West Somerset Strategic Flood Risk Assessment Level 2 (2010)

Home , Monksilver, River Avill, Washford River

West Somerset Council Level 2 Strategic Flood Risk Assessment

Final Report October 2010

Prepared for

West Somerset Council Level 2 SFRA

Revision Schedule

Level 2 SFRA October 2010

Rev Date Details Prepared by Reviewed by Approved by

01 May 2010 Vs1 Nick Bosanko Dr Rob Sweet Jon Robinson Flood Risk Specialist Senior Flood Risk Associate Director Specialist

02 October 2010 Vs2 – Updated Nick Bosanko Tom Edwards Jon Robinson with EA & WSC Flood Risk Specialist Senior Flood Risk Associate Director comments Specialist Dr Rob Sweet Senior Flood Risk Specialist

Scott Wilson The Crescent Centre Temple Back This document has been prepared in accordance with the scope of Scott Wilson's appointment with its client and is subject to the terms of that appointment. It is addressed Bristol to and for the sole and confidential use and reliance of Scott Wilson's client. Scott Wilson BS1 6EZ accepts no liability for any use of this document other than by its client and only for the purposes for which it was prepared and provided. No person other than the client may copy (in whole or in part) use or rely on the contents of this document, without the prior written permission of the Company Secretary of Scott Wilson Ltd. Any advice, opinions, Tel: 0117 917 1214 or recommendations within this document should be read and relied upon only in the Fax: 0117 930 0342 context of the document as a whole. The contents of this document do not provide legal or tax advice or opinion.

Table of Contents Glossary & Abbreviations ...... 1 1 Introduction ...... 3 1.1 Overview ...... 3 1.2 Aim of Level 2 SFRA ...... 4 1.3 Level 2 SFRA Objectives...... 4 1.4 Level 1 SFRA Critique ...... 4 2 Context...... 6 2.1 Study Area...... 6 2.2 Hydraulic Modelling ...... 6 2.3 Flood Hazard...... 6 2.4 Flood Zone 3b (Functional Floodplain)...... 7 2.5 Additional Considerations ...... 7 3 Minehead...... 9 3.1 Location...... 9 3.2 Topography ...... 9 3.3 Flood Risk and Flood Risk Management ...... 10 3.4 Aims ...... 12 3.5 Methodology...... 12 3.6 Hydraulic Model Results – Tidal Overtopping ...... 14 3.7 Hydraulic Model Results – Tidal Overtopping & Breach...... 16 3.8 Focused Assessments...... 17 3.9 Limitations ...... 18 3.10 Fluvial Flood Risk ...... 18 3.11 Flood Zone 3b (Functional Floodplain)...... 19 3.12 Access & Egress ...... 19 3.13 Flood Mitigation ...... 20 3.14 Recommendations...... 22 4 Williton ...... 24 4.1 Location...... 24 4.2 Topography ...... 24 4.3 Overview of Flood Risk and Flood Risk Management...... 25 4.4 Aims ...... 25 4.5 Methodology...... 25 4.6 Hydraulic Model Results ...... 26

West Somerset Council Level 2 SFRA

4.7 Limitations ...... 30 4.8 Flood Zone 3b (Functional Floodplain)...... 30 4.9 Access & Egress ...... 31 4.10 Flood Mitigation ...... 31 4.11 Recommendations...... 32 5 Watchet ...... 33 5.1 Location...... 33 5.2 Topography ...... 33 5.3 Overview of Flood Risk and Flood Risk Management...... 34 5.4 Aims ...... 34 5.5 Methodology...... 34 5.6 Hydraulic Model Results ...... 35 5.7 Limitations ...... 35 5.8 Flood Zone 3b (Functional Floodplain)...... 37 5.9 Access/Egress...... 37 5.10 Flood Mitigation ...... 37 5.11 Recommendations...... 37 6 Sequential Approach to Site Allocation...... 39 6.1 Exception Test...... 39 7 Site Specific Flood Risk Assessment Guidance...... 40 7.1 Site Specific FRA Requirements...... 40 7.2 Site Vulnerability and Site Layout ...... 41 7.3 Finished Floor Levels...... 41 7.4 Raising Ground Levels ...... 42 7.5 Surface Water Management ...... 43 8 Residual Risk Mitigation...... 44 8.1 Flood Resilience and Resistance Measures ...... 44 8.2 Safe Access and Egress...... 45 8.3 Flood Warning and Evacuation Plans ...... 45 9 Policy Guidance ...... 47 10 Summary and Conclusions ...... 49 10.1 Minehead...... 49 10.2 Williton...... 50 10.3 Watchet ...... 51 10.4 Site Allocation...... 51

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10.5 Maintenance and Update...... 52 11 Appendices...... 53

West Somerset Council Level 2 SFRA

Glossary & Abbreviations

Term Definition

1D Hydraulic Model Simulates the flow of water, including water levels, within the river channel.

2D Hydraulic Model Simulates multi-directional flows, normally used to model the floodplain.

Event that on average will occur once every 100 years. Also expressed as an event, 1 in 100 year event which has a 1% probability of occurring in any one year or 1% annual exceedence probability (AEP).

AOD Above Ordnance Datum

Climate change. When included as part of a flood event return period scenario, it means that that scenario includes the anticipated affects of climate change. For tidal CC events, this will result in a higher sea level of approximately 1m over 100 years and for fluvial events, it incorporates a 20% increase in river flow. These climate change values are based upon information within PPS25.

EA Environment Agency

ENPA Exmoor National Park Authority

Infrastructure such as floodwalls and embankments used to protect land from flooding. Flood defence Flood defences are normally designed to a specific standard of protection (design standard).

Floodplain Area adjacent to river, coast or estuary that is naturally susceptible to flooding.

FRA Flood Risk Assessment

Fluvial flooding Flooding by a river or a watercourse.

This zone comprises of land assessed as having a less than 1 in 1000 annual Flood Zone 1 probability of river of sea flooding in any year (0.1%).

This zone comprises land assessed as having between a 1 in 100 year and 1 in 1000 Flood Zone 2 year annual probability of river flooding (1% - 0.1%) or between a 1 in 200 year and a 1 in 1000 year annual probability of sea flooding (0.5% - 0.1%) in any year.

This zone comprises land assessed as having a 1 in 100 or greater annual probability Flood Zone 3a of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.

This zone comprises land where water has to flow or be stored in times of flood. Local Planning authorities should identify in their SFRAs areas of Functional Floodplain and its boundaries accordingly, in agreement within the Environment Agency. The Flood Zone 3b – Functional identification of Functional Floodplain should take account of local circumstances and Floodplain not be defined solely on rigid probability parameters. But 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, should provide a starting point for consideration and discussions to identify the functional floodplain.

The core of the updated planning system (introduced by the Planning and Compulsory Local Development Purchase Act 2004). The LDF comprises the Local Development Documents, including Framework (LDF) the development plan documents that expand on policies and provide greater detail. The development plan includes a core strategy, site allocations and a proposals map.

An element of development design which may be used to manage flood risk or avoid Mitigation measure an increase in flood risk elsewhere.

PPS25 Planning Policy Statement 25: Development and Flood Risk. This is the current

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guidance document used to inform development of areas subject to flood risk.

Risk is a factor of the probability or likelihood of an event occurring multiplied by Risk consequence: Risk = Probability x Consequence. It is also referred to in this report in a more general sense.

SFRA Strategic Flood Risk Assessment

SMP Shoreline Management Plan

Sustainable Drainage Systems. Methods of management practices and control SuDS structures that are designed to drain surface water in a more sustainable manner than some conventional techniques.

STW Sewage Treatment Works

WSC West Somerset Council

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1 Introduction

1.1 Overview

The latest revision of Planning Policy Statement 25: Development and Flood Risk (PPS25)1 was published in March 2010, which emphasises the active role Local Planning Authorities (LPAs) have in ensuring flood risk is considered in strategic land use planning. PPS25 requires LPAs to undertake a Strategic Flood Risk Assessment (SFRA) as part of their evidence base for the Local Development Framework (LDF) process and to use the findings of SFRAs to inform strategic land use planning.

The Level 1 SFRA2 for West Somerset Council (WSC) was completed in 2009 and incorporated Exmoor National Park. The Level 1 report has allowed WSC and Exmoor National Park Authority (ENPA) to help determine the significance of flood risk for each potential source of flooding across the entire administrative area for spatial planning purposes and to undertake the Sequential Test as set out in PPS25.

WSC Core Strategy3 identifies the statutory requirement to provide 2,500 dwellings, 4 residential Gypsy pitches and 5 hectares of additional employment land during the period 2009 to 2026. Flood risk is relatively extensive across parts of West Somerset, and when combined with the housing needs and other development constraints, availability of suitable land for development is limited. Consequently, a Level 2 SFRA has been prepared for West Somerset. The aim of the Level 2 SFRA is to provide more detailed information on flood risk at the key areas identified for development as part of the Sequential Test, to help determine how or if development can be undertaken in a safe and sustainable way, in accordance with the principles of PPS25.

WSC Core Strategy identifies three strategic development areas in West Somerset where new development is likely to be concentrated. This Level 2 SFRA focuses on these strategic development areas, which are located at Minehead, Williton and Watchet and are considered by WSC to be the only areas suitable for (large) development on sustainability grounds. This is because they are the only settlements to offer a reasonable range of existing services and facilities on which it is possible to encourage sustainable patterns of development and existence amongst potential future residents. All three settlements have significant constraints to further expansion through development from flooding. This limits possible future strategies and/or options and must therefore be examined in further detail.

This Level 2 SFRA presents the methodology and findings of a hydraulic modelling and flood risk mapping exercise, which has investigated the strategic flood risk associated with fluvial or tidal sources at the three strategic development areas. The Level 2 SFRA provides flood depth and hazard mapping to inform the strategic land allocation process.

However, it is important to note that the Level 2 SFRA flood maps for each strategic development area only consider flood risk from one (but considered to be the most significant) source. The Level 1 SFRA flood maps provide flood risk information associated with each potential source of flooding and must therefore be cross referenced by the Council during the site allocation process etc.

1Communities and Local Government (2010) ‘Planning Policy Statement 25: Development and Flood Risk’, TSO: London. 2 West Somerset Council and Exmoor National Park Authority (2009) Strategic Flood Risk Assessment. Prepared by Scott Wilson. 3 West Somerset (2010) Core Strategy Options Paper.

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1.2 Aim of Level 2 SFRA

The aim of the Level 2 SFRA is to provide supplementary information to the WSC Level 1 SFRA on flood risk issues specific to the three strategic development areas. This Level 2 SFRA (and accompanying GIS data) should be used by WSC in conjunction with the Level 1 SFRA to help assess the development opportunities at the strategic development sites.

1.3 Level 2 SFRA Objectives

The aim of this Level 2 SFRA will be achieved through the following general objectives:

• Identify the flooding mechanism at each strategic development site; • Identify the depth of flooding within each strategic development site; • Indicate the flood hazard within each strategic development site; • Advise on general safe access and egress opportunities from each strategic development site; • Identify the impact of climate change upon flood risk; • Identify where specific mitigation measures may improve development opportunities; • Guidance on application of the Exception Test, where required; • Guidance on residual risk mitigation; • Guidance on site specific Flood Risk Assessments.

1.4 Level 1 SFRA Critique

The Level 1 SFRA study area covers all the land within the administrative boundary of WSC, which covers an approximate area of 945 km2 and includes 490km2 associated with Exmoor National Park (Appendix A).

A considerable amount of data was collected from a number of stakeholders in order to determine the main sources of flooding within the study area. The predominant and most widespread flood risk throughout the study area is from fluvial sources. Tidal flood risk is also significant, but isolated to coastal areas, where the coastal zone is flat and low-lying, such as at Minehead. Historic flood incidents associated with groundwater, surface water and sewers were also identified within the study area.

The Level 1 SFRA was based upon fluvial and tidal Flood Zone Maps provided by the Environment Agency. The Flood Zone Map shows the estimated extent of Flood Zones 2 and 3 ignoring the presence of flood defences for all Main Rivers, watercourses with identified critical drainage problems and coastal areas.

The Environment Agency Flood Zone Maps do not differentiate between Flood Zone 3a (high probability) and Flood Zone 3b (Functional Floodplain). Consequently, a precautionary approach was adopted to identify Flood Zone 3b, see Section 2.4. In addition, the effects of climate change on Flood Zone extent is not generally available from the Environment Agency and therefore a pragmatic approach was undertaken to identify utilising surrogates (i.e. Flood

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Zone 2 under present day conditions is used as a surrogate for Flood Zone 3a accounting for climate change).

The Level 2 SFRA incorporates hydraulic modelling of fluvial and tidal sources, which include consideration of flood defences, where they exist. This is particularly important at Minehead where major coastal defences align the sea front. Hydraulic modelling also allows a more accurate representation of Flood Zone 3b to be derived for fluvial watercourses, which has been undertaken at Williton and Watchet. Flood extent, depth and hazard mapping has been undertaken at each strategic development site to inform the strategic land allocation process.

The Level 2 SFRA provides no further information with respect to other sources of flood risk (i.e. groundwater, surface water and sewers) within the strategic development areas and therefore the Level 1 SFRA should be consulted in this regard.

The Level 2 SFRA hydraulic modelling also derives the flood extent associated with the anticipated affects of climate change at each of the three strategic development areas (i.e. Minehead, Williton and Watchet). This therefore replaces the associated flood maps outlined in the Level 1 SFRA at the three strategic development areas.

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2 Context

2.1 Study Area

West Somerset is located on the north coast of South West England, along the Severn Estuary. As discussed in Chapter 1, this Level 2 SFRA considers three strategic development areas within the settlements of Minehead, Williton and Watchet. At this stage, WSC has identified general areas within each of these settlements, which are likely to be considered for future development and are summarised below:

• Land to the south east of Minehead; • Land to the west of Williton; and, • Watchet Paper Mill.

