EB13-West-Somerset-Council-Level-2-Strategic-Flood

Transcription

West Somerset Council
Level 2 Strategic Flood Risk Assessment
Final Report
October 2010
Prepared for
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
Flood Risk Specialist
Dr Rob Sweet
Senior Flood Risk
Specialist
Jon Robinson
Associate Director
02
October 2010
Vs2 – Updated
with EA & WSC
comments
Nick Bosanko
Flood Risk Specialist
Tom Edwards
Senior Flood Risk
Specialist
Dr Rob Sweet
Senior Flood Risk
Specialist
Jon Robinson
Associate Director
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
to and for the sole and confidential use and reliance of Scott Wilson's client. Scott Wilson
accepts no liability for any use of this document other than by its client and only for the
purposes for which it was prepared and provided. No person other than the client may
copy (in whole or in part) use or rely on the contents of this document, without the prior
written permission of the Company Secretary of Scott Wilson Ltd. Any advice, opinions,
or recommendations within this document should be read and relied upon only in the
context of the document as a whole. The contents of this document do not provide legal
or tax advice or opinion.
© Scott Wilson Ltd 2010
Scott Wilson
The Crescent Centre
Temple Back
Bristol
BS1 6EZ
Tel: 0117 917 1214
Fax: 0117 930 0342
www.scottwilson.com
Level 2 SFRA
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
2.5
Flood Zone 3b (Functional Floodplain)............................................................................ 7
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
3.5
Aims ............................................................................................................................. 12
Methodology ................................................................................................................. 12
3.6
Hydraulic Model Results – Tidal Overtopping ............................................................... 14
3.7
3.8
Hydraulic Model Results – Tidal Overtopping & Breach................................................ 16
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
4.3
Topography .................................................................................................................. 24
Overview of Flood Risk and Flood Risk Management................................................... 25
4.4
Aims ............................................................................................................................. 25
4.5
Methodology ................................................................................................................. 25
4.6
Hydraulic Model Results ............................................................................................... 26
Level 2 SFRA
4.7
4.8
Limitations .................................................................................................................... 30
4.9
Access & Egress .......................................................................................................... 31
4.10
4.11
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
5.9
Access/Egress.............................................................................................................. 37
5.10
5.11
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
10.4
Watchet ........................................................................................................................ 51
Site Allocation............................................................................................................... 51
Level 2 SFRA
10.5
Maintenance and Update.............................................................................................. 52
11
Appendices.................................................................................... 53
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.
1 in 100 year event
Event that on average will occur once every 100 years. Also expressed as an event,
which has a 1% probability of occurring in any one year or 1% annual exceedence
probability (AEP).
AOD
Above Ordnance Datum
CC
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
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
Flood defence
Infrastructure such as floodwalls and embankments used to protect land from flooding.
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.
Flood Zone 1
This zone comprises of land assessed as having a less than 1 in 1000 annual
probability of river of sea flooding in any year (0.1%).
Flood Zone 2
This zone comprises land assessed as having between a 1 in 100 year and 1 in 1000
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.
Flood Zone 3a
This zone comprises land assessed as having a 1 in 100 or greater annual probability
of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the
sea (>0.5%) in any year.
Flood Zone 3b – Functional
Floodplain
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
identification of Functional Floodplain should take account of local circumstances and
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.
Local Development
Framework (LDF)
The core of the updated planning system (introduced by the Planning and Compulsory
Purchase Act 2004). The LDF comprises the Local Development Documents, including
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.
Mitigation measure
An element of development design which may be used to manage flood risk or avoid
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
Risk is a factor of the probability or likelihood of an event occurring multiplied by
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
SuDS
Sustainable Drainage Systems. Methods of management practices and control
structures that are designed to drain surface water in a more sustainable manner than
some conventional techniques.
STW
Sewage Treatment Works
WSC
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1
1.1
Introduction
Overview
1
The latest revision of Planning Policy Statement 25: Development and Flood Risk (PPS25)
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.
2
The Level 1 SFRA 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.
3
WSC Core Strategy 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.
1
Communities and Local Government (2010) ‘Planning Policy Statement 25: Development and Flood Risk’, TSO: London.
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.
2
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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,
2
2
which covers an approximate area of 945 km and includes 490km 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
2.1
Context
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 HECRAS 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
Extreme
Dangerous for all
Significant
Dangerous for most people
Moderate
Dangerous for some e.g.
children
Low
Caution
Colour
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
3.1
Minehead
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
Copyright West Somerset (2010). All rights reserved. WSC Licence No. 100023932 (2010)
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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.
4
