which results in a 100yr event at the bottom of the catchment. It is therefore necessary to run an appropriate number of different storms through the model.

Initially five locations were chosen, as listed below;

♦ Copford Hall GS

♦ Stisted GS

♦ Appleford GS

♦ Guithavon GS

♦ Location just upstream of Braintree on the River Brain.

The results from these initial model runs for this phase of the study showed that simulating design events to each of these 5 locations is sufficient to describe the flood levels for the catchment.

At each of these 5 locations, a flood frequency curve has been developed. To derive the flood levels for a given return period (and for a given model set up), an event of the appropriate magnitude (ascertained from the flood frequency curves) is run through the model to provide peak flows at each of the 5 locations.

6.2 Methodology

To derive the flood frequency curves the AMAX data was recalculated for the gauges within the catchment and the records updated for the gauges outside the catchment. With the AMAX data updated, the most appropriate curve was fitted to the data to provide the flood frequency curve.

There were 8 main steps that were undertaken to derive the flood frequency curve.

1. Collate level and flow maxima data

For the gauging stations within the catchment, level maxima were collected from the Agency. These data were available in digitised format back to 1978 (approx.); prior to 1978 chart records were scrutinised. For all the stations that were to be used in the pooling groups, flow maxima data were requested as appropriate to bring FEH AMAX data up to date.

2. Derive Level/Discharge relationship

Spreadsheets have been set up to provide look up tables for the full range flows at the gauging stations. The BS3680 (Part 4G: Flat V weirs) equations were modified and ‘calibrated’ to the Agency modular ratings. Modifications to the equations were necessary to account for the differences between the standard Flat V weir design and the design of the Essex profile weirs (principally vertical wingwalls vs. sloping wingwalls). The extended equations within BS3680

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Blackwater Modelling Report Flood Risk Mapping Study Design Model Runs

(Part 4G), to calculate drowned flow, were then applied to provide a lookup table for the full range of flows.

3. Incorporate discharge look up tables in to the Hydraulic Model.

The look up tables were directly included in the Mike 11 model as Tabular Structures (these type of structures in Mike 11 allow for a look up table such as the ones calculated to be used for the levels and flows at the gauging station).

4. Run Hydraulic model for range of events

Results were extracted from the calibrated model for the calibration period, therefore providing flows and levels at the gauging stations for a range of events. Several additional arbitrary events were run through the model to provide a larger data set.

5. Extract peak flows and levels

The peak flows and levels for each event (all peaks within the calibration period and all the peaks of the arbitrary events) were extracted from the model results and plotted. The charts show that there is a unique relationship between peak flows and levels (as an example see Figure 6-1).

33.8

33.6

33.4

33.2

33

32.8 Event Peaks

32.6 Peak Level (mAOD)

32.4

32.2

32

31.8 0 102030405060 Peak Flow (m3s-1)

Figure 6-1 Plot of Peak Flow against Peak Level for all events within the calibration period at Stisted Gauging Station

6. Recalculation of flow AMAX

For each level maxima a flow was read off the chart produced in step 5.

7. Derivation of Flood Frequency Curve

FEH frequency analysis techniques were used to fit a best line fit to the AMAX data as described in Section 6.3.

8. Design Event simulation

Design event inflows were then calculated to provide the appropriate peak flows at each location based on the respective flood frequency curves. The results from these model runs have then been used to develop the flood extent maps.

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6.3 Flood Frequency Curves

The flood frequency curves for each location are shown in Figure 6-2. Three different approaches have been taken to derive these flood frequency curves for the catchment, based on the recommendations within FEH. Details of these approaches are given in sections 6.3.1, 6.3.2 and 6.3.3.

Return Period

1. 1 1. 5 2 5 10 25 50 75 10 0 50 0 100 0 70

60

50

) Copford -1 s

3 40 Stisted Appleford 30 Guithavon Flow (m Flow Braintree 20

10

0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 Plotting Position

Figure 6-2 Blackwater Flood Frequency Curves

6.3.1 Approach 1: Copford Hall, Stisted, Appleford

The length of the record available for each of these gauging stations is between 30 and 40 years. In order to estimate a flood event with a return period of 100 years, with a record of this length, FEH recommends the use of pooling analysis, including the subject site record within the pooling group (Vol. 4, Chapter 8). This methodology has therefore been used at Copford, Stisted and Appleford.

6.3.2 Approach 2: Guithavon Valley

The level of urbanisation of the catchment to Guithavon Valley gauging station is given as 0.058 (5.8%). The FEH recommends that any catchment with urbanisation greater than 0.025 (2.5%) should not be included in any pooling analysis (Vol. 4, Chapter 6), instead pooling analysis should be undertaken as if the location were ungauged. This flood frequency curve is then adjusted to account for urbanisation. This approach was undertaken but found to provide a relatively flat flood frequency curve. A comparison of the two largest events at Guithavon Valley (Oct 2001 and Oct 2000) to the flood frequency curve indicates that both events would be greater than the 1000 year event (see Figure 6-3)

Single site analysis was therefore undertaken to derive a flood frequency curve. It was thought that the extreme nature of the October 2001 event may skew any flood frequency curve to over predict flows for a given return period, therefore single site analysis was undertaken using the AMAX data series with and without the October 2001 event (see Figure 6-3).