The three study areas identified above are discussed in turn in the following Chapters.

2.2 Hydraulic Modelling

The principle flood source at Minehead is tidal. Two-dimensional (2D) hydraulic modelling of tidal flood sources was undertaken in 2006 at Minehead for the New Horizons Healthplex Development. This hydraulic modelling focused on the proposed development site and the surrounding settlement of Minehead. As part of the Level 2 SFRA a new 2D hydraulic modelling study has been undertaken based upon the current revision of PPS25 and has been extended to consider a wider area towards Marsh Street. This has considered the impact of residual tidal flood risk associated with overland flood routing following overtopping and breaching events of the coastal flood defences, which has been used to derive flood extent, depth and hazard mapping. This has included analysis of present day and climate change scenarios for a range of tidal events.

In 2009 the Environment Agency completed the Williton Flood Mapping Study, which aimed to improve the existing Flood Zone Maps. This was undertaken using a 1D/2D hydraulic modelling approach in ISIS-TUFLOW software. As part of the Level 2 SFRA the hydraulic modelling has been re-run to derive flood extent, depth and hazard mapping of Williton for present day and climate change scenarios for a range of fluvial events.

The Washford River flows through Watchet before discharging into the sea. In 2008 the Environment Agency completed the Washford River Flood Zone Compliance Assessment, which included Watchet. This was undertaken using 1D hydraulic modelling approach in HEC- RAS software. As part of the Level 2 SFRA flood levels have been extracted from the hydraulic modelling to derive flood extent, depth and hazard mapping, at the Watchet Paper Mill for present day and climate change scenarios for a range of fluvial events.

A more detailed discussion of hydraulic modelling can be found in the following Chapters.

2.3 Flood Hazard

Flood hazard is a function of both the flood depth and flow velocity. Each element generated by the flood mapping process has been assigned one of four hazard ratings, which are shown in

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Table 2-1, which includes an identification of the colour coordination with respect to flood hazard mapping.

Table 2-1: Flood hazard categories Hazard Category Description Colour Extreme Dangerous for all Significant Dangerous for most people Dangerous for some e.g. Moderate children Low Caution

These have been designated in accordance with the Defra/Environment Agency research documentation (FD2320/TR2) and are derived using the formula shown in Figure 2-1:

Figure 2-1: Hazard formula

2.4 Flood Zone 3b (Functional Floodplain)

The Level 1 SFRA presents the extent of Flood Zone 3b (Functional Floodplain) and identifies the methodology applied to derive it. Where the standard of protection of flood defences has an annual probability of 5% or greater (as defined in the NFCDD), the adjacent floodplain was not be considered Flood Zone 3b. Additionally, developed areas where there is existing infrastructure and solid buildings were not considered to be Flood Zone 3b. All remaining areas of Flood Zone 3a were therefore assumed to equal the extent of Flood Zone 3b, until proven otherwise.

This methodology was suitable for the purposes of the Level 1 SFRA and applied a precautionary approach. This was based on guidance within current policy (PPS25) and was agreed with the Environment Agency, more information can be found within the Level 1 SFRA.

2.5 Additional Considerations

Flood risk is one of numerous considerations that need to be accounted for as part of the spatial planning process. Other considerations include Exmoor National Park boundary and the position of Minehead Sewage Treatment Works (STW), which are specific to West Somerset and more general considerations, such as the availability of previously developed land, environmental designations, or access and sustainability issues. Consequently, whilst large parts of West Somerset are not located within the floodplain, upon reflection of the other considerations, land availability becomes limited and therefore development of areas located

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within the floodplain are considered, to investigate if they are appropriate to develop whilst being safe and sustainable for their lifetime.

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3 Minehead

3.1 Location

Minehead is located on the coast of the Severn Estuary. At this stage precise development allocations have not been determined and the strategic development area is considered to be to the south east of the existing settlement. Minehead and the approximate extent of the study area are shown in Figure 3-1.

3.2 Topography

The study area consists of a flat and low-lying coastal zone, which is relatively isolated in a topographical context; surrounding areas are more elevated, resulting in a distinct low-lying coastal plateau. Ground levels within the study area are relatively uniform between approximately 5-6m AOD. The topography of study area is also identified in Figure 3-1. Various topographical features can be seen including the sand dunes and earth embankment coastal defences southeast of the Minehead, the West Somerset Railway and the A39. The coastal defences that align the sea front of Minehead are not visible. The STW, located between Minehead and Marsh Street, is also shown to be elevated compared to surrounding land.

Figure 3-1: Approximate location of study area

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3.3 Flood Risk and Flood Risk Management 3.3.1 Tidal Flood Risk

The principle flood source at Minehead is tidal. The area is protected by formal coastal defences, which extend along the sea front from the Esplanade to the Minehead Golf Club clubhouse. These defences consist of sloping revetments with a re-curved wave return wall. Beyond the settlement (in a south east direction) the sea defences constitute sand dunes and earth embankments. These features are shown in Figure 3-2 and Figure 3-3.

Figure 3-2: Sloping Revetment opposite the Settlement of Minehead

Figure 3-3: Shingle bank and sand dunes opposite Minehead Golf Club

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The NFCDD (National Flood and Costal Defence Database), supplied by the Environment Agency identifies a minimum revetment height of 8.70m AOD. This provides protection greater than the 1 in 1000 year return period event (i.e. 7.42m AOD). The sand dunes and earth embankments are of more variable elevation, some of which are greater than 10m AOD. However, the NFCDD suggests that minimum elevations are in the order of 7.14m AOD and therefore are likely to experience some overtopping under the 1 in 1000 year return period event.

The addition of the anticipated affects of climate change result in estimated peak tidal levels rising by approximately 1m over the next 100 years, as presented within Annex B of PPS25. Consequently, increased overtopping of the flood defences would be expected to occur in the future.

Tidal flood risk at Minehead and the surrounding area is considered to be a residual risk due to the benefit offered by the coastal defences. The residual flood risk is a consequence of the risk associated with overtopping of or a breach in the flood defences, which is described in further detail in Section 3.5.

The recently released North Devon and Somerset Shoreline Management Plan4 (SMP2) identifies the recommended policy for Minehead, The Warren and Dunster Beach (which makes up the study area) is to ‘hold the line’ of the existing coastal defences during the short to medium term. However, a secondary defence wall is recommended within the study area, which may form the main defence line in the long term, as part of a managed realignment policy for The Warren and Dunster Beach.

The hold the line recommendation is maintained for the Minehead sea front in the medium and long term. However, the recommendation for the study area in the medium to long term is one of managed realignment, as maintaining the existing defence line is likely to become technically unsuitable during this period. The secondary defence line constructed in the short term would then become the primary flood defence line along this frontage.

The predicted sea level rise due to climate change (as defined within PPS25) suggests that peak tidal levels are likely to be 1m higher than existing conditions along the Severn Estuary. This will result in significant implications for coastal areas, where overtopping of existing defences will occur more frequently. This is considered within the policy recommendations of the North Devon and Somerset SMP2 and is included in the hydraulic modelling and is discussed below.

3.3.2 Fluvial Flood Risk

The River Avill is classified as a ‘Main River’ and flows through the study area. It splits between its natural course and a flood relief channel just upstream of the A39, both of which discharge into the sea. These features are indicated by the Main River line on Figure 3-1. The flood relief channel conveys flows at times of greater discharges and therefore reduces flows in the natural watercourse which provides protection to areas downstream of the split. The flood relief channel offers a significant conveyance capacity and therefore whilst extreme high tides will propagate up the channel, it is not considered to cause fluvial flooding through tide locking.

The natural course of the River Avill drains into the sea beneath the coastal defences via a pipe, which connects to a control chamber on the landward side of the defences. The control

4 See www.ndascag.org/

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chamber is intended to prevent tide locking by allowing for a positive hydraulic gradient to force discharge into the sea, even under extreme tidal conditions. Photographs of this structure are shown in Figure 3-4. There are a number of other smaller watercourses that discharge into the River Avill which flow through the study area that constitute a relatively small potential source of fluvial flood risk.

Figure 3-4: Side on and inside view of control chamber

The severity of fluvial flooding in the study area is considered to be less significant compared to tidal flooding and therefore it is not considered in detail within this Level 2 SFRA. However, a brief discussion is included within Section 3.10.

3.4 Aims

The specific aim for the study area is to determine its susceptibility to tidal flooding with inclusion of the benefit offered by the coastal defences (i.e. to analyse residual flood risk), by determining the flood extent, depth and hazard associated with overtopping and breaching by extreme tidal conditions of the coastal defences. Therefore, this will refine the mapping provided within the Level 1 SFRA by considering the benefit offered by the flood defences. This is achieved through 2D hydraulic modelling and discussed in further detail below.

3.5 Methodology

A 2D hydraulic model was constructed to analyse residual tidal flood risk at Minehead and the surrounding area. A topographical representation of the study area was constructed, which included various features, such as the coastal defences, watercourses, the West Somerset Railway and various roads, including Seaward Way and the A39. Ground levels were defined using LiDAR (Light Detection and Radar) data, which allowed overland flow routing to be calculated by the hydraulic modelling software (DHI MIKE21-HDFM).

Residual flood risk can result in flooding in two key ways:

1. Overtopping – when tidal levels exceed the level of the flood defences and spill onto the land behind; and,

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2. Defence breach – when a flood defence fails and allows water to flow through the defence onto the land behind.

Both of the flooding mechanisms described above could occur at the same time, which is dependant upon whether the tidal levels are of sufficient height. The hydraulic modelling allows for this mechanism. Peak still water tidal levels have been provided by the Environment Agency for extreme tidal conditions, which are summarised in Table 3-1. It is understood that these tidal levels may be subject to review by the Environment Agency in the near future.

Table 3-1: Extreme tidal still water levels Return Period Year Still Water Level 1 in 200 year 2010 7.21m AOD 1 in 1000 year 2010 7.42m AOD 1 in 200 year+climate change5 2110 8.21m AOD

The 1 in 200 year+climate change (CC) return period event includes 100 years of climate change, which translates to a 1m increase in sea level rise, as defined in current policy (PPS25). The still water levels represent the peak tidal levels (excluding the impact of waves) of a high tide encountered for the specific return period. A tidal curve was derived for each return period which includes this peak tidal level and the subsequent low tide, approximately 6 hours later. A detailed description of the methodology can be found within Appendix B. This includes information upon depth and hazard results processing and mapping.

The tidal curve represents the hydraulic model boundary condition. It was run for four subsequent high to low tide events. The hydraulic model performs numerous calculations at each interval of time to determine the movement of tidal water. The movement of tidal water is controlled by topography, for example, where the tidal water level exceeds the coastal defence crest height, floodwater will spill onto the land behind the defences and spread out as water continues to spill. However, if the tidal water level does not exceed the coastal defence crest height, tidal water will be held back within the seaward side of the defence. If a breach were to occur, floodwater would penetrate the land behind the flood defence. This forms the basis of the hydraulic modelling, which is intended to mimic the actual flood mechanism.

A single breach location was applied, the location of which is included in Appendix B.

The following hydraulic model scenarios have been undertaken:

• 1 in 200 year tidal event overtopping scenario – 20106 • 1 in 1000 year tidal event overtopping scenario – 2010 • 1 in 200 year tidal event overtopping scenario – including 100 years of climate change (i.e. 1 in 200+CC) • 1 in 200 year tidal event overtopping and defence breach scenario – 2010 • 1 in 1000 year tidal event overtopping and defence breach scenario – 2010 • 1 in 200 year tidal event overtopping and defence breach scenario – including 100 years of climate change (i.e. 1 in 200+CC)

5 Including the anticipated affects of climate change, which is defined in PPS25. 6 The date of 2010 represents the particular return period event occurring without any future impact of climate change upon sea level

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Flood depth and hazard mapping has been prepared for each of the above scenarios. This will allow consideration of general access and egress routes as well as identification and guidance on required mitigation measures.

3.6 Hydraulic Model Results – Tidal Overtopping

This section provides the results of the 2-D hydraulic modelling at Minehead and refers to Figures 1A, 1B, 1F and 1G in Appendix C, which illustrate the flood extent, depth and hazard associated with each overtopping scenario identified above, where relevant.

The hydraulic model was run for four consecutive tidal cycles, which is discussed in more detail within Appendix B. Peak overtopping was experienced on the second and largest of the tidal cycles, which is when the peak storm surge coincides with the highest astronomical tide.

A general overview of the flood propagation, depth and hazard is discussed below, based upon the three overtopping scenarios. For the purpose of this section ‘present day’ refers to 2010 and ‘climate change’ refers to 2110.

3.6.1 Present Day

The 1 in 200 year event peak still water level was not sufficient to overtop the flood defences, therefore no flooding was observed under this scenario. Under the 1 in 1000 year event some overtopping was observed, which was restricted to two isolated locations. The first was at the mouth of the River Avill and the second was within the car park for Dunster Beach (to the north of Sea Lane End). This is shown below (Figure 3-5) from an extract of the flood depth map. Whilst the resultant flood extents are limited, overtopping at the mouth of the natural course of the River Avill is able to propagate further inland by flowing up the river and spilling out of bank, as shown on Figure 1A, Appendix C. Tidal water also propagates upstream of the River Avill Flood Alleviation Channel but is contained within the banks. General flood depths experienced outside of the watercourses (i.e. the River Avill Flood Alleviation Channel and its natural route) range between 0.2 and 0.5m.

The shallow flood depths experienced under the present day overtopping 1 in 1000 year event translate to a low or moderate flood hazard, except in the watercourses where greater water depths are experienced, as shown in Figure 1F, Appendix C.

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Figure 3-5: Overtopping locations under the 1 in 1000 year event

With the addition of climate change the impact upon overtopping flood extent, depth and hazard is significant. The addition of climate change results in an increase in tidal water level of 1 m. Under the 1 in 200 year+CC overtopping scenario a third overtopping location is observed, within golf club, to the north of the River Avill.

