Taunton Deane Borough Council Strategic Flood Risk Assessment

FINAL

May 2011

Taunton Deane Borough Council The Deane House Belvedere Rd Taunton TA1 1HE

JBA Office

Aberdeen House South Road HAYWARDS HEATH West Sussex RH19 1NG JBA Project Manager

Alastair Dale Revision History

Revision Ref / Date Issued Amendments Issued to Ralph Willoughby-Foster TDBC Planning First Draft 22/11/2010 N/A Nigel Smith Significant changes to Ralph Willoughby-Foster all sections in light of TDBC Planning Second Draft 08/02/2011 Environment Agency Nigel Smith Comments Environment Agency Consideration of Norton Fitzwarren Ralph Willoughby-Foster Dam, inclusion of TDBC Planning Final Report 31/05/2011 Flood Map for Surface Nigel Smith Water and minor Environment Agency corrections to text Contract

This is a study commissioned by Taunton Deane Borough Council relating to prepare a Strategic Flood Risk Assessment. This document has been prepared in response to a commission awarded by Taunton Deane borough Council in August 2010.

Prepared by ...... Oliver Francis MEng MSc Reviewed by ...... Alastair Dale BSc PGDip MIAHR Purpose

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

Copyright

© Jeremy Benn Associates Limited 2011 Carbon Footprint

577g

A printed copy of the main text in this document will result in a carbon footprint of 454g if 100% post-consumer recycled paper is used and 577g if primary-source paper is used. These figures assume the report is printed in black and white on A4 paper and in duplex. JBA is a carbon neutral company and the carbon emissions from our activities are offset.

Contents

1. Introduction ...... 2 1.1 About this report ...... 2 1.2 SFRA objectives ...... 3 1.3 How to find what you need in the SFRA ...... 3 1.4 Scope of assessment ...... 4 1.5 Approach ...... 7 1.6 Consultation ...... 9 1.7 Legislation and Regulation ...... 9 2. Understanding flood risk in the Borough ...... 10 2.1 Why flood risk is an issue ...... 10 2.2 How flood risk is assessed ...... 19 2.3 Understanding flooding in the Borough ...... 23 2.4 Possible responses to flooding ...... 29 2.5 Policy considerations ...... 30 3. Mapping and risk based approach ...... 33 3.1 Summary of mapping for all sources of flood risk ...... 33 3.2 Other relevant flood risk information ...... 34 3.3 Sequential approach ...... 35 3.4 Sequential test ...... 35 3.5 Exception test ...... 38 4. Overview of future development ...... 39 4.1 Overview and vision for new development ...... 39 4.2 Extent and type of development ...... 39 4.3 Timing of development ...... 41 5. Strategic assessment of future development ...... 42 5.1 Summary of flood risk issues ...... 42 5.2 Development over next 5 years ...... 42 5.3 Longer term development up to 2028 ...... 42 6. Strategic responses ...... 44 6.1 Introduction ...... 44 6.2 Review and assessment of options ...... 44 6.3 Description of strategic options considered ...... 48 6.4 Requirements for Development ...... 51 6.5 Generic guidance on applicable SUDs techniques ...... 53 7. Summary assessment of development sites ...... 56 7.1 Introduction ...... 56 7.2 Summary tables and FRA requirements ...... 56 8. Strategic management plan ...... 57 8.1 Reasons why a plan is needed ...... 57 8.2 Key issues ...... 57 8.3 The Plan ...... 58 Appendices...... I A. Update of historical flood database ...... II B. Assessment of Norton Fitzwarren Dam ...... III

C. Section 7 Maps ...... IV D. Summary of Modelling Work ...... XXXIX E. Flood Defences within the borough ...... XLII F. Surface Runoff Calculations ...... XLIV G. Procedure for Flood Zone Challenges ...... XLV H. Mapping ...... XLVII

1. Introduction

1.1 About this report This version of the Taunton Deane Borough Council SFRA replaces the previous document "Taunton Deane Strategic Flood Risk assessment Main Report - Final - September 2007". The primary objective for updating the previous version of the SFRA was to prepare a document that was compliant with the latest guidance described in the Planning Policy Statement 25 (PPS25) Practice Guide1. The key issues being: The information on sequential testing was out of date; The flood modelling needed to be updated to reflect recent changes; and The flood outlines needed to be updated to reflect the latest master planning proposals. The report contains information on flood zones and an assessment of risks from all sources of flooding and also provides more detailed information on the nature of flood hazards that exist in areas that do flood. In addition the strategic responses that should be considered to address the effect of proposed development allocations are described to address conditions as they are now and as they will be in the future. The study area is outlined in Figure 1-1. Figure 1-1: Assessment Area and extent of river hazard mapping

1 Planning Policy Statement 25: Development & Flood Risk Practice Guide (Communities and Local Government, December 2009)

TDBC SFRA v3.2 (FINAL).docx 2

1.2 SFRA objectives SFRAs should be a key part of the evidence base to help inform the allocation of development in a local plan area through the preparation of Local Development Documents. The primary objective of the SFRA is that is should form part of the evidence base of the Local Development Framework to inform Core Strategy allocations and ensure that they are in accordance with PPS252. In order to achieve this, the Practice Guide1 states that SFRAs need to provide sufficient detail on all types of flood risk to enable the Local Planning Authority (LPA): to apply the Sequential and, where necessary, Exception Tests in determining land use allocations; Fully understand flood risk from all sources within its area and also the risks to and from surrounding areas in the same catchment; Inform the Sustainability Appraisal so that flood risk is fully taken account of when considering options and in the preparation of LPA land use policies; Prepare appropriate policies for the management of flood risk within LDDs; Identify the level of detail required for site-specific flood risk assessments in particular locations; Determine the acceptability of flood risk in relation to emergency planning capability; To meet these objectives it will also be a requirement for those preparing information for assessment and testing of flood risk to understand the assessment process and the specific characteristics of the flooding that affects the Borough. The SFRA should also: Identify strategic measures required to address the effects of proposed development; and Influence and provide evidence that assists when making decisions on windfall planning applications. Thus the report provides the reader with an understanding of flood risk and how risks can be managed in the future.

1.3 How to find what you need in the SFRA Use Table 1-1 to find the information you need. Table 1-1: SFRA Report layout Section Description of contents (this section) - defines objectives, describes the background of the study area, outlines the 1. Introduction approach adopted and the consultation performed Gives a general introduction to the assessment of flood risk and describes the general characteristics of the flooding affecting the 2. Understanding flood risk in the Borough assessment area. It also summarises the responses that can be made to flood risk together with policy and institutional issues that should be considered Contains a summary of the results of the assessment and describes mapping that 3. Mapping and risk based approach should be used for Sequential and Exception testing

2 Planning Policy Statement 25: Development and Flood Risk (Communities and Local Government, March 2010)

TDBC SFRA v3.2 (FINAL).docx 3

Summarises the development proposals for 4. Overview of future development different time frames throughout the plan period Summarises the influential flood risk issues associated with future development and 5. Strategic assessment of future development describes how these might affect flood risk both in the next five years and over a longer time frame Describes the responses required so that flood 6. Strategic responses risk is not increased Tabulated information is given on specific 7. Summary assessment of development sites requirements for respective locations across the Borough 8. Strategic Flood Management Plan The objectives of the plan and the key issues.

1.4 Scope of assessment 1.4.1 Hierarchy The over arching aim of planning policy on development and flood risk is to ensure that flood risk is taken into account at all stages of the planning process. The Secretary of State has announced his intention to abolish Regional Spatial Strategies. The role of Regional Flood Risk Appraisals will also be reduced in future. The new landscape for the assessment of flood risk is now as shown in Figure 1-2 (Figure 2.2 in the Practice Guide1 now being modified in response to the changes in the planning process)

TDBC SFRA v3.2 (FINAL).docx 4

Figure 1-2: Key documents and strategic planning links - Flood Risk - (©JBA) EU EC "Floods" Directive

National Flood & Water Management Act Flood Risk Regulations Planning Acts Statutory National Strategy for National Flood Risk and Coastal Erosion Planning Risk Management Policy County / Catchment Catchment Flood Preliminary Flood Management Plan Risk Assessment (PFRA) & significant Shoreline * Can be flood risk areas Management Plan harmonised with Flood & Water Flood Risk and Management Flood Hazard Act Statutory Local Flood Risk Mapping Requirements Management Strategy (Local) Flood Risk Management Plan*

Surface Water Local Management Plan* **Also influenced by requirements of the River Basin Strategic Management Local Development Framework (Plan) Flood Risk Plan Including: Assessment Urban Extensions SPD; Infrastructure Delivery Plan; Green Infrastructure Plan; Emergency planning; and Water Cycle Sustainability, climate change & environment Strategy**

Site Planning Applications Flood Risk Assessments

Planning Decisions

Legend: Responsibilities are indicated using colour coding as follows: European National Local Planning Environment Agency / Developer Union Government Authority LLFA/ Maritime Local Authorities

Figure 1-2 shows that The Flood Risk Regulations 20093 and The Flood and Water Management Act 2010 (Commencement No. 3 and Transitional Provisions) Order 20114 introduce a wider requirement for the exchange of information and the preparation of strategies and management plans than existed previously. The SFRA contains information

3 Statutory Instruments, 2009 No. 3042, ENVIRONMENTAL PROTECTION -The Flood Risk Regulations 2009 4 Statutory Instruments, 2011 No. 694 (C. 25), Local Government, And Wales Coast Protection, England And Wales Environmental Protection, England And Wales Flood Risk Management, England And Wales Water Industry, England And Wales - The Flood and Water Management Act 2010 (Commencement No. 3 and Transitional Provisions) Order 2011

TDBC SFRA v3.2 (FINAL).docx 5

that should be referred to in responding to the Flood Risk Regulations and the formulation of local flood risk management strategies and plans. As previously it is also linked to the preparation of Catchment Flood Management Plans, Shoreline Management Plans and Surface Water Management Plans and Water Cycle Strategies. It should be recognised that there is also a requirement for decisions to be based on sustainability appraisals and the information in the SFRA should be used to inform this process at local level. In Taunton Deane the Local Development Documents include Supplementary Planning Documents (SPDs) that describe the design requirements for strategic Urban Extensions in Taunton. It is the intention to adopt the Core Strategy in July 2012 and to adopt the Urban Extensions SPD in 2013 The Urban Extension SPD includes requirements for physical infrastructure, including surface water attenuation and Sustainable Drainage Systems (SUDS) for the SPD strategic sites. Those preparing planning applications for development on land within the Strategic Sites shall also have regard to the physical infrastructure strategic provisions described in the SFRA.

1.4.2 Responsibilities The new and emerging responsibilities under the Flood and Water Management Act and the Flood Risk Regulations are summarised in Table 1-2. Table 1-2: Roles and responsibilities Risk Management Strategic Level Operational Level Authority (RMA) Environment Agency National Statutory Strategy Main rivers Reporting and general Sea supervision Reservoirs

and for these flood sources prepare and publish: PFRA; Significant Flood Risk Areas; Flood Risk and Hazard Maps; and Flood Risk Management Plan (or exercise "Exception") Lead Local Flood Input to National Statutory Surface Water Authority Strategy Groundwater and other sources of Formulate and implement flooding local flood risk management strategy And for these sources prepare and publish: PFRA; Significant Flood Risk Areas; Flood Risk and Hazard Maps; and Flood Risk Management Plan (or exercise "Exception") District Councils Input to National and Ordinary watercourse and Sea (with Internal Drainage Board Local Statutory Strategies EA approval)

Thus those making use of flood risk information described in the Taunton Deane Borough Council SFRA should also make reference to and be aware of: The Catchment Flood Management Plan (CFMP); The National Statutory Strategy for Flood and Coastal Erosion Risk [as laid before Parliament in May 2011]; The Local Statutory Flood Risk Management Strategy [to be published by (SCC) in 2011]; The Surface Water Management Plan prepared by SCC [due to be issued in 2011];

TDBC SFRA v3.2 (FINAL).docx 6

The PFRA prepared by SCC [by Dec 2011]; The Water Cycle Strategy [due to be prepared by TDBC in 2011/2012]; Parrett Estuary Flood Risk Management Strategy (Environment Agency, Feb 2009); and Lower Parrett and Tone Options Report (Environment Agency, 2004). A number of the above documents have not been completed at the time of preparation of this SFRA. Thus it will be imperative that those using this SFRA in the future also make reference to the other relevant documents. The key issues from the River Parrett CFMP are summarised in Section 2 of this SFRA whilst the Lower Parrett and Tone Options Report is cited in Section 6. Following the introduction of the Flood and Water Management Act and the Flood Risk Regulations the responsibility for the formulation of the Surface Water Management Plan (SWMP) now lies with Somerset County Council. When preparing the SWMP SCC should use the information in the SFRA to assist with the understanding of flood risk, the identification of Critical Drainage Areas and ensure that specific flood risk management measures, identified in the SFRA, are included in the SWMP. The SFRA will also be used to inform TDBC's Urban Extensions SPD. Requirements for on- site surface water attenuation schemes will be included in the SPD.

1.5 Approach 1.5.1 General assessment of flood risk The SFRA adopts the flood risk management hierarchy advocated in the Practice Guide1 as summarised in Figure 1-3. Figure 1-3: Flood risk management hierarchy

Step 1 Step 2 Step 3 Step 4 Step 5

Assess Avoid Substitute Control Mitigate

Appropriate Apply the Apply the e.g. SUDS, e.g. Flood Flood Risk Sequential Sequential design, resilient Assessment approach Test at site flood construction level defences

This hierarchy underpins the risk based approach and must be the basis for making all decisions involving development and flood risk. When using the hierarchy account shall be taken of: The nature of the flood risk (the source of the flooding); The spatial distribution of the flood risk (the pathways and areas affected by flooding); Climate change impacts; and The degree of vulnerability of different types of development (the receptors). Site allocations should reflect the application of the Sequential Test using the maps and guidance in this SFRA. The information in this SFRA should be used as evidence and where necessary reference should also be made to relevant evidence in the documents described in Section 1.4 of this chapter. The flood zone maps and flood risk information on other sources of flooding contained in this SFRA should be used to apply the Sequential Test. Where other sustainability criteria outweigh flood risk issues, the decision making process should be transparent. Through their Core Strategy TDBC does not propose to allocate any major allocations in high flood risk areas (Flood Zone 3). Where sites are being considered in high flood risk areas, due to over-arching socio-economic reasons, information from this

TDBC SFRA v3.2 (FINAL).docx 7

SFRA should be used to justify these decisions. To that end this report contains detailed information on the level of flood hazard in the centre of Taunton. The basis for all decision making on flood risk is to first understand the risk and then identify responses to that risk so that it is effectively managed. The SFRA provides detailed information that must be supplemented where necessary with more detailed information contained in the other relevant documents described in this chapter. 1.5.2 Technical assessment of flood hazards The technical assessment of risk has been performed by using and enhancing computer models under the direction of the Environment Agency. In particular, to prepare this version of the SFRA: A more detailed, linked 1Dimensional - 2Dimensional (1D-2D) model has been developed to improve the understanding of flood risk along the River Tone and in particular in the centre of Taunton. This model has been used to: o Improve the understanding of the influence of flood water levels at the confluence between the River Tone and the River Parrett; o Increase the resolution of the estimated extent of the Flood Zones; o Improve the understanding of the influence of the storage capacity within the "Moors" to the east of Taunton; o Improve the understanding of the flood mechanisms, the flood velocities, the flood depths in the centre of Taunton (using the 2D model output); o To examine the feasibility of strategic measures to address flood risk. More detailed models (using JFlow+) have been developed for the tributaries affected by future development; The hydrological assessment of the catchment has been reviewed; and Mapping showing the Areas Susceptible to Surface Water Flooding (AStSWF) and the Flood Map for Surface Water (FMfSW) have been used to identify locations where there is a surface water flood risk. The AStSWF mapping was prepared for the Environment agency in January 2009 and the Flood Map for Surface Water, a 2nd generation flood map, was made available in December 2010. It should be noted that these mapping products are 'different' and it is incorrect to regard the later mapping as an update of the original product. Since the completion of the previous version of the SFRA a number of development proposals have been brought forward in the area. Where possible the results of the assessments prepared for these proposals have been incorporated into the technical assessment that supports this version of the SFRA. The proposals brought forward in the study area since the last version of the SFRA was issued are: Long Run Farm Flood Storage Scheme; Third Way Crossing of the River Tone; and Norton Fitzwarren Flood Alleviation Dam on the Halsewater. Information on the Long Run Farm Flood Storage Scheme has been taken from modelling undertaken for the Long Run Farm Flood Risk Assessment and incorporated into the updated modelling of the River Tone. Information on the changes to levels on the left bank of the River Tone due to construction of the Third Way Crossing were provided by Somerset County Council and have been included in the modelling. The bridge itself has not been modelled at this time as there are a number of other changes on the right bank in Tangier that are ongoing. It is anticipated that these changes will be modelled at a future date and checks should be made to determine whether this revised output is available. A detailed representation of the Norton Fitzwarren Dam has not been included in the modelling for this study, although an assessment on its effects on flood levels in Taunton has been made using

TDBC SFRA v3.2 (FINAL).docx 8

the model prepared to support the Flood Risk Assessment5. This can be found in Appendix B. 1.5.3 Scope of assessment This version of the SFRA contains flood risk information that satisfies the requirements of a Level 1 and Level 2 SFRA. The Practice Guide1 advises that: "A Level 1 SFRA should be sufficiently detailed to allow application of the Sequential Test (annex D table D.1 of PPS25) and to identify whether development can be allocated outside high and medium flood risk areas, based on all sources of flooding, not just river and coastal, or whether application of the Exception Test is necessary. The information may also be used to assess how any environmental objectives relating to flooding, as defined in the Sustainability Appraisal, may be affected by additional development. A Level 1 SFRA may principally be a desk-based study making use of existing information."; and that "The Level 2 SFRA corresponds to the ‘increased scope’ SFRA referred to in paragraph E6 of PPS25. The principal purpose of a Level 2 SFRA is to facilitate application of the Sequential and Exception Tests. More detailed information is required where there is deemed to be development pressure in areas that are at medium or high flood risk and there are no other suitable alternative areas for development after applying the Sequential Test. This more detailed study should consider the detailed nature of the flood hazard, taking account of the presence of flood risk management measures such as flood defences. This will allow a sequential approach to site allocation to be adopted within a flood zone (paragraphs 17 and D4 of PPS25). It will also allow the policies and practices required to ensure that development in such areas satisfies the requirements of the Exception Test, to be identified for insertion into the LDD." It should be noted that the detailed information for Level 2 has been prepared only for the main part of Taunton's urban area. Information on the level of flood hazard has been prepared only for the area of Taunton from Bishops Hull along the river Tone to the M5 as shown in Figure 1-1.

1.6 Consultation The following parties (external to TDBC) have been consulted during the preparation of this version of the SFRA: The Environment Agency; Somerset County Council; British Waterways; Natural England; Parrett IDB Wessex Water

1.7 Legislation and Regulation This version of the SFRA was prepared during a time when considerable changes were being introduced as a consequence of the Flood and Water Management Act 2010 and the Flood Risk Regulations 2009. It has not been possible to capture or describe all the implications of the changes resulting from the new legislation and regulation and so users of this SFRA must check the current status when using this document.

