Flood Risk Assessment and Drainage Strategy
Land South of Branston, Burton-upon-Trent
Proposed Development at Branston
St Modwen Developments
August 2011
Halcrow Group Limited
Red Hill House, 227 London Road, Worcester WR5 2JG
tel 01905 361 361 fax 01905 361 362
halcrow.com
Halcrow Group Limited has prepared this report in accordance with
the instructions of client St Modwen Developments for the client’s sole and specific use.
Any other persons who use any information contained herein do so at their own risk.
© Halcrow Group Limited 2011
Flood Risk Assessment and Drainage Strategy
Proposed Development at Branston
St Modwen Developments
August 2011
Flood Risk Assessment and Drainage Strategy
Document history
Flood Risk Assessment and Drainage Strategy
Proposed Development at Branston
St Modwen Developments
This document has been issued and amended as follows:
Version Date Description Created by Verified by Approved by
1.0 24/08/11
Draft Final for Comment J Parkin S Price P Robinson
Flood Risk Assessment and Drainage Strategy
Contents
Executive Summary
1 Introduction 1 1.1 Terms of Reference 1
1.2 Flood Risk Assessments under PPS25 1
1.3 Data Received 2
1.4 Scope of the Report 2
1.5 Consultation 3
1.6 Climate Change 3
2 Existing Site 4 2.1 Site Location 4
2.2 Site Description and Land Use 4
2.3 Watercourse Description 4
2.3.1 Tatenhill Brook 4
2.3.2 River Trent 5
2.4 Existing Structures 5
2.5 Existing Surface and Foul Water Arrangements 5
2.6 Section 2.6 Geo-environmental considerations 6
3 Existing Flood Risk 7 3.1 Probability of Fluvial Flooding 7
3.1.1 Environment Agency Flood Zones 7
3.2 Historic Fluvial Flooding and Flood Mechanisms 8
3.3 Main Street Surface Water Pumping Station and Culvert 9
4 Hydrological and Hydraulic Modelling – Existing Conditions 11
4.1 Overview 11
4.2 Existing Model 11
4.3 Hydraulic Model Review and Improvements 11
4.3.1 Model Improvement – Schematisation 11
4.3.2 Model Improvement – Additional Survey Data 12
4.3.3 Model Node Labelling Convention 12
4.4 Model Improvement - Hydrology 12
4.4.1 FEH Rainfall Runoff 13
4.4.2 FEH Statistical Pooled Analysis 13
4.4.3 ReFH 13
4.4.4 Preferred Method for the FRA 13
4.5 Existing Scenario Modelling Results 14
Flood Risk Assessment and Drainage Strategy
5 Proposed Development & Modelling 15 5.1 Summary of Proposed Development 15
5.2 Vulnerability of Flooding 15
6 Proposed Development and Flood Risk 19 6.1 Modelled Flood Events 19
6.2 Proposed Development and Fluvial Flood Risk - Summary 21
6.2.1 Dominant River Trent Scenario 21
6.2.2 Dominant Tatenhill Brook Scenario 22
6.2.3 Dominant River Trent Scenario – Climate Change 24
6.2.4 Dominant Tatenhill Brook Scenario – Climate Change 25
6.2.5 UK Flood Hazard 27
6.2.6 Flood Hazard at the Proposed Branston Development 28
6.2.7 Impounded Water Bodies 30
6.2.8 Groundwater 30
6.2.9 Pluvial Flooding / Overland Flows 31
7 Surface Water Drainage 32 7.1 General Introduction 32
7.2 Geological Environment 32
7.3 Surface Water Runoff Rates 33
7.4 Sustainable Urban Drainage Systems 33
7.4.1 Infiltration techniques 34
7.4.2 Source Control techniques 34
7.4.3 Pollution Prevention 34
7.5 Site Drainage 36
7.6 Section 7.7 Ground condition considerations 37
8 Proposed Foul Water System 38
9 The Sequential Test 39 9.1 The Sequential Approach 39
9.2 The Sequential Test Background Information 39
9.3 Proposed Branston Site Sequential Test & Exception Test 39
10 Conclusions 40 10.1 Conclusions 40
11 References 42
Glossary 43
List of Figures 44
Flood Risk Assessment and Drainage Strategy
List of Tables 45
Appendix
Appendix A Proposed Development Outline Plan
Appendix B Modelling & Flood Outlines B.1 Hydrology Technical Note
Appendix C Surface Water Drainage Details
Flood Risk Assessment and Drainage Strategy
All maps in this document are reproduced from Ordnance Survey material with the
permission of Ordnance Survey on behalf of the Controller of Her Majesty's
Stationery Office © Crown copyright. Unauthorised reproduction infringes Crown
copyright and may lead to prosecution or civil proceedings. Licence Number:
AL100017424, (2011).