With the addition of climate change the length of coastal defence that experiences overtopping increases dramatically, for example at the mouth of the River Avill, the overtopping length increases from just a few metres (under present day conditions) to approximately 600 m. However, no overtopping occurs of the sloping revetments along Minehead sea front. Parts of Minehead experience flooding which are therefore associated with overtopping under a climate change scenario attributed to the lower lying sand dunes and earth embankments distant from the settlement (i.e. ‘back door’ flooding).

The West Somerset Railway influences the spread of floodwater, which tends to direct floodwater in a north westerly direction towards Butlins holiday camp. Significant areas to the seaward side of the West Somerset Railway become inundated including the Butlins holiday camp prior to spilling across the railway into agricultural fields to the northwest of the STW. This occurs 65 minutes from the onset of overtopping. During the subsequent 45 minutes the area between the West Somerset Railway and Seaward Way becomes inundated and as floodwater continues to spill over the coastal defences, Seaward Way also overtops. This causes flooding of the low-lying areas to the south and west of Seaward Way, which includes areas of developed and undeveloped land.

Maximum flood depths tend to occur towards the seaward side of the West Somerset Railway, at approximately 2.0-2.5m depth. Generally, flood depth becomes shallower with distance inland (and hence distance from the overtopping areas), which can be seen on Figure 1B, Appendix C. Inundation is compartmentalised with the West Somerset Railway and Seaward Way effectively forming cascade features that overtop in turn.

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This trend is also evident in terms of flood hazard. Extreme flood hazard is limited to the area seaward of the West Somerset Railway. Inland of the West Somerset Railway, only limited areas of Extreme flood hazard are evident and the dominant flood hazard is Significant. This can be seen on Figure 1G, Appendix C.

3.7 Hydraulic Model Results – Tidal Overtopping & Breach

This section provides the results of the 2-D hydraulic modelling at Minehead and refers to Figures 1C, 1D, 1E, 1H, 1I and 1J in Appendix C, which illustrate the flood extent, depth and hazard associated with each overtopping and breach scenario identified in Section 3.5.

The hydraulic model was run for four consecutive tidal cycles, which is discussed in more detailed within Appendix B. The breach was set to coincide with the first tidal cycle, to include the impact of four subsequent high tides following through the breach and into the study area. Floodwater drains back out through the breach as tide retreats. Peak flow through the breach was experienced on the second and largest of the tidal cycles, when the storm surge coincides with the highest astronomical tide. Peak flood depths, velocity and therefore hazard also tend to occur under this second tidal cycle.

A general overview of the flood propagation, depth and hazard is discussed below. For the purpose of this section ‘present day’ refers to 2010 and ‘climate change’ refers to 2110.

3.7.1 Present Day

Whilst the 1 in 200 year event peak still water level was not sufficient to overtop the flood defences, with the inclusion of a breach significant flood inundation was observed. Following the breach, the principal flow path of floodwater in a north westerly direction towards Butlins holiday camp and Minehead is observed. Floodwater overtops the West Somerset Railway 13 hours after the breach occurs, which is during the second and largest tidal cycle. Subsequently, as floodwater continues to flow through the breach, the area between the West Somerset Railway and Seaward Way becomes inundated. However, Seaward Way does not become overtopped, which restricts flood inundation further inland.

Similar to the overtopping scenario described in section 3.6.2 flood depths to the seaward side of the West Somerset Railway tend to be the greatest, with maximum depths in the order of 2.0m AOD in parts of the golf course. Maximum flood depths between West Somerset Railway and Seaward Way are approximately 1.5m AOD, which can be seen in Figure 1C, Appendix C.

The same flood mechanism occurs under the 1 in 1000 year overtopping and breach scenario. However, peak tide levels are 0.21m higher and consequently additional floodwater flows through the breach. Some overtopping is also experienced, as discussed in Section 3.6.1. The more elevated tidal level causes the area to the south and west of Seaward Way to become inundated (see Figure 1D, Appendix C) and general depths experienced are slightly greater compared to that quoted above for the 1 in 200 year overtopping and breach scenario.

Significant flood hazard dominates the majority of the inundated area under the 1 in 200 year overtopping and breach scenario. However, an Extreme flood hazard is evident directly inland of the breach (within the Rapid Inundation Zone, shown from an extract of the flood hazard map in Figure 3-6). Extreme flood hazard is also evident in parts of the golf course, within the River Avill and the moat that surrounds Butlins holiday camp. A similar trend is evident for the 1 in 1000 year overtopping and breach scenario, except the Extreme flood hazard extent is more

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widespread. Flooding occurs beyond Seaward Way under this scenario, which is associated with a variation in flood hazard, including Low, Moderate and Significant. Flood hazard is shown on Figure 1H and 1I, Appendix C.

Figure 3-6: Illustration of rapid inundation zone flood hazard (see Section 2.3 for the flood hazard classification

With this inclusion of climate change this forms the most onerous scenario. This scenario incorporates the most elevated tidal level and allows more floodwater to enter the study area through overtopping and a breach. Consequently, the flood extent observed is the largest of all the scenarios investigated. The flood mechanism is similar to that described above, with the general movement of floodwater in a north westerly direction, which is influenced by both the West Somerset Railway and Seaward Way. Generally, maximum flood depths are 0.5m deeper than the climate change overtopping scenario discussed in Section 3.6.2, with maximum depths in the order of 3m. Flood depth and hazard maps can be found in Appendix C, Figure 1E and 1J, respectively.

The revetments that align the sea front of Minehead protect the settlement from flooding even under climate change conditions. However, ‘back door’ flooding is observed at Minehead associated with overtopping and possible breach of the sand dunes and earth embankments distant from the settlement.

3.8 Focused Assessments

Appendix D includes a summary of data extraction points for various locations through the around surrounding Minehead. The data extraction provides more detail with respect to flood level, flood depth, flood hazard and illustrate the time of inundation for various parts of the study area. This information can be used to determine the variation in flood risk between opposing areas of land to the south east of Minehead.

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3.9 Limitations

The selection of a breach location is a relatively subjective exercise. The breach location included within the hydraulic modelling, as discussed in Appendix B, was based upon current best practice and using a conservative approach. Selection of a single, conservative, breach location is generally the accepted approach for the assessment of residual flood risk.

It is important to understand that a vast number of circumstances could occur during a breach event which could result in a different flooding mechanism affecting the study area. For example, a second breach could occur, the resultant breach width may be greater than that included in the hydraulic model or a breach may occur in a different location. Each of the scenarios would potentially result in a different flooding mechanism within the study area and form a more widespread extreme flood hazard zone.

However, the breach location was located within the sand dunes and embankments, which are the most likely to breach. It is likely that a different breach location would result in the same general flood mechanism, with flow moving in a northwest direction and result in similar cascade overtopping of the West Somerset Railway and Seaward Way.

The hydraulic modelling does not include the impact of waves, which could exacerbate overtopping into the study area, by increasing water level and eroding the sand dunes and embankments. This should be considered within a site specific FRA. It also does not account for natural drainage processes of floodwater with time, such as via the River Avill.

The hydraulic modelling does not include for the presence of the culverts beneath the West Somerset Railway and Seaward Way. This may be an appropriate consideration for site specific FRAs that are subject to tidal flood risk.

3.10 Fluvial Flood Risk

As discussed in Section 3.3, the River Avill Flood Alleviation Channel effectively removes fluvial flood risk associated with this watercourse from a large area to the north of the A39. However, a residual flood risk remains, if for example, the natural course of the River Avill becomes blocked and floodwater spills out of bank. Furthermore, a flood risk is posed by the ordinary watercourses that drain into the River Avill within the study area. Nevertheless, fluvial flooding is not considered to be a significant flood risk at Minehead and the surrounding area, when compared to tidal flood risk. This can be illustrated by a comparison of floodwater volume likely to be generated by each source, as shown in Table 3-2.

Table 3-2: Volume of floodwater storage Return Period Source Volume of Floodwater7 1 in 200 year Fluvial 550,000m3 1 in 200 year Tidal 1,963,000m3 1 in 200 year +CC Fluvial 661,000m3 1 in 200 year +CC Tidal 6,348,000m3

7 The fluvial volume is based upon a catchment size of 6.6km2 taken from the FEH CD-ROM v2. This was then multiplied by rainfall depth (based upon a storm duration of 6.5 hours), also from the FEH CD-ROM v2, to determine the volume. The tidal volume was calculated using Mapinfo Vertical Mapper and includes the total volume spilling into the study area through the breach and overtopping, where relevant.

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Tidal floodwater volumes are significantly larger compared to the volumes potentially generated from fluvial flooding (i.e. through rainfall). The tidal scenarios identified in Table 3-2 are inclusive of the breach, which allow a large volume of floodwater to enter the study area. Under the tidal climate change scenario (i.e. 1 in 200 year +CC) long stretches of the coastal defences experience overtopping, which increase the volume of floodwater stored with the study area. Consequently, 10 times more floodwater storage is experienced within the study area compared to the same return period fluvial event. Furthermore, the fluvial flood volumes assume a 100% runoff proportion and do not account for floodwater that will drains into the sea. The fluvial estimates presented in Table 3-2 are therefore considered to be an overestimate.

Fluvial flooding is not considered to be significant when compared to tidal flood risk in the Minehead area, at a strategic scale. However, at the local scale, fluvial flooding is important and may result in relatively widespread flooding across the flat study area. It must therefore be considered as part of a site specific FRA (Flood Risk Assessment).

The Environment Agency has produced ‘Areas Susceptible to Surface Water Flooding’ maps which are distributed to LPAs in a similar way to the Environment Agency Flood Maps. The surface water maps illustrate the areas susceptible to surface water flooding. However, they also provide an indication of the areas at risk from fluvial flooding. This is because surface water flooding is generated within the river basin catchment and tends to route through and accumulate within the lower lying areas, like a river and its floodplain.

The surface water maps for Minehead suggest that it is predominately susceptible to low or intermediate level of surface water flood risk. A very limited area is considered to be susceptible to a high level of surface water flood risk.

3.11 Flood Zone 3b (Functional Floodplain)

Based upon the methodology described in Section 2.4 no Flood Zone 3b extent was derived for Minehead, as part of the Level 1 SFRA. This was based upon the presence of coastal defences that align the seafront. This designation has not changed following completion of the Level 2 SFRA.

No Flood Zone 3b has been derived for the fluvial flood risk because the River Avill is not considered to represent a significant flood risk, due to the presence of the flood alleviation channel, see Section 3.3. However, the ordinary watercourses that discharge into the River Avill are likely to experience flooding under extreme conditions and therefore are likely to be associated with a Flood Zone 3b extent. This must be considered as part of a site specific FRA, where relevant (see Chapter 7).

3.12 Access & Egress

The study area is surrounded by more elevated land, as shown in Figure 3-1. A number of roads allow access to this elevated land, such as the A39, Seaward Way and some minor roads within residential areas. The flood maps included within Appendix C illustrate that these roads generally offer opportunities for safe access and egress form parts of the study area. However, under climate change conditions, flooding of the roads is more widespread.

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Areas upon the landward side of Seaward Way, between Stephenson Road and Marsh Lane, are generally located within the edge of the floodplain. This area can be accessed via a number of roads that are located outside of the floodplain and offer safe access and egress from this general area, even under the more extreme scenarios.

Seaward Way itself offers access and egress opportunities for the land between Seaward Way and the West Somerset Railway. Seaward Way becomes overtopped under extreme scenarios and is associated with a significant flood hazard under the 1 in 200 year overtopping and breach scenario. Generally, under its current elevation, it cannot be relied upon for safe access and egress.

The area to the west of the West Somerset Railway is significantly affected by tidal flood risk and safe access and egress from this area is unlikely to be achievable.

Opportunities for access and egress routes should be considered on a site specific basis. The impact from fluvial flooding should also be considered.

3.13 Flood Mitigation

Development should be steered to the areas found to be at low risk of tidal flooding, however if this is not found to be achievable, mitigation may need to be considered to enable the development of areas more prone to tidal flooding. On this basis, the hydraulic modelling has suggested that in terms of residual tidal flood risk the most appropriate location for development will be to the landward side of Seaward Way. This is because the modelling has shown that these areas will:

1. Only experience significant flooding under the more extreme tidal scenarios investigated;

2. Benefit from the attenuation of floodwater, which is delayed by the elevated West Somerset Railway and Seaward Way, under the more extreme scenarios; and,

3. Generally offer safe access and egress routes due to position upon the edge of the floodplain.

However, should development be steered into these areas, flood mitigation must be considered to ensure that they are safe under the more extreme tidal scenarios.

With the area subject to residual tidal flood risk, the areas between Seaward Way and the West Somerset Railway offer the next most favourable location for development. However, significant flood mitigation and opportunities for safe access and egress must be considered before the allocation of this area. The area to the east of the West Somerset Railway, north of Lower Marsh Farm is unlikely to be suitable for allocation, unless commitments can be made for significant flood protection infrastructure.

The coastal defences along Minehead seafront should not become overtopped under extreme tidal conditions. However, the impact of waves (which was not included in the hydraulic modelling) may result in some residual flooding from spray overtopping defences. The hydraulic modelling has shown that major overtopping will occur to the southeast of Minehead where the sand dunes and earth embankments are at lower elevations. The standard of protection offered by the sand dunes and earth embankments is less than the formal defences

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within Minehead. The sand dunes and earth embankments are therefore considered more likely to overtop and/or breach.

The North Devon and Somerset SMP28 identifies the recommended policy for Minehead, The Warren and Dunster Beach (which makes up the study area) is to ‘hold the line’ of the existing coastal defences during the short to medium term. However, a secondary defence is recommended within the study area, which may form the main defence line in the long term, as part of a managed realignment policy for The Warren and Dunster Beach.

Without some form of investment or intervention existing development at Minehead will be highly vulnerable to tidal flood risk. The improvement of the standard of the existing coastal defences could offer strategic benefits to both Minehead and the surrounding area, including any future development.