The recently released North Devon and Somerset Shoreline Management Plan (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 http://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
2110
8.21m AOD
1 in 200 year+climate change
5
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 – 2010
6
• 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
6
Including the anticipated affects of climate change, which is defined in PPS25.
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
3.6.2
Climate Change
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
3.7.2
Climate Change
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 Floodwater
1 in 200 year
Fluvial
550,000m
1 in 200 year
Tidal
1,963,000m
1 in 200 year +CC
Fluvial
661,000m
1 in 200 year +CC
Tidal
6,348,000m
7
3
3
3
3
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
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.
8
The North Devon and Somerset 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 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
9
and displacement of floodwater (tidal 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
10
approximately a 0.2m rise in water level across the flooded area . 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
http://www.ndascag.org/
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.
9
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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
4.1
Williton
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.
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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
11
event affected approximately 50 properties .
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 +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 +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
-
6
No impact
No impact
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7
8
No impact
No impact
No impact
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
The definition of Flood Zone 3b is described within the PPS25, but elaborated within the PPS25
12
Practice Guide . 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
13
report for Williton , 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:
Zone 1);
of land raising required, and therefore reduce any floodplain compensation that may be
required;
• A detailed study should be commissioned to evaluate the costs and benefits of the
opportunities associated a flood diversion channel and/or a flood storage area. However,
former studies suggested that some potential schemes were not economically viable;
• Any proposed ground raising should consider the implications upon displacement of
floodwater and the impact upon key flow paths and provide the appropriate level for level
floodplain compensation if required;
• At the local scale the impact of surface water flooding should be considered.
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5
5.1
Watchet
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.
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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
14
upstream of the paper mill .
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 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
5.6.3
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
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
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
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:
Zone 1);
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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.environmentagency.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.
15
The Environment Agency website provides standing advice on the requirement of FRAs for
developers and LPAs. Annex E of PPS25 provides an identification of the general requirements
16
for the assessment of flood risk. The PPS25 Practice Guide 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
London
16
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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
17
increase the risk to existing properties .
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
18
developments can be significant .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
London
London
18
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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
19
and local Government in ‘Improving the Flood Performance of New Buildings’ (May 2007) .
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
19
Feature
Considerations To Improve Flood Proofing
External
Walls
Careful consideration of materials: use low permeability materials to limit
water penetration if dry proofing required. Avoid using timber frame and
cavity walls. Consider applying a water resistant coating. Provide fittings for
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.
Floors
Avoid use of chipboard floors. Use concrete floors with integrated and
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,
Fixtures and
Services
If possible, locate all fittings, fixtures and services above design flood level.
Avoid chipboard and MDF. Consider use of removable plastic fittings. Use
solid doors treated with waterproof coatings. Avoid using double-glazed
http://www.floodforum.org.uk/improvingfloodresilienceofnewbuildings.pdf
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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
20
register with the Environment Agency Floodline Warnings Direct (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.environmentagency.gov.uk/homeandleisure/floods/34681.aspx?area=112FWT3T1A
http://www.environmentagency.gov.uk/homeandleisure/floods/34678.aspx?type=Fwacode&term=112FWF3A2A
20
http://www.environment-agency.gov.uk/homeandleisure/floods/38289.aspx
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http://www.environmentagency.gov.uk/homeandleisure/floods/34678.aspx?type=Fwacode&term=112FWFMON20X
The Flood Plan should be prepared in consultation with the Environment Agency and Somerset
21
Local Authorities Civil Contingencies Partnership . 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
http://www.somerset.gov.uk/irj/public/council/departments/department?rid=/guid/e098bebf-764c-2c10-56a0-be374f2821c2
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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
22
operations in the bed of, or within 20 metres of the top of a bank of, a main river ;
•
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
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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.
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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.
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11
Appendices
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Appendix A
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Appendix B
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Appendix C
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Appendix D
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EB13-West-Somerset-Council-Level-2-Strategic-Flood

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