All maps orientated with North at the top of the image.
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Executive Summary
Halcrow Group Ltd were commissioned by St Modwen to produce a Flood Risk
Assessment (FRA) for a proposed development at Branston, south of Burton-
upon-Trent. The FRA has been produced in accordance with Planning Policy
Statement 25 (PPS25), and will be used to support an outline planning
application for the development of a mixed use site.
This document has been prepared using a range of relevant data sets to inform
the Flood Risk Assessment. The River Trent Strategy hydraulic model was
purchased from the Environment Agency for use in this commission. The
Environment Agency have been consulted at key stages of the project, and
provided additional data sets and guidance including topographic and structural
survey data plus information on the Main Street surface water pumping station.
Additional channel, topographic and structural survey was also commissioned
specifically for this study.
The proposed development site comprises three sites of undeveloped land. The
first is a large site to the south of Branston (approximately 1.2km2 in area) with
two additional smaller sites within Branston, the first being north of Main Street
between the A38 roundabout and the B5018 (approximately 0.15km2) and a
second south of Main Street (approximately 0.03km2). The proposed
development site has been sequentially tested and demonstrated to be
appropriate for this location.
The River Trent flows to the east of the larger site, whilst the Tatenhill Brook
flows through the site, entering from the west. The channel survey data was
used to create a linked 1D2D ISIS-TUFLOW hydraulic model of the study area.
This was nested within the existing River Trent Strategy model (which was
modified for this commission by improving floodplain representation with 2D
domains to ensure that the wider results upstream and downstream from the
proposed development understood).
During previous historic flood events, the River Trent has been recorded flowing
underneath / through the railway embankment and into the lower lying land of
the Tatenhill Brook channel. From here, flood water backs up this channel and
floods the A38 dual carriageway.
The FRA assessed flooding from all sources and indicated that during the
existing conditions, some areas of the large site to the west of the railway line
were shown to be within fluvial Flood Zone 3. As part of the development
proposals, some of these areas were simulated to be raised 600mm above the 1%
Annual Exceedance Probability (AEP) (1 in 100 year) flood event level. To
provide compensation, the existing Tatenhill Brook channel was widened and
lowered, in addition to creating a new storage / drainage area. Finished floor
levels for properties will be set at 600mm above the 1% AEP (1 in 100 year) +
Climate Change flood level.
A River Trent backwater has also been incorporated into the design, providing a
balance in water levels either side of the railway line to the north and a refuge
area for fish in times of flood. A flood embankment has also been located around
the existing properties on the eastern side of the A38.
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The proposed design scenario incorporates a raised embankment across the line
of the Tatenhill Brook channel. This removes the flood flow route of the River
Trent and significantly reduces flooding to the A38 (in addition to the frequency,
with the approximate onset of flooding reducing from a 1 in 40 year event to a 1
in 50 year event).
The increased storage and drainage ditches have been designed to act as a
Sustainable Urban Drainage System, accommodating surface run-off volumes (in
addition to the fluvial flow).
Through the hydraulic modelling, the proposed development has been
demonstrated to cause no additional increase in flooding at the site, or upstream
and downstream for both a dominant River Trent scenario and a dominant
Tatenhill Brook scenario (including with allowances made for the possible effects
of Climate Change). A Hazard analysis and mapping study has also been
undertaken to recommended published guideline standards, and has shown not
to increase risk to the surrounding area.