Opportunities could be sought for developer contributions towards the improvement and maintenance of the coastal defences along this stretch of coastline. This would provide a mechanism to secure delivery of a secondary defence, protect the study area and reduce the risk of the back door flooding found to affect the existing settlement of Minehead (see Section 3.6 and 3.7). This could be sufficient to enable development of the areas at risk from tidal flooding. However, improving coastal defences may not remove the residual risk associated with a breach and therefore other flood mitigation measures should be considered.

Further details on the issues relating to developer contributions to flood risk management can be found in Annex G of PPS25.

Raising of ground levels within new development sites may also be necessary to protect from residual tidal flood risk. However, this will offer no benefit to neighbouring sites and may therefore be a less sustainable approach compared to the more strategic opportunities associated with the improvement to existing coastal defences. Raised access routes may also be necessary to provide safe access and egress routes. However, the impact upon flow paths and displacement of floodwater (tidal9 and fluvial) must be considered and compensated for, where necessary.

An assessment was undertaken to determine the impact of raising a 20ha site above the tidal flood level. The area considered was located to the west of Seaward Way. This was investigated for the 1 in 200 plus climate change event, which was found to result in approximately a 0.2m rise in water level across the flooded area10. In terms of current policy (i.e. PPS25) this is not an acceptable impact and is likely to be objected to by the Environment Agency. Therefore a viable strategic flood mitigation solution must be identified before the site allocation process, for any sites which may require major ground raising in the tidal floodplain. If allocation of large areas of development continues to be sought in the tidal floodplain, it is recommended that West Somerset Council undertake a new study to investigate a viable strategic flood mitigation solution to minimise any detrimental impact upon floodwater displacement and floodplain flow paths.

The cumulative impact of displacement from new development has not been considered at this stage. However, if displacement of floodwater can be mitigated where necessary, the cumulative impact should be negligible.

8 www.ndascag.org/ 9 PPS25 Practice Guide states that if there is a finite volume of water able to pass into a defended area following a failure of the defences, then a new development, by displacing some of the flood water, will increase the risk to existing properties 10 This impact upon water level was not calculated using hydraulic modelling and is only an approximate estimation.

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Flood warning procedures are already in place to raise awareness of imminent storm surges in the Severn Estuary, which can allow people to evacuate the area before the risk of flooding occurs. Generalised flood mitigation measures are discussed in Chapter 7 and 8.

3.14 Recommendations

The list below highlights the key recommendations for any future development in the study area, where identified to be at risk of tidal flooding from the hydraulic modelling undertaken within the Level 2 SFRA:

• Where possible, development should be steered outside of the floodplain (i.e. into Flood Zone 1); • Where development cannot be steered outside of the floodplain, development should be sequentially located by steering less vulnerable development (car park/amenity areas) towards land which experiences most significant flood depths. This may reduce the amount of land raising required, and therefore reduce the displacement of tidal floodwater; • Due to the flat study area, any location adjacent to the existing soft coastal defences should be considered within a rapid inundation zone with an extreme flood hazard classification, until it can be proved otherwise; • Flood Risk Assessment Guidance for New Development Phase 2. R&D Technical Report FD2320/TR2 (October 2005) should be used as an early tool for the identification of flood hazard from overtopping or a breach of coastal defences; • Depending on other constraints, future development should be set back as much as possible from the existing coastal defences, where existing flood risk conditions (depth, hazard and time of inundation) tend to be less severe. This may also improve opportunities for safe access/egress during a tidal flood event; • A policy of managed realignment is identified for The Warren and Dunster Beach in the North Devon and Somerset SMP2 and therefore any new development near the coastal frontage at these locations would not be sustainable; • Opportunities should be sought to help deliver the strategic flood defence proposals outlined in the North Devon and Somerset SMP2, which could be constructed in a way to help protect existing and new development at Minehead; • Developer contributions may help fund the construction of a secondary defence line, as identified in the North Devon and Somerset SMP2; • Any proposed ground raising should consider the implications upon displacement of floodwater, including the cumulative impact of displacement from subsequent development, as well as the impact upon key flow paths. It is recommended that a new study is undertaken by West Somerset Council to investigate a viable strategic flood mitigation solution to minimise any detrimental impact upon floodwater displacement and floodplain flow paths, associated with mass ground raising; • A detailed study should be commissioned to evaluate the costs and benefits of the opportunities associated with improving existing coastal defences, compared with site by site land raising works; • Where possible, future development should be positioned alongside existing infrastructure to improve opportunities for safe access/egress during a tidal flood event;

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• As part of a site specific FRA the impact of waves, fluvial and surface water flooding should be considered and appropriate mitigation measures development, where necessary.

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4 Williton

4.1 Location

Williton is located approximately 2km inland from the coast. The strategic development area is the land surrounding the existing settlement. Williton and the approximate location of the study area are shown in Figure 4-1.

4.2 Topography

Williton lies near the confluence of two streams with relatively small catchments that drain steep sided valleys. The Monksilver Stream runs through the centre of the village and the Doniford Stream flows close to the eastern edge of the village.

The town is situated within a relatively large basin of the catchments associated with the Monksilver Stream and the Doniford Stream. Ground levels fall in an easterly direction towards the Doniford Stream, from approximately 35m AOD in the west, to approximately 20m AOD adjacent to the Doniford Stream to the east of the settlement.

Figure 4-1: Approximate location of study area

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4.3 Overview of Flood Risk and Flood Risk Management

The principle flood source at Williton is fluvial. The Monksilver Stream flows through Williton in a north east direction to meet with the Doniford Stream, just beyond the settlement and the West Somerset Railway. The catchments rapid response to rainfall makes Williton prone to flooding.

There is a history of flooding at Williton that includes significant floods in 1960, and more recently on 30th October 2000 and 7th December 2000. The flooding problem is primarily due to alteration of the river channel through Williton, and the construction of many bridges and culverts through the town, resulting in significant hydraulic restrictions. The December 2000 event affected approximately 50 properties11.

A small ditch runs perpendicular to the Monksilver Stream just upstream (i.e. to the west) of the settlement and flows in a clockwise direction around Williton. This essentially allows some floodwater to bypass Williton and to join back with the Monksilver Stream downstream of Williton. This flooding mechanism results in some shallow flooding of the area to the west and north of Williton. For the context of this report the ditch will be known as the West Williton Ditch.

There are no formal defences maintained by the Environment Agency in the study area. This has been confirmed by inspection of the NFCDD.

4.4 Aims

The source of data used to create the Level 1 SFRA flood maps has recently been revised using 1D-2D hydraulic modelling to provide a more accurate representation of fluvial flood risk. The specific aim for the study area is to provide the flood extent, depth and hazard mapping associated with fluvial flooding based upon this revised hydraulic modelling. This includes the derivation of Flood zone 3b (Functional Floodplain) for the study area. This will be achieved through application of the existing 1D-2D hydraulic modelling and is discussed in further detail below.

4.5 Methodology

A 1D-2D hydraulic model for Williton was made available by the Environment Agency. The hydraulic model incorporated the Monksilver Stream, the Doniford Stream and the West Williton Ditch.

The results (or outputs) of the hydraulic model were not provided by the Environment Agency and therefore the model has been re-run for the purposes of this study. The model parameters have not been adjusted and therefore outputs will be consistent with that used for Environment Agency Flood Zone Maps. The following model scenarios were run:

• 1 in 20 year return period • 1 in 100 year return period • 1 in 1000 year return period • 1 in 100 year return period +CC

11 Babtie Brown & Root (2003) Williton Flood Defence Scheme Options Summary Report.

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Whilst the 1 in 20 year return period scenario was not provided by the Environment Agency, the boundary conditions were scaled to allow this scenario to be assessed.

During a site walkover, a structure within the West Williton Ditch was observed that was not included within the hydraulic model. The structure consists of a 420mm diameter pipe through an earth embankment, several metres downstream of the spilt from the Monksilver Stream. The location and photograph of the structure is shown in Figure 4-2. It is anticipated that inclusion of the structure will reduce flow downstream and through the West Williton Ditch (i.e. to the north). It may therefore affect flood extent associated with the ditch. A number of additional scenarios have been undertaken to illustrate the impact of inclusion of the structure, including:

• 1 in 20 year return period with structure • 1 in 100 year return period with structure • 1 in 1000 year return period with structure • 1 in 100 year return period +CC with structure Figure 4-2: Location (red circle) and photograph of structure

4.6 Hydraulic Model Results

This section presents the results of the 1D-2D hydraulic modelling at Williton and refers to Figures 2A to 2H in Appendix C, which illustrate the flood extent, depth and hazard associated with each scenario. A general overview of the flood propagation, depth and hazard is discussed below.

4.6.1 Flood Mechanism

The results of the hydraulic model illustrate that flooding is first experienced downstream of Williton at the confluence of the Monksilver Stream and the Doniford Stream. Subsequently, flooding is observed upstream at the Long Street (A39) Bridge. The next area to experience flooding is along the reach of the West Williton Ditch, which is exacerbated when small access structures become surcharged and force water out of bank to spill across the fields in a north easterly direction. At the same time Williton experiences flooding from Monksilver Stream, which is also exacerbated by limited bridge/culvert capacity. The maximum flood extent is relatively widespread, affecting large parts of Williton and the surrounding area.

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Flow into the area to the west of Williton (i.e. via the West Williton Ditch) is controlled by the channel conveyance capacity at the spilt of the Monksilver Stream and the West Williton Ditch. However, under the 1 in 1000 year event floodwater spills out of bank at this split and inundates the floodplain. Therefore flow into the area to the west of Williton is not just defined by the channel conveyance capacity and more widespread flooding is observed adjacent to the West Williton Ditch.

4.6.2 Flood Depth

The general flood mechanism described above is appropriate for all of the scenarios identified in Section 4.5. Figure 2A to 2D, Appendix C, show the flood extent and depth maps for the study area, based upon the hydraulic model as received from the Environment Agency. Whilst the flood extents are relatively widespread through the study area, the flood depths observed on the floodplain are minimal.

Typical depths of floodwater have been extracted for various locations within the study area. These locations are shown in Figure 4-3. The data extraction points have been positioned based upon areas that experience flooding.

Figure 4-3: Data Extraction Points

The flood depths for each of the data extraction points are shown in Table 4-1.

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Table 4-1: Flood depths (mm and m) at various data extraction points ID 1 in 20 year 1 in 100 year 1 in 100 year +CC 1 in 1000 year 1 <0.01m <0.01m <0.01m <0.01m 2 0.02m 0.04m 0.04m 0.05m 3 - - <0.01m 0.02m 4 - 0.03m 0.04m 0.01m 5 - 0.06m 0.08m 0.16m 6 0.03m 0.05m 0.05m 0.19m 7 0.08m 0.13m 0.01m 0.31m 8 0.62m 0.87m 0.93m 1.36m

Typical flood depths are very small in the area to the west of Williton (ID 1, 2 and 3). The majority of this area experiences very shallow flooding as water flows across the surface in a northeast direction. The majority of the area is subject to flooding of less than 0.01m similar to that identified for data extraction point 1.

The flood maps included within Appendix C show that the area to the north of Williton does not experience significant flooding, except under the 1 in 1000 year event, but is largely contained by roads which provide flood pathways.

There is more variation with the area to the east of Williton, with some areas adjacent to Doniford Stream experiencing relatively deep flooding, especially upstream of High Bridge. Flooding downstream of High Bridge and to the west of the West Somerset Railway tends to be shallower.

The area to the south of Williton is elevated above the floodplain and is not subject to fluvial flooding.

With the inclusion of the structure at the upstream end of the West Williton Ditch the flood extent and associated depths have changed, especially within the area to the west of Williton. Figures 2E to 2H, Appendix C, show the resultant flood extent and depth maps for the study area. An extract of the flood maps is also included in Figure 4-4 for the area to the west of Williton where the most significant impact is observed.

The impact of including the structure upon average depths is shown in Table 4-2 with respect to flood inundation of the study area.

Table 4-2: Flood depths and storage volumes within the study area (with structure) ID 1 in 20 year 1 in 100 year 1 in 100 year +CC 1 in 1000 year 1 - - - No impact 2 No impact 3 - - No impact 4 - No impact No impact 5 - No impact 6 No impact No impact No impact

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7 No impact No impact No impact 8 No impact No impact No impact No impact

The green highlighted areas show where the flood depth has become reduced due to the addition of the structure, whilst the red highlight illustrates where flood depth has become greater. If there was no impact observed this is also referenced. The impact upon flood depth was limited a maximum of ±0.2m and is not considered significant.

The reduction in flood depth is observed only in the area to the west of Williton. This is not surprising as inclusion of the structure is thought to reduce flow into this area. Data extraction point 1 is entirely removed from the floodplain by addition of the structure. This is further illustrated in Figure 4-4. A small increase in flood depth is observed elsewhere as more flow is diverted down the Monksilver Stream.

The impact upon flood extent is more discernable compared to the impact upon depth, especially in the area to the west of Williton, which is a result of the flat topography in this area.

Figure 4-4: Comparison of 1 in 20 year flood extent with (right) and without (left) structure

Under the 1 in 1000 year return period event, floodwater spills into the area to the West of Williton over the floodplain, by-passing the West Williton Ditch. Consequently, the inclusion of the structure makes no discernable difference to flood depth at the data extraction points under this extreme scenario.

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4.6.3 Flood Hazard

The predominant flood hazard within the study area is Low. This Low flood hazard is applicable to all of the scenarios discussed above. However, more significant flood hazard categories are associated with the area to the east of Williton under the larger return periods (i.e. the 1 in 1000 year event). Figure 2A to 2H also illustrate the flood hazard for the various return period events.

4.7 Limitations

The 1D-2D hydraulic model approach is the most sophisticated approach commercially available for analysis of fluvial flood risk, which allows accurate representation of flood pathways across the floodplain. However, all hydraulic modelling is a simplification of a complex interaction of natural processes and is associated with as degree of error. Furthermore, the hydraulic modelling does not include the impact of blockage or any sensitivity testing, which should been considered as part of a site specific FRA.