The flood frequency curve constructed using the AMAX data including the October 2001 (see Figure 6-3) is considerably steeper than that constructed without using the October 2001 event. The former curve would suggest that the October 2001 event was less than a 1 in 100 year event. This is considerably lower than is thought to be the case therefore the flood frequency curve derived without including the October 2001 event has been adopted for this study.

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Return Period 1.1 1.5 2 5 10 25 50 100 200 500 1000 60

50

40

30

Flow (m3s-1) Flow October 2001 Flow 20 October 2000 Flow

10

0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 Plotting Position Pooled Analysis for Urban Catchment Single Site excluding Oct 01 event Single Site including Oct 01 event

Figure 6-3 Guithavon Valley Flood Frequency Curves

6.3.3 Approach 3: Braintree on the River Brain

The location on the River Brain, upstream of Braintree is ungauged, therefore a pooling analysis was undertaken. The Guithavon Valley site was used as a donor site for the estimation of Qmed at the subject site.

6.4 Design Rainfall & Antecedent Conditions

6.4.1 Rainfall Profile

Sensitivity analyses were undertaken to identify the rainfall profile that resulted in the highest peak flows and levels. The FEH includes either a summer or a winter profile. Both these profiles were run through the model for a range of return periods (100 year) and the results reviewed to determine which profile should be used.

The difference in the peak flows and levels was minimal (insignificant in terms of level), with the summer profile giving marginally higher flows. The summer profile was therefore adopted for the design rainfall.

6.4.2 Critical Rainfall Duration

Rainfall of a given return period can produce a wide range of estimated design floods depending on the storm duration and the antecedent catchment wetness. A storm of critical duration to each of the five locations listed in section 6.1 was derived using the standard equations given in FEH (Vol. 4, Chapter 3).

To determine the actual critical duration to each location, sensitivity analyses was carried out on the storm duration. Design events were run through the model for storms of theoretical duration, and for durations equal to the theoretical ±6 hours. The results from this showed that the theoretical durations resulted in the peak levels at each location. Table A-1 shows the critical durations adopted.

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Blackwater Modelling Report Flood Risk Mapping Study Design Model Runs

Critical Duration Location (hrs) Appleford 33.8 Copford 17.8 Stisted 24.4 Guithavon 17.2 Braintree on Brain 14.3 Table A-1 Critical Duration at each Location

6.4.3 Antecedent Conditions

The initial conditions of the root zone store (L) in the rainfall runoff models equates to the antecedent conditions of the catchment and therefore have a significant influence on the magnitude of a flood event. During the calibration period the state of the root zone store is dynamic, responding to the rainfall and evapotranspiration. The design events are single events however and an initial value of L needs to be determined.

For the purposes of this study the initial conditions have been determined through an analysis of the state of the root zone store during the calibration period. For each sub-catchment the median value of L has been determined and these values have been set as the initial conditions for the respective sub-catchment. The median value has been used to provide an ‘average’ catchment condition.

The initial conditions used for this study are shown in Table A-1. Sub-catchment Initial Conditions (L%) 1 0.684 2 0.689 3 0.668 4 0.596 Table A-1 Initial Conditions

6.4.4 Design Rainfall Depths

Design rainfall depths have been derived using the FSR rainfall runoff method as described in the Flood Estimation Handbook (FEH) Volume 4.

Depth-duration-frequency tables were produced using the standard catchment-average FEH method (FEH Vol. 2) for a series of points located along the river Blackwater and the river Brain. The catchment descriptors for the 4 gauged sites in the catchment and 1 ungauged sites are shown in Appendix A.

6.5 Design Conditions for Hydraulic Models

The initial conditions for the hydraulic model were determined by running a small rainfall event though the model and hot starting the design event simulations from this result file. The rainfall event which was used represented a relatively small event producing a small flow through the model. This flow was significantly lower than that required to obtain out of bank/bank full flow. The purpose of this activity was to fill up the river system to a point where all the dead storage in the channels were full (i.e. water levels were at sill level at all weirs and culverts and water level behind dynamic structures were up to their prescribed retention levels).

6.6 Description of Design Runs

As part of this study, design events were run for the following return periods; 10yrs; 25yrs; 50yrs; 100yrs; 200yrs; 1000yrs and 100yrs +20% (accounting for climate change). The peak flows and levels for each of the Gauging Stations and the river Brain at Braintree for each of the design events are shown in Table A-1 and Table A-2.