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1 Introduction
1.1 Terms of Reference
Halcrow Group Ltd has been commissioned by St Modwen to produce a Flood
Risk Assessment (FRA), in accordance with Planning Policy Statement 25
(PPS25), for the proposed redevelopment at Branston, south of Burton-upon-
Trent. The FRA has been prepared to support a planning application for the
development of a mixed use site. An outline plan is contained in Appendix A.
1.2 Flood Risk Assessments under PPS25
Government Policy with respect to development and flood risk in England is
contained within the Department of Communities and Local Government PPS25
‘Development & Flood Risk’. PPS25 was published in December 2006 and
revised in March 2010 and supersedes the Planning Policy Guidance Note 25
(PPG25). It introduces the Sequential and Exception tests, based on Flood Zones
as outlined in PPS25.
PPS25 has been developed to reflect the general direction set out in ‘Making
Space for Water’, the government’s strategy to shape flood risk management
policy over the next 20 years. It is set, therefore, in the context of new
Government priorities for sustainable development, and drivers for change such
as climate change and increased development pressures, including those in areas
at risk of flooding.
PPS25 stresses the importance of taking account of the consequences, and not just
the probability, of future flooding events. It clarifies the Sequential Test as a risk-
based approach to be applied at all stages of the planning process, to steer new
development toward areas with the lowest probability of flooding.
Under the Environment Agency’s standing advice for PPS25
(http://www.environment-agency.gov.uk/research/planning/33098.aspx/)
proposed sites larger than 1 hectare located in Flood Zones 1, 2 and/or 3 must
have an FRA completed and consultation with the Environment Agency should
take place. The principal reasons for this are to:
• Consider the principles of sustainable drainage of surface water
• Consider the effect of any works on watercourses or flood defences
• Demonstrate recognition of any known drainage problems
The flood risk and drainage proposals outlined in this report have been designed
to meet the requirements of PPS25, East Staffordshire Local Planning Authority,
and the Environment Agency. This report deals with flood risk from all sources
(fluvial, surface water, groundwater and sewers), includes the outcomes of
relevant consultations with the Environment Agency and Severn Trent Water
Ltd and has developed outline details of the proposed disposal of surface water
from the Site.
PPS25 states that a Strategic Flood Risk Assessment (SFRA) should be carried out
by the Local Planning Authority to inform the preparation of Local Development
Documents (LDDs), having regard to catchment-wide flooding issues which
affect the area. Both a Level 1 and Level 2 SFRA has been produced for this area
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(the East Staffordshire Level 1 SFRA, Feb 2008, Haskoning UK Ltd and the East
Staffordshire Level 2 SFRA, Aug 2008, Haskoning UK Ltd). These documents
have been consulted in preparation of this FRA and have been referenced in this
report where necessary.
1.3 Data Received
The data available in performing the FRA and compiling this report is as follows:
• River Trent Strategy Model: Burton Model (Improved Trent Model 3) –
Purchased by Halcrow and supplied by the Environment Agency
• Topographic Survey data of the site – Greenhatch Group, Drawing
15050_OGL 24/11/10
• Tatenhill Brook Channel Survey and Additional Topographic Survey –
Monument Geomatics July 2011
• Branston Railway Culvert Survey Data, May 2009 – Supplied by the
Environment Agency
• LiDAR Data of the site and surrounding area – purchased by Halcrow for
use in this commission
• Tatenhill Trent & Mersey Canal Culvert Dimensions – supplied by British
Waterways for this commission – May 2011
• Tatenhill A38 Culvert Dimensions – supplied by the Environment Agency
for use in this commission – May 2011
• Main Street Surface Water Pumping Station information – supplied by the
Environment Agency for use in this commission – July 2011
1.4 Scope of the Report
The flood risk assessment carried out comprises the following:
• A description of the existing site (Section 2)
• An analysis of the existing flood risk in the site (Section 3)
• Description of the proposed development (Section 5)
• An assessment of the hydrology and hydraulic modelling for existing
(Section 4) and proposed sites (Section 5)
• Potential flood risk with the proposed development, both off-site and on-
site (Section 6)
• Surface Water Management (Section 7)
• Proposed Foul Water System (Section 8)
• Planning recommendations for the proposed development, including the
Sequential and Exception tests (Section 9)
• Conclusions (Section 10)
• References (Section 11)
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Drawings, plans and tables are illustrated sequentially in the text or contained
within appendices where indicated.