4.8 Flood Zone 3b (Functional Floodplain)

The definition of Flood Zone 3b is described within the PPS25, but elaborated within the PPS25 Practice Guide12. It is the Practice Guide definition which is described below in relation to the study area. The key part of the definition is ‘land where water has to flow or be stored in times of flood.’

However, the PPS25 Practice Guide states that the definition in PPS25 allows flexibility to make allowances for local circumstances and whilst Flood Zone 3b should not be defined on rigid probability parameters, the general accepted return period to derive Flood Zone 3b is the 1 in 20 year return period event.

Whilst the results of the hydraulic modelling have illustrated a relatively extensive Flood Zone 3b extent (based upon a 1 in 20 year return period event) it is significantly smaller compared to that derived from the Level 1 SFRA methodology. It is therefore recommended that the Flood Zone 3b designation within the Level 1 SFRA for this location should be superseded by the findings of this study.

Inclusion of the structure downstream of the split between the Monksilver Stream and the West Williton Ditch results in a reduction in Flood Zone 3b extent for the area to the west of Williton. Flood depths in this area were found to be extremely shallow, generally less than 0.1m. However, inclusion of the structure makes no significant difference to other parts of Williton.

Development should be steered to lowest risk areas first, where development can not be located within Flood Zone 1, then development should be located based on flood risk vulnerability. Whilst areas of Flood Zone 3b have been identified, in some locations these depths are limited and therefore opportunities exist for the construction of flood control infrastructure within Flood Zone 3b to mitigate flood risk and enable development. In addition, reconfiguration of Flood Zone 3b may provide wider benefits to existing development within Williton the reduction of flood risk.

12 Communities and Local Government (2009) ‘Planning Policy Statement 25: Development and Flood Risk – Practice Guide’, TSO: London

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4.9 Access & Egress

The area to the west of Williton is generally associated with a low flood hazard, even under the most extreme scenarios. Consequently, a number of safe access and egress routes are likely to exist in this area. The area to the north of Williton is located outside of the floodplain and North Road will provide a safe access and egress route.

Upstream of the A39 and High Bridge, to the east of Williton significant flood hazards are observed under the 1 in 1000 year event. The area downstream of the High Bridge and west of the West Somerset Railway (i.e. around the Roughmoor Industrial Estate) is generally associated with lower flood hazard categories. Access routes from these areas become submerged and safe access and egress may not be achievable.

Opportunities for access and egress routes should be considered on a site specific basis.

4.10 Flood Mitigation

There are various areas around Williton that are shown to be located outside of the fluvial floodplain. Development should be steered to these low risk areas, however if this is not found to be appropriate, mitigation may need to be considered to enable the development of areas more prone to fluvial flooding.

The opportunity exists to provide flood risk infrastructure to manage flooding, which could help enable new development and potentially alleviate flood risk to existing built development in Williton. A number of previous studies that investigated flood alleviation opportunities for existing development at Williton include Travers (1978), the Williton Flood Mitigation Appraisal (1993), and the Williton Pre-Feasibility Study (1995). However, none of the studies indicated an economically viable scheme to protect the existing settlement.

The flood alleviation opportunities included a series of options. However, only two of these options are considered to represent a strategic investment that will provide widespread benefits to new development in Williton. These two options were a flood diversion channel around Williton and an upstream storage area. If one of these options were taken forward, it should also offer an improvement to the protection of existing development in Williton.

The Environment Agency has provided a more recent flood defence scheme options appraisal report for Williton13, which was completed in January 2003. A number of options were evaluated and whilst there was no clear preferred option, a diversion channel around Williton appeared to be the most likely. The report states that further qualification of this option would be provided through a risk assessment, when the project is revisited. However, it is understood that no further progress has been made.

The diversion channel was identified at a similar location to the West Williton Ditch. This is considered to be the most appropriate general position of a diversion channel and could reduce flood risk throughout Williton. However, floodwater cannot simply be diverted around Williton without consideration of the impact downstream. Therefore, storage of floodwater must also be provided, whether adjacent to the diversion channel and/or elsewhere.

13 Babtie Brown & Root JV (2003) Williton Flood Defence Scheme Options Summary Report. Rev 01.

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Babtie Brown & Root JV (2003) Williton Flood Defence Scheme Options Summary Report (Rev 01), suggested that the most appropriate location for flood storage was identified as the area immediately upstream of Williton. Upstream storage of floodwater could form an effective alleviation strategy to help enable development at various locations throughout Williton.

It is recommended that both of these options are taken forward for further investigation. This should use the existing ISIS-TUFLOW hydraulic model to help evaluate the effectiveness of the two options.

Level for level floodplain compensation must be undertaken if any new development sites are considered to result in a displacement of fluvial floodwater.

Flood warning procedures are already in place to raise awareness of imminent flooding from the Monksilver Stream, which can allow people to evacuate the area before the risk of flooding occurs.

Some more general flood mitigation measures are discussed in Chapter 7 and 8.

4.11 Recommendations

The list below highlights the key recommendations at the study are with respect to any future development in the areas identified to be at risk of fluvial flooding from the hydraulic modelling analysed within the Level 2 SFRA:

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5 Watchet

5.1 Location

Watchet is a small coastal village located approximately 10km east of Minehead. The study area in Watchet consists of the Watchet Paper Mill, which is located approximately 500m inland and is situated above the tidal limit. The approximate location of Watchet Paper Mill is identified on Figure 5-1. In Watchet the (larger) development options are realistically reduced to one (i.e. the Paper Mill) because of local development constraints and alternative areas are not considered to be suitable in sustainability terms. Therefore, this section considered this single development option, known as the Watchet Paper Mill.

5.2 Topography

The topography of Watchet Paper Mill slopes in a north easterly direction, from a maximum of approximately 18m AOD to a minimum of approximately 13m AOD. The Washford River flows through the site and is associated with lower elevations, especially in the northeast corner of the study area, where it runs through a 5-6m depth ravine, where minimum river bed levels are approximately 9.5m AOD.

Figure 5-1: Approximate location of study area

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5.3 Overview of Flood Risk and Flood Risk Management

The study area is sufficiently elevated to remove any direct tidal flood risk. However, Environment Agency Flood Zone Maps suggest that the study area is subject to fluvial flood risk, associated with the Washford River. Two mill leats flow through the study area, which potentially introduce additional flood pathways and have not been considered by the Environment Agency Flood Zone Maps.

The Washford River has a relatively impermeable catchment and steep gradient, which result in rapid response times to the onset of rainfall.

The Washford River experienced a large flow event in during October 1960, where buildings, roads and land at Watchet, including Watchet Paper Mill, were reportedly flooded. After this event major work including channel widening and regrading, wall raising and the construction of new floodwalls was undertaken. At Watchet Paper Mill the channel conveyance capacity was improved in 1961, and in 1968 a new culvert and channel were built with a gabion cascade upstream of the paper mill14.

The NFCDD identifies a small raised flood defence embankment just upstream of the paper mill. It also identifies a number of privately owned culverts located throughout the study area, which are associated with the mill leats.

5.4 Aims

The hydraulic modelling used to derive the Level 1 SFRA flood maps is considered to be the best available information. However, the specific aim for the study area is to provide the flood extent, depth and hazard mapping, to supplement the information included within the Level 1 SFRA. Flood Zone 3b (Functional Floodplain). These aims will be achieved through use of the existing 1D hydraulic modelling and is discussed in further detail below.

5.5 Methodology

The 1D (HEC-RAS) hydraulic model of the Washford River was made available by the Environment Agency. The results (or outputs) of the 1D hydraulic model are in the form of flood levels at cross sections through the watercourse. These were included within the hydraulic model when provided by the Environment Agency. However, the 1 in 20 and 1 in 1000 year return period events were not included. The associated peak flows were provided by the Environment Agency for the 1 in 1000 year scenario and the boundary conditions were scaled for the 1 in 20 year scenario. The hydraulic model was then re-run to determine flood levels for all of the scenarios required. These were:

• 1 in 20 year return period • 1 in 100 year return period • 1 in 1000 year return period • 1 in 100 year return period +CC

14 Mott MacDonald (2008) Washford River - West Somserset Streams Flood Zone Compliance. 240422/04/A

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The flood extents have been mapped based upon Environment Agency procedures. This involves projection of flood levels from the hydraulic model onto the LiDAR Digital Terrain Model (DTM), using Mapinfo Vertical Mapper.

5.6 Hydraulic Model Results

This section presents the results of the 1D hydraulic modelling at Watchet and refers to Figure 3A to 3F, Appendix C, which illustrate the flood extent, depth and hazard associated with each scenario identified. A general overview of the flood propagation, depth and hazard at Watchet Paper Mill is discussed below.

5.6.1 Flood Mechanism

The results of the hydraulic model indicate that no out of bank flooding is experienced under the 1 in 20 year scenario. However, flooding is experienced under the remainder scenarios. Under the 1 in 100 year and 1 in 100 year +CC scenarios flood inundation is limited to less than half of the study area. Only a small section of the Washford River overtops its banks under the 1 in 100 year scenario, whilst the 1 in 1000 year scenario results in surcharging at each culvert through the site and various reaches of the watercourse spill overbank. This exacerbates flood inundation causing flooding of the majority of the study area.

5.6.2 Flood Depth

Figure 3A to 3C, Appendix C, show the flood extent and depth maps for the study area, based upon the hydraulic model received from the Environment Agency.

Typical depths of floodwater to the north of the main paper mill building in the car park are shown in Table 5-1. Other parts of the study area experience greater depths, which are in the order of 1.6m for the 1 in 100 year scenario, which is consistent with a depression in the ground level. Figure 5-2 is an extract of the flood depth maps which illustrates the difference in flood depth and extent between the 1 in 100 year and the 1 in 1000 year event.

Table 5-1: Typical flood depths in the study area Return Period Year Average Depth (m) 1 in 100 year 2010 0.38 1 in 100 year +CC 2110 0.52 1 in 1000 year 2010 0.59

5.6.3 Flood Hazard

The predominant flood hazard within the study area is Significant, which is applicable to all of the scenarios discussed above. Flood hazard maps can be found at Figure 3D to 3F, Appendix C.

5.7 Limitations

Whilst the existing modelling provides sufficient detail to inform strategic planning decisions it is recommended that further work be undertaken to better represent floodplain flow mechanisms. The existing 1D hydraulic model is considered appropriate at this time and for this current

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purpose. The flood depth and hazard maps presented within the Level 2 SFRA are considered appropriate for the strategic nature of the assessment. However, once the site becomes available, the existing 1D hydraulic model is not considered to be sufficient to accurately represent the floodplain flow paths at Watchet Paper Mill. Furthermore, it does not consider interaction of River Washford with the Mill-Race.

The flood mapping process used within this study has applied the approach adopted by the Environment Agency for flood mapping purposes at this location, where modelled flood levels are extrapolated across the floodplain. This conservative approach is likely to overestimate the extent of the floodplain due to not fully representing overbank flow paths and floodplain topography.

It is recommended that if further modelling is undertaken for the study area, the use of a linked 1D-2D hydraulic model. This will represent floodplain flow paths and deliver a more accurate flood extent.

Figure 5-2: Flood extent of the 1 in 100 year (top) and 1 in 1000 year (bottom) event

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5.8 Flood Zone 3b (Functional Floodplain)

The hydraulic modelling indicates that the Watchet Paper Mill is not located within Flood Zone 3b, based upon a 1 in 20 year event. It is therefore recommended that the Flood Zone 3b designation within the Level 1 SFRA for this location should be superseded by the findings of this study.

5.9 Access/Egress

Access to the Wachet Paper Mill consists of a relatively steep road down the valley side from Brendon Road. The majority of the access road is elevated above the extreme peak water levels and will therefore provide a safe access/egress route from the site. However, the hydraulic modelling has suggested that large parts of the site are inundated, especially under the 1 in 1000 year event. Therefore, as part of more detailed work, consideration should be given to how site users can reach the access route via a safe route from more distant parts of the site. This could be achieved if the floodplain is formalised into a two-stage channel, allowing the developed part of the site to remain dry, as discussed in Section 5.10.

5.10 Flood Mitigation

The Watchet Paper Mill is unlikely to offer any strategic flood mitigation opportunities like those discussed for Minehead and Williton. Following the completion of a linked 1D-2D hydraulic model, as discussed in Section 5.7, appropriate flood mitigation requirements will be clearer.

A two stage channel could be created alongside the Washford River, which would allow for additional watercourse capacity and allow for the formalisation of the floodplain within a defined area, rather than the current uncontrolled spread of floodwater over the study area. This must be achieved without any detriment to third parties by the displacement of floodwater etc. This could be combined with sequential positioning of proposed development to steer more vulnerable parts outside of the floodplain.

Whilst flood warning should not be used as a stand alone measure to mitigate flood risk, the Environment Agency have flood warning procedures in place to raise awareness of imminent flooding from the River Washford (see chapter 8), which can allow people to evacuate the area before the risk of flooding occurs.

Generalised flood mitigation measures are discussed further within Chapter 7 and 8.

5.11 Recommendations

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of land raising required, and therefore reduce any level for level floodplain compensation that may be required; • It is recommended that a linked 1D-2D hydraulic model of the study area is constructed when the site becomes available for development to verify the flood mapping included within this report; • The linked 1D-2D hydraulic model can be used to evaluate the opportunities associated with the formalisation of the floodplain within a two-stage channel, forming a defined and managed zone alongside the River Washford. This should also improve access opportunities; and, • The impact of surface water flooding should be considered.

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6 Sequential Approach to Site Allocation

The WSC Core Strategy identifies three strategic development areas for future growth within West Somerset, located at Minehead, Watchet and Williton. The results from the hydraulic modelling work, undertaken as part of this Level 2 SFRA, can be used to inform the sequential approach to site allocation within these settlements.