5 Norton Fitzwarren Dam - Residual Flood Risk Assessment, Hyder Consulting, December 2006

TDBC SFRA v3.2 (FINAL).docx 9

2. Understanding flood risk in the Borough

2.1 Why flood risk is an issue 2.1.1 Historic Flooding The existing database of flooding events from all sources of flood risk, previously compiled by TDBC, has been updated for this version of the SFRA. New information was requested from all the relevant Risk Management Authorities as described in Appendix A. The historic flooding is most relevant to understanding flood risk and river flooding is the most serious issue. Notable river flood events on the River Tone have included: 27th October 1960 - Resulted in severe flooding and lead to the re-alignment of the river and subsequent installation of flood defences; 11th July 1968 - Large summer storm resulted in flooding of areas in Norton Fitzwarren, Ham, and ; 27th January 1984; 20th January 1999; 30th October 2000; and 8th December 2000 The six events listed above are amongst the most severe events recorded in the borough, there have been a number of smaller more recent events (Dec 2004, Jan 2008, and Feb 2009). Of those events 10th February 2009 was the most significant event being driven somewhat by snowmelt. Photographs from the 1960 and 2000 events have been provided by the Environment Agency and are presented in Figure 2-1 below.

Figure 2-1: Photographs of historic flooding in Taunton Flooding in Taunton - 27th October 1960 Location:

Railway Bridge on Station Road

TDBC SFRA v3.2 (FINAL).docx 10

Location:

Junction of Station Road and Bridge Street

Flooding in Taunton - 30th October 2000 Location:

The Bridge

TDBC SFRA v3.2 (FINAL).docx 11

Location:

French Weir Recreation Ground

2.1.2 Topography, hydrology, geology, and soils The boundary of TDBC encompasses an area of 462 square kilometres. The borough area can be split into two main geographic regions: The "uplands" comprise the headwaters of all the watercourses draining into Taunton and comprise the majority of the borough. They include parts of the in the northwest, the to the north and the to the south. The terrain is typified by rolling countryside, with some steep escarpments particularly around the Blackdown Hills. The highest point within the borough is Wills Neck (384m) in the Quantock Hills, although outside of the borough the Brendon Hills Rise to over 400m. The terrain defines the drainage in the area, which is typically characterised by meandering, mostly natural (i.e. unmodified) watercourses which are largely confined to the floodplain. The topography of the uplands has a significant influence on the nature of flooding within the catchment. The "lowlands" form part of the and Moors. They comprise the substantial portion of land to the east of the borough. They are characterised by the low lying, flat topography, much of which extends down to an elevation that is only just above sea level. The area is intersected by a network of man-made drainage channels called „rhynes‟, which being flat can flow in either direction. The tidal limit extends over 30km from the coast up to Newbridge Sluice on the River Tone. The tidal influence, along with the low lying character of this area, has a significant influence on the nature of flooding and the flow and water level in watercourses. Figure 2-2 shows the topography and principal morphological features. It illustrates that Taunton lies in a "bowl" surrounded by higher ground and that Wellington is situated at a higher elevation (approx 60m compared to 30m for Taunton) and thus is less vulnerable to flooding from the Somerset Levels and Moors. The topography of TDBC reflects both the geological history of the area, and the long history of human intervention and modification of the natural drainage systems in the region.

TDBC SFRA v3.2 (FINAL).docx 12

Figure 2-2: Topographical and hydrological features (Contains Ordnance Survey data © Crown copyright and database right 2010)

The River Tone is the principal hydrological feature within the borough and collects and conveys the majority of runoff generated. The River Tone rises at Beverton Pond situated outside of the borough in the Brendon Hills just within the boundary of National Park. The source of the Tone is at an elevation of 370m AOD but is very steep falling approximately 150m in just 3km (a gradient of 1 in 20) as it enters Reservoir. The reservoir is managed by Wessex Water and impounds the water of 5 other streams and has a capacity of 5 million cubic metres. The river continues as the main outfall from the dam at Clatworthy and continues at a shallower but still steep gradient of approximately 1 in 100 flowing in a southerly direction to Bridge where it reaches an elevation of 100m AOD. Here the river begins to flatten out and starts meandering, passing Greenham before turning North then East. The river then passes Wellington and Bradford-on-Tone before crossing Roughmoor into Taunton. The river drops over French Weir passing through Tangier on its way through the town centre. There is another weir at Firepool where the Taunton- Canal leaves the river whilst the Tone continues flowing in an easterly direction under the M5 into the Somerset Moors and Levels. By this stage the rivers gradient has become very shallow (less than 1 in 1000). The river turns to the northeast and before passing through the sluice gate at Newbridge which is the limit of the tidal influence. The river passes alongside Currymoor which acts as a washland receiving floodwaters from the Tone. The Tone then meets the River Parrett at having flowed 51km from its source. The Parrett subsequently conveys flows through Bridgwater to the coast at Bridgwater Bay. More details on the hydrology of the Parrett can be found in the River Parrett CFMP6. There are a small number of watercourses within the TDBC area that do not drain into the River Tone. To the east of the borough, Old Rhyne drains directly to the River Parrett. To the south of the borough, with sources in the Blackdown Hills: the River Culm

6 The River Parrett Catchment Flood Management Plan, Environment Agency 2008

TDBC SFRA v3.2 (FINAL).docx 13

drains to the catchment of the ; the River Otter drains to the South Devon coast; and the is a tributary of the which drains to the Parrett Catchment. The primary focus of this SFRA is the urban area of Taunton. There are a number of tributaries of the Tone that have an influence on flooding within Taunton and its surrounding areas. Figure 2-3 illustrates how the centre of Taunton is affected by flooding from these watercourses. The most important tributaries in terms of the quantity of water they convey are: Hillfarrance Brook; Halsewater; and Back Stream. There are a number of smaller watercourses which also affect flood risk in the centre of Taunton, these are the: Black Brook; The Stockwell Stream; Sherford Stream Galmington stream; The Mill Stream; Norton Brook Mill Lease Stream; Kingston Stream; Priorswood Stream; Maiden Brook; Allen's Brook; and Dyers Brook. The Priorswood Stream, Maiden Brook, Allen's Brook, and Dyers Brook all drain to a larger collector watercourse (a continuation of the Kingston Stream)) via three separate culverts that flow beneath the Taunton to Bridgwater Canal and then discharge to the River Tone. There are a number of other watercourses which drain to the Tone downstream of Taunton but these are considered to pose a lesser risk to flooding in the town centre. A map of the catchments used in constructing the hydraulic model for this SFRA is shown in Figure 2-4. The boundaries shown were obtained from version 3 of the FEH CD-ROM and occasionally do not match the actual drainage area of the watercourses. The catchment divides between the Back Stream, Mill Lease Stream, Kingston Stream, and Maiden Brook are poorly defined to the north of Taunton. In the absence of more conclusive information values from the FEH CD-ROM have been taken as correct. The Black Brook catchment required manual amendment as the FEH CD-ROM apportioned a significant area to the Stockwell Stream. Table 2.1 provides a description of the watercourse for the respective codes described in Figure 2-3 and gives the statistical estimate for peak flows of an event with a 1 in 100 year probability. It should be noted that the peak flow estimates for the respective watercourses cannot be simply summated to produce estimates at additional locations. For example combining catchments T1, HFB1, T3, OS1, and INT1 is equivalent to the catchment area of the Bishops Hull gauging station but a simple arithmetic summing of the peak flows generated by each of the watercourses would give a significantly higher estimate of 158.1 m3/s than the statistical estimate at the gauge of 127.9 m3/s. This is because a storm resulting in a 1 in 100 year event on all the tributaries would produce a flood at Bishops Hull significantly rarer than a 1 in 100 year event. Appendix D describes more information on the estimation of flows along with a summary of the modelling work undertaken for this SFRA.

TDBC SFRA v3.2 (FINAL).docx 14

Figure 2-3: Watercourses affecting flood risk in the borough (Contains Ordnance Survey data © Crown copyright and database right 2010)

TDBC SFRA v3.2 (FINAL).docx 15

Figure 2-4 Catchments used in the modelling of the River Tone

Table 2-1: Summary of modelled catchments Name Watercourse Location Area 100 year (km2) statistical flow estimate (m3/s) T1 River Tone Upstream of confluence 127.66 87.0 with Hillfarrance Brook HFB1 Hillfarrance Brook Upstream of confluence 49.31 36.9 with River Tone T3 West Buckland Upstream of confluence 17.46 16.8 Tributary with River Tone OS1 Stream Upstream of confluence 3.88 3.2 with River Tone INT1 River Tone Intervening area upstream 5.34 4.2 of Bishops Hull Gauge HALS1 Halsewater Upstream of confluence 41.41 16.2 with Back Stream BS1 Back Stream Upstream of confluence 46.82 14.9 with Halsewater MS1 Mill Lease Stream Upstream of confluence 1.59 1.2 with River Tone GS1 Galmington Stream Upstream of confluence 7.54 8.2 with Mill Stream (River Tone) SS1 Sherford Stream Upstream of confluence 14.56 18.1 with Mill Stream (River Tone) INT2 River Tone Intervening area between 5.12 2.8 Bishops Hull and Taunton Market Gauges T7a Kingston Stream Upstream of confluence 3.36 2.3 (and Priorswood with Allens brook

TDBC SFRA v3.2 (FINAL).docx 16

Stream) T7 Maiden Brook Upstream of confluence 13.33 7.6 with Kingston Stream AB1 Allens Brook Upstream of confluence 2.40 1.8 with Kingston Stream DB1 Dyers Brook Upstream of confluence 2.90 1.5 with Kingston Stream SW1 Black Brook Upstream of confluence 6.78 5.7 with Broughton Beck BB1 Broughton Beck Upstream of confluence 14.90 13.8 with Black Brook BB2 Henlade Stream Upstream of confluence 5.14 4.1 with Black Brook TF1 Thornfalcon Stream Upstream of confluence 5.31 4.3 with River Tone T9 Ham Tributary Upstream of confluence 3.61 3.1 with River Tone T10 Creech St Michael Upstream of confluence 8.09 3.8 Tributary with River Tone T14 Creech Heathfield Upstream of confluence 2.08 0.9 Tributary with River Tone Currymoor n/a Upstream of Currymoor 9.42 3.7 Pumping station INT3 River Tone Intervening area between 8.56 2.7 Taunton Market Gauge and Knapp Bridge

The nature of the catchment is such that the steep watercourses in the upland portions of the catchment will respond rapidly to rainfall, especially those comprising bedrock channels. The configuration of the catchment means that floodwaters will reach the centre of Taunton at different times. The upper portion of the Tone is very fast responding but the presence of will provide significant attenuation of that response. The reservoir is very close to the source of the River Tone and the reaches immediately downstream remain steep and can produce significant runoff. The smaller contributing watercourses in the upper reaches of the Tone will respond quickly themselves but significant attenuation of the peak flows will be provided by the significant length of the river channel upstream of Taunton. In theory this means that floodwater peaks produced in the Halsewater, Back Stream and Hillfarrance catchments, which are also relatively steep and responsive to runoff, are likely to reach Taunton before the main peak flow in the Tone. In practice gauged level and flow data from a gauge on the Halsewater downstream of the confluence with the Back Stream shows that peaks on the Halsewater have occurred at the same time as those recorded at Bishops Hull on the Tone. It is thought that this could be due to the significant degree of interaction between the Tone and Halsewater floodplains. This would imply that floodwaters from the Tone that are out of channel could be responsible for the peaks observed on the Halsewater, rather that the watercourses having a similar response time. The construction of the Norton Fitzwarren Dam, which has been assessed in Appendix B, has altered the response of the Halsewater to larger events by providing significant attenuation and made the relative timings of peaks on the Back Stream, Halsewater, and Tone more complex. The Back Stream should now respond the quickest, whilst the flood peak on the Tone may now arrive at the confluence before the flood peak from the Halsewater. This is not the case for smaller events for which the dam provides minimal attenuation and the interaction between the floodplains will likely mean that peaks observed at the Halsewater gauging station will remain strongly influenced by the Tone. The relative sizes of the different catchments means that despite the peak on the Halsewater being slowed it has no influence on the timing of the combined peak that could be experienced in the centre of Taunton. The Northern tributaries of Taunton: The Mill Lease Stream, Kingston Stream, Priorswood Stream, Maiden Brook, Allen's Brook, and Dyers Brook will also respond rapidly as they have a considerable gradient. The Southern tributaries: Galmington Stream, Sherford Stream, Stockwell Stream, and Black Brook will also respond rapidly. They are equally steep in their

TDBC SFRA v3.2 (FINAL).docx 17

headwaters but do flatten considerably as they approach Taunton, with the Sherford Stream supporting a floodplain in Vivary Park. Many of these tributaries have catchments that contain a considerable urban area. Combining this with their relatively short lengths mean, that for events where rainfall is uniform across the catchment, their floodwaters would be expected to reach the centre of Taunton more rapidly than the other watercourses. The timing of flood peaks does not always remain constant as it is dependent on the physical properties of a rainfall event: magnitude, duration, spatial variability and direction of travel. It will also be affected by conditions experienced in the time period before flooding was experienced in each catchment. The typical response of the catchments described in the preceding paragraphs would be considered to be for an event where rainfall was generally uniform across the catchment. Taunton however is not always subject to a 'typical flood'. The presence of the levels and moors means that the town can be affected by very long duration events where flows in the Tone become locked either due to the tide, river flood levels in the Parrett, or both. In this scenario the timing of flood peaks can be very different as water levels in the Tone can hold back peaks from the tributaries. Wellington is most affected by another tributary of the River Tone also known as the Back Stream or Westford Stream (this is not the same watercourse that flows from the Quantock Hills to the north of Taunton). The CFMP6 states that the River Tone experiences the greatest fall in elevation from headwater to lowlands of all its neighbouring watercourses. The River Tone channel longitudinal profile also varies from convex in the upper catchment to a concave profile further downstream. The convex profile is characteristic of a Devonian bedrock dominated channel and reflects where the River Tone flows down through the Brendon Hills and is unable to incise the channel bed. Further downstream a more typical concave profile resumes, indicative of a lowland channel. The geology of the Tone catchment is characterised by Devonian, Permian, Triassic, Jurassic, Cretaceous, and Quaternary deposits. The oldest rocks are Devonian slates, dark red sandstones and shales which form the Brendon and Quantock Hills enclosing the vale of Taunton Deane to the west and north. Next are the Permian red sandstones and pebble beds of the high vale in the west, followed by the Triassic sandstones and mudstones of the lower vale in the centre around Taunton. To the south east of Taunton lie the Jurassic blue lias, limestones, and clays. The younger Cretaceous rocks to the south of Taunton comprise the upper greensand scarp slope of the Blackdown Hills which enclose the southern side of the vale. The most recent Quaternary deposits are the river valley alluvium and the peat of the Somerset levels and moors in the east. The relatively permeable upland geology includes mudstone, siltstone, sandstone, and basal sediment deposits. There are also a few isolated areas of Limestone deposits and a band of pebble beds. Geological maps can be referenced within the CFMP6 for more detail. In the river valleys and flood plains the solid geology is overlain by alluvium (clay, silt and sand) and river terrace drift deposits (sand and gravel). The runoff from these areas can be quite variable due to the nature of the associated soils. Generally the soft siltstone/ fine grained landscape characteristic of the upper Tone catchment is vulnerable to structural soil degradation and as such has the potential to increase flooding and cause pollution. The TDBC area does not include any major aquifers and therefore groundwater flooding is not considered to be a major risk. The soils map included within the CFMP6 shows that the majority of soils within TDBC are freely draining loamy or sandy soils. The sources of the Tone, Hillfarrance and Halsewater are all on Acid Heath or Moorland soils whilst an extensive area in the catchments of the southern tributaries, and to the southeast of Taunton, is dominated by cracking clay and seasonally waterlogged, impermeable soils. Areas with high groundwater levels can be found along the Tone from just upstream of Taunton to the confluence with the Parrett and in the Blackdown Hills.

TDBC SFRA v3.2 (FINAL).docx 18

A Geomorphological Assessment of the River Tone7 prepared by Black & Veatch for Project Taunton divided the Tone into three distinct regimes: The Upper Tone: Here the channel is typically confined by bedrock. Steeper gradients, incised channels and narrow natural floodplains give a rapid response to flood events and high potential for sediment transport and low potential for sediment deposition; The Middle Tone: The channel is less confined typically with vegetated banks and tree cover although through Taunton they are increasingly modified with bank protection, revetments and embankments. The gradient is moderate and the floodplains are wider with some modification and hard engineering in urban areas. This gives a reduced response to flood events but maintains a high potential for sediment transport. The potential for deposition of sediments remains low except in the modified reaches through Taunton; and The Lower Tone: The channel banks are vegetated and heavily managed; they are largely embanked towards the confluence with the Parrett. The gradient becomes very slack and the channel is heavily modified, the floodplain is managed by a significant number of rhynes creating artificial connections. There is little potential for sediment transport and significant deposition occurs in this reach.

2.2 How flood risk is assessed 2.2.1 Definitions A Flood is now formally defined in the Flood and Water Management Act as including cases where land not normally covered by water becomes covered by water and can be the result of water emanating from a number of sources. PPS 25 states that flood risk is the combination of the statistical probability of a flood occurring and the scale of its potential consequences, whether inland or on the coast and includes consideration of development located outside of the river and tidal flood risk areas. Thus it is possible to define flood risk as:

Flood Risk = (Probability of a flood) ● (scale of the consequences) On that basis it is useful to express the definition as follows:

Flood = Probability X Consequences Risk Flood Hazard X Receptor X Receptor Magnitude presence Vulnerability

Using this definition it can be seen that: Increasing the probability or chance of a flood increases the flood risk. Thus in situations where the probability of a flood being experienced is increasing gradually over time (say as a consequence of increased flood frequency due to climate change effects) then the magnitude of the risk will increase. Thus in locations affected by climate change effects the flood risk will increase if no action is taken; The scale of the consequences can increase the flood risk. The scale of the consequences can be increased by: o Flood Hazard Magnitude - If the direct hazard posed by the depth of flooding, the velocity of the flow, the speed of onset or the rate of rise in flood water is

7 River Tone Geomorphological Assessment DRAFT, Black & Veatch April 2010

TDBC SFRA v3.2 (FINAL).docx 19

increased then the consequences of flooding are increased [and so is the flood risk]; o Receptor presence - The consequences of a flood will be increased if there are more receptors affected. Thus the consequences can be increased if the extent and frequency of flooding affects more people, property or infrastructure. Additionally the consequences will be increased [and so is the flood risk] if there is new development that increases the probability of flooding (for instance by causing an increase in the volume of runoff from new paved areas) or increases the density of infrastructure in areas known to flood; and o Receptor vulnerability - The consequences will be increased if the vulnerability of the people, property or infrastructure is increased. For example old or very young people are more vulnerable if there is a flood and hence if they were present in greater numbers the consequences of a flood would be increased [and so is the flood risk]. The risk must be assessed for flooding from all main sources and these are flooding from: The sea; Main rivers; Reservoirs; Surface runoff from the land and surface water from drainage systems (caused by heavy rainfall); Ordinary watercourses; Groundwater; and Artificial sources. 2.2.2 Using SFRA risk information Figure 2-5 shows that the SFRA contains information that should be used strategically for planning operations and responses in advance of flooding. It also provides information on the effects of flood events (due to failure or overtopping of defences) that can be used to plan tactical operations in advance of a flood. The SFRA flood risk data should be updated following flood events.