1.5 Consultation
Consultation has formed a key part of the FRA, and in preparing this report the
following Environment Agency teams have been consulted:
• Flood Risk Mapping & Data
• Development & Flood Risk
• Asset System Management
Information from these parties has helped capture information on the risk of
flooding from all sources, and the condition of local assets, upon which
appropriate recommendations for the site have been made.
The approach and methodology for this FRA has been developed under
guidance of the Environment Agency, adhering to their appropriate
methodologies and assessments.
1.6 Climate Change
In accordance with Table B2 of PPS 25 (reproduced in Table 1.1 below) the design
of the proposed development will be carried out with an allowance for climate
change dependent upon its design life. For a development with design life until
between 2055 and 2085, an allowance for a 20% increase in rainfall intensity will
be made. For a design life until between 2085 and 2115 the corresponding
allowance is 30%. Residential developments are normally assumed to have a
design life until between 2085 and 2115.
The potential impacts of Climate change on the Tatenhill Brook and River Trent
is also taken into consideration, allowing for a 20% increase in peak river flows.
Table 1-1: Reproduction of Table B2 PPS25, Annex B – giving precautionary sensitivity ranges for taking climate change into account
Parameter 1990 to 2025
2025 to 2055
2055 to 2085
2085 to 2115
Peak rainfall
intensity
+5% +10% +20% +30%
Peak river Flow +10% +20%
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2 Existing Site
2.1 Site Location
The proposed development site comprises areas of land to the south of Branston,
near Burton-upon-Trent - Ordnance Survey Nation Grid Reference (OS NGR) SK
2200,2056, and two additional sites within Branston, the first being north of Main
Street between the A38 roundabout and the B5018 (OS NGR 4222,3212), with the
second south of Main Street (OS NGR 4222,3210) – as shown in Figure 2.1, with
larger versions contained in Appendix A for clarity.
Figure 2-1: Site Location – Development site demonstrated by red polygons
2.2 Site Description and Land Use
The large section of the site to the south of Branson is approximately 1.2km2 in
area, and is of undeveloped land. This site is divided by the railway line running
south-west to north-east, with the western section further divided by the
Tatenhill Brook which flows into the site underneath the A38. Both the area east
and the west of the railway comprise of former pulverised fuel ash (PFA) – a
legacy of lagoons from the Drakelow Power Station, which was used to reinstate
the former gravel workings on this site. The existing land use is for grazing.
The sites located to the north and south of Main Street consist of undeveloped
land and are approximately 0.15km2 and 0.03km2 in area respectively.
2.3 Watercourse Description
2.3.1 Tatenhill Brook
The Tatenhill Brook is shown to have a catchment area of 15km2 from the FEH
CD-ROM v3. The brook flows from the Head of Main River (HoMR) at Tatenhill
in a southerly, and south-westerly direction before reaching the Branston Water
Park area. Here the brook flows to the south of the Water Park, and underneath
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the Trent & Mersey Canal via a twin-barrel culvert. Upon exiting, it flows
through a short open section (350m) before flowing underneath the A38 where it
emerges at the western boundary of the proposed site. From here the brook
flows across the site in a predominately southerly direction, before turning at the
railway embankment and flowing north-east (for clarity in this report, the turn at
the railway embankment will be referred to as ‘the elbow’). Approximately
500m from here, the brook splits, with some flow going underneath the railway
and out to the River Trent (the official line of the Tatenhill Brook) and the
majority of the flow continuing adjacent to the railway embankment, before
flowing underneath the railway at the northern end of the site.