The Exception Test should be applied only after the Sequential Test has been applied and in the circumstances shown in PPS25 Table D.1, firstly, when more vulnerable development and essential infrastructure cannot be located in Flood Zone 1 or 2 and secondly, where highly vulnerable development cannot be located in Flood Zone 1.

6.1 Exception Test

When considering the future development outlined within the WSC Core Strategy, the Exception Test is only likely to be required for more vulnerable development (residential, hotels and educational establishments) proposed within Flood Zone 3a when accounting for climate change.

PPS25 states that for the Exception Test to be passed, three main criteria must be satisfied in order for the development to be considered acceptable:

• Part A – It must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk, informed by a SFRA where one has been prepared. If the Development Plan Document (DPD) has reached the ‘submission’ stage – see Figure 4 of PPS12: Local Development Frameworks – the benefits of the development should contribute to the Core Strategy’s Sustainability Appraisal; • Part 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; • Part C - A FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and, where possible will reduce the flood risk overall.

The outputs of this Level 2 SFRA can be used by WSC to assess where a development may satisfy Part C of the Exception Test at the strategic level. The presented information can be used by WSC to refine the strategic land allocation process. Clear demonstration for the satisfaction of Part C of the Exception Test should be achieved through a robust and thorough site specific FRA prepared in consultation with the LPA and the Environment Agency.

For successful application it is important that the arguments presented for justification through the Exception Test are in line with current policies of the LPA.

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7 Site Specific Flood Risk Assessment Guidance

A site specific Flood Risk Assessment (FRA) aims to refine available information and minimise these risks through site design, layout and where required, mitigation. This chapter presents the principle requirements for site specific FRAs for submission with planning applications within the study area.

7.1 Site Specific FRA Requirements

Once a development/site is considered to be appropriate for a development type, a site specific FRA is likely to be required to accompany the planning application. The Level 1 SFRA identifies the circumstances when a FRA is required. The key principle of a site specific FRA for developments located in the floodplain is to demonstrate how the site and occupants will be safe for the development lifetime without exacerbating flood risk elsewhere. The site specific FRA must identify the Flood Zone designation of the site, available at www.environment- agency.gov.uk. The presence of Flood Zone 3b (Functional Floodplain) must also be considered. This will then define what further information is required, which should be agreed in consultation and partnership with the LPA and the Environment Agency.

Where an Environment Agency Flood Zone Map has not been defined for watercourse, within or nearby a particular site, further investigation will be required to confirm the flood risk associated with the watercourse.

The Environment Agency website15 provides standing advice on the requirement of FRAs for developers and LPAs. Annex E of PPS25 provides an identification of the general requirements for the assessment of flood risk. The PPS25 Practice Guide16 should also be consulted for guidance on how to assess flood risk appropriately.

Further to PPS25 and Environment Agency guidance documents, the following information provides some additional and specific requirements of site specific FRA’s within the Minehead and Williton. No specific requirements have been identified for the Watchet Paper Mill because it is unknown when the site may become available.

7.1.1 Minehead

If the site is located within the residual flood risk zones presented within this Level 2 SFRA or within the Environment Agency Flood Zone Map (i.e. Flood Zone 2 or 3), a specific hydraulic modelling study may be necessary. If required, this should consider how the hydraulic model can be tailored to the particular site and may include the following considerations listed below:

1. More appropriate breach location;

2. Width of breach (dependant upon location i.e. for hard defences a smaller breach width should be applied);

3. Impact of ongoing infrastructure improvements in Minehead on flood flow pathways;

15 http://www.environment-agency.gov.uk/research/planning/82584.aspx 16 Communities and Local Government (2009) ‘Planning Policy Statement 25: Development and Flood Risk – Practice Guide’, TSO: London

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4. The impact of storm action upon the effective level of existing soft coastal defences; and,

5. Inclusion of culverts beneath the West Somerset Railway and Seaward Way.

If the site is found to be subject to residual tidal flood risk the impact of displacement of floodwater should be considered. In order to minimise displacement of floodwater development should be sequentially located by steering less vulnerable development (car park/amenity areas) towards land which experiences most significant flood depths. This may reduce the amount of land raising required, and therefore reduce any floodplain compensation that may be required.

7.1.2 Williton

Williton experiences fluvial flooding which spills out of bank from the Monksilver Stream and across the settlement. Any new development should have no detrimental impact upon these flow paths. There should also be no detrimental impact upon storage of floodwater within a site and room should be made available for floodwater.

Many of the areas that surround Williton are poorly drained and therefore the use of infiltration SuDS may not be appropriate to manage surface water generated by new development. The use of SuDS and effective management of surface water should be considered within a site specific FRA.

7.2 Site Vulnerability and Site Layout

WSC and developers should use the Level 2 SFRA flood risk mapping (Sections 3 to 5 and Appendix C) at the master planning stage to, where possible, sequentially located development based on flood risk vulnerability classification (PPS25 Table D.2, Reference 1), to areas of lowest risk e.g. residential developments should be restricted to lowest risk areas and open space areas or parking could be placed on lower ground with a higher probability of flooding exists.

Structures such as bus or bike shelters, park benches and refuse bins (and associated storage areas) located in areas with a high flood risk should be flood resilient and be firmly attached to the ground.

7.3 Finished Floor Levels

Where developing in flood risk areas is unavoidable, a common method of mitigating flood risk to people is to ensure floor levels are raised above the 1 in 100 year plus climate change flood level for fluvial flood events derived for the immediate vicinity of the site (i.e. relative to the extent of a site along a watercourse as flood levels are likely to vary with increasing distance downstream) and the 1 in 200 year plus climate change flood level for tidal events.

An additional freeboard allowance of approximately 0.3m should be included in the derivation of an appropriate finished floor level as a precautionary approach to take account for any uncertainty associated with climate change effects or model error, for example.

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Further consultation with the Environment Agency will be required during the undertaking of any detailed site specific FRA. The impact upon the displacement of floodwater must be considered, as discussed below.

7.4 Raising Ground Levels

Ground levels can be raised to reduce flood depths to acceptable levels or to elevate the site above the particular flood level identified. For example, ground levels adjacent to a river could be lowered to increase the storage capacity, whereas land set back from the river could be raised. This approach could increase green open space adjacent to riverside areas and reduce potential flood risk to the development set back from the river.

Developers should consult the Environment Agency and WSC when considering feasible flood alleviation options. A site specific FRA would have to demonstrate that raising ground levels or constructing a flood wall would not pose an increased flood risk to the development or to any existing buildings at risk from flooding, via the displacement of floodwater.

7.4.1 Flood Storage Capacity

The site specific FRA should include calculations to demonstrate the impact that mitigation options have on floodplain storage volumes, and show how the design and layout of the development mitigates the impact of floodwater displacement.

Loss of flood storage capacity in defended area from both tidal and fluvial sources should be considered. If there is a finite volume of water able to pass into a defended area following a failure of the defences, then a new development, by displacing some of the flood water, will increase the risk to existing properties17.

In undefended coastal areas, raising the ground is less likely to impact on maximum water levels from tidal sea flooding and provision of compensatory storage may not always be necessary. There are few circumstances where provision of compensatory flood storage or conveyance will not be required for undefended fluvial floodplain areas. This is because, whilst single developments may have a minimal impact, the cumulative impact of many such developments can be significant18.In this case a level for level floodplain compensation strategy is likely to be required, to minimise the impact upon flood storage.

A site specific FRA should demonstrate that the development design and layout has been carefully planned to ensure that loss of floodplain storage is minimised. For example, development should be sequentially located by steering less vulnerable development (open space areas or parking) towards land which experiences most significant flood depths. This would reduce the amount of land raising required, and therefore reduce potential loss of floodplain storage.

17 Communities and Local Government (2009) ‘Planning Policy Statement 25: Development and Flood Risk – Practice Guide’, TSO: London 18 Communities and Local Government (2009) ‘Planning Policy Statement 25: Development and Flood Risk – Practice Guide’, TSO: London

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7.5 Surface Water Management

Surface water management arrangements for new developments should be such that the volumes and peak flow rates of surface water leaving a developed site are no greater than the rates prior to the proposed development, unless specific off-site arrangements are made and result in the same net effect. PPS25 recommends the use of Sustainable Drainage Systems (SuDS) to be incorporated into the development at the design stage. This will ensure that flood risk to third parties is not increased.

Control of surface water at source is recommended through SuDS features, this may include infiltration through the use of soakaways or infiltration basins, attenuation using balancing ponds or tanked systems or a combination of techniques to effectively manage runoff including an allowance for climate change. Attenuation and water capture could also be achieved at a property level using source control measures such as rainwater harvesting and green roofs. These techniques should not be relied upon as stand alone systems, but should be viewed as additional storage that contributes to runoff control.

Further guidance on which SuDS techniques are appropriate under different circumstances is provided in CIRIA publication C697, The SUDS Manual (2007). The Level 1 SFRA also provided additional information.

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8 Residual Risk Mitigation

Residual risks are those that remain with flood mitigation measures, such as flood defences, in place. Land to the southeast of Minehead is the only strategic development site that is located behind flood defences and is therefore at risk of flooding through failure or overtopping and are therefore subject to residual risk. Flood defences help to protect other parts of West Somerset including Doniford and Blue Anchor.

8.1 Flood Resilience and Resistance Measures

PPS25 Annex E states that where there is a low probability of limited shallow depth water entry, but not severe inundation to buildings, the use of flood-resilient construction may be considered.

Within the design of buildings in areas where the probability of flooding is low or in areas where other flood risk management measures have been put in place, guidance has been outlined in paragraphs 6.29 to 6.35 of the PPS25 Practice Guide and by the Department of Communities and local Government in ‘Improving the Flood Performance of New Buildings’ (May 2007)19.

Flood proofing is a technique by which buildings are designed to withstand the effects of flooding. There are two main categories of flood proofing; dry proofing and wet proofing. Dry proofing methods are designed to keep water out of the building, and wet proofing methods are designed to improve the ability of the property to withstand the effects of flooding once the water has entered the building.

Further guidance is also provided in the CIRIA Research Project 624 ‘Development and Flood Risk: Guidance for the Construction Industry’ (2004). Table 8-1 summarises recommendations made within Table A3.6 of the report for flood proofing measures which can be incorporated within the design of buildings (subject to compliance with Building Regulations).

Table 8-1: Flood Proofing Options Feature Considerations To Improve Flood Proofing

Careful consideration of materials: use low permeability materials to limit water penetration if dry proofing required. Avoid using timber frame and External cavity walls. Consider applying a water resistant coating. Provide fittings for Walls flood boards or other temporary barriers across openings in the walls (dry proofing).

Internal Walls Avoid use of gypsum plaster and plasterboard; use more flood resistant linings (e.g. hydraulic lime, ceramic tiles). Avoid use of stud partition walls. Avoid use of chipboard floors. Use concrete floors with integrated and Floors continuous damp proof membrane and damp proof course. Solid concrete floors are preferable; if a suspended floor is to be used, provide facility for drainage of sub-floor void. Use solid insulation materials. Fitting, If possible, locate all fittings, fixtures and services above design flood level. Fixtures and Avoid chipboard and MDF. Consider use of removable plastic fittings. Use Services solid doors treated with waterproof coatings. Avoid using double-glazed

19 www.gov.uk/government/publications/flood-resilient-construction-of-new-buildings

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window units that may fill with flood water. Use solid wood staircases. Avoid fitted carpets. Locate electrical, gas and telephone equipment and systems above design flood level. Fit anti-flooding devices to drainage systems.

8.2 Safe Access and Egress

The ability for occupants and users of a development to gain safe access and egress to higher ground outside of the floodplain during a flood event is of primary concern. It is also important to consider whether emergency services will be able to access the development to provide assistance during a flood event.

For less vulnerable development (i.e. retail or leisure) it is considered that safe access and egress from the site will be desirable during times of extreme floods. For more vulnerable development (i.e. residential, hotel or educational), it is considered that safe access and egress from the will be essential during times of extreme floods from each residential unit to an area outside of the floodplain. New properties within a ‘dry island’ of the floodplain will also require safe access due to potential disruption to essential services (i.e. gas and water etc.) that may be experienced during a flood event.

It is necessary to ensure that proposed road levels are such that emergency access and egress routes are maintained or where possible constructed to the 1 in 100 year +CC event, as a minimum.

Details of how this will be achieved should be described in a site-specific FRA and investigate the feasibility of safe access routes both within and beyond the proposed development.

8.3 Flood Warning and Evacuation Plans

Fluvial flood risk from the Washford River and Monksilver Stream are the predominant source of flood risk to Watchet and Williton, respectively. Tidal flood risk is the predominant source of risk for Minehead. The Environment Agency flood warning service monitors rainfall, river levels and tides to forecast the possibility of flooding in the area. They aim to provide a minimum of two hours warning prior to the onset of a flood event.

The advanced warning provided by the Environment Agency indicates that it is feasible for WSC to formulate and implement a Flood Plan. The Flood Plan should set out specific actions based on the level of flood warning. All residents and business should be encouraged to register with the Environment Agency Floodline Warnings Direct20 (Tel. 0845 988 1188) to receive early alerts of a possible flood event.

The status of the flood warning for the three strategic development areas can be seen on the Environment Agency website links shown below:

http://www.environment- agency.gov.uk/homeandleisure/floods/34681.aspx?area=112FWT3T1A

http://www.environment- agency.gov.uk/homeandleisure/floods/34678.aspx?type=Fwacode&term=112FWF3A2A

20 www.gov.uk/sign-up-for-flood-warnings

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http://www.environment- agency.gov.uk/homeandleisure/floods/34678.aspx?type=Fwacode&term=112FWFMON20X

The Flood Plan should be prepared in consultation with the Environment Agency and Somerset Local Authorities Civil Contingencies Partnership21. The plan should also be reviewed at regular intervals to ensure it is based on the most up to date information and still recommends the most appropriate actions.