Figure 2-5: How the information in the SFRA can be used throughout the flooding timeline

Assess Avoid or Control or Tactical response Post event risk reduce risk mitigate risk to flood event recovery support

Before a flood During a flood After a flood

The assessment of flood risk in the SFRA is primarily based on the following three types of information: 1. Flood Zones (Refer to Figure 2-7) The SFRA includes maps that show the flood zones. These zones describe the land that would flood if there were no defences present. PPS25 identifies the following Flood Zones and these are used in the Taunton Deane Borough SFRA: Zone 1 Low Probability

TDBC SFRA v3.2 (FINAL).docx 20

This zone comprises land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%). Zone 2 Medium Probability This zone comprises land assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (0.1% - 1%) or between 1 in 200 and 1 in 1000 annual probability of sea flooding (0.1% – 0.5%) in any year. Zone 3a High Probability This zone comprises land assessed as having a greater annual probability of river flooding (>1.0%) or a greater annual probability of flooding from the sea (>0.5%) in any year. Zone 3b The Functional Flood Plain This zone comprises land where water has to flow or be stored in times of flood. SFRAs should identify this Flood Zone (land which would flood with an annual probability of 1 in 20 (5%) or greater in any year or is designed to flood in an extreme (0.1%) flood, or at another probability to be agreed between the LPA and the Environment Agency, including water conveyance routes.

Zone 3b is a critical area to define. Table D1 of PPS252 has been included in Figure 2-6 below for additional information:

Figure 2-6: Definition of Flood Zone 3b (Table D1 of PPS25)

The preference when allocating land is whenever possible to place all new development on land in Zone 1. Since the Zones identify land that is not reliant on flood defences then placing development on Zone 1 land means that in future there is no commitment to spending

TDBC SFRA v3.2 (FINAL).docx 21

money on flood banks or flood alleviation measures and not committing future generations to costly long term expenditure that would become increasingly unsustainable as the effects of climate change increase. However, the runoff from development on Zone 1 land can potentially cause an increase in the probability of flooding to existing downstream development. Information in the SFRA should be used to address these issues. Figure 2-7: Definition of Flood Zones

2. Actual flood risk If it has not been possible for all future development to be situated in Zone 1 then a more detailed assessment is needed to understand the implications of locating proposed development in zones 2 or 3. This is accomplished by considering information on the “actual risk” of flooding. The assessment of actual risk takes account of the presence of flood defences and provides a picture of the safety of existing and proposed development. It should be understood that the standard of protection afforded by flood defences is not constant and it is presumed that the required minimum standards for new development are:

Residential development should be protected against and flooding with an annual probability of river flooding of 1% in any year; and Residential development should be protected against flooding with an annual probability of tidal (sea) flooding of 0.5% in any year.

The assessment of the actual risk should take the following issues into account:

The level of protection afforded by existing defences might be less than the appropriate standards and hence may need to be improved if further growth is contemplated; The flood risk management policy for the defences will provide information on the level of future commitment to maintain existing standards of protection. If there is a conflict between the proposed level of commitment and the future needs to support growth then it will be a priority for the Flood Risk Management strategy to be reviewed; The standard of safety must be maintained for the intended lifetime of the development (assumed to be 100 years for residential development). Over time the

TDBC SFRA v3.2 (FINAL).docx 22

effects of climate change will erode the present day standard of protection afforded by defences and so commitment is needed to invest in the maintenance and upgrade of defences if the present day levels of protection are to be maintained; and The assessment of actual risk can include consideration of the magnitude of the hazard posed by flooding. By understanding the depth, velocity, speed of onset, rate of rise, and duration of flooding it is possible to assess the level of hazard posed by flood events from the respective sources. This assessment will be needed in circumstances where consideration is given to the mitigation of the consequences of flooding or where it is proposed to place lower vulnerability development in areas that are at risk from inundation. Those using the Taunton Deane SFRA should refer to the Environment Agency's National Flood and Coastal Defence Dataset (NFCDD), or other relevant datasets describing assets and features, for details on the standard of protection of defences. The mapping in the SFRA also provides analyses that show the effect of defences at Taunton and details of the defences using the data available at the time. A map and table of defences present within the borough of Taunton Deane is provided in Appendix E. This data should always be checked with information available from the Environment Agency and the Lead Local Flood Authority (Somerset County Council).

3. Residual risk The residual risk refers to the risks that remain in circumstances where measures have been taken to alleviate flooding. It is important that these risks are quantified to confirm that the consequences can be safely managed. The residual risk can be: The effects of a flood with a magnitude greater than that for which the defences, or management measures have been designed to alleviate. This can result in over topping of flood banks, failure of flood gates to cope with the level of flow or failure of pumping systems to cope with the incoming discharges; or Failure of the defences or flood risk management measures to perform their intended duty. This could be breach failure of flood embankments, failure of flood gates to operate in the intended manner or failure of pumping stations.

The assessment of residual risk demands that attention be given to the vulnerability of the receptors and the response to managing the resultant flood emergency. In this instance attention should be paid to the characteristics of flood emergencies and the roles and responsibilities during such events.

2.3 Understanding flooding in the Borough 2.3.1 Introduction The following techniques have been used to assess the probability and magnitude of flooding: An inventory of historic flooding has been updated from that prepared for the previous issue of the SFRA. This information is provided in Appendix A; Sea flooding has been assessed by examining the effect of the tide levels on the water levels in the River Parrett and the lower reaches of the River Tone. In particular consideration has been given to the potential impact of tide levels and also the effect of sea level rise on the predicted river levels. A 1 Dimensional model (ISIS) has been used to examine these effects and the results from this model have been used to prepare the mapping issued with the SFRA. It should be noted that the analysis of the tide and sea level rise effects have been simplified, which is acceptable for the preparation of mapping outputs. However, when examining and developing strategic options involving the operation and management of "the Moors"

TDBC SFRA v3.2 (FINAL).docx 23

the model should be enhanced so that it can take account of the daily fluctuations in water levels; River flooding has been assessed by performing a hydrological assessment using the techniques described in the Flood Estimation Handbook (FEH) and the estimates previously prepared have also been reviewed. The FEH techniques have been used to prepare estimates of potential runoff volumes from proposed future development. The flows predicted using the FEH techniques have been used in the 1 Dimensional River model, but this analysis has been augmented with a combined 1Dimensional - 2Dimensional model (ISIS- TUFLOW) for the reach of the River Tone from Bishops Hull to the M5. The model results have informed the preparation of the flood zone information, predictions of the actual risk of flooding and the consequences of over topping of defences and breach failure of defences in Taunton. Flooding of the smaller tributaries has been prepared using a 2 Dimensional model (JFlow+); Surface water flooding has been assessed using both the 1st generation "Areas Susceptible to Surface water Flooding" (AStSWF) mapping issued by the Environment Agency to Local Authorities in January 2009 and the 2nd generation Flood Map for Surface Water (FMfSW) issued in December 2010. The FMfSW uses improved data, and a more realistic representation of conditions affecting flooding for particular conditions. However, it should be recognised that the results have some limitations that are linked to the generic assumptions made in the modelling. The main limitation is that the map uses model results that assume a national average drainage capacity for urban areas as the actual drainage capacity in each part of England and Wales is unknown. In places where drainage capacity is much less than the average the older AStSWF map may show a more appropriate flood extent. In places where the drainage capacity is significantly higher than the average then the FMfSW could overestimate surface water flood risk. The Environment Agency has stated that it is important that Lead Local Flood Authorities work with their partners to review, agree and record what surface water flood data best represents local conditions. This is known as locally agreed surface water information. At Taunton Deane the locally agreed surface water information is prepared by Somerset County Council (SCC). Users of this SFRA should check with SCC to establish whether the locally agreed information is available, as when published it will replace the AStSWF mapping and the FMfSW. Groundwater flooding has not been assessed using models and no detailed assessment has been prepared as the preliminary desk top review prepared in the previous issue of the SFRA stated that it was not considered to be a significant risk within this catchment. A brief investigation of British Geological Survey (BGS) records reveals that there are some minor aquifers within the borough. The majority of these are at a significant depth below ground. However, there may be a small number of locations within the borough where groundwater could pose a risk under exceptional circumstances; Flooding from reservoirs has not been addressed in detail this SFRA as it is not considered to be a significant risk with respect to proposed future development and the requirements of PPS 25. It should be noted that the Environment Agency published reservoir inundation mapping in 2009 showing the flood outlines from reservoir failure. Originally for reservoir owners and local authorities Reservoir Flood Mapping was made available to the public on the Environment Agency website in December 2010. Detailed maps showing the level of flood hazard are available via the National Resilience extranet to category 1 and category 2 responders (as defined by the Civil Contingencies act). The two notable reservoirs that should be considered, as appropriate, are the Clatworthy Reservoir and the "Norton Fitzwarren Dam". There are also several smaller reservoirs in the borough including the Westford, Luxhay, Blagdon, and Leigh reservoirs; Ordinary watercourse flooding has either been assessed using the detailed 1D model, the JFlow + model or can be inferred from the flood extents shown on the AStSWF and FMfSW mapping; and

TDBC SFRA v3.2 (FINAL).docx 24

Artificial sources of flooding affecting Taunton could emanate from the Taunton Bridgwater Canal, but this feature has been included in the linked 1D- 2D (ISIS- TUFLOW) model and has thus been assessed in conjunction with the river flooding estimates. 2.3.2 Description of principal flood areas and mechanisms Fluvial flooding from the River Tone is a complex process as there are a significant number of tributaries in the vicinity of Taunton (see 2.1.2). The complexity is further increased by the effects of the tide, levels in the River Parrett and storage volumes in the moors, all of which combine in the lower reaches of the Tone and can result in impedance of flows in the watercourse. For consideration of the flood mechanisms the Tone will be split into three zones: Upstream of Taunton The Tone upstream of Taunton is a sizeable catchment which is capable of generating high peak flows. In the upper reaches of the Tone the river is constrained within narrow valleys and areas of flood risk are the areas in the immediate vicinity of the river. Flooding occurs when the channel capacity is exceeded, velocities are likely to be high due to the gradients of the channel but the rapid response of the catchment means that flooding in these areas is short-lived. After the Tone turns to the northeast as it reaches Greenham the valley becomes less constrained and the gradient of the watercourse slackens. This causes the land alongside the river to become flatter and a more defined floodplain begins to develop. In these areas the extent of flooding is increased and the velocities of floodwater are likely to be lower. The primary mechanism of flooding remains insufficient capacity of the channel to convey the flood flows. This is an entirely natural process and the capacity of the channel will vary along the watercourse, although it is often considered that floodwaters will begin to enter the floodplain during an event with a 2 year return period or greater. Along these reaches the duration of inundation by floodwaters increases but there remain relatively few properties at risk. There are a number of properties at risk of flooding on the outskirts of Wellington although many are affected by the Westford stream. By this point the size of the Tone catchment is large enough that flooding on tributaries will be affected by water levels on the main river with the potential that tributaries can become influenced/locked when flows in the Tone are sufficiently high. An upstream storage reservoir located in the village of Westford is considered a defence as it offers some protection to properties downstream. After passing Tonedale sewage works the floodplains widen still further and the extent of flooding is quite significant. There is a very extensive floodplain upstream of Bradford-on- Tone where Haywards Water joins the Tone. In this area a flood bank offers some protection to the settlement of Tone Green as the Tone is constricted by Bradford Bridge. The floodplain is again extensive at the confluence of the Tone, the Hillfarrance Brook, and the watercourse draining West Buckland. In these areas water can remain on the floodplain for a significant period of time. The gradients along the watercourses are not steep and generally velocities are not high. Floodwaters can enter the floodplain where the natural channel does not have the capacity to convey the flood flows. This can occur on some or all of the watercourses at a confluence. This is because there is often connectivity between the watercourses either from historic channels or agricultural ditches and drainage. Flooding of the tributaries can be due to their locking from high water levels on the Tone. The Hillfarrance Brook is a major left bank tributary with a catchment area of 49km2. This is significant compared with the area of the Tone upstream of this tributary (128km2). The physical properties of the Hillfarrance suggest that it should be quicker to respond than the Tone. The Hillfarrance Brook has historically been the cause of considerable flood risk in the village of Hillfarrance and as such flood defences are present and flood diversion channel taking water away from the village centre has been constructed. Continuing downstream an extensive floodplain remains. A second major tributary the Norton Brook, which itself is formed from the Halsewater and Back Stream, joins the left bank.

TDBC SFRA v3.2 (FINAL).docx 25

These catchments have a combined area of 88km2 compared with 204km2 on the Tone upstream of Norton Brook. The physical properties of these catchments are also such that they should respond faster than the Tone itself making it unlikely for peaks on the tributaries to be co-incident with those on the Tone during a storm that effects the whole catchment in a similar way. Hydrographs from Bishops Hull gauging station on the Tone and the Halsewater gauging station for the 2000 event and other recent large events suggest that in practice the response is more complex. The telemetry data from both gauging stations show peaks occurring at the same time implying a similar response time. However there are significant connections between the two watercourses across the floodplain and floodwaters from the Tone can cross the floodplain and enter the Norton Brook possibly explaining the coincident peaks. The newly constructed Norton Fitzwarren Dam with a capacity of 700,000m3 will provide significant attenuation for larger flood events and will slow the response of the Halsewater. The Dam is controlled by a penstock which is set to allow the smaller events to pass without much attenuation. This will alter the flood regime in this area making it much more complex. Currently there are flood defences protecting localised areas of Norton Fitzwarren but generally the locality remains at risk of flooding both from the Norton Brook, Halsewater, and Back Streams. Additional flood alleviation works associated with the dam are currently under consideration at the time of preparation of this version of the SFRA. Beyond the confluence with the Norton Brook, there are two areas of open land before the urban area of Taunton flanks the Tone: Roughmoor to the North, and Long Run Farm to the South. Long Run Farm is now an operational flood storage scheme which was built to provide compensatory storage for land raising schemes forming part of the re-development of Taunton. Long Run Farm is designed to flood under fairly modest flood events and has an operational capacity of approximately 40,000m3. Taunton Urban Area The Tone encounters the urban area of Taunton at Frieze Hill on the edge of Roughmoor. Development has encroached on to the Tone floodplain and the watercourse heavily altered to meet the needs through the various stages of development at Taunton. The river is joined by a small heavily culverted left bank tributary, the Mill Lease Stream in this area. Passing Frieze Hill the river is controlled by French Weir, this structure was built to maintain water levels in the Mill Stream an artificial mill channel. The Mill Stream is joined by the Galmington Stream and Sherford Streams in the Tangier district. This area is currently being extensively re-developed including the opening of a new river crossing, the Third Way Bridge. The Mill Stream runs through Goodland Gardens rejoining the Tone upstream of The Bridge. Downstream, in the vicinity of the Cricket Ground an outfall for the culverted Stockwell Stream joins the Tone on it's right bank. The Tone continues through Taunton to Firepool where another weir across the river maintains water levels for the Taunton-Bridgwater Canal which branches off from the river's left bank. Downstream of Firepool the Tone has a two-stage channel which formed part of the flood alleviation works constructed in the 1960s. These modifications increase channel capacity and enable the river to convey water away from the town centre quickly. In this area the development becomes sparser and does not come as close to the river. Just upstream of the A38 crossing the Tone has a high level overflow which allows water to be channelled into the pond next to Heron Gate. Beyond this a considerable area of flood plain on the left bank re-emerges extending up to the railway line. The Canal follows a route slightly to the north of the Tone and in the vicinity of Priorswood Lane a watercourse (a continuation of the Kingston Stream) collects the flows from the Kingston and Priorswood Streams before being culverted beneath the canal and flowing adjacent to the canal's right bank. The Maiden Brook goes under the canal and joins this watercourse by the recycling centre. The watercourse follows the canal as far as Bathpool where it turns towards the Tone and is joined by Allen's Brook, which is culverted beneath the canal. In Bathpool the canal has a spillway incorporated into its right bank allowing excess water to enter the Kingston Stream/Allen's Brook. To the east, the Dyers Brook flows beneath the canal (siphon) by the swing bridge before joining the Kingston Stream/Allen's Brook just downstream of the canal spillway. The combined

TDBC SFRA v3.2 (FINAL).docx 26

watercourse continues alongside a housing estate, under the A38 into the south of Bathpool, then under the railway, before discharging into the Tone through a large double flap outfall. The Taunton urban area is the primary area for flood risk within the borough as there is considerable development along the waterfront. The town has a comprehensive flood defence scheme that was implemented in the 1960s following severe floods and subsequently the defences were raised further in the 1990s. The original flood protection works also involved channel modifications that increase channel conveyance through the town. Within the town the Tone is joined by a number of tributaries which can be split into the Northern and Southern tributaries (see Section 2.1.2). The Northern and Southern tributaries comprise a number of watercourses with catchments significantly smaller than that of the Tone. The largest two are: the Maiden Brook, a Northern tributary rising in the Quantock Hills; and the Sherford Stream, a Southern tributary rising in the Blackdown Hills. Both of these watercourses have steep gradients whilst the other tributaries have more moderate gradients. The Northern tributaries are generally more permeable than the Southern tributaries but the Southern tributaries have a more extensive floodplain as the land to the immediate south of Taunton is flatter than the corresponding land in the north. Both sets of tributaries have a very flashy response compared with the Tone due to their small size, notable gradients and significant urban fraction. The much flashier response means that for many rainfall events their flood peak will have passed long before that on the Tone but elevated water levels on the Tone can prevent them from discharging in longer duration events and those with high antecedent water levels. Despite their size the tributaries can often be the cause of considerable flooding which can be exacerbated by returns from surface water drainage systems and extensive sections of the watercourse being culverted. The Sherford Stream and Black Brook/Stockwell Stream system have a reasonably sized floodplain area in Vivary Park. The Northern tributaries pass underneath the Taunton-Bridgwater Canal which can interact with them in larger flood events. There are numerous mechanisms of flooding in Taunton and it is not always clear which mechanisms are prevalent in a given area. The first mechanism of flooding is exceedance of channel capacity and overtopping of banks. This can be subdivided into overtopping of undefended and defended areas. In Taunton there are a number of flood defences and these primarily consist of walls and raised defences. The defences run along the left bank of the Tone from Frieze Hill to the former Livestock Market next to Priory Bridge. On the right bank one line of defences extends from the end of Longrun Lane and run along the southern edge of the Tone at French Weir. A second line starts at Goodland Gardens and extends to the end of Priory Park development in Firepool. There is also a small defence line in the northwest corner of the former livestock market. The standard of protection (SOP) of defences within Taunton is indicated within Figure E-1 in Appendix E. It is important to note that the values of SOP shown by the shading are the design values from when each scheme was undertaken not the current protection they offer. Detailed modelling, carried out in conjunction with this SFRA, has identified the following: On the left bank of the Tone the majority of the defences offer a SOP of 100 years or more, however there are some exceptions: Sections of the flood wall at the eastern end of Clarence Street and at the telephone exchange overtop for events with a return period between 50 and 75 years; sections of the wall in front of the Lidl superstore also overtop for events between 50 and 75 year return periods; and the section of defence in front of Poundstretcher next to The Bridge overtops for events less than a 50 year return period. The remainder of the defences along the left bank have a 100 year SOP or better however floodwaters do get behind them even for events with return periods less than 50 years because water can exit the Tone at the undefended livestock market overtopping the small defence at the corner of Canal Road and Priory Bridge Road, which has an SOP less than 50 years, and then flows down Station Road. It should be noted that the re-development of the livestock market site will involve raising existing land levels and this flow pathway should be eliminated. On the right bank the situation is more complicated. The defences along the cricket ground have a SOP of greater than 200 years. Downstream of Priory Bridge the defences have a few sections where the SOP reduces to 75-100 years. Upstream of the cricket ground very few of the defences have an SOP equal to 100 years. On average these sections of defence