2.3.2 River Trent
For this commission, the upstream extent of the River Trent is located at Walton
Bridge. From here the Trent flows in a north-easterly direction through a
predominantly rural reach, passing the Branston site on the left bank, before
continuing on, flowing around the Drakelow nature reserve, and Branston Golf
Course, before continuing in a north-easterly direction towards the Drakelow
railway viaduct. From here the River Trent flows on towards Burton town centre
where the flow is mostly contained by a continuous flood defence along the left
bank and high ground on the right.
2.4 Existing Structures
There are a number of structures on the Tatenhill Brook upstream of the site that
require consideration for this flood risk assessment. The first is where the brook
flows underneath the Trent & Mersey Canal. Dimensions of this structure were
obtained from the British Waterways, with additional survey details obtained by
Monument Geomatics as part of this commission. Downstream of this, the
Tatenhill Brook flows underneath the A38 culvert. Details of the dimensions
were obtained from the Environment Agency, and from the Monument
Geomatics survey.
Within the site itself, there are a number of structures underneath the railway
embankment. These are a combination of culverts, and underpasses. Two are
located at the elbow, where the Tatenhill Brook meets the railway line and turns
to flow in a north-easterly direction. There are three further openings at intervals
to the north, which are ring protected by bunds on the eastern side of the
railway. The final opening is at the downstream end of the site, where the
Tatenhill Brook flows through. Details of these structures were obtained from
existing survey provided by the Environment Agency, and from the specially
commissioned Monument Geomatics survey.
All of these structures has been represented in the hydraulic model
schematisation to ensure that the existing conditions scenario accurately reflects
the study area.
2.5 Existing Surface and Foul Water Arrangements
There are no existing surface water or foul sewer arrangements on the proposed
development sites. For the western main site to the south of Branston, any
surface water would drain naturally to the Tatenhill Brook by infiltration
(limited due to the PFA) or via overland flow. The eastern section to the railway
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line would naturally drain towards the Tatenhill Brook Channel or the River
Trent.
2.6 Section 2.6 Geo-environmental considerations
As part of this planning application three geo-environmental audits have been
carried out on the site with the following findings of relevance to the
hydrological environment:
See document: Phase 1 Geo-Environmental Audit, Land North of Main Street
PJF116/2/12382 V:01 Halcrow Group Ltd July 2011.
Possible sources of contamination may be present, identified due to historic uses
of the site. These include pesticides and hydrocarbons (from spills) in the field
areas. Also, due to the presence of previous development in the south of the site,
there may be localised hydrocarbon spills from vehicles or made ground
including building debris following previous changes in building layout. The
preliminary risk assessment undertaken for the Land North of Main Street shows
that, for the intended future use of the site as residential development, the risks
to ground waters are generally low.
See document: Branston East (see Phase 1 Geo-Environmental Audit ref
PJF112/22/12373, Halcrow July 2011 and Geo-environmental Planning
Statement), Branston West (PJF116/22/12354 Halcrow May 2011).
A development proposal that reduces the flood risk to the site would reduce the
risk posed by the underlying Pulverised Fuel Ash (PFA) to controlled waters.
During re-development it will be important to ensure that appropriate site
controls are put in place in order to prevent any earthworks and general
construction activity from creating potential pathways for any potential
contaminants or actually causing pollution.
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3 Existing Flood Risk
3.1 Probability of Fluvial Flooding
The site is at risk of flooding from both the Tatenhill Brook and the River Trent.
Figure 3.1 details these two watercourse in relation to the development site.
The probability of fluvial flooding is discussed in this report in relation to return
period years. For instance a 1 in 100 year flood event indicates that the
probability of a flood event of this magnitude occurring in any given year is 1%.