Site specific Flood Plans should be provided for developments located in areas which are designed to flood, such as ground floor car parking or amenity areas, to ensure site users are safe during a flood event. Flood warning signs highlighting the flood risk and clearly marked flood evacuation routes should be included in the design and layout of the development. A site specific FRA should include details of an adequate maintenance regime to ensure flood warning signs are kept visible and flood evacuations routes are kept clear.

21 www.somerset.gov.uk/how-the-council-works/somerset-local-authorities-civil-contingencies-partnership/

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9 Policy Guidance

For the purpose of development control, detailed policies will need to be set out by WSC to ensure that flood risk is taken account of appropriately during the planning process. This chapter provides guidance on the preparation of such policies for sites, including requirements and conditions to be considered at the planning stage.

• WSC should adopt any revised Flood Zone Mapping Flood held by the Environment Agency and the mapping prepared as part of this Level 2 SFRA (see accompanying GIS layers); • In accordance with PPS25, development should be sequentially located based on flood risk vulnerability classification (PPS25 Table D.2, Annex D), to areas of lowest risk. Opportunities to increase biodiversity and improve amenity value (e.g. pedestrian / cycle routes along the river) should be sought in areas of higher risk adjacent to the river; • Development should be safe throughout its life and emergency vehicular access should be achievable above the 1 in 100 year + CC flood level; • Where development is proposed within an area at risk of flooding, an evacuation plan should be prepared in liaison with the Environment Agency and Somerset County Council emergency planners. The Flood Plan should set out specific actions based on the level of flood warning; • A development should not increase flood risk on site or elsewhere, and where possible, opportunities should be taken to decrease overall flood risk; • Floodplain compensation should be undertaken for loss of floodplain storage. A site specific FRA should demonstrate that loss of floodplain storage through the displacement of floodwater will not increase flood risk to third parties; • Basements should not be used for habitable purposes in areas at risk of flooding. Where an underground car park is proposed, it is necessary to ensure that access points and any venting or other penetrations are situated at least 0.3m above the 1 in 100 year + CC event; • Development should be set back appropriately from all watercourses to allow appropriate access for routine maintenance and emergency clearance; • The Environment Agency should be consulted on development involving any works or operations in the bed of, or within 20 metres of the top of a bank of, a main river22; • Development should not have a detrimental impact on the water environment through changes to water chemistry or resource and this should be ensured through the use of drainage systems which limit the occurrence of pollution to the water environment; • SuDS should be implemented to ensure that runoff from the site (post development) is either to greenfield runoff rates where the site is undeveloped at present or provide betterment, where possible, where the site is previously developed. This should include space set-aside within the confines of the site to accommodate SuDS;

22 Introduced by Statutory Instrument 2006 No.2375 “The Town and Country Planning (General Development Procedure) (Amendment) (No.2) (England) Order 2006”. Available at www.opsi.gov.uk/si/si2006/uksi_20062375_en.pdf

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• Developments should look to incorporate water re-use and minimisation technology for example green roofs and rainwater harvesting. This will aid developments in the adoption of source control SuDS as part of PPS25 requirements.

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10 Summary and Conclusions

To ensure a holistic approach to flood risk management and ensure that flooding is taken into account at all stages of the planning process, the findings of this Level 2 SFRA should be incorporated into the emerging LDF for WSC and read in conjunction with the 2009 Level 1 SFRA.

The WSC Core Strategy identifies three strategic development areas for future growth within West Somerset, located at Minehead, Watchet and Williton. This Level 2 report has focused on these general areas, which have been identified as requiring further investigation in terms of flood risk.

Flood risk maps have been produced for the various scenarios at each strategic development site to produce flood extent, depth and hazard mapping to elaborate upon the conclusions that can be drawn from the Level 1 SFRA, which will allow WSC to make informed planning decisions with regard to flood risk. It is considered that each of the strategic development areas could be suitable for future development; however significant consideration is required with respect to the mitigation and management of flood risk, which is summarised below. The outputs of the Level 2 SFRA have demonstrated that it is unlikely that any of the three strategic development areas can be developed without some form of flood mitigation.

10.1 Minehead

The study area consists of land to the south east of Minehead. 2D hydraulic modelling was undertaken to determine the susceptibility of the study area to tidal flooding with inclusion of the benefit offered by the coastal defences (i.e. to analyse residual flood risk), associated with overtopping and breaching by extreme tidal conditions of the coastal defences. Therefore, unlike the Level 1 SFRA flood maps, the benefit offered by the flood defences is considered.

Whilst the coastal defences were found to protect the study area from the 1 in 200 year overtopping event, some very limited overtopping was experienced under the 1 in 1000 year overtopping event. However, with the inclusion of the anticipated affects of climate change upon tidal levels (1 in 200 year + CC) the majority of the study area became inundated with significant flood depths observed. This was a result of overtopping of the sand dunes and embankments to the south east of Minehead. The formal coastal defences that align the sea front at Minehead were not overtopped under climate change conditions. However, Minehead did experience ‘back door’ flooding, which was due to overtopping of the sand dunes and embankments to the south east, distant from the settlement. The West Somerset Railway and Seaward Way were found to offer some benefit to area on the landward side, through delaying the onset of flooding.

The inclusion of a breach in the coastal defences also had significant implications for flood risk within the study area, with flood depth, hazard and the time of inundation becoming significantly more onerous. However, the impact of waves has not been considered.

Generally, the hydraulic modelling has suggested that the parts of the study area more distant from the coastal defences are more suitable for future development. This is because flood depth, hazard and the time of inundation tends to be less onerous, helped by the defensive action provided by the West Somerset Railway and Seaward Way. One of the limitations of the hydraulic modelling was to ignore the presence of the culverts beneath the West Somerset

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Railway and Seaward Way. These culverts are relatively small and inclusion was not considered appropriate for the nature and scale of this study. However, they should be considered as part of a site-specific FRA.

Existing infrastructure is already in place alongside Seaward Way and safe access/egress of areas adjacent to this road may be more easily achieved. Flood mitigation can offer improved opportunities for development, whether located close or distant from the costal defences, but would require appropriate assessment in terms of the impact upon flood risk to third parties.

Without some form of investment or intervention in the existing coastal defences the existing development of Minehead will be very vulnerable to tidal flood risk, due to back door flooding. Therefore, opportunities could be sought for developer contributions towards the improvement and maintenance of the coastal defences along this stretch of coastline. This would offer the opportunities required to protect the study area to enable future development as well wider benefits where it may reduce the risk of backdoor flooding found to affect the existing settlement of Minehead.

A viable strategic flood mitigation solution must be identified before site allocation in the tidal floodplain, where mass ground raising is required. West Somerset Council should undertake a new study to investigate a viable strategic flood mitigation solution to minimise any detrimental impact upon floodwater displacement and floodplain flow paths.

However, an effective procedural approach with regard to potential developer contributions must be devised and agreed with all relevant stakeholders. Further details on the issues relating to developer contributions to flood risk management can be found in Annex G of PPS25.

However, improving coastal defences will not remove the residual risk associated with a breach and therefore other flood mitigation measures should be considered. Mitigation could be achieved by ground raising on a site by site basis, if necessary, which may offer a more achievable solution in the short term.

Consideration must be given towards the cumulative impact of displacement of tidal floodwater from subsequent new development. However, if displacement of floodwater can be mitigated where necessary, the cumulative impact should be negligible.

Tidal flooding is considered to be the most significant source of flood risk at Minehead. The River Anvil flood alleviation channel helps to significantly reduce the risk associated with fluvial flooding. However, fluvial flood risk must still be considered as part of a site specific FRA.

10.2 Williton

The study area consists of Williton and the surrounding area. The source of data used to create the Level 1 SFRA flood maps at Williton has recently been revised using 1D-2D hydraulic modelling to provide a more accurate representation of fluvial flood risk. The hydraulic modelling was used for the Level 2 SFRA to prepare flood extent, depth and hazard mapping associated with the various local watercourses.

Various areas around Williton are not located within the fluvial floodplain. Development should be steered into these areas, where possible. Where this is not possible, development should be

D126878 October 2010 50 West Somerset Council Level 2 SFRA

sequentially located by steering less vulnerable development (car park/amenity areas) towards land which experiences most significant flood depths etc.

The construction of a strategic flood alleviation scheme could help enable benefit new development and reduce the existing flood risk associated with the settlement of Williton. It is recommended the opportunities identified within this report are taken forward for further investigation.

During a review of the hydraulic model it was observed that a flow control structure that would restrict flow and hence flooding within the the area to the west of Williton was missing. This was included within the hydraulic model to prepare a comparable set of flood maps. Inclusion of the structure was found to some benefits to this area in terms of a reduction in flood extent in the area to the West of Williton.

The extent of Flood Zone 3b identified within this Level 2 SFRA should override that identified in the Level 1 SFRA at Williton.

10.3 Watchet

The study area consists of the Watchet Paper Mill. The hydraulic modelling used to derive the Level 1 SFRA flood maps is considered to be the best available information. This was used to provide the flood extent, depth and hazard mapping, to supplement the information included within the Level 1 SFRA.

The study area was not found to be located within Flood Zone 3b, which overrides the conclusions of the Level 1 SFRA. However, flooding was found to be relatively widespread under the 1 in 100 year and 1 in 1000 year event.

Whilst the existing modelling provides sufficient detail to inform strategic planning decisions it is recommended that further work be undertaken once the site becomes available to better represent floodplain flow mechanisms. This will help to identify an appropriate flood mitigation strategy, which could incorporate the construction of a two stage channel and sequential positioning of the development.

The extent of Flood Zone 3b identified within this Level 2 SFRA should override that identified in the Level 1 SFRA at the Watchet Paper Mill.

10.4 Site Allocation

The three strategic development areas are located within an area at risk of flooding. Therefore, if development is to be allocated within these areas, WSC have to fulfil parts a) and b) of the Exception Test, as outlined in PPS25. This would also require a site specific FRA (i.e. part c) of the Exception Test) to demonstrate how the site and occupants will be safe for the development lifetime without exacerbating flood risk elsewhere.

Site allocation can also be sequentially located within development sites, with the most vulnerable parts of the development being steered to the lowest flood risk zones.

D126878 October 2010 51 West Somerset Council Level 2 SFRA

10.5 Maintenance and Update

SFRAs should be considered as ‘live’ documents where regular review and monitoring should be undertaken to ensure that the best available data on flood risk issues is being used to inform WSC planning decisions.

It is recommended that an Environment Agency data request is undertaken on an annual basis to identify additional flooding information (from all flood sources) and flood risk management information (e.g. new flood alleviation schemes or flood warning advice).

It is recommended that during the Annual Monitoring Report process, a review of existing Planning Policy Statements or associated guidance is undertaken to identify where significant updates may require significant revision of the SFRA.

D126878 October 2010 52 West Somerset Council Level 2 SFRA

11 Appendices

D126878 October 2010 53 West Somerset Council Level 2 SFRA

Appendix A

D126878 October 2010 A

West Somerset Council Level 2 SFRA

Appendix B

D126878 October 2010 B Methodology

This Appendix includes the methodology for the 2D overtopping and breach hydraulic modelling which has been undertaken for Minehead and the surrounding area as part of the Level 2 SFRA. Hydraulic models have been provided for Williton and Watche, which were prepared for Environment Agency Flood Zone Maps and therefore it is assumed that are of appropriate standard for the purposes of the Level 2 SFRA.

Extreme Water Levels

Peak tidal levels were provided by the Environment Agency are presented within Table 1. The table also illustrates the climate change flood level associated with the 1 in 200 year return period event (i.e. 0.5% AEP+CC). The impact of climate change upon tidal level was calculated based upon the rates of sea level rise presented within PPS25, for the period up to 2110.

Table 1: Extreme tidal still water levels Return Period Year Still Water Level 0.5% AEP1 2010 7.21m AOD 0.1% AEP 2010 7.42m AOD 0.5% 2110 8.21m AOD AEP+CC2

Tidal curve

The MIKE21 breach model requires an extreme tidal curve to be input to represent the changes in water level with time during each return period scenario. The extreme tidal curve for each return period scenario is created from two components; an astronomical tide and a surge residual tide. The astronomical tide is assumed to be independent of the metrological conditions.

Mean Spring Tidal Water levels were extracted from the Admiralty Tidal Tables and applied to a sine curve with a 12-hour cycle. The published tidal data for Minehead was extracted.

The surge component is simulated by a regular half-sinusoidal water level increase with assumed storm duration of 40 hours. In order to achieve the worst case scenario the storm surge peak coincides with the second astronomical high tide in the simulation.

The water levels during a tidal flood event were generated by a summation of the astronomical tide levels and the storm surge residual. The 0.5% AEP event tidal curve used for the hydraulic modelling is shown in Figure 1 below.

Digital Terrain Map

A key component of the modelling report is the representation of topography throughout flood prone areas within the study area. LiDAR data and OS maps were used to identify the areas likely to be affected by flooding from the breach.

The platform used for the generation of the Digital Terrain Model (DTM) was the GIS package MapInfo Professional (version 9.5) with the addition of Vertical Mapper (version 3.1) to process raster data containing 3D information.

1 Annual Exceedence Probability otherwise known as the 1 in 200 year return period event 2 Including the anticipated affects of climate change, which is defined in PPS25. The topographical information for the modelling is primarily based on LiDAR data. LiDAR data was provided by the Environment Agency in two formats; a Digital Surface Model (DSM) that includes vegetation and buildings and a DTM, which is filtered to remove the majority of buildings and vegetation.