TDBC SFRA v3.2 (FINAL).docx 27

have an SOP of around 50 years. The defences in Tangier are not necessarily overtopped from the Tone. The Mill Stream exceeds its channel capacity at relative low return period events (5-10 year return period around the confluence with the Sherford Stream). The defences are almost certainly being overtopped from the landside in some locations. Again the redevelopment of Tangier will involve land-raising and some of these issues are likely to cease following the implementation of the development proposals. There is however one area of concern; defences at the riverside car park off Coal Orchard appear to overtop at events of less than a 50 year return period. For the undefended sections of watercourses in Taunton the majority of overtopping is to areas of open or recreation ground which have been set aside for use as floodplain. However there are notable areas of flood risk to properties: in Tangier around the confluences of the Mill Stream with both Sherford Stream and Galmington Stream; around Mountfields Road and Mountfields Avenue adjacent to the Black Brook; and parts of Bathpool. Another significant mechanism of flooding within Taunton will be the blockage of structures. There are a significant number of culverted channels within the urban area which may be prone to blockage. These tend to be located on the smaller watercourses but if they become blocked by debris they can considerably exacerbate the flood risk to adjacent areas. A number of the affected watercourses have trash screens installed to reduce this risk. An additional mechanism of flooding within Taunton is surface water flooding. This has been assessed across the borough with reference to both the AStSWF and FMfSW mapping issued by the Environment Agency. Across the borough surface water flooding is largely co- incident with areas of fluvial flood risk. In the Taunton urban area the AStSWF mapping shows that surface water affects considerable areas surrounding the Black Brook and Galmington Stream which are not at risk of flooding from the watercourse itself. There are also additional areas associated with transport infrastructure: a linear area to the north of the railway line between Norton Fitzwarren and Taunton; and an area on the western side of the M5 south of Black Brook. Surface Water flood risk as evidenced by the FMfSW is notably less extensive as it makes allowances for sewer capacities. An outline consideration of the differences between the two mapping sources can be found in Section 3.1. More detailed information on the surface water mapping methods and the reasons for the differences in Environment Agency publications8910. Downstream of the M5 As the Tone makes its way further downstream the floodplain becomes wider again as it is less constrained by development. There are a number of flood defences to protect the settlements of , Ham, and Creech St Michael that are at risk of flooding. The flood defence schemes for Ham and Creech villages were improved to a 1 in 100 year standard under capital schemes. The Ham scheme was completed in 2008 and Creech St Michael in 2009/10. Beyond Ham Bridge significant floodplain storage areas can be utilised. Within these areas known as the Moors the Parrett IDB maintains a number of Rhynes and drainage ditches. Further downstream the Moors have significant embankments and include pumping stations managed by the Environment Agency. Pumping stations are used to try and manage water levels within the Moors so that they can be used to store floodwaters. This can only be done when the river is not in flood and cannot finely control water levels. It can take many days or weeks to pump out the Moors following a flood event. It is within the Moors section that the tidal limit of the Tone at Newbridge is located. The tidal influence on the Tone along with fluvial levels in the River Parrett can exert a significant locking effect on the River Tone. This could result in elevated water levels in Taunton on the rare occasions when all floodplain storage in the Levels and Moors has been fully utilised. This is considered to be a very low probability event, and the effect on the water levels due to locking is quite small but the

8 Using Surface Water Flood Risk Information - Guidance for Local Resilience Forums, Regional Resilience Teams, Local Planning Authorities and Lead Local Flood Authorities v1 November 2010 9 What are Areas Susceptible to Surface Water Flooding - Guidance for Local Resilience Forums, Regional Resilience Teams, Local Planning Authorities and Lead Local Flood Authorities v1 November 2010 10 What is the Flood Map for Surface Water Guidance for Local Resilience Forums, Regional Resilience Teams, Local Planning Authorities and Lead Local Flood Authorities v1 November 2010

TDBC SFRA v3.2 (FINAL).docx 28

consequences of this event in combination with a fluvial event on the Tone are magnified as the defences in Taunton are sensitive to relatively small changes in water level. Summary Within the borough as a whole there are a number of mechanisms by which flooding can occur. Within the urban areas surface water flooding resulting from intense pluvial events is potentially a significant source of flood risk depending on the capacity of the sewer system. Fluvial flooding is most likely outside of Taunton in the undefended areas although there are relatively few properties at risk. Within Taunton fluvial flooding may occur as a result of structures such as bridges and culverts becoming blocked or due to overtopping of the defences. Overtopping of the defences is likely only in the largest flood events or if a breach occurs. Although modelling has demonstrated how defences can be circumvented. Tidal flooding resulting from an exceptional high tide in Bridgwater Bay is unlikely if river levels are within a normal range as the defences on the lower Tone are raised above the peak tide level. However an extreme tide will certainly exacerbate any flooding if it coincides with the Tone being in flood. Across the borough one key consideration in determining flood risk for the borough is event duration. For very short durations (< 5 hours) the primary flood risk is likely to be surface water flooding, for moderate durations (10-25 hours) the primary flood risk is likely to be fluvial with peak flows overtopping bank levels. For long duration events (>40 hours), the primary risk is again fluvial but this time as a volumetric concern. Flood waters will potentially fill storage areas above capacity and raise water levels above defences. This may be heightened by tidal effects or river levels in the Parrett. Considerations for a critical duration have suggested that for the overtopping of banks due to peak flows the a duration of around 17 hours is critical for Taunton, however work undertaken by Atkins for the Lower Parrett and Tone Flood Management Strategy (LPTFMS) has suggested a duration of around 50-55 hours is critical for the moor storage areas, it is therefore durations of this length that may result in overtopping of river banks due to excessive flood volume. The LPTFMS also considered a 250 hour duration event which is the critical duration for the whole Parrett system. It is evident that longer duration events will need to be considered when assessing mitigation measures for proposed development. 2.3.3 Key flooding issues for Taunton Deane Borough Council Key flooding issues for Taunton that need to be addressed are: Areas of Taunton where defences are not of the required standard Mechanism for updating modelling following redevelopment of town centre sites to ensure that current flood risk is up to date. Prevention of blockage at key structures Longer duration events (River Parrett flows)

2.4 Possible responses to flooding 2.4.1 Assess The first response to flooding must be to understand the nature and frequency of the risk. The assessment of risk is not just performed as a "one off" during the process, but rather the assessment of risk should be performed during all subsequent stages of responding to flooding. 2.4.2 Avoid The sequential approach requires that the first response is to avoid the hazard. If it is possible to place all new growth in areas at a low probability of flooding then the flood risk management considerations will relate solely to ensuring that proposed development does not increase the probability of flooding to others. This can be achieved by implementing SUDS systems and other measures to control and manage run-off. In some circumstances it

TDBC SFRA v3.2 (FINAL).docx 29

might be possible to include measures within proposed growth areas that reduce the probability of flooding to others and assist existing communities to adapt to the effects of climate change. In such circumstances the growth proposals should include features that can deliver the necessary levels of mitigation so that the standards of protection and probability of flooding are not reduced by the effects of climate change. In Taunton consideration should be given not only to the peak flows generated by new development but also to the volumes generated during longer duration storm events.

2.4.3 Substitute, control and mitigate These responses all involve management of the flood risk and thus require an understanding of the consequences (the magnitude of the flood hazard and the vulnerability of the receptor). In theory there are opportunities to reduce the flood risk by lowering the vulnerability of the proposed development. For instance changing existing residential land to commercial uses will reduce the risk provided that the residential land can then be located on land in a lower risk flood zone. In practice Government has placed a requirement on local authorities to prepare urban capacity studies with the objective of giving “priority to re-using previously developed land within urban areas, bringing empty homes back into use and converting existing buildings in preference to the development of Greenfield sites” (PPS3, CLG). This stated preference is more likely to involve the changing uses from a lower vulnerability to a higher one but does not introduce a presumption that previously developed land is necessarily a more appropriate location for new development in flood risk areas. Flood risk management responses in circumstances where there is a need to consider growth or regeneration in areas that are affected by a medium or high probability will include: Strategic measures to maintain or improve the standard of flood protection so that the growth can be implemented safely for the lifetime of the development (must include provisions to invest in infrastructure that can adapt to the increased chance and severity of flooding presented by climate change); Design measures so that the proposed development includes features that enables the infrastructure to adapt to the increased probability and severity of flooding whilst ensuring that new communities are safe and that the risk to others is not increased (preferably reduced); Flood resilient measures that reduce the consequences of flooding to infrastructure so that the magnitude of the consequences is reduced. Such measures would need to be considered alongside improved flood warning, evacuation and welfare procedures so that occupants affected by flooding could be safe for the duration of a flood event and rapidly return to properties after an event had been experienced. It would be necessary to address the necessary commitment and provisions for the long term management and maintenance of all measures to control and mitigate flooding, where they have to be deployed.

2.5 Policy considerations The principal policy document that influences the SFRA is the River Parrett Catchment Flood Management Plan (CFMP). The key objective of a CFMP is to develop complementary policies for long-term management of flood risk within the catchment that take into account the likely impacts of changes in climate, the effects of land use and land management, deliver multiple benefits and contribute towards sustainable development (CFMP: volume I – policy guidance, 2004).

As part of the CFMP process each CFMP area was divided up into broad areas (known as „policy units‟), which represent areas of similar characteristics, similar flood mechanisms and similar flood risks. Each policy unit was then assessed to decide which policy will provide the most appropriate level and direction of flood risk management both now and in the future.

TDBC SFRA v3.2 (FINAL).docx 30

One of six standard flood risk management policies has been applied to each policy unit: Policy 1 – No active intervention (including flood warning and maintenance). Continue to monitor and advice. Policy 2 – Reduce existing flood risk management actions (accepting that flood risk will increase over time). Policy 3 – Continue with existing or alternative actions to manage flood risk at the current level. Policy 4 – Take further action to sustain the current level of flood risk into the future (responding to the potential increases in risk from urban development, land use change and climate change). Policy 5 – Take further action to reduce flood risk. Policy 6 – Take action to increase the frequency of flooding to deliver benefits locally or elsewhere (which may constitute an overall flood risk reduction, e.g. for habitat inundation). The proposed CFMP policies affecting the borough are summarised in Figure 2-5 Figure 2-5 CFMP Policies

Figure 2-5 shows that the important policies with respect to proposed future development in Taunton are: On the River Tone catchment upstream and to the north of Taunton policy 3 is the applicable policy. This means that the flood risk management proposals involve

TDBC SFRA v3.2 (FINAL).docx 31

continuing the existing or alternative actions to manage flood risk at the current level. Thus it is possible for risk management measures to be implemented so that current levels of flood risk are not exacerbated; On the River Tone within Taunton policy 5 is applicable. Policy 5 requires that further flood risk management actions are taken to reduce flood risk; and On the River Tone catchment downstream of Taunton policy 6 is applicable. This policy requires that further action is taken to increase the frequency of flooding to deliver benefits locally or elsewhere (which may constitute an overall flood risk reduction, e.g. for habitat inundation). It will be important that strategic flood risk management measures in response to potential increases of flood risk as a consequence of proposed future development are consistent with these policies. The policies in the CFMP have been prepared using evidence that assess the current conditions and estimate the effects of future changes due to climate change. It is notable that the predicted future change in average annual flood damage in Taunton results in damages increasing from £1.915 million to £5.213 million. This significant increase in damage explains the policies along the River Tone and demonstrates that it will be necessary to make significant investment in flood risk management measures to address the reduction in the standard of protection afforded by the flood defences as a consequence of climate change effects (increased river flows).

TDBC SFRA v3.2 (FINAL).docx 32

3. Mapping and risk based approach

3.1 Summary of mapping for all sources of flood risk Mapping of fluvial flood risk presented in this SFRA has come from multiple sources. The primary source has been results from the 1D-2D model commissioned by the Environment Agency to accompany this study. The model is an upgrade of the model used to produce flood outlines for the previous SFRA. It now includes a 2D model domain between Bishops Hull and the M5. The 1D domain extends from Bradford-upon-Tone down to the confluence with the River Parrett and includes an extent along the key tributaries joining the River Tone. In addition to this model, TDBC commissioned some supplementary modelling of the upstream sections of the tributaries using JBA's JFLOW+ modelling software. These results have been used to augment the flood outlines from the 1D-2D model. For areas outside of the modelled extents the fluvial mapping has been taken from existing maps of Flood Zones 2 and 3. Mapping of surface water flood risk has been taken from both the Areas susceptible to surface water flooding (ASTSWF) mapping issued in January 2009 and the second generation of surface water mapping called the Flood Map for Surface Water (FMfSW) issued in December 2010. The ASTSWF map shows areas that are susceptible to surface water flooding. It has been produced using a simplified method that excludes: underground sewerage and drainage systems, and smaller over ground drainage systems; buildings. It uses a single rainfall event. Therefore, it only provides a general indication of areas which may be more likely to suffer from surface water flooding. The map provides three bandings, indicating „less‟ to „more‟ susceptible to surface water flooding. The bands are displayed in shades of purple. The FMfSW is a map which shows areas that are considered to be at risk from surface water flooding. It has been produced using a simplified approach to replicate the effect of underground sewerage and drainage systems and smaller over ground drainage systems. It uses simplified rainfall information that is different to that used to generate the AStSWF mapping. Therefore, it only provides a general indication of areas which are at risk from surface water flooding and is not appropriate for use at the individual property level. It shows areas that are likely to flood in storms with a 1 in 200 and 1 in 30 chance of occurring in any year. For each storm, the map shows areas which are likely to flood to a depth greater than 0.1m and areas which are likely to experience deeper flooding, greater than 0.3m. The map is displayed in shades of purple. The Environment Agency has produced a guidance document8 on the use of surface water flood risk information that gives a detailed consideration of both mapping datasets and their limitations. Flood Map for Surface Water is considered the Environment Agency‟s primary surface water flood map. Areas Susceptible to Surface Water Flooding should now only be used as a supporting tool to Flood Map for Surface Water where, following the procedure set out in the guidance, Areas Susceptible to Surface Water Flooding has been assessed and agreed as best representing local conditions. It should be noted that within Taunton Deane there are considerable differences between the two datasets. The issue of the differences should be addressed when Somerset County Council as Lead Local Flood Authority finalise the 'locally agreed surface water information' which considers the two sources of mapping and determines which (if any) best represents the local conditions. This exercise also uses local data to inform decision making. In addition to the fluvial mapping an analysis of 3 breach locations within Taunton has been undertaken. At each of these locations the 100 year return period event was run with a 20m wide breach in the defence included. The three locations chosen were: Clarence Street, The Cricket Ground and Pollards Way.

TDBC SFRA v3.2 (FINAL).docx 33

Hazard mapping has also been produced for the areas of Taunton covered by the 2D domain. The hazard rating is calculated directly within the TUFLOW modelling package and utilises the classifications of hazard presented in DEFRA R&D Technical Note FD2320: Flood Risk Assessment Guidance for New Development. All of the mapping can be found in Appendix H and is presented in the following structure: Flood Zones: Outlines of flood extent for the following cases: o Zone 3b (functional floodplain) - 20 year return period, defended case o Zone 3a - 100 year return period, undefended case o Zone 2 - 1000 year return period, undefended case o Climate change - 100 year return period with 20% increase in flows, undefended case All sources of Flooding o Areas susceptible to surface water flooding map o Flood Map for Surface Water - 1 in 30 year return period o Flood Map for Surface Water - 1 in 200 year return period No suitable mapping is available for risks of flooding from groundwater or reservoir inundation. Actual flood risk: Flood outlines for defended case o 100 year return period o 100 year return period with 20% increase for climate change Residual flood risk o Hazard mapping for the defended case . 100 year return period . 100 year return period with 20% increase for climate change . 1000 year return period . 3 breach scenarios o Flood extents for the defended case . 1000 year return period . 3 breach scenarios

3.2 Other relevant flood risk information The mapping prepared for this version of the SFRA provides information on: The extent of flooding; the depth of flooding; the velocity of flood water; and The hazard from floodwater It should be noted that it will also be possible for users of this SFRA to refer to other relevant information on flood risk, as this is published and becomes available. Other information that should be referred to includes: The Preliminary Flood Risk Assessment (PFRA) and Significant Flood Risk Areas (to be published for local flood risk in December 2011); The Surface Water Management Plan (SWMP) (predicted to be available in 2011); National Flood and Coastal Defences Dataset (NFCDD) or equivalent (available now); and National Receptor Dataset (NRD) (available now). Information produced by the Environment Agency on how to challenge Flood Maps and Flood Zones included within the SFRA is included in Appendix G.

TDBC SFRA v3.2 (FINAL).docx 34

3.3 Sequential approach The general concepts used to apply sequential approach are described in Chapter 2 of this SFRA. Planning Policy Statement 25: Development and Flood Risk2 and the accompanying Practice Guide1 set out the desired approach for avoidance of development in flood risk areas. It is often the case that it is not possible for all new development to be allocated on land that is not at risk from flooding. In these circumstances the Flood Zone maps (that show the extent of inundation assuming that there are no defences) are too simplistic. A greater understanding of the scale and nature of the flood risks are required. To help achieve this, more detailed modelling of a range of extreme storm events and failure of flood management operational features has been undertaken (discussed in Section 2). The ability to manage flood risk for new development must consider a wide range of issues, which includes how any evacuation of the occupants would be handled, how the new development fits in with the existing flood management provision and, should there be an event, how quickly the wider area would recover and return to normal. Some areas, either through natural or artificial topography, are easier to integrate flood management measures into the new development, without causing a significant alteration in its design and its place setting. These measures can have the potential to cause an alteration to the flood risk to adjacent property or in flood cells on the opposite bank.

3.4 Sequential test The Sequential Test must be performed when considering the placement of future development and for planning application proposals. Again details of the test are described in PPS 25 and the accompanying Practice Guide. The Practice Guide gives detailed instructions on how to perform the test. These instructions on how to perform the Test should be used with the following information from the SFRA: Identify the area to be assessed (including alternatives) on the flood Zone Maps that are provided with this assessment; Establish the risk of flooding from other sources again using the Maps in this SFRA; and Follow the instructions given in Chapter 4 of the Practice Guide. The Practice Guide gives specific guidance on the application of the Sequential Test in relation to allocation of land, individual planning applications, windfall sites, renewable energy projects, redevelopment of an existing single property, and change of use. The Sequential Test is used to direct all new development (through the site allocation process) to locations at least risk of flooding, giving highest priority to Flood Zone 1. Before the sites being considered in this SFRA can be allocated for development TDBC must complete the Sequential Test to determine whether these sites are appropriate as strategic allocations given the flood risks associated with them. The output from the Strategic Housing Land Availability Assessment (SHLAA) will be critical evidence in this process. If these sites do not pass the Sequential Test they should not be allocated and alternative sites should be brought forward. Where the Sequential Test alone cannot deliver acceptable sites, the Exception Test will need to be applied. The Environment Agency (2009)11 recommends that the following approach is used by local planning authorities to apply the Sequential Test to planning applications located in Flood Zones 2 or 3. The same approach should also be used for the LDF site selection process, which is undertaken at the larger borough scale. There are three stages to the test, as follows: Stage 1 – Strategic application & development vulnerability

11 Environment Agency (2009) Demonstrating the flood risk (PPS25) Sequential Test for Planning Applications, PPS25 FRSA (national) version 2.0 Advise issued on 27 January 2009

TDBC SFRA v3.2 (FINAL).docx 35

Stage 2 – Defining the evidence base Stage 3 – Applying the Sequential Test

Stage 1 – Strategic application & development vulnerability

The Sequential Test can be considered adequately demonstrated if both of the following criteria are met: The Sequential Test has already been carried out for the site (for the same development type) at the strategic level (development plan) in line with paragraphs D5 and D6 of PPS25; and The development vulnerability is appropriate to the Flood Zone (see table D1 of PPS25)

1.A Has the Sequential Test already been carried out for this development at the development plan level? If yes, reference should be provided to the site allocation and Development Plan Document (DPD) in question. 1.B Is the flood risk vulnerability classification of the proposal appropriate to the Flood Zone in which the site is located according to Tables D1 and D3 of PPS25? The vulnerability of the development should be clearly stated. Finish here if the answer is „Yes‟ to both questions 1.A. and 1.B. Only complete Stages 2 and 3 if the answer to either questions 1.A and 1.B is „No‟.