Current Environment Agency guidance stipulates that for consistency this
should be referred to as the 1% Annual Exceedance Probability (AEP) event (1 in
100 Year).
Figure 3-1: River Trent shown in dark blue, Tatenhill Brook channels shown in light blue
3.1.1 Environment Agency Flood Zones
The Environment Agency publishes Flood Risk Zone Maps which relate to the
likelihood and magnitude of fluvial flooding in a given area. The existing Flood
Zones held by the Environment Agency for the Branston area (shown in Figure
3.2) indicate areas of fluvial flood risk across the site.
The darker blue shading represents Flood Zone 3a – classified as having a High
Probability of flooding (which equates to a > 1% AEP event). The lighter cyan
shading represents Flood Zone 2 – classified as having a Medium Probability of
flooding (between a 0.1% and 1% AEP event). The areas outside of this are
classified as Flood Zone 1 – Low Probability.
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Figure 3-2: Environment Agency Flood Zone Maps – Blue = Flood Zone 2, Cyan = Flood Zone 3 (hatched area shows areas benefitting from existing formal Environment Agency defences)
As can be seen in Figure 3-2, the site to the east of the railway line resides in
Flood Zone 3 (highly vulnerable), whilst the majority of the site to the west of the
railway line lies within Flood Zone 2 (less vulnerable). There are areas of the
western section of the site in Flood Zone 3, but these are mainly areas of lower
ground following the path of the Tatenhill Brook.
The smaller sites to the north and south of Main Street, are located in the
Environment Agency category of Flood Zone 3 Areas Benefitting from Defences
(ABDs).
3.2 Historic Fluvial Flooding and Flood Mechanisms
During large flood events, such as that experienced in 2000, the flow mechanisms
that affect the site are complex, and need to be understood to enable an FRA to
be undertaken correctly.
From the knowledge gained during this study, and from discussions with the
Environment Agency, it is understood that flood water from the River Trent
flows into the area to the west of the railway line, initially through the openings
that are not protected by ring-banks. As the water levels on the River Trent rise,
the flood water has been seen to flow over the railway embankment / through
the ballast in the vicinity outlined in Figure 3-4, as documented by the
Environment Agency.
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Figure 3-3: Area of known flowpath from the River Trent across the railway line (shown as blue hatched area)
The flood water from the River Trent crosses the railway line, then fills the low
lying land along the route of the Tatenhill Brook. This water doesn’t drain from
the western side of the railway due to the ring-banks on the railway culverts, and
therefore forms a water surface against the railway embankment, that has a
different elevation to that created by the River Trent hydraulic gradient on the
eastern side of the railway. This results in a differential on the hydrostatic load
against the railway embankment in the downstream / northern section of the site.
3.3 Main Street Surface Water Pumping Station and Culvert
There is a Local Authority (East Staffs Borough Council) owned and maintained
surface water pumping station located off Main St at OS NGR 422141, 321008.
This pumping station protects residential and commercial properties in the
village of Branston. The structure consists of a penstock and chamber containing
a float. When the River Trent reaches a pre-defined level, the penstock
automatically closes to prevent backing up of water into an ordinary watercourse
which could cause localised property flooding. If there is a rise in water levels in
the ordinary watercourse behind the closed penstock, another gauge will trigger
to the pump to automatically over-pump this water into the Trent. Whilst the
pump is currently owned and operated by East Staffs Borough Council, the
automated penstock is owned and operated by the Environment Agency, as is
the culvert downstream of here to the outfall on the Tatenhill Brook at OS NGR
422510, 321013.
It is believed that there may be a connection from the Branston Water Park to
Tatenhill Lane and Court Farm Lane, which would then drain to this pump. This
could therefore act as a flood flow route during large flood events. However,
after liaising and discussing this issue with the Environment Agency it is
believed that when a large event occurs on the Tatenhill Brook or River Trent –
the surrounding floodplain in this area is large enough to accommodate the
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associated volume before depths would reach the invert level of the connection
pipe.
Figure 3-4 shows the location of the pumping station, Environment Agency
maintained culvert, and outfall (confirmed by information provided by the
Environment Agency for use within this commission).
Figure 3-4: Location of Local Authority Pump (green star) and Environment Agency Maintained Culvert (green line)
The Environment Agency Asset System Management team believes that the
pumping station has not exceeded capacity to date.