Figure 1: The 0.5% AEP event tidal curve

8.00 Spring Tide Storm Surge Tidal Water Level 6.00

4.00

2.00

- 0 5 10 15 20 25 30 35 40 45 50 Tidal Level AOD) (m

-2.00

-4.00

-6.00 Time (hours)

Flood Cell Definition

Integral to the modelling methodology is the definition of flood cells. Flood cells are typically defined by prominent topographic features (relative to the flood source), which serve to constrain the movement of floodwater. The flood cell must be of sufficient size to represent floodwater movement which may influence the nature of flooding at the site. The study area consists of a low-lying coastal zone surrounded by more elevated ground and therefore the definition of the flood cell extent is easy to achieve. The flood cell used within the hydraulic modelling was based upon an extent considered to be sufficient to capture the entire inundation area resulting from overtopping and breach of the coastal defences. The flood cell is shown in Figure 2.

Breach and Overtopping Characteristics

In this case, the flood defence crest level is not considered to be sufficient to prevent overtopping from tidal flooding. Therefore, floodwater will enter the study area (or flood cell) via overtopping and through the breach, where appropriate. Floodwater may also drain from the study area via the breach location, but no further drainage (i.e. through watercourses discharging into the sea) is considered within the model, therefore providing a conservative estimation of flood depth and extent.

The breach location used within the hydraulic modelling is located within the sand dunes / embankments that align the Minehead Golf Club, see Figure 2. The breach location is positioned directly opposite the study area and represents the closest position. Other potential breach locations were investigated, but the breach location chosen was considered to represent the worst case scenario for the site, based upon a combination of position, depth and existing flow paths. The flood conditions (i.e. time of inundation, flood extent, depth of flooding) that may be experienced in the event of a breach in the flood defence wall are a function of the breach dimensions, time required to repair the breach (exposure duration) and tidal conditions.

The breach width and exposure duration is determined on the type of defence. Flood defences are categorised as either, ‘Hard Defences’ or ‘Earth Embankments’. The flood defences in this area are classified as an ‘Earth Embankment’ and as such, the breach width adopted is consistent with current guidelines. No breach has been included in the ‘Hard Defences’ that align the Minehead sea front.

The breach in the flood defence wall was set to occur at the peak of the first tidal cycle and 3 subsequent tidal cycles have been included in the analysis.

The base level of the breach has been set to the lowest elevation of the land directly behind (landward) the flood defence. The elevation of land behind the flood defence wall has been established by interrogation of the 1m resolution LiDAR DTM. The invert level of the breach was set to 5.2m AOD. The details of the breach characteristics used in the modelling are presented in Table 2.

Table 2: Breach characteristics Breach Width Breach Invert Level (m Exposure Easting Northing (m) AOD) Time 299674 145699 100 5.2 4 tidal cycles

It is important to note that the current condition of the defences has not been used as a criterion on which to base the breach dimensions or location. No assessment of the probability of failure has been undertaken in this study. However, the sand dunes / embankments are considered more likely to breach compared to the revetments that align the Minehead sea front.

Hydrodynamic model setup

To assess flood propagation in events where the flood defences are breached, hydraulic modelling analysis has been undertaken using the two-dimensional hydraulic modelling software MIKE21-HDFM (version 2009).

Model and software selection

The model used to quantify the residual flood risk at the site was required to:

• Accommodate the effects of a flood flow (propagation of a flood wave and continuous change of water level); • Simulate the hydraulics of the flow at the breach of the defences; and • Generate detailed information on the localised hydraulic conditions over the floodplain area in order to evaluate flood depths.

MIKE21-HDFM simulates water level variations and flows for depth-averaged unsteady two- dimensional free-surface flows. MIKE21 is specifically oriented towards establishing flow patterns in complex water systems, such as coastal waters, estuaries and floodplains. The MIKE21 hydraulic modelling software is developed by the Danish Hydraulic Institute (DHI) Water and Environment. MIKE21-HDFM is a new modelling system based on a flexible mesh approach. The flexible mesh model has the advantage that the model resolution can be varied across the model area. The model utilises the numerical solution of two-dimensional shallow water equations. Figure 2: Flood cell, DTM and breach location

Model extent and resolution

A MIKE21 flexible mesh has been developed using the MIKE21 program, Mesh Generator. The mesh generator creates a mesh from triangular elements covering the flood cell shown in Figure 2.

The element size in the mesh is varied throughout the model domain depending upon the complexity of the floodplain and any topographic features identified as important to flood propagation.

To represent the hydraulics around each breach with a relatively high level of accuracy, a comparatively small element size has been applied in the vicinity of breaches. The breach itself is represented with a minimum of 4 elements across its width as shown in Figure 3. A typical area of elements in the vicinity of a breach is 50 to 100m2. Further from the breach location the mesh is less resolved with a larger maximum element area.

Some parts of the study area are either urbanised or associated with man-made features (e.g. roads, railway embankments, flood defences). Urban areas and structures within the floodplain have the potential to affect the free flow of floodwater. Various roads and the West Somerset Railway have been included in the mesh for the consideration of the impact upon the flow of floodwater.

Control lines were added to the mesh to force it to follow the alignment of the features ensuring the elevations of important features are picked up during the mesh generation as shown in Figure 3. The control-lines of linear man-made features were schematised by reference to the DTM and 1:10,000 OS maps. The crest levels of linear features, such as flood defences, road embankments and railway embankments, have been established by interrogation of the DTM. It should be noted that some of the features described above have been identified through a desktop analysis only and have not been verified on the ground. Figure 3: Example of Mike21 flexible mesh

Hydraulic Roughness

Hydraulic roughness represents the conveyance capacity of the vegetative growth, bed and bank material, channel, sinuosity and structures of the floodplain. Within the MIKE21 model, hydraulic roughness is defined by the dimensionless Manning’s ‘n’ roughness coefficient.

The Manning’s value of urban (0.07) and rural (0.04) areas has been specified within the hydraulic model, based upon available literature (e.g. Chow, 1979).

Model time-step

The model time step interval is very important with respect to the numerical stability of the hydraulic model. The time step adopted in the MIKE21 models was chosen to ensure stability of the hydraulic models. The stability of the model is defined by two stability criteria, namely the courant number and the CFL stability condition.

In order to ensure numerical stability the courant number was kept smaller than 0.50 during the entire simulation whilst the maximum CFL stability condition was less than 1.0.

Boundary conditions

The MIKE21 breach model requires a boundary condition to be defined. This is a time dependent tidal water level boundary located seaward of the coastal defences, which replicates the extreme water level during a tidal flood event and provides the important input of water volumes to the model. The generation of the extreme tidal boundary conditions is discussed above.

Simulations

Six scenarios was simulated to provide information on the floodwaters which may affect the site in the event of a breach in the flood defences and to quantify the potential impact at the site. The scenario modelled was the:

• 0.5% AEP overtopping scenario • 0.1% AEP overtopping scenario • 0.5% AEP+CC overtopping scenario • 0.5% AEP overtopping and defence breach scenario • 0.1% AEP overtopping and defence breach scenario • 0.5% AEP+CC overtopping and defence breach scenario

West Somerset Council Level 2 SFRA

Appendix C

D126878 October 2010 C

West Somerset Council Level 2 SFRA

Appendix D

D126878 October 2010 D

Data Extraction Summary

A total of 8 data extraction points are identified on figure above. Time series data has been extracted from the hydraulic model for each scenario and for each data extraction point. These have been summarised in the following tables, which included maximum water surface elevation, depth, velocity, time of inundation and maximum flood hazard. The time of inundation relates to the time when the area floods subsequent to the onset of coastal defence overtopping or subsequent to the defence breach failure.

Table A: Data Extraction Point 1 Surface Elevation Water Velocity Time to Hazard Scenario (m AOD) Depth (m) (m/s) Inundation (h) 0.5% AEP OT1 - - - - - 0.1% AEP OT - - - - - 0.5% AEP+CC OT 6.47 1.07 0.36 13.58 Significant 0.5% AEP OT&B2 - - - - - 0.1% AEP OT&B 6.50 1.11 0.36 14 Significant 0.5% AEP+CC 7.79 2.40 0.36 12.75 Extreme OT&B

Table B: Data Extraction Point 2 Surface Elevation Water Velocity Time to Hazard Scenario (m AOD) Depth (m) (m/s) Inundation (h) 0.5% AEP OT - - - - - 0.1% AEP OT - - - - - 0.5% AEP+CC OT 6.50 1.16 0.51 13.08 Significant 0.5% AEP OT&B 6.34 1.00 0.36 13.42 Significant 0.1% AEP OT&B 6.52 1.18 0.37 13.08 Significant 0.5% AEP+CC 7.79 2.46 0.35 1.5 Extreme OT&B

Table C: Data Extraction Point 3 Surface Elevation Water Velocity Time to Hazard Scenario (m AOD) Depth (m) (m/s) Inundation (h) 0.5% AEP OT - - - - - 0.1% AEP OT - - - - - 0.5% AEP+CC OT 6.46 0.67 0.30 13.58 Significant 0.5% AEP OT&B - - - - - 0.1% AEP OT&B 6.50 0.71 0.34 14 Significant 0.5% AEP+CC3 7.80 2.01 0.56 12.75 Extreme OT&B

1 OT - Overtopping 2 OT&B – Overtopping and breach 3 CC – Climate change Table D: Data Extraction Point 4 Surface Elevation Water Velocity Time to Hazard Scenario (m AOD) Depth (m) (m/s) Inundation (h) 0.5% AEP OT - - - - - 0.1% AEP OT - - - - - 0.5% AEP+CC OT 6.50 1.10 0.54 13 Significant 0.5% AEP OT&B 6.34 0.94 0.38 13.33 Significant 0.1% AEP OT&B 6.52 1.13 0.40 13 Significant 0.5% AEP+CC 7.79 2.40 0.37 1.42 Extreme OT&B

Table E: Data Extraction Point 5 Surface Elevation Water Velocity Time to Hazard Scenario (m AOD) Depth (m) (m/s) Inundation (h) 0.5% AEP OT - - - - - 0.1% AEP OT - - - - - 0.5% AEP+CC OT 6.59 1.19 0.55 12.16 Extreme 0.5% AEP OT&B 6.39 0.99 0.59 0.16 Significant 0.1% AEP OT&B 6.52 1.12 0.63 0.16 Extreme 0.5% AEP+CC 7.76 2.36 1.02 0.08 Extreme OT&B

Table F: Data Extraction Point 6 Surface Elevation Water Velocity Time to Hazard Scenario (m AOD) Depth (m) (m/s) Inundation (h) 0.5% AEP OT - - - - - 0.1% AEP OT - - - - - 0.5% AEP+CC OT 6.61 1.60 0.79 12.08 Extreme 0.5% AEP OT&B 6.39 1.37 0.40 0.5 Significant 0.1% AEP OT&B 6.52 1.51 0.46 0.66 Significant 0.5% AEP+CC 7.77 2.76 0.64 0.33 Extreme OT&B

Table G: Data Extraction Point 7 Surface Elevation Water Velocity Time to Hazard Scenario (m AOD) Depth (m) (m/s) Inundation (h) 0.5% AEP OT - - - - - 0.1% AEP OT - - - - - 0.5% AEP+CC OT 6.45 0.89 0.17 13.58 Significant 0.5% AEP OT&B - - - - - 0.1% AEP OT&B 6.36 0.80 0.14 13.92 Significant 0.5% AEP+CC 7.78 2.39 0.38 12.66 Extreme OT&B

Table H: Data Extraction Point 8 Surface Elevation Water Velocity Time to Hazard Scenario (m AOD) Depth (m) (m/s) Inundation (h) 0.5% AEP OT - - - - - 0.1% AEP OT - - - - - 0.5% AEP+CC OT 6.51 1.12 0.23 13 Significant 0.5% AEP OT&B 6.35 0.96 0.29 13.58 Significant 0.1% AEP OT&B 6.52 1.13 0.30 13.25 Significant 0.5% AEP+CC 7.78 2.39 0.23 1.5 Extreme OT&B

The discussion below provides a brief commentary with respect to the results of the data extraction tables.

There is no data presented for 0.5% AEP OT (overtopping) and 0.1% AEP OT scenarios because floodwater was either not sufficient to overtop the defences (0.5% AEP OT) or the flood extent was not significant and did not affect the data extraction point locations (0.1% AEP OT). Therefore, there is only one overtopping scenario which affects the data extraction points (0.5% AEP+CC OT), which is with the inclusion of climate change.

The surface elevation observed at each of the data extraction points is relatively consistent between individual scenarios. This suggests that the peak water level is reached at each data extraction point and an approximate equilibrium observed across the study area.

The flood depth from one data extraction point to another varies significantly, which is dependant upon the ground level at the extraction point. It should be noted that there is some topographical variation in the proximity of each data extraction point and the least elevated area was chosen for the data extraction point location. This approach was adopted to illustrate the approximate maximum flood depths.

Maximum velocities tend to be higher for data extraction point 5 and 6, which can be attributed to their position which is closest to the breach location and the costal defences.

Time to inundation varies significantly between data extraction points and each scenario. The location of the data extraction point has a significant impact upon time of inundation, with distance increasing the time of inundation. The effects of Seaward Way and the A39 also tend to increase the time of inundation for those extraction points located beyond these features.

The first tidal curve results in only a small amount of overtopping during the 0.5% AEP+CC OT scenario, which does not affect any of the data extraction points. However, the second (and largest) tidal curve, when the storm surge is at its peak causes sufficient water to overtop the coastal defences and spread over the coastal zone affecting the data extraction points. Consequently, time of inundation for the 0.5% AEP+CC OT scenario is associated with a relatively long period, which starts when the first overtopping incident is observed (in the first tidal cycles) to the second, when the data extraction point experiences flooding.

Time of inundation associated with the breach scenarios is very short (less than 0.5 hours) for the data extraction points close to the coastal defences and breach location (i.e. points 5 and 6). The West Somerset Railway and Seaward Way influence the onset of inundation and consequently the time of inundation is extended to extraction points beyond these features, by approximately 45 minutes. However, there is some variation between data extraction points and individual scenarios.

Maximum flood hazard identified in the tables above for the various data extraction points is either significant or extreme, which can be attributed to the low-lying ground levels at each data extraction point. Low or moderate flood hazard only tends to be located towards the edges of the flood extent.