Stage 2 – Defining the evidence base 2.A State the geographical area over which the test is to be applied. 2.B If greater or less than the borough boundary justify why the geographical area for applying the test has been chosen.

Identify the geographical area of search over which the test is to be applied – this will usually be over the whole of the borough but may be reduced where justified by the functional arrangements of the development (e.g. catchment area for a school or doctors surgery) or relevant objectives in the LDF. Equally, in some circumstances it may be appropriate to expand the search area beyond the borough for uses that have a sub- regional, regional or national market.

2.C Identify the source of reasonable available sites, either: Background / evidence base documents (state which), or if not available Other sites known to TDBC that meet the functional requirements of the application Identify the source of „reasonably available‟ alternative sites – these sites will usually be drawn from the evidence base / background documents that have been produced to inform the emerging LDF. For example, an important source of information from housing sites and employment land will be provided by the SHLAA and the Employment Land Review (ELR). In the absence of background documents, „reasonably available‟ sites would include any sites that are known to TDBC and that meet the functional requirements of the application in question, and where necessary, meet the LDF Policy criterion for windfall development (see below)

Windfall sites

TDBC SFRA v3.2 (FINAL).docx 36

Windfall sites are those which have not been specifically identified as available in the Development Planning Process. They comprise sites that have unexpectedly become available. Within settlement limits windfall sites will be previously-developed. Government policy in PPS3 para. 59 advises that LPAs should not normally rely on windfall sites to meet housing needs. The Environment Agency recommend that the acceptability of windfall applications in flood risk areas should be considered at the strategic level through a policy setting out broad locations and quantities of windfall development that would be acceptable or not in Sequential Test terms. Evidence on this position should be provided as support to the soundness of the Core Strategy. Guidance on determining the housing potential of windfall (where justified) for broad locations can be found in paras. 50-52 of Strategic Housing Land Availability Assessments, Practice Guide to PPS3. In the absence of flood risk windfall policy, it may be possible (where data is sufficiently robust) for the LPA to apply the Sequential Test taking into account historic windfall rates and their distribution across the district relative to Flood Zones. Where historic and future trends evidence indicate that housing need in the district through windfall can be met largely/entirely by development outside high flood risk areas, this may provide grounds for factoring this into the consideration of „reasonably available‟ alternative sites at the planning application stage.

2.D State the method used for comparing the flood risk between sites, whether it is this SFRA or an alternative (e.g. Environment Agency flood map, site specific flood risk assessment) as new information becomes available.

Stage 3 – Applying the Sequential Test

Compare the reasonably available sites identified under stage 2 with the application site. Sites should be compared in relation to flood risk; development plan status; capacity; and constraints to delivery including availability, policy restrictions, physical problems or limitations, potential impacts of the development, and future environmental conditions that would be experienced by the inhabitants of the development. 3.A State the name and location of the reasonably available site options being compared to the application site 3.B Indicate whether flood risk on the reasonable available options is higher or lower than the application site. State the Flood Zone or SFRA classification for each site. 3.C State whether the reasonably available options being considered are allocated in the Development Plan. Confirm the status of the plan. 3.D State the approximate capacity of each reasonably available site being considered. This should be based on: the density policy within a Local Development Document (LDD) the current Strategic Housing Land Availability Assessment for the borough past performance 3.E Detail any constraints to the delivery of identified reasonably available options; for example, availability within a given time period or lack of appropriate infrastructure i.e. flood defences which protect the site through its design lifetime. This part of the test should include recommendations on how these constraints should be overcome and when.

Sequential Test conclusion Are there any reasonably available sites in areas with a lower probability of flooding, which would be appropriate to the type of development or land use proposed?

TDBC SFRA v3.2 (FINAL).docx 37

Next step

Exception Test – Where necessary, the Exception Test should now be applied in the circumstances set out by table D.1 and D.3 of PPS25. Applying the sequential approach at the site level – In addition to the formal Sequential Test, PPS25 sets out the requirements for developers to apply the sequential approach (see para. 14 and D8) to locating development within the site. The following questions should be considered: Can risk be avoided through substituting less vulnerable uses or by amending the site lay-out? Has the applicant demonstrated that less vulnerable uses for the site have been considered and reasonably discounted? Can layout be varied to reduce the number of people or flood risk vulnerability or building units located in higher risk parts of the site?

3.5 Exception test The Exception Test is performed to ensure that more vulnerable property types, such as residential development are not located in areas at high risk of flooding where development would not be safe. Again PPS 25 and the accompanying Practice Guide gives detailed information on how the Test should be performed. The Test involves satisfying the following three components: 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; b. The development should be on developable previously developed land or, if it is not on previously developed land, 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 the flood risk elsewhere, and, where possible, will reduce flood risk overall.

The advice and guidance given in PPS 25 should be used in conjunction with the mapping issued in this version of the SFRA. The Practice Guide gives specific guidance on: The identification of wider sustainability benefits; How to determine what is safe; and Access and egress requirements.

When considering development in areas that are protected by flood defences consideration should also be given to the residual risk that is either a result of the failure or overtopping of defences. This SFRA provides information on the level of hazard (Hazard mapping) that would affect people, property and infrastructure if the existing flood defences failed (due to breaching) or if an event exceeded their original design standard. The methods used to generate the hazard mapping are as described in the PPS 25 Practice Guide. The existing town centre of Taunton is particularly vulnerable to residual risk events. This information can also be used by those preparing for flood emergencies or requiring tactical information during a flood event.

TDBC SFRA v3.2 (FINAL).docx 38

4. Overview of future development

4.1 Overview and vision for new development The proposed developments within Taunton Deane were set out in the borough council's Core Strategy which was released for consultation in January 2010. The strategy identifies the potential areas for future development up to 2028 (originally 2026). The scope of the strategy at that time was the provision of approximately 21,800 new homes and 34ha of employment land within the borough. Following the government's announcement of its intention to abolish Regional Spatial Strategies, the council has commissioned revised economic and demographic projections and the number of homes now proposed up to 2028 is in the order of 17,000.

4.2 Extent and type of development The majority of the proposed allocation sites are located around Taunton, with a smaller number around Wellington. In addition a number of smaller sites are proposed around the rural centres of: , Bishops Lydeard, Cotford St Luke, Creech St Michael, Milverton, , and Churchinford. As outlined there have been a number of changes to the allocations since the Core Strategy went for consultation in 2010. The strategic employment site east of Junction 25 of the M5 and the sites at Norton Fitzwarren, Bishops Hull and Killams are no longer being allocated in the Core Strategy. Some of these sites may be brought forward within the plan period in the Site Allocations DPD which will follow completion of the Core Strategy. In Taunton the Core Strategy allocates mixed use urban extensions at Monkton Heathfield and Nerrols. /Trull and Staplegrove are identified as broad locations for growth after 2016. The development areas shown in Figure 4-1 are those which were the subject of the Core Strategy and Small Site Allocations public consultation in January and February 2010. In Wellington the Core Strategy allocates strategic urban extensions at Longforth and Cades/Jurston, together with a strategic employment site south of Chelston. These can be seen in Figure 4-2. For the urban extensions the majority of the land within the development areas is previously undeveloped. Only the western part of the Longforth development involves significant re- development.

TDBC SFRA v3.2 (FINAL).docx 39

Figure 4-1: Development Areas around Taunton (Contains Ordnance Survey data © Crown copyright and database right 2010)

Figure 4-2: Development Areas around Wellington (Contains Ordnance Survey data © Crown copyright and database right 2010)

TDBC SFRA v3.2 (FINAL).docx 40

4.3 Timing of development The timing of these developments is critical to any strategic consideration of flood risk. The following timetable for development at Taunton is anticipated: : Within the next 5 years Up to 1,000 dwellings at Monkton Heathfield (allocated in the Local Plan); 400 dwellings at Nerrols (phase 1 on southern part of the site); and 125 dwellings at Bishops Hull (planning permission granted) The remainder of the plan period 4,000 dwellings at Monkton Heathfield; 500 dwellings at Nerrols; Between 1,000 and 2,000 dwellings at Comeytrowe/Trull; Between 500 and 1,500 dwellings at Staplegrove; and Sites identified in the Site Allocations DPD. The Local Plan allocated 1,000 dwellings at Monkton Heathfield and planning permission has been granted for 900 homes, for which a drainage strategy and FRA has been approved. As such this has not been considered as part of the strategic assessment of future development in Section 5 of the SFRA.

TDBC SFRA v3.2 (FINAL).docx 41

5. Strategic assessment of future development

5.1 Summary of flood risk issues The main flood risk issue resulting from the proposed developments will be an increase in impermeable areas within the catchment. It is considered that if the volume of runoff from this increase in impervious area is not mitigated in some way that there will be an increase in the flood risk either at the point of development or a receptor downstream.

5.2 Development over next 5 years Of the sites proposed for development within the next 5 years, Monkton Heathfield already has an approved FRA and agreed drainage strategy. The total area of developments within the 5 year plan is approximately 0.7km2 (70 hectares). In order to assess the effect of these developments the additional runoff from these sites due to the increase in impervious area must be calculated. A copy of the calculations can be found in Appendix F. Monkton Heathfield should be excluded from the calculations because of its agreed drainage strategy. Accordingly the remaining area of proposed development in the next 5 years is approximately 33 hectares comprising phase 1 of Nerrols and the Bishops Hull development. Taking a 1 in 100 year return period rainfall event for both the duration critical to Taunton for peak flows (17 hours) and the duration critical for the entire River Tone system (50 hours) it is possible to estimate the increase in runoff using some basic assumptions. For the 17 hour event the rainfall depth for a 100 year event, taken from the FEH CD-ROM, is approximately 90mm in Taunton. For the 50 hour event it is 127mm. For the lifetime of the development PPS25 stipulates that an increase in precipitation of 30% must be considered to allow for the effects of climate change giving revised values of 117mm and 165mm respectively. For the development sites within the Taunton Deane area the pre-development runoff volumes will vary between 25% and 45% of the rainfall depending on the part of a site in question. In order to simplify the calculations a value of 35% will be assumed. The highest runoff rates are likely during or just after development and it is reasonable to assume that runoff would be 70% of the rainfall (although it is recognised that this would depend on the density of development). This puts the increase in runoff volume due to development at 13,500 m3 for the 17 hour event. For the 50 hour event this increases to 19,100 m3. These values are indicative as to the amount of storage required to compensate for the developments. A more detailed assessment of post development runoff should be undertaken either, as part of an FRA, or as part of the master-planning process for each individual development before detailed proposals of how to provide this storage volume are proposed. The required storage volume could be provided on site or as part of a larger more strategic scheme across the borough. Even though Monkton Heathfield has had a drainage scheme approved, the scheme should be assessed to see if is broadly in line with the above suggestion. If the volume provided by this scheme is not sufficient, consideration should be given to making up the shortfall in future phases of Monkton Heathfield.

5.3 Longer term development up to 2028 The cumulative total development footprint for all of the areas highlighted within the Core Strategy as presented in Figure 4-1 and Figure 4-2 is approximately 7.7 km2 (770 hectares). The development footprint of 770 ha is the total area of sites taken from the 2010 Core Strategy. Subsequent revisions to the Core Strategy have significantly reduced the number of dwellings proposed for Comeytrowe/Trull down from the 6,000 the red line boundary could

TDBC SFRA v3.2 (FINAL).docx 42

contain to between 1,000 and 2,000 in the period to 2028. Beyond the plan period it is still anticipated that Comeytrowe/Trull would continue to provide the main area of growth for Taunton. The calculations provided will continue to consider the Comeytrowe/Trull site as shown in the red line boundary even though development would not occur until after 2028. The cumulative development footprint of 7.7 km2 represents an increase in development area of 7.0 km2 in addition to the development being brought forward in the next 5 years. . This would result in the need to find 287,000 m3 of additional storage for the 17 hour event and 404,000 m3 of storage for the 50 hour event to mitigate the increased surface runoff. It is apparent that when considering volumes of this magnitude a borough wide strategic solution could prove to be more cost effective as well as providing scope to reduce the flood risk to the remainder of Taunton in accordance with its CFMP policy area. Consideration of which storm duration to select for providing storage is problematic using the technical information available at the time of preparation of this SFRA. The 17 hour storm duration is the one that will produce the largest peak flows in the centre of Taunton, whilst the 50 hour duration is that which the storage areas in the Moors are most sensitive to. It has been considered that in order to account for residual risk (see discussion within Sections 2.2 and 2.3) and in the absence of undertaking a more extensive analysis that the 50 hour duration is the most appropriate when considering runoff volumes that might affect flood risk in the centre of Taunton. If required it would be possible to undertake a more detailed analysis of the response of the River Tone to different storm durations. This could be achieved by performing a continuous simulation across the Parrett catchment. Continuous simulation would utilise both the Tone and Parrett models and would provide a detailed understanding of how the systems interact. It may also be possible to undertake a joint probability analysis if there are sufficient level and flow data across both the Tone and Parrett catchments. It is considered that continuous simulation would provide the most conclusive answer.

TDBC SFRA v3.2 (FINAL).docx 43

6. Strategic responses

6.1 Introduction This version of the SFRA considers the strategic responses that are needed to address the key flood risk issues in the Borough so that the Infrastructure Development Plan (IDP) can include specific infrastructure measures that must be delivered. The focus of the assessment has been on the potential effect of new development to the north and south of Taunton. The assessment of the responses falls into two categories, namely: Those elements of infrastructure that should be implemented alongside development proposed within the next 5 years; and Infrastructure required in support of future development planned up to 2028. The measures have been assessed using the technical analyses performed during the preparation of the SFRA modelling and mapping. In particular this technical assessment has included: Consideration of the actual and residual risk to existing property in Taunton using the 1D - 2D model; Consideration of the effect of the increased volumes of runoff generated by proposed future development, as has been described in Chapter 4 of this report; Consideration of the effect of future increases in upstream flows and downstream water levels in the River Parrett as a consequence of the effects of climate change. The extent to which it is possible to link proposed future development to strategic infrastructure is very much dependent on the status of planning permissions for the land identified for development. The SFRA assessment has identified that there is uncertainty in the analysis used to predict effects. However, it is proposed that the measures include identification of further actions and implementation of measures to reduce this uncertainty.

6.2 Review and assessment of options 6.2.1 Summary of existing situation The River Tone collects runoff from the catchment upstream of Taunton and conveys flows through the town in an easterly direction. Significant new areas of development are proposed to the north and to the south of the existing urban area of Taunton. These areas will generate additional volumes of runoff that will flow along existing watercourses through Taunton and subsequently discharge to the River Tone. There are a number of influential watercourses that flow through the centre of Taunton and discharge to the River Tone. It is worth noting that the Black Brook does not convey flow to the town centre and flows beneath the M5 to discharge to the River Tone downstream of Taunton. The Bridgwater & Taunton canal is connected to the River Tone to the east of the town centre and links to the River Parrett at Bridgwater. The M5 is a major feature that is constructed in the flood plain of the River Tone to the East of Taunton. The River Tone flows through "the Moors" to the east of the M5. The Moors provide significant storage capacity during periods when the outfall to the River Parrett is tide locked during periods of high flow and high tide. At the confluence between the River Tone and the River Parrett water levels are influenced by tide levels in Bridgwater Bay. This effect is magnified during periods of high flow in the River Parrett. Figure 6-1 illustrates the influential features.

TDBC SFRA v3.2 (FINAL).docx 44

Figure 6-1: Schematic Configuration of Tone and Parrett system affecting Taunton

Bridgwater M5 Bay

Bridgwater

NORTH

Bridgwater & Taunton Canal "The Moors" River Parrett (flood storage)

local northern catchments Taunton

River Tone

Black Brook

local southern catchments

6.2.2 Flood Risk Management Issues The following aspects will determine the scale and scope of strategic options to address flood risk. Reference should be made to Figure 6-1 for the naming conventions given to the features described: The primary issue with respect to the proposed new development is the additional volumes of runoff that will be generated. New development will reduce the storage that exists in natural systems by increasing the efficiency with which water runs off the land (this is a prerequisite within the design of urban infrastructure to prevent local ponding and flooding - which would potentially be a nuisance). Conventionally it is possible to mitigate potential adverse effects of additional volumes by controlling the magnitude of the peak flow so that the volumes of water in the receiving river systems are not exacerbated during the critical peak flow period; The local watercourses in Taunton receiving flows from the proposed new development in the "local northern catchments" and the "local southern catchments" are already affected by flooding during high probability events (events with a 1% annual chance of occurrence). Thus proposed development will have to incorporate mitigation measures to prevent exacerbation of flooding to existing infrastructure "downstream" during local flood events; Water levels in the River Tone through the centre of Town are critical. Results from the hydraulic modelling analyses indicate that there are sections of existing defences

TDBC SFRA v3.2 (FINAL).docx 45

in the town centre that fail to provide the 100 year SOP aimed for. The defences which overtop are identified in Section 2.3.2. Overtopping of defences and flooding of undefended areas results in extensive flooding in the centre of Taunton as shown in Figure 6-2. Flood outlines are likely to change following completion of the Third Way Bridge and other redevelopments in Tangier and the Town Centre, and these changes will need to be included into an updated model to re-evaluate the SOP of the defences. Figure 6-2: Existing flood risk in the centre of Taunton (100 year return period with defences) [Contains Ordnance Survey data © Crown copyright and database right 2010]

The results from the analyses of the River Parrett indicate that water levels at the confluence with the Tone can be elevated for significant durations during events with a 50% annual chance of occurrence. The duration of these elevated water levels is estimated to be in the order of 300 hours, as shown in Figure 6-3 (i.e. - in the order of two weeks); Tide levels in Bridgwater Bay influence the predicted water levels at the confluence between the River Tone and the River Parrett; The flood storage capacity of the "the Moors" is significant, but the emptying time following an event that fills them is substantial, since the pumps have limited capacity in relation to the total volume of water; The Bridgwater & Taunton canal has limited capacity to convey flow; and The increased reliance on the receiving watercourses in Taunton to discharge increased volumes of water from proposed future development will make inundation more severe if culverts or channels become blocked. Hence it will be necessary for consideration to be given to the blockage risk along watercourses and where necessary measures installed to alleviate existing problems or prevent generation of problems not yet identified.