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4 Hydrological and Hydraulic Modelling – Existing Conditions
4.1 Overview
To improve the understanding of the exiting flood risk and flood mechanism to
the site, and to provide a base model to compare proposed scenarios against, a
hydrological and hydraulic model improvement exercise was required.
4.2 Existing Model
The Environment Agency River Trent Burton Model (B&V June 2005) was
purchased for this commission. To ensure that the existing flow paths, active
floodplain and flow transfer to areas of Burton-upon-Trent were retained, the
model was used in it’s entirety. The section of River Trent represented by the
Burton Model is from Walton Bridge, Walton on Trent (approximately 1km
upstream of the Branston Site), to Sarsons Bridge (NGR SK 3890 2752) giving a
modelled length of approximately 30km.
After an initial assessment, it was judged that both the upstream and
downstream extents and boundaries are of sufficient quality and distance from
the site to enable this model to be utilised for this Flood Risk Assessment.
From the upstream extent at Walton, the Trent flows in a north-easterly direction
through a predominantly rural reach, passing the Branston site on the left bank,
before continuing on around the Drakelow nature reserve, and Branston Golf
Course. From here it continues to flow in a north-easterly direction towards the
Drakelow railway viaduct, and on towards the town centre where the flow is
mostly contained by a continuous flood defence along the left bank and high
ground on the right.
4.3 Hydraulic Model Review and Improvements
A review of the River Trent Burton model was undertaken to assess the
suitability for this commission. As a result of this review, a number of
improvements were required which are listed below, and then discussed in
detail in the subsequent sections:
• Improve the representation of flow paths across the floodplain and
interactions between the Tatenhill and Trent floodplain areas
• Improve the simulation of more extreme flood events (100yr+CC and 1000)
• Improve the determination of depth and velocities through the study area
with the adoption of 2D floodplain representation
4.3.1 Model Improvement – Schematisation
To ensure that flood flow paths and flooding mechanisms were correctly
modelled, and that the implications of changes to the floodplain from the
proposed development are understood, the 1D ISIS River Trent Burton Model,
purchased from the Environment Agency, was therefore re-schematised into a
1D2D model.
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This was accomplished by using the ISIS-TUFLOW software packages
(Environment Agency approved industry standards for studies such as this).
This approach links the ISIS (version 3.5) 1D hydraulic modelling software and
the TUFLOW (version 2009-07-AF-iSP) 2D hydraulic modelling software
packages. The 1D and 2D portions of this hydraulic model were connected
appropriately using both HX and SX lines to allow a seamless transfer of flow,
stage and velocity between and within the two domains.
The 2D representation was nested inside of the existing 1D ISIS River Trent
Burton Model to ensure that flows were not lost and were correctly transferred
downstream. This would allow a direct comparison between the existing
conditions and proposed scenarios, at the site and both upstream and
downstream – to ensure that the recommendations of PPS25 were met.
The 2D floodplain representation includes derivation of depth and velocity
suitable for calculation of hazard. Whilst simpler approaches to estimation of
depth and velocity are available, notably using average estimates from 1D
methods, a full 2D hydrodynamic approach was adopted for the Branston FRA
study due to the complexity of the floodplain geometry and potential
interactions between floodplains around Branston. The ISIS 1D representation of
the channel network was retained because it offers the most flexible tool for
representing the complex channel network and structure details.
A 10m grid size was adopted for this study to stabilise and expedite the iterative
hydraulic model simulations. The 10m grid size was selected as an acceptable
balance between model run times and accuracy.
4.3.2 Model Improvement – Additional Survey Data
To improve the hydraulic model in this location, and to enable an existing
conditions vs. proposed scheme comparison, a 1D channel representation of the
Tatenhill Brook was required. In order to achieve this, cross section channel data
was required at a number of key locations, in addition to structure details. For
the purposes of this study, a newly commissioned survey was undertaken by
Monument Geomatics to obtain Tatenhill Brook channel section information, in
addition to additional structure details to improve the model schematisation. A
plan of the cross section locations are contained in Appendix B.