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Return Period 10 25 50 100 200 1000 Appleford 18.54 22.74 26.35 30.36 34.59 45.53 Copford 17.42 21.92 24.41 27.65 31.61 41.66 Guithavon 6.57 8.37 9.71 12.49 14.20 22.76 Stisted 25.12 29.98 35.36 40.13 45.37 54.33 Braintree on Brain 3.18 4.05 5.00 5.57 6.42 8.58 Table A-1 Peak Flows for each of the Design Events

Return Period 10 25 50 100 200 1000 Appleford 16.22 16.37 16.46 16.53 16.58 16.68 Copford 59.24 59.42 59.59 59.80 60.04 60.57 Guithavon 16.93 17.03 17.09 17.19 17.26 17.52 Stisted 33.24 33.30 33.38 33.47 33.56 33.72 Braintree on Brain 47.62 47.76 47.92 48.13 48.50 49.64 Table A-2 Peak Levels for each of the Design Events

6.7 Conclusions

♦ The derivation of the flood frequency curves were carried out in accordance with the preferred methodologies of FEH.

♦ The Flood Frequency Curves provide an accurate representation of the catchment by using different approaches for each location.

♦ The critical duration analysis showed that the critical duration was the same as the theoretical duration.

♦ The critical duration uses the summer profile from FEH as this produced more conservative values although the winter and summer values were very similar.

♦ The initial conditions were derived by taking an average value from the rainfall runoff modelling results. This methodology provides the best representation of the initial conditions in the catchment.

♦ The peak flow and levels for each of the Design events run suggest that the volume range of flooding is very little, this corresponds with the flood extents which show very little change in the flood extents, as the return period changes. This also shows the nature of the steep sided narrow catchment.

♦ Confidence in the extents of the design events has been proved through the assessment of verification.

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7 Flood Extent Mapping

7.1 Introduction

This section outlines the approach taken to produce the flood maps for this study.

Two sets of maps have been produced for this study; the first set is

♦ Level 1 maps for a Section 105 study (100 year flood outline) and has been constructed following the National Section 105 Specification (Ref: 11);

♦ the second set of maps presents the flood extents for the 100 year extent and four other flood extents modelled (25 year, 50 year, 100 year + climate change, 1000 year).

Figure 7-1 provides a flow chart presenting the general approach adopted to construct the flood extent maps. Project specific details are given in the remainder of this section.

Manipulation of model results DEM creation and analysis Review of Flood Extent

1 Identify sections of the model that will provide accurate flood levels

2 Extract maximum flood level for each cross section from model 6 Import raw topographic results file data into GIS package (eg Lidar, photo- grammetry, topo survey) 3 Georeference each cross section, creating an x, y, z matrix of flood 10 Plot the flooded area levels 7 Interpolate topo data onto OS background using triangulated maps and LiDAR interpolation method to contours and undertake 4 Import x, y, z information create DEM common sense check into GIS package to provide the appropriate flood level along the 8 Review DEM to remove entire length of each anomalies 11 Correct flooded area cross section based on common sense checks

59Interpolate the x, y, z Subtract the DEM matrix of flood levels surface from the 12 Merge flood envelope to using linear triangular interpolated flood existing flood envelopes interpolation method in surface. All positive up and downstream of GIS package to create results represent a the study reach flood surface flooded area

Figure 7-1 Flow chart describing Flood Mapping methodology

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7.2 Mapping Approach

7.2.1 Flood mapping within the study reach

As described in Section 6 for any given return period, design events have been run through model for a range of storms; the duration of each storm representing the critical duration storm for different sections of the catchment. Five different duration events have been simulated for each return period providing the peak flood levels along five reaches as shown in Figure 7-2.

Figure 7-2 Flood extent reach map

To develop a single flood extent for the catchment (for each respective return period) the five flood extents were cropped at the appropriate location and merged together.

7.2.2 Flood mapping outside the study reach

Reaches in the catchment that are outside the study reach have not been mapped with the exception of the 100 year extent; the Agency’s existing 100 year indicative flood extent has been merged onto the 100 year flood extent produced from this study.

7.2.3 Population of floodplain databases

The results of the HD modelling were used to create a flood level database. Flood levels from the 10yr, 20yr, 50yr, 100yr, 100yr+20%, 200 and the 1000yr modelled events were entered into the database by geo-referenced model node. This database is in MapInfo, ArcView and Excel format.

Note that care should be taken when using this information. In particular the model schematic should be referenced to ensure that the most appropriate model node has been selected.

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7.3 Flood Extent Maps

A full set of the flood extent maps is given in Appendix F: and Appendix G: . As stated above two sets of maps are included, the first showing the 100 year flood extent, the second showing all the flood extents modelled. Each set of maps consist of 13 maps.

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Braintree District Council D111532 Strategic Flood Risk Assessment

Braintree Figures Appendix

Strategic FRA July 2008