TDBC SFRA v3.2 (FINAL).docx 46

Figure 6-3: Water levels at the confluence between the River Tone and River Parrett

The most challenging issue with respect to potential exacerbation flood risk at the centre of Taunton is the duration of the flood event that should be considered. Whilst statistically it can be shown that the peak flow in the River Tone at Taunton is generated by a rainfall event with a duration of 17 hours, it can be observed that the River and tide levels in the River Parrett can influence conditions for much longer periods (statistically it is not unreasonable to consider events with a duration in the order of 2 weeks). It should also be remembered that the hydrological analyses used to generate the flood events present statistical scenarios that are very often not matched by real events that cause actual flooding. Thus it is misleading to suggest that a flood with a 1% annual chance of occurrence will be observed only during an event with a duration of 17 hours. It is probable that the flood event will actually be caused by a rainfall event with a more complex profile, possibly involving unusual antecedent conditions (the conditions in the month or months prior to the time when the flood is experienced). In addition the consequences with respect to flood water levels are complicated by the fact that the water levels in the River Parrett can affect the performance of the flood storage capacity in "the Moors" and hence water levels in Taunton. 6.2.3 Policy considerations The complexity of the interactions between surface runoff and river levels in the River Tone and River Parrett would suggest that a much more detailed suite of analyses should be performed to refine the estimates. This would provide more certainty with respect to the relative influence of the various factors affecting flood risk and more certainty on the frequency of flooding. However, the advantages of this approach are constrained by: Uncertainty and scope of available data. To have confidence in the results from a more detailed analysis would demand access to more extensive, high quality data.

TDBC SFRA v3.2 (FINAL).docx 47

Unfortunately the data that exists would not provide the appropriate level of detail; and The time to perform the analyses would not fit within the programme for the SFRA. Thus a more pragmatic approach will need to be adopted, although this must be consistent with policy context described in PPS 25. The policy on development and flood risk requires that residual risk should be included in the assessment of flood risk and it is stated that the analyses should: Include the assessment of the remaining (known as "residual") risk (see Annex G) after risk reduction measures have been taken into account and demonstrate that this is acceptable for the particular development or land use." The approach described in Annex G of PPS 25 also includes provisions for developer contributions. Thus for the circumstances pertaining to new development at Taunton it is possible for new urban development to be implemented to the north and south of the town that includes mitigation measures (SUDS and local attenuation) to prevent local exacerbation of risk. However, there remains a residual risk that the runoff from the development can exacerbate flooding in the centre of Taunton as a consequence of the increased volumes of water generated by the development. Thus the strategic options describe measures that could be included to address the additional volume of runoff (the residual risk). The fundamental policy issue is accordingly to decide whether the potential exacerbation of flooding as a consequence of the increased discharge volumes at the centre of Taunton is acceptable. The focus of this question should be on the potential increase to water levels in the River Tone that could cause the defences to be overtopped, rather than the incremental increase to flood levels after the failure of defences. The outline summary of the review of the CFMP, included in this report indicates that it would not be inconsistent to consider actions along the Tone.

6.3 Description of strategic options considered On the basis of the assessment of the influential surface runoff considerations the key issues are: To reduce flood risk in the centre of Taunton; and Identifying measures that do not cause increased volumes of flood water to contribute to the town centre during events with extensive durations. To date the following options have been identified: Option 1 - Upstream Storage The residual risk of the increased volumetric discharge at Taunton is addressed by the implementation of measures to increase the volume of available flood storage upstream of the centre of town. Thus the additional volume made available upstream compensates for the additional volumes being discharged at the centre of town. The storage would involve the modification of flood plains so that volume was made available to store additional flood water during the peak of the flood event. This concept has been put into practice in Northampton. The measures at Northampton primarily involved the implementation of raised retention areas (bunds) on the flood plain to retain additional water in the flood plain, but not interfere with the flow in the river channel. The Taunton Flood Defence Improvements report produced by Black & Veatch identified a number of locations within the borough where storage areas could be sited. The focus of that study was looking at building dams to impound water and the sites considered were either on tributaries or too far upstream on the Tone and did not offer a significant reduction in flood levels in Taunton. For: Technically feasible; Easy to demonstrate effectiveness; Tried and trusted method; Simple to construct; Potential to reduce peak flows as well as mitigating volume; Peak reduction could be designed to provide third party betterment; A strategic solution.

TDBC SFRA v3.2 (FINAL).docx 48

Against: Cost of construction; Questions of ownership and adoption issues; Availability of land; Timescale of delivery; Breach/failure of asset and risk to third parties; Water Framework Directive implications; Time to peak implications with other Tone tributaries. Conclusion: The scheme has merit and is suitable for further consideration. A potential site has already been identified. The scheme has the potential to significantly reduce flood risk in the centre of Taunton. Option 2 - Diversion of flows using Bridgwater and Taunton Canal This option would involve implementing measures to improve the discharge of water from the centre of town. The flood water conveyed from the town centre would be discharged to "the Moors" system to the east of the M5. This would potentially create a marginal increase in water levels in "the Moors" storage areas, but would compensate for additional volumes of water discharging to the centre of Taunton. The Bridgwater & Taunton canal is an asset that could potentially facilitate this process on the basis that it gradually removed flood water from the centre of Taunton at a relatively low flow rate. It is worth noting that a similar scheme was considered in the 1960s but that a proposed cost of £1.7m was substantially more expensive than the channel improvements and defence construction undertaken. For: Making use of a pre-existing asset. Against: Cost of upgrading canal; Increased flood risk to Bathpool; Political issues in transferring more water to the levels and moors. Conclusion: Scheme is too expensive and likely to meet considerable objections. It is unlikely that this option would offer much reduction in flood risk to the centre of Taunton and should not be taken forward. Option 3 - Diversion of flows using Black Brook An initial assessment indicates that it might be feasible to improve the flow capacity of the Black Brook so that it could convey a greater quantity of flood water. The Black Brook conveys flows to the east of the M5 and hence it could potentially reduce the volume of flood water contributing to the centre of town. The upper reaches of the catchment also contain a location where the flows can be split between adjacent watercourses (the Stockwell and Sherford watercourses). It is possible that a proportion of the flow from the Sherford catchment could be diverted to the Black Brook and hence further reduce the volume of flow reaching the town centre. The potential effect of an increase in flow volume on the flood plain to the east of the M5 would need to be examined to ascertain whether there was a need for any further measures to mitigate potential adverse impacts due to increased volumes in the Black Brook. For: Potentially a cheaper solution. Against: Increased flood risk to third parties; Feasibility is difficult to demonstrate; May affect operation of attenuation ponds. Conclusion: Difficult to show that it could offer sufficient volume reduction or reduce flood risk to town centre and the increased third party risk means this option should be discounted. Option 4 - Enhanced mitigation measures on development sites It would be possible to consider the provision of additional storage areas within the proposed development sites that could accommodate the volumes of water associated with the residual risk. This would require bigger volumes and areas of storage to be made available to accommodate runoff from a long duration event. It has been suggested that this additional storage could be located in the country parks and corridors alongside watercourses identified in the masterplanning. The outline assessment performed for this version of the SFRA suggests that it would be prudent to explore the potential opportunities. The practicality in terms of the spatial extent of the areas, respective catchment areas and the operational implications would need to be examined. For: Easily deliverable; Tried and trusted Methods; Cost effective; Biodiversity and amenity

TDBC SFRA v3.2 (FINAL).docx 49

Against: Future ownership and maintenance; Land take may be significant (long duration events). Conclusion: Any enhanced scheme would have to meet Environment Agency requirements for a good SUDS management train through a development to ensure high water quality and amenity values. This remains a sensible approach for developments brought forward in the next 5 years. This option could offer some reduction in flood risk to the centre of Taunton and should be taken forward. Option 5 - Defence Improvements in Taunton It was proposed that the risk of flooding in Taunton can be reduced if the flood defences which do not offer an appropriate standard of protection (SOP) are upgraded. Section 2.3.2 identified the key stretches of defences which could be improved as being in the vicinity of: the telephone exchange, The Bridge (left bank), and Coal Orchard car park. Other potential weak points are defences in Tangier and the Old Livestock Market, however these are undergoing redevelopment including land-raising and would need to be reassessed once complete. Originally it was suggested that improving just the defence by the telephone exchange may provide a cost effective way to reduce flood risk however, a more detailed investigation revealed that only a co-ordinated improvement of defences could deliver a reduction in flood risk as a piecemeal approach just shifts the problem to the next weakest element of the defences. For: Effectiveness easily demonstrated; Reduces existing flood risk in line with CFMP policies. Against: Sustainability issues (climate change); Very High cost both capital and maintenance; Flood risk from breaching defences increases; Timescale of implementing. Conclusion: If an upstream storage option cannot be realised this may well be the only other viable option to reduce flood risk in Taunton significantly. It would however be a very expensive option but nonetheless should be considered further. Option 6 - Online storage on Tone tributaries It is possible to consider the use of strategic attenuation features on the principal tributaries contributing to the River Tone. The previous version of the SFRA identified potential strategic storage attenuation features such as flood storage reservoirs that could be constructed on some of the tributaries that discharge to the River Tone at Taunton. These have not been investigated further on the basis that they were placed in locations with properties in close proximity downstream. This would pose a risk to life if the dam were to fail and as such would make it necessary for the 1 in 10,000 year flood to be passed safely. In view of the high design standards required to pass this flood event the cost of these structures would likely be prohibitive and it is considered that they could not be implemented economically. For: Technically feasible Against: Expensive; May require significant land take; Issues of biodiversity, etc. from Water Framework Directive (WFD); Risk to third parties (dam failure); Adoption issues; Timescale of delivery. Conclusion: This option could prove problematic, there are siting and sizing issues to address and if they have to be constructed as Category A reservoirs they will involve a high capital cost. Maintenance costs and ownership questions will also prove difficult. It would be possible to provide the required attenuation and some reduction of flood risk in Taunton. A number of similar schemes have been investigated in previous studies in the borough and have not been taken forward and therefore this option should not be taken forward. "Linked" Strategic options There are two strategic schemes which have already been implemented in Taunton: The first was the construction of the Norton Fitzwarren Dam on the Halsewater and the associated channel improvements to support re-development of the Old Cider Works; and the second was the implementation of enhanced floodplain storage at Long Run Farm to mitigate land raising in the town centre required for the regeneration plans.

TDBC SFRA v3.2 (FINAL).docx 50

The effect of the Norton Fitzwarren Dam on river flows has not been taken into account in the flood zone mapping or actual flood risk predictions prepared for this SFRA as the final phase of works are not complete and the entirety of the Halsewater will be remapped at a later date. A review of the effects of the Norton Fitzwarren Dam on flood levels in Taunton can be found in Appendix B. It was found that the dam provides negligible reductions in water level in the centre of Taunton and does not alter the timing of those peak levels. The Long Run Farm flood storage area has been considered in the mapping and description of actual flood risk presented in this SFRA. The Long Run Farm scheme was designed solely to provide compensatory storage and is utilised very early during a flood event. Any proposed strategic scheme needs to be aware of how Norton Fitzwarren Dam and Long Run Farm operate in order that the new scheme does not compromise their operation or result in co-incident peaks due to the attenuation of the existing schemes. It therefore becomes an important to consider how the strategic options link with existing and proposed options. One option that was considered whilst preparing this SFRA was to investigate the management of the Level and Moors downstream of Taunton. This option was not considered further based on the findings of the Lower Parrett and Tone Flood Management Strategy (LPTFMS) prepared by Atkins which had considered a number of management options. There is still scope for considering changes to the management of the Levels and Moors in the longer term (beyond the end of the plan period in 2028). It is appreciated that the system is very complicated, hampered by a lack of available data, and that it would be contentious to suggest changes which may result in increased flooding of these areas but this would be in line with the CFMP policy and could provide a long term solution as the progressive effects of climate change worsen the situation for Taunton. Additionally Somerset County Council has advised that Sedgemoor District Council has long term plans to investigate the potential of a Barrage on the River Parrett to reduce flood risk at Bridgwater in response to the threat posed by rising sea levels as a consequence of climate change. The barrage is a tidal surge barrier and it is intended to be constructed in about 30 years time. Sedgemoor DC has already begun developer contributions. This proposal may not in itself reduce flood risk at Taunton but the operation of such a barrage could affect strategic proposals implemented in "the Moors" and Levels. Thus the potential influence of a barrage should be considered when investigating and analysing the opportunities to introduce strategic measures in the lower reaches of the River Tone.

6.4 Requirements for Development The requirements for the proposed developments will be in terms of infrastructure that will have to be implemented in order for the development of Taunton to remain sustainable. TDBC have set out along a sustainable path by aspiring to locate all of their proposed allocations within Flood Zone 1 as they have enough land suitable for development. This clearly demonstrates that they are applying the sequential test for their preferred sites as they are avoiding areas of higher flood risk. There is however areas of floodplain located within some of the larger proposed allocations, particularly Monkton Heathfield and Comeytrowe. Due to the amount of land with Flood Zone 1 within the allocations TDBC will seek to avoid development within the floodplain through the master-planning process by applying the sequential test within the site boundary. These areas of floodplain and river corridor should be protected and set aside by designating as green infrastructure areas. 6.4.1 Preferred infrastructure requirements - The next 5 years In the short term the most appropriate strategic option that should be implemented is limited to: (option 4) The provision of "residual risk" storage within proposed future development so that the volume surface runoff discharged is not increased over a storm duration of 50 hours. The cost of this option is closely linked to the drainage provisions identified within development being brought forward. There is some flexibility with respect to the details of the

TDBC SFRA v3.2 (FINAL).docx 51

provision. The most economical option would be to utilise public open space, recreation, amenity or green infrastructure areas to store water during the long duration, residual risk events. If it is not possible to design development that can make use of planned open areas then it will be necessary to identify bespoke areas of land that can be used for the sole purpose of storing surface runoff during residual risk events. In the latter circumstances the cost of implementation will include the potential effect on the density of development that can be implemented. Planning submissions for all development brought forward up to 2016 should contain detailed proposals that clearly identify the land that will be used for the "residual risk" storage together with details of how the storage systems will operate and be maintained.

6.4.2 Preferred infrastructure requirements - Up to 2028 In the longer term there is a need to consider the impact of runoff from more extensive areas of future development, as described in chapter 4. For the longer term it is appropriate to: Reduce the uncertainty associated with the prediction of flood risk in the centre of Taunton; and Identify strategic measures that not only address the requirements associated with additional future development, but which are also integrated with the wider Flood Risk Management needs of the catchment. This SFRA study has identified two potential options in the longer term. These are: Option 1 - Upstream storage Option 5 - Defence improvements in Taunton The feasibility of Option 1 has been assessed by considering the provision of additional storage in the flood plain upstream of the River Tone upstream of Taunton. Figure 6-4 shows how this scheme could provide the required reduction in volume of water reaching Taunton. Figure 6-4: Performance of proposed upstream storage location

TDBC SFRA v3.2 (FINAL).docx 52

Whilst the initial assessment performed for the SFRA demonstrates that an upstream storage option can provide the required storage volumes for the proposed developments, further assessment should be performed to: Provide further detail on the implications of increasing the existing flood storage capacity on the River Tone upstream of Taunton; and The infrastructure requirements and costs associated with these provisions must be included in future revisions to the Local Plan. Option 1 also provides the potential to offer a significant reduction to flood risk in Taunton if it were to be designed in such a way that the storage would not fill early in the event so as to permit the optimum reduction in peak flows. Option 5 is a default option. It is unlikely to ever be a preferred option but in the event that Option 1 cannot be implemented it becomes the only scheme capable of reducing the residual risk from the proposed developments. It would still require suitable on-site mitigation measures to ensure that runoff does not exceed greenfield rates for the new developments as it does not provide any additional storage. Consideration would also need to be given to the potential effects on flood risk at locations downstream of Taunton. The EA has also identified supplementary measures for consideration as part of the longer term plan. Whilst these may not offer any benefits in their own right they should be considered as part of any strategic scheme. The EA have proposed installing additional gauges on the River Parrett and Lower Tone in order to verify the findings of their LPTFMS study. The installation of these gauges would boost understanding of how the Moors operate as well as being used for flood warning. The EA feel that this would be of benefit in assisting TDBC's long term infrastructure proposals. Additionally the EA has identified improvements to Firepool and French Weirs as something that should be considered in the long term plan. This would almost certainly form part of Option 5 if it had to be taken forward but should also be considered alongside Option 1.

6.5 Generic guidance on applicable SUDs techniques Reference should be made to the Surface Water Management Plan prepared by SCC (predicted to be issued in 2011). All FRAs carried out in Taunton Deane borough should take account of surface water flooding. This should include all sites within Environment Agency Flood Zone 3a and 2, sites greater than 1ha in Flood Zone 1 and sites greater than 0.5ha in Critical Drainage Areas (CDAs). Managing surface water flooding should consider the same management hierarchy of assess, avoid, substitute and control as outlined in PPS25 with the aim of reducing risk by controlling water at the source (through SUDS) and considering flood mechanisms during exceedance events (i.e. development of flow paths). During the SWMP, each future development site discussed in this SFRA should be assessed to include: Risk assessments - assessing surface water risks Options - identify and prioritising mitigation measures Implementation - secure funding for implementation Review Phases The final SWMP, expected to be finalised in 2011, will be used to inform the delivery of Taunton Deane's Core Strategy, and should include appropriate policies on flooding and surface water drainage. All development proposed in Taunton Deane should refer to these documents for guidance and recommendations. As well as adhering to the guidance provided, FRAs should consider the following issues with regards to managing surface water as outlined in PPS25.

TDBC SFRA v3.2 (FINAL).docx 53

"How surface water is currently managed on site, how it is currently functioning and how it is to be undertaken in the new development All sewers that will subsequently be adopted by the sewerage undertaker must be designed and built in accordance with the requirements of Sewers of Adoption Sewers should be designed to ensure that no flooding occurs above ground level for events with a return period of 30 years For events with a return period in excess of 30 years, surface flooding of open space such as landscaped areas or car parks is acceptable for short periods No flooding of property should occur as a result of a one in 100 year storm event Developed rate of runoff into a watercourse, or another receiving body, should be no greater than the existing rate of runoff for the same event Developers are, however, strongly encouraged to reduce runoff rates from previously-developed sites as much as is reasonably practicable. Volumes of runoff should also be reduced wherever possible using infiltration and attenuation techniques.12" Using information provided in the SUDS Manual13 on design criteria, Table 6-1 has been prepared in order to identify available SUDS techniques when designing for a range of hydraulic conditions and objectives. These criteria are purely based on flood risk and others such as water quality, amenity and ecology should also be considered at a site level. Table 6-1: SUDS Hydraulic Design Criteria Criteria Design Event Design Objective Available Techniques Protect against 1 in 100/200 Control risks to people and Preservation of flooding from year event property riverside buffers and watercourse natural floodplain Site 1 in 10/30 No flooding on site, except Adequate site Protect against year event where planned and drainage and flapped flooding from drainage approved outfalls system Site 1 in Control risks to people and Subsurface storage, 100/200 year property increase flood levels event and retention ponds Protect against Site 1 in Planned flood routing and Open channels such flooding from overland 100/200 year temporary storage as swales or use of flows event, short accommodation on site road network duration events Catchment 1 in Attenuation storage to The majority of SUD 1 year event control 1 year site discharge techniques will help rate to ≤ 1 in 1 year achieve this aim Protect receiving greenfield peak rate drainage Catchment 1 in 100/200 year site discharge Retention and system/watercourse 100/200 year rate to ≤ 1 in 100/200 year detention from rate of discharge event greenfield peak rate All events Where possible, interception Source control storage to prevent runoff from first 5mm of rainfall Protect receiving Catchment 1 in Where possible, long term Infiltration and source drainage 100 year event storage/infiltration to control control system/watercourse 1 in 100 year discharge from volume of volume to ≤ 1 in 100 year discharge greenfield volume. Usually applied to 6hr even but in light of critical durations for the Tone this will need to be extended significantly.