4.3.3 Model Node Labelling Convention
The node labelling convention adopted by B&V for the River Trent was retained
to enable comparison of results with earlier and concurrent studies.
4.4 Model Improvement - Hydrology
To provide flow data for the hydraulic model, flow estimates were required for
both the Tatenhill Brook and the River Trent. The River Trent flows were
derived from the Environment Agency River Trent Burton Model and remain
unaltered.
The existing River Trent Burton model did not have a specific inflow
representing the Tatenhill Brook (instead, flows from this brook were
schematised as part of a wider lateral catchment). Therefore, for this FRA
Tatenhill Brook design flows were required.
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Existing hydrological analysis for the Tatenhill Brook existed from a study
produced by JBA in 2007. Since this report was produced, the Flood Estimate
Handbook (FEH) (an industry standard and Environment Agency approved
hydrological estimation methodology software package) has released an updated
version (FEH CD-ROM v3) which indicates improved flow paths and catchment
areas for the Tatenhill Brook. In addition, it now contains the Revitalised
Rainfall Runoff methodology. Therefore, a review of the JBA hydrology in
addition to a new investigation was required for this study, to ensure that it
meets PPS25 standards (utilising the most relevant and recent data sets).
For this commission, various methodologies have been considered. All are a
standard approach for an un-gauged catchment:
4.4.1 FEH Rainfall Runoff
This method has received criticism for overestimating flows, although it is still
considered the best approach in some scenarios. For this catchment, the peak
flows are thought to be overestimated.
4.4.2 FEH Statistical Pooled Analysis
Analysis relies on a representative pooled group of stations that are
hydrologically similar to the subject site. Due to the small catchment area of
Tatenhill Brook (15km2) the pooling group was significantly heterogeneous, this
reduces confidence in the peak flow estimates.
4.4.3 ReFH
The hydrograph has been derived using catchment descriptors only as no
gauged data exists. The peak flows have been estimated using a storm duration
of 5.15 hours, which is the calculated critical storm duration for peak levels.
4.4.4 Preferred Method for the FRA
The choice of method is a subjective one. Figure 1-1 in Appendix B shows the
Flood Frequency Curves for the methods outlined above. In the absence of any
data with which to verify flows, we recommend adopting the peak flow
estimates from the ReFH method (as detailed in Appendix B Table 1-1). This is
within the confidence limits and for a rural catchment such as this, based on our
experience, we would deem that they are realistic and appropriate for a Flood
Risk Assessment of this nature. Table 4-1 details the preferred Peak Flow
Estimates for a range of return periods.
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Table 4-1: Preferred Peak Flow Estimates for Tatenhill Brook
Return Period
AEP Event
Return Period
(1 in x Year)
ReFH Methodology
Tatenhill Peak Flow (m3s-1)
5% 20 9.1
4% 25 13.0
1% 100 13.0
0.1% 1000 23.4
As the proposed designs for the Branston site included flood storage areas, it was
important to assess storm duration of the Tatenhill Brook in relation to volume
sensitivity.
The Critical Storm Duration for the Tatenhill catchment has been calculated to be
5.15 hours (providing the largest peak flow for the catchment). In order to assess
the volumes from the Tatenhill brook, a longer storm duration was necessary to
assess total volumes. A duration of 24 hours was considered an appropriate
estimate. It is less than the Trent hydrograph, but considered realistic (Appendix
B provides additional details and supporting information).
4.5 Existing Scenario Modelling Results
The results show that the improved hydrological and hydraulic model is
providing an estimation that matches well with the existing knowledge of flood
risk and flow routes in this location.
The improved schematisation of the hydraulic model has resulted in a number of
improvements in the results that it is producing, when compared to the existing
1d hydraulic model for this area.
A key difference is the flow route from the River Trent, flowing along the
Tatenhill Brook channel, towards and over the A38, continuing north-eastwards
over Branston Road, towards the land west of Branston (for the