12 Communities and Local Government (2009) Planning Policy Statement 25: Development and Flood Risk - Practice Guide 13 CIRIA (2007) The SUDS Manual Table 3.5

TDBC SFRA v3.2 (FINAL).docx 54

Using information produced within this SFRA, the risk to each site and its receiving watercourse/drainage system has been assessed at a high level provided in Table 6-1. Each site risk identified should be then linked to criteria in Section 7 to identify appropriate SUDS techniques suitable for managing that risk.

TDBC SFRA v3.2 (FINAL).docx 55

7. Summary assessment of development sites

7.1 Introduction A high-level assessment of the following potential development sites has been undertaken: 1) Monkton Heathfield 2) Priorswood / Nerrols 3) Staplegrove 4) Norton Fitzwarren / Ford Farm 5) Bishops Hull 6) Comeytrowe / Trull 7) Killams 8) Wellington (Cades, Jurston & Longforth) The assessment includes: flood zone mapping; mapping of water depths for the 100 year and 100 year plus climate change events (where available); and mapping of hazard rating (where available). The assessments consider the following: Type of development proposed; Is the site located outside flood zone 1 Depth of flooding possible at the site Are access routes affected Implications of Climate Change Assessment of Hazard The assessment also allows for the restatement of TDBCs development goals and the need for consideration of a strategic solution for mitigating increased flood risk from development.

7.2 Summary tables and FRA requirements The site summary tables can be found in Appendix C

TDBC SFRA v3.2 (FINAL).docx 56

8. Strategic management plan

8.1 Reasons why a plan is needed The analysis prepared for the TDBC SFRA has shown that: From 2015 up to 2028 it is proposed that there will be a significant increase in the volume of runoff that affects the volume of water in the River Tone that will potentially cause an increase in flood levels that could cause the flood defences to be over topped. This would result in flooding of substantial urban areas in the centre of Taunton; Over a longer time frame, but within the expected lifetime of development the effect of climate change (increased river flows and higher tide levels affecting the River Parrett) will increase the flood risk in the centre of Taunton. The operation of the flood storage in the Moors and the timing of events that elevate water levels in the River Parrett critically affects the chances of a flood being experienced at Taunton; and A substantial area of Taunton is protected by raised defences. If these banks fail or are overtopped the resulting level of hazard in the Centre of Taunton is high and the flooding extensive.

8.2 Key issues In preparing the SFRA the following issues have been identified. In developing a future strategy to manage flood risk it will be important that these issues are addressed as a priority: There is an urgent need to improve the data available on flood water levels, flows and the effect of tide levels at the confluence between the River Parrett and the River Tone. Collecting this data will improve the ability to prepare models to identify appropriate strategic measures and will improve the prediction of flood water levels during a flood event; There is a need to improve the data available on the operation of the flood storage areas within the Moors although it is unlikely that operational procedures will be changed at the present time. It will be important to fully understand the potential implications on the River Tone of constructing a barrage on the River Parrett; An assessment should be made of the quality of the hydrometric data available for the Hillfarrance Brook and the Halsewater particularly their rating curves. Despite recent changes to the rating curve, Bishops Hull is not accurate at recording high flows. We would recommend either: full 2D modelling of the gauging station to improve the rating; or re-locating the gauge; or using the gauge at Taunton Market in preference for which a rating is currently being validated. Improved understanding is required to address the uncertainty with respect to the timing of flows from the major tributaries of the River Tone upstream of Taunton; In view of the high level of hazard consideration should be given to the development of a more detailed flood emergency plan, based on the information that is now available as a consequence of preparing the SFRA; There is a need to identify the cost, feasibility and long term commitment required to strategic measures that will be needed as evidence for future revisions of the IDP. Unless the issues identified above are addressed it will not be possible to prepare proposals that are robust when the subject of scrutiny or examination; The plan must be coordinated to meet the requirements of other plans and policies, both existing and emerging; and Specific sources of funding should be sought to deliver the required outcomes

TDBC SFRA v3.2 (FINAL).docx 57

8.3 The Plan Wherever possible the activities required to prepare the plan should be coordinated with ongoing duties and responsibilities of relevant authorities. Many of the outcomes relate to activities that are performed by the Environment Agency. In the first instance a programme should be developed to identify outcomes and sources of funding. The primary objective of the plan should be to inform the TDBC Core Strategy and Infrastructure Delivery Plan in accordance with PPS12. It should also set out the intention of TDBC to apply a tariff in order to obtain developer contributions to ensure that the proposed infrastructure can be successfully delivered. TDBC has included the preferred strategic option (Option 1 - Upstream Storage) in their Infrastructure Delivery Plan (IDP) being prepared by Three Dragons and Roger Tym & Partners. Included within the plan is an initial cost estimate based on the scheme investigated for Section 6.4.2. It is proposed that this scheme would be funded through the Community Infrastructure Levy (CIL) and the details of how this could be applied are being considered. TDBC has also commissioned an inception report to investigate whether or not the proposed site of the upstream storage is feasible in light of the constraints and requirements detailed in Section 6 of this SFRA. The inception report will culminate in the completion of an FRM7 form to apply for funding for a full pre-feasibility study of the upstream storage option.

TDBC SFRA v3.2 (FINAL).docx 58

Appendices

TDBC SFRA v3.2 (FINAL).docx I

A. Update of historical flood database

In the previous version of the SFRA, Black & Veatch produced a database of historic flood events. This database was prepared using data provided by: The Environment Agency, Taunton Deane Borough Council, and Wessex Water. It contains data pertaining to all types of flooding within the borough and contains over 2000 records from the last 100 years. As part of producing this Level 2 SFRA it was necessary to update the database and its associated GIS files. A map of all the flood records is shown in Figure A-1 (below). The majority of the flooding in the database is not attributed a source but it is evident from the figure below that a significant amount of the historic events have occurred outside Flood Zones 2 and 3. Figure A-1: Historic flood events in the borough

TDBC SFRA v3.2 (FINAL).docx II

B. Assessment of Norton Fitzwarren Dam

As discussed in Section 1.5.3 a flood storage area has been constructed upstream of Norton Fitzwarren on the Halsewater. The Norton Fitzwarren Dam was constructed in 2008 as part of a flood alleviation scheme constructed to allow the redevelopment of the former Taunton Cider Site. The Dam is a 450m long earth bank with a clay core, the maximum height of the structure is over 5m and it can impound over 700,000m3 of flood water. Flood waters are only impounded once the capacity of a 2.5m x 2m box culvert is exceeded and the dam is designed not to overtop in the 100 year return period event plus an additional 20% allowance for climate change. A HEC-RAS model of the structure and a 7km stretch of the Halsewater, including the Norton Brook was produced by Hyder Consulting to support the Flood Risk Assessment of the dam. The modelling assessment included a baseline version of the model (before the construction of the dam), and versions that included the proposed dam so that the effects could be evaluated. Although the dam was constructed in order to facilitate development around Norton Fitzwarren it has been necessary as part of this SFRA to consider whether or the dam offers any residual benefit to flooding in the centre of Taunton. In order to assess this design flows for the 20, 50, 75 and 100 year return period events were passed through the HEC-RAS model for both the baseline and "with dam" scenarios. The outflows from the HEC-RAS model were then used in an ISIS model of the Tone developed for this SFRA and compared to investigate the effects on flood water levels in Taunton. It was found that whilst the Norton Fitzwarren Dam offers attenuation to the Halsewater this does not have a significant impact in the centre of Taunton. Reductions in levels and flows for each return period are shown in Table B-1 (below) for 3 locations in Taunton: downstream of French Weir, Priory Bridge, and downstream of Firepool. It was also found that the dam did not apparently alter the timing of flood peaks in Taunton.

Table B-1 Reduction in level and flows in Taunton due to construction of Norton Fitzwarren Dam Return Period Variable Downstream of Downstream of Downstream French Weir Priory Bridge of Firepool 20 Level (m) -0.015 -0.010 -0.010 Flow (m3/s) -0.9 -1.0 -1.0 50 Level (m) -0.030 -0.018 -0.016 Flow (m3/s) -1.9 -2.0 -2.0 75 Level (m) -0.031 -0.017 -0.016 Flow (m3/s) -1.9 -2.1 -2.0 100 Level (m) -0.037 -0.020 -0.016 Flow (m3/s) -2.2 -2.6 -2.4

There are several reasons why the dam would not provide any significant benefit to the centre of Taunton, notably the distance between Taunton and the dam and more importantly the fact that the peak flows on the Halsewater are significantly less than the flows on the Tone upstream of Taunton.

TDBC SFRA v3.2 (FINAL).docx III

C. Section 7 Maps

TDBC SFRA v3.2 (FINAL).docx IV

1) Monkton Heathfield Flood Zones

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx V

Current Flood Risk (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx VI

Future Flood Risk (100 year + climate change)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx VII

Hazard Rating (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx VIII

Summary of Risk Majority of development area is within Flood Zone 1except section along Dyer's Brook Fluvial and surface water flooding are the predominant risks Depths below 0.3m except in immediate vicinity of watercourse where depths up to 1m are possible. In channel depths may exceed 2m. Fluvial flood extents increase slightly under the climate change scenario. No defences within development area Hazard Classification Majority of site has a low hazard rating (<0.75) Areas of significant hazard largely confined to the immediate vicinity of the channel and around culverts Access and egress Minor Flooding to Yallands Hill and Blundell Lane possible in the vicinity of Dyer's Brook Flood Zones suggest flooding to both the A38 and M5 in the vicinity of the site. Flood Risk Implications Total development area of 2.1 km2 (210 ha) comprising 1 main area and 5 smaller for Development allocations. Majority of land is undeveloped, although some properties within main site. Proposed for residential mixed use with some open spaces TDBC to insist all development located within Flood Zone 1 Potential for a maintenance corridor along the watercourse in accordance with TDBC's Green Infrastructure proposals Consideration of the peak flows on the Tone and their durations required when considering drainage design Soils in the northern portions of the main site judged to have slightly impeded drainage, whilst southern portion is consider free draining. Climate change does not increase extent of fluvial flood risk significantly, <5cm increase in depths on average Assessment of runoff should include allowance for climate change effects New or re-development must seek opportunities to reduce overall level of flood risk at the site for example by: o reducing volume and rate of runoff o relocating development to zones with lower flood risk, o creating space for flooding. New developments should adopt exemplar source control SuDS techniques to reduce the risk of frequent low impact flooding due to post development run-off. Types of Development Where Greenfield land within Flood Zone 3 is to be developed, PPS25 states that the Sequential and Exception Tests must be passed for all types of development. Where in Flood Zone 3a only less vulnerable or water compatible development suitable. Where in Flood Zone 3b only water-compatible uses and essential infrastructure are appropriate. These areas should be kept as open space where possible. Mitigation measures will need to be taken to ensure that water flows are not impeded and flood risk is not increased elsewhere to allow development to proceed. FRA Issues guidance Additional modelling may be required for a site specific FRA.

Infrastructure Strategic Solutions for Taunton as outlined in Section 6 of this SFRA are required to Requirements enable this site. Consideration must be given to the magnitude of peak flows and the total volume of runoff generated

TDBC SFRA v3.2 (FINAL).docx IX

2) Priorswood/Nerrols Flood Zones

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx X

Current Flood Risk (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XI

Future Flood Risk (100 year + climate change)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XII

Hazard Rating (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XIII

Summary of Risk Majority of development area is within Flood Zone 1except section along Maiden Brook Fluvial and surface water flooding are the predominant risks Depths below 0.5m except in immediate vicinity of watercourse where depths up to 1.5m are possible Fluvial flood extents increase under the climate change scenario. No defences within development area Hazard Classification Majority of site has a low hazard rating (<0.75) Areas of significant hazard largely confined to the immediate vicinity of the channel Hazard higher upstream of the A2359 (Yallands Hill) culvert Access and egress No existing highways within site, only a farm track. Surrounding roads not inundated Risk of flooding to A2359 if culvert blocked Flood Risk Implications Total development area of 0.41 km2 (41 ha) for Development Majority of land is undeveloped with exception of Nerrols Farm. Proposed for residential mixed use with some open spaces TDBC to insist all development located within Flood Zone 1 Potential for a maintenance corridor along the watercourse in accordance with TDBC's Green Infrastructure proposals Consideration of the peak flows on the Tone and their durations required when considering drainage design Soils in the northern part of the site judged to have slightly impeded drainage, whilst southern section is considered free draining. Climate change does increase the extent of fluvial flood risk slightly, a 5cm increase in depths on average but higher locally upstream of culvert. Assessment of runoff should include allowance for climate change effects New or re-development must seek opportunities to reduce overall level of flood risk at the site for example by: o reducing volume and rate of runoff o relocating development to zones with lower flood risk, o creating space for flooding. New developments should adopt exemplar source control SuDS techniques to reduce the risk of frequent low impact flooding due to post development run-off. Types of Development Where Greenfield land within Flood Zone 3 is to be developed, PPS25 states that the Sequential and Exception Tests must be passed for all types of development. Where in Flood Zone 3a only less vulnerable or water compatible development suitable. Where in Flood Zone 3b only water-compatible uses and essential infrastructure are appropriate. These areas should be kept as open space where possible. Mitigation measures will need to be taken to ensure that water flows are not impeded and flood risk is not increased elsewhere to allow development to proceed. FRA Issues guidance Additional modelling may be required for a site specific FRA.

Infrastructure Strategic Solutions for Taunton as outlined in Section 6 of this SFRA are required to Requirements enable this site. Consideration must be given to the magnitude of peak flows and the total volume of runoff generated

TDBC SFRA v3.2 (FINAL).docx XIV

3) Staplegrove Flood Zones

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XV

Current Flood Risk (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XVI

Future Flood Risk (100 year + climate change)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XVII

Hazard Rating (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XVIII

Summary of Risk Entire site is within Flood Zone 1 Surface water flooding is the predominant risks Fluvial flood risk is negligible. No defences within development area Hazard Classification Entire development site has no hazard rating Neighbouring watercourses have low hazard except in the immediate vicinity of the channel Access and egress No roads within the site boundaries are inundated. Surrounding roads: Hope Corner Lane and Corkscrew Lane may suffer inundation to depths of less than 0.5m where the Kingston and Mil Lease Streams pass under them. Flood Risk Implications Total development area of 0.62 km2 (62 ha) for Development Majority of land is undeveloped with exception of a few properties on the edge of Staplegrove Proposed for residential mixed use with some open spaces TDBC to insist all development located within Flood Zone 1 Potential for a maintenance corridor along the watercourse in accordance with TDBC's Green Infrastructure proposals Consideration of the peak flows on the Tone and their durations required when considering drainage design The majority of the site is comprised of soils judged to have slightly impeded drainage. Climate change does not increase the extent of fluvial flood significantly, < 5cm increase in depths on average. Assessment of runoff should include allowance for climate change effects New or re-development must seek opportunities to reduce overall level of flood risk at the site for example by: o reducing volume and rate of runoff o relocating development to zones with lower flood risk, o creating space for flooding. New developments should adopt exemplar source control SuDS techniques to reduce the risk of frequent low impact flooding due to post development run-off. Types of Development Where Greenfield land within Flood Zone 3 is to be developed, PPS25 states that the Sequential and Exception Tests must be passed for all types of development. Where in Flood Zone 3a only less vulnerable or water compatible development suitable. Where in Flood Zone 3b only water-compatible uses and essential infrastructure are appropriate. These areas should be kept as open space where possible. Mitigation measures will need to be taken to ensure that water flows are not impeded and flood risk is not increased elsewhere to allow development to proceed. FRA Issues guidance Additional modelling may be required for a site specific FRA.

Infrastructure Strategic Solutions for Taunton as outlined in Section 6 of this SFRA are required to Requirements enable this site. Consideration must be given to the magnitude of peak flows and the total volume of runoff generated

TDBC SFRA v3.2 (FINAL).docx XIX

4) Norton Fitzwarren/Ford Farm Flood Zones

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XX

Current Flood Risk (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXI

Future Flood Risk (100 year + climate change)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXII

Summary of Risk Entirety of proposed development area is within Flood Zone 3, mostly Flood Zone 3b Fluvial and surface water flooding are the predominant risks Depths above between 0.5m and 1.5m in the eastern part of the site. Up to 2m in vicinity of the channel. Fluvial flood extents increase under the climate change scenario. Norton Fitzwarren Dam and associated flood alleviation schemes provide flood defence but SFRA modelling does not include at present. Hazard Classification No explicit representation of hazard available Significant flood depths likely to result in significant hazard rating in eastern portion of site Higher velocities around culverts will increase hazard Access and egress No roads exist within the site boundary. Main road north of the site, B3277innundated to a depth of up to 1m in places. Flood Risk Implications Development area of 0.16 km2 (16 ha) for Development Majority of land is undeveloped with exception of a few properties along the B3277 Proposed for residential mixed use with some open spaces TDBC to insist all development located within Flood Zone 1 Potential for a maintenance corridor along the watercourse in accordance with TDBC's Green Infrastructure proposals Consideration of the peak flows on the Tone and their durations required when considering drainage design Soils in the west of the site are classified as having impeded drainage, whilst the remainder are naturally wet with high water tables. Climate change does not increase the extent of fluvial flood significantly, < 5cm increase in depths on average. Assessment of runoff should include allowance for climate change effects New or re-development must seek opportunities to reduce overall level of flood risk at the site for example by: o reducing volume and rate of runoff o relocating development to zones with lower flood risk, o creating space for flooding. New developments should adopt exemplar source control SuDS techniques to reduce the risk of frequent low impact flooding due to post development run-off. Types of Development Where Greenfield land within Flood Zone 3 is to be developed, PPS25 states that the Sequential and Exception Tests must be passed for all types of development. Where in Flood Zone 3a only less vulnerable or water compatible development suitable. Where in Flood Zone 3b only water-compatible uses and essential infrastructure are appropriate. These areas should be kept as open space where possible. Mitigation measures will need to be taken to ensure that water flows are not impeded and flood risk is not increased elsewhere to allow development to proceed. FRA Issues guidance Additional modelling may be required for a site specific FRA, particularly to consider the effects of Norton Fitzwarren Dam and associated flood alleviation works. Infrastructure Strategic Solutions for Taunton as outlined in Section 6 of this SFRA are required to Requirements enable this site. Consideration must be given to the magnitude of peak flows and the total volume of runoff generated

TDBC SFRA v3.2 (FINAL).docx XXIII

5) Bishops Hull Flood Zones

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXIV

Summary of Risk Entire development area is within Flood Zone 1 Surface water flooding is the predominant risk No fluvial flood risk No flood defence present Hazard Classification No representation of hazard available Hazard would be expected to be low throughout the site Access and egress No roads exist within the site boundary. Adjacent roads not at risk of flooding. Flood Risk Implications Development area of 0.14 km2 (14 ha) for Development Land is entirely undeveloped Proposed for residential mixed use TDBC to insist all development located within Flood Zone 1 Soils in the site are classified as having slightly impeded drainage. Assessment of runoff should include allowance for climate change effects New or re-development must seek opportunities to reduce overall level of flood risk at the site for example by: o reducing volume and rate of runoff o relocating development to zones with lower flood risk, o creating space for flooding. New developments should adopt exemplar source control SuDS techniques to reduce the risk of frequent low impact flooding due to post development run-off. Types of Development Where Greenfield land within Flood Zone 3 is to be developed, PPS25 states that the Sequential and Exception Tests must be passed for all types of development. Where in Flood Zone 3a only less vulnerable or water compatible development suitable. Where in Flood Zone 3b only water-compatible uses and essential infrastructure are appropriate. These areas should be kept as open space where possible. Mitigation measures will need to be taken to ensure that water flows are not impeded and flood risk is not increased elsewhere to allow development to proceed. FRA Issues guidance Additional modelling may be required for a site specific FRA.

Infrastructure Strategic Solutions for Taunton as outlined in Section 6 of this SFRA are required to Requirements enable this site. Consideration must be given to the magnitude of peak flows and the total volume of runoff generated

TDBC SFRA v3.2 (FINAL).docx XXV

6) Comeytrowe/Trull Flood Zones

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXVI

Current Flood Risk (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXVII

Future Flood Risk (100 year + climate change)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXVIII

Hazard Rating (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXIX

Summary of Risk Majority of development area is within Flood Zone 1 Fluvial and surface water flooding are the predominant risks Depths below 0.5m except in immediate vicinity of watercourse where depths up to 2m are possible Fluvial flood extents increase slightly under the climate change scenario. No defences within development area Hazard Classification Majority of site has a low hazard rating (<0.75) Areas of significant hazard largely confined to the immediate vicinity of the channel Just upstream of Horts Bridge out of channel hazard is increased Access and egress Access roads are inundated even during lower return period events. Potential for Dipford Road and Sweethay lane to be flooded to depths >1m. Flood Risk Development area of 2.9 km2 (290 ha) Implications for Largely undeveloped land Development Proposed for residential mixed use with some open spaces TDBC to insist all development located within Flood Zone 1 Potential for a maintenance corridor along the watercourse in accordance with TDBC's Green Infrastructure proposals Consideration of the peak flows on the Tone and their durations required when considering drainage design Soils across the site judged to have slightly impeded drainage Climate change does not increase extent of fluvial flood risk significantly 5cm increase in depths on average Assessment of runoff should include allowance for climate change effects New or re-development must seek opportunities to reduce overall level of flood risk at the site for example by: o reducing volume and rate of runoff o relocating development to zones with lower flood risk, o creating space for flooding. New developments should adopt exemplar source control SuDS techniques to reduce the risk of frequent low impact flooding due to post development run-off. Types of Where Greenfield land within Flood Zone 3 is to be developed, PPS25 states Development that the Sequential and Exception Tests must be passed for all types of development. Where in Flood Zone 3a only less vulnerable or water compatible development suitable. Where in Flood Zone 3b only water-compatible uses and essential infrastructure are appropriate. These areas should be kept as open space where possible. Mitigation measures will need to be taken to ensure that water flows are not impeded and flood risk is not increased elsewhere to allow development to proceed. FRA Issues guidance Additional modelling may be required for a site specific FRA.

Infrastructure Strategic Solutions for Taunton as outlined in Section 6 of this SFRA are Requirements required to enable this site. Consideration must be given to the magnitude of peak flows and the total volume of runoff generated

TDBC SFRA v3.2 (FINAL).docx XXX

7) Killams Flood Zones

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXXI

Current Flood Risk (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXXII

Future Flood Risk (100 year + climate change)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXXIII

Hazard Rating (100 year)

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXXIV

Summary of Risk Majority of development area is within Flood Zone 1 Fluvial and surface water flooding are the predominant risks Depths below 0.5m except in immediate vicinity of watercourse where depths up to 2m are possible Fluvial flood extents increase slightly under the climate change scenario. No defences within development area Hazard Classification Majority of site has a low hazard rating (<0.75) Areas of significant hazard largely confined to the immediate vicinity of the channel Area of significant hazard on Killams Lane just outside site boundary Access and egress Currently there are no roads passing through the site Adjacent roads suffer minor flooding even at moderate return periods Killams Lane floods to depths of 0.5m. Flood Risk Implications Development area of 0.23 km2 (23 ha) for Development Previously undeveloped land Proposed for residential mixed use with some open spaces TDBC to insist all development located within Flood Zone 1 Potential for a maintenance corridor along the watercourse in accordance with TDBC's Green Infrastructure proposals Consideration of the peak flows on the Tone and their durations required when considering drainage design Soils across the site judged to have slightly impeded drainage. Climate change does not increase extent of fluvial flood risk significantly, <5cm increase in depths on average Assessment of runoff should include allowance for climate change effects New or re-development must seek opportunities to reduce overall level of flood risk at the site for example by: o reducing volume and rate of runoff o relocating development to zones with lower flood risk, o creating space for flooding. New developments should adopt exemplar source control SuDS techniques to reduce the risk of frequent low impact flooding due to post development run-off. Types of Development Where Greenfield land within Flood Zone 3 is to be developed, PPS25 states that the Sequential and Exception Tests must be passed for all types of development. Where in Flood Zone 3a only less vulnerable or water compatible development suitable. Where in Flood Zone 3b only water-compatible uses and essential infrastructure are appropriate. These areas should be kept as open space where possible. Mitigation measures will need to be taken to ensure that water flows are not impeded and flood risk is not increased elsewhere to allow development to proceed. FRA Issues guidance Additional modelling may be required for a site specific FRA.

Infrastructure Strategic Solutions for Taunton as outlined in Section 6 of this SFRA are required to Requirements enable this site. Consideration must be given to the magnitude of peak flows and the total volume of runoff generated

TDBC SFRA v3.2 (FINAL).docx XXXV

8) Wellington Flood Zones

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right 2010. All rights reserved. Ordnance Survey Licence number 100019499.

TDBC SFRA v3.2 (FINAL).docx XXXVI

TDBC SFRA v3.2 (FINAL).docx XXXVII

Summary of Risk Majority of development areas are within Flood Zone 1 Surface water flooding is the predominant risk Fluvial flood risk limited to the northernmost portion of the Cades site No flood defences present Hazard Classification No representation of hazard available Hazard would be expected to be low throughout the site Access and egress Jurston Lane is situated within the Jurston site but it is not at risk of flooding. Adjacent roads not at risk of flooding Flood Risk Implications Development area of 1.12 km2 (112 ha) for Development Land is largely undeveloped excluding western part of Longforth development and isolated farms elsewhere Proposed for residential mixed use TDBC to insist all development located within Flood Zone 1 Soils in the site are classified as having impeded drainage. Assessment of runoff should include allowance for climate change effects New or re-development must seek opportunities to reduce overall level of flood risk at the site for example by: o reducing volume and rate of runoff o relocating development to zones with lower flood risk, o creating space for flooding. New developments should adopt exemplar source control SuDS techniques to reduce the risk of frequent low impact flooding due to post development run-off. Types of Development Where Greenfield land within Flood Zone 3 is to be developed, PPS25 states that the Sequential and Exception Tests must be passed for all types of development. Where in Flood Zone 3a only less vulnerable or water compatible development suitable. Where in Flood Zone 3b only water-compatible uses and essential infrastructure are appropriate. These areas should be kept as open space where possible. Mitigation measures will need to be taken to ensure that water flows are not impeded and flood risk is not increased elsewhere to allow development to proceed. FRA Issues guidance Additional modelling may be required for a site specific FRA.

Infrastructure Strategic Solutions for Taunton Deane as outlined in Section 6 of this SFRA are required Requirements to enable this site. Consideration must be given to the magnitude of peak flows and the total volume of runoff generated

TDBC SFRA v3.2 (FINAL).docx XXXVIII

D. Summary of Modelling Work

D.1 Existing river models There is a long history of river models of the River Tone and its tributaries through Taunton. In 2004, Symonds developed a single River Tone catchment model from all the pre-existing hydraulic models in the area, as part of a Section 105 flood mapping study for the Environment Agency. This was further developed in 2007 by Black and Veatch, for the Taunton Flood Defence Improvements Study and Taunton Flood Management Masterplan Study. At this time the extents of the model were reduced and the hydrological inputs adjusted. Both of these models are 1D models (built using ISIS software) and both were supplied to JBA Consulting to use for the SFRA.

D.2 Aims of modelling for SFRA There were two main aims of the river modelling carried out for the SFRA. 1. Update the existing river model to include the latest data and information available in the area; 2. Link the 1D model through Taunton to a 2D model of the floodplain to enable a better understanding of flow routes and flood mechanisms in the town.

D.3 Updating the hydrology and tidal information The hydrology within the model was revised to take account of revisions to the procedures for applying the statistical method within the Flood Estimation Handbook (FEH). The hydrological analysis included a review of the locations of inflows. The majority of existing locations were retained but some simplification of inflows on tributaries such as the Galmington Stream was undertaken and the intervening areas were reassessed. The new flow estimates were produced for a full range of return periods (2, 5, 10,20,50,75,100, 100+climate change, 200, and 1000 years) using the statistical method. This method takes into account the observed data available at local gauging stations such as Bishops Hull, Halsewater, and Milverton. To maintain consistency with the previous model, hydrograph shapes were produced using the FEH Rainfall Runoff method and scaling the peak flows to the statistical estimates. Improvements were also made to the downstream boundary of the Tone model by utilising modelled levels from the Lower Parrett and Tone Flood Management Strategy Model (Atkins, 2003), which allowed for improved representation of the tidal conditions.

D.4 Updating the river model Building a 1D/2D linked model The 1D model through Taunton has been linked to a 2D model of the floodplain (using the ISIS-TUFLOW software) to better represent the interaction between the river and the floodplain, and flow routes across the floodplain. The 2D part of the model covers the centre of Taunton, including the River Tone between Silk Mills Road (A 3065) and the M5, part of the Galmington Stream, part of Sherford Stream, part of Black Brook, and the canal area. Ground levels for the floodplains have been taken from up to date LIDAR data, unless additional information was available (see next section for details). The ground model grid resolution is 5m. Significant details such as raised flood defences and changes to ground levels as a result of new development were built into the model as additional features. OS

TDBC SFRA v3.2 (FINAL).docx XXXIX

Mastermap data was used to provide detail to the model about the type of ground surface, and the location of buildings and roads, in order to represent their effect on flow. Beyond these extents the model is still in 1D, but various improvements have been made based on the model review. The Black and Veatch model has been re-extended, and tributaries added back in, including parts of Allens Brook and Mill Brook. A check was carried out for model cross sections where the extent of the floodplain was not fully represented (leading to water levels reaching beyond the edge of the data, known as 'glass-walling'). These sections were extended using LIDAR data where necessary. A separate version of the model was built with the formal raised flood defences removed. Incorporation of new data and information Since the previous models were built, new LIDAR ground level data, several new surveys and sources of data and information have become available. A summary of what has been included in the new model is given below: A survey of flood defence crest levels (spot levels every 10m along crests) was carried out by Halcrow for the Environment Agency in 2008, following recommendations by the Black and Veatch study. The survey showed significant differences from the levels used in the original model. These up to date crest levels have been used in the new model. A survey of the canal was provided by British Waterways and incorporated into the model to improve the representation of the canal between Firepool and Bathpool. New ground levels were incorporated according to the recent works carried out at the Somerset County Cricket Ground and the Long Run Farm development flood mitigation scheme. The ground levels of the floodplains downstream of Taunton were improved in the model using recent LIDAR ground data. Storage of water in these floodplains during an event (particularly when combined with a high tide) has a significant effect on flooding in Taunton. Information on pumping rules was provided by the Environment Agency and included. However, on the basis of the results from the model review it was identified that there were several locations where the model representation could be improved, but this was not possible at the following locations for reasons stated: Several footbridges were identified as missing but no survey data was available for them so they were not added into the model. The Third Way Bridge was not modelled in detail as associated redevelopments in Tangier are ongoing. Information on the bridge's embankments on the left bank was however used to update defence information. Knapp Bridge was not included in the original model due to stability issues that could not be resolved by the present study. The Norton Fitzwarren Dam was not modelled in detail as the associated flood alleviation scheme was not complete at the time the SFRA modelling was undertaken.

D.5 Modelled events The improved 1D/2D linked River Tone model was run for the following events, both with and without the formal flood defences: 20 year 100 year 100 year with climate change 1000 year

TDBC SFRA v3.2 (FINAL).docx XL

D.6 Summary of results The model shows that flooding is relatively limited to the Tangiers area in the 20 year return period, and generally the raised defences successfully alleviate flooding to properties in most of the town. In the 50 year return period event floodwater overtops the defences at the former livestock market allowing water to flood the town in other locations from behind the defences. There is also some localised flooding from overtopping of the defences in the vicinity of The Bridge. At the 75 year return period this localised flooding connects with the flood waters leaving the livestock market. For the 100 year event the defences are overtopped on both banks in central Taunton. Further overtopping occurs in the 100 year with climate change and 1000 year events, notably on the right bank at Firepool and in the canal area. Discussion with the Environment Agency and Council suggests that this pattern of flooding is realistic. A comparison of results with the original model shows that predicted water levels are similar. However, the incorporation of updated survey data means the new modelled outlines show some significant differences from the existing Flood Zones through Taunton, for example at the Cricket Ground, and the area south of the canal. The flood outlines also clearly show the advantages of modelling the floodplain through Taunton using a 2D technique, as routes around areas of high ground and along roads are incorporated, while defences are clearly shown holding back flood water.

TDBC SFRA v3.2 (FINAL).docx XLI

E. Flood Defences within the borough

There are a significant number of raised defences within Taunton Deane these are shown along with Flood Storage Areas in Figure E-1 (below). The longest sections of defences are surrounding the flood storage areas in the Somerset Levels and Moors. There are also significant lengths of defence in the centre of Taunton and protecting the settlements of Ham, Ruishton, and Creech St Michael. Additionally there are defences for the flood alleviation channel in Hillfarrance, a flood bank in Tone Green, the Norton Fitzwarren Dam, a flood storage area in Westford, and a number of small localised defences. In total as of May 2011 there are 244 raised defence assets and 7 flood storage areas within the borough. Details of the defences and flood storage areas can be obtained on request from the Environment Agency. The Standard of Protection (SOP) indicated by the shading in Figure E-1 is a value from when the structure was designed and not the current standard of protection which will likely have changed.

TDBC SFRA v3.2 (FINAL).docx XLII

310000 311000 312000 313000 314000 315000 316000 317000 318000 319000 320000 321000 322000 323000 324000 325000 326000 327000 328000 329000 330000 331000 332000 333000 334000 335000 336000 337000 338000 339000 340000 341000 342000 343000 344000 345000 346000 347000 348000 349000

133000 133000 North

132000 132000

131000 131000 KEY 130000 130000

129000 129000 Raised Defences - Design SOP (years) Unknown

128000 128000 1 2

127000 127000 3 5

126000 126000 20 25

125000 125000 40 50

124000 124000 60 70

123000 123000 75 80

122000 122000 100 150

121000 121000 200 Flood Storage Area

120000 120000

KEY PLAN 119000 119000

118000 118000

117000 117000

116000 116000

115000 115000

114000 114000

113000 113000

112000 112000

111000 111000

Contains Ordnance Survey data © Crown copyright and database right 2010. This map is reproduced from Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty's Stationery Office Crown copyright. Unauthorised reproduction infringes Crown copyright 110000 110000 and may lead to prosecution or civil proceedings. Taunton Deane Borough Council. 100019499 (2010).

109000 109000

108000 108000 Rev. Modifications Date Drawn Checked Approved

Aberdeen House South Road 107000 107000 Haywards Heath West Sussex RH16 4NG United Kingdom

106000 106000 t +44 (0)1444 473 652 TAUNTON DEANE BOROUGH COUNCIL f +44 (0)8458 627 772 THE DEANE HOUSE BELVEDERE ROAD e [email protected] TAUNTON 105000 105000 SOMERSET Other offices at Atherstone, Doncaster, Edinburgh, TA1 1HE Newcastle upon Tyne, Newport, Northallerton, Saltaire, Skipton, Tadcaster, Wallingford & Warrington

104000 104000 for TANUTON DEANE BOROUGH COUNCIL

103000 103000 RAISED DEFENCES

102000 102000

This document is the property of Jeremy Benn Associates Ltd. It shall not be reproduced in whole or in part, nor disclosed to a third party, without the permission of Jeremy Benn Associates Ltd. 101000 101000 Scale: Drawn BG 22/06/2011 1:60,000 Checked OF 22/06/2011 Approved AD 22/06/2011 100000 100000 Digital File Name: 2010s4327_RAISED_DEFENCES.MXD Drawing Number: Sheet No.: Status: Rev.:

2010s4380_RAISED_DEFENCES_01 1 of 1 FINAL A 99000 99000 310000 311000 312000 313000 314000 315000 316000 317000 318000 319000 320000 321000 322000 323000 324000 325000 326000 327000 328000 329000 330000 331000 332000 333000 334000 335000 336000 337000 338000 339000 340000 341000 342000 343000 344000 345000 346000 347000 348000 349000 Original @ A1

F. Surface Runoff Calculations

Runoff Calculations - Taunton Deane SFRA 17 Hour 50 Hour Storm Duration Storm Duration Notes: Rainfall Depth (mm) (100 year Return Period) 90 127 Rainfall Depth (mm) with climate change allowance (+30%) 117 165 Average value across the borough individual development sites vary Pre-development (Greenfield) runoff (%) 35 35 between 30 and 40% Conservative value assumed in absence of data on proportion of impermeable areas within Post Development runoff (%) 70 70 developments

Storage Volume = Increase in Runoff

Development in next 5 years Exludes sites with agreed drainage Area of development (ha) 33 33 strategies Pre-development Runoff Volume (m3) 13513.5 19057.5 Post-development Runoff Volume (m3) 27027 38115 Required Storage (m3) 13,500 19,100

Additional Development upto 2027 Area of development (ha) 700 700 Pre-development Runoff Volume (m3) 286650 404250 Post-development Runoff Volume (m3) 573300 808500 Required Storage (m3) 287,000 404,000

TDBC SFRA v3.2 (FINAL).docx XLIV

G. Procedure for Flood Zone Challenges

TDBC SFRA v3.2 (FINAL).docx XLV

SFRA Flood Map and Flood zone Challenge Decision Tree Important Note: We would strongly advise that all SFRA challenges are submitted and What type of SFRA Are they challenging the flood outline approved by both the LPA and the EA prior to the application of PPS25 Challenge has been shown in the Level 1 SFRA and EA flood Sequential Test. submitted? map or is it a SFRA 3a/3b zone challenge? As it can take time to update the SFRA maps the challenger must provide a revised map showing the approved flood map or flood zone.

The EA is the Project The LPA's SFRA Officer is the Project Manager for approving this SFRA Flood Map Challenge SFRA Zone 3a/3b Challenge Manager for approving this challenge in challenge. conjunction with the EA.

Has the challenge been Is the challenge supported by credible evidence to demonstrate that a SFRA zone challenge can supported by extensive No hydrological/hydraulic No be achieved? modelling?

EA to contact Challenger to LPA SFRA project Manager to contact inform them what Challenger to inform them what information is required. information is required. Yes Yes

Has the EA The EA Flood Map and LPA SFRA LPA to consult the EA on approved the No map will remain as they currently the 3a/3b challenge challenge? stand.

LPA SFRA project Manager to contact Has a satisfactory challenge been approved by both the LPA and EA in accordance with the Yes Challenger to inform them, with No reasons, why the challenge has been functional floodplain definitions stated in PPS25 unsucessful . and the Practise Guide? This will be included in the next quarterly EA flood map update and This should then be fed back into the returned to the LPA in SFRA flood map as part of the annual The revised map will be included in the usual CD format. monitoring update or at the earliest annual monitoring update of the SFRA opportunity. maps by the LPA or at the earliest Yes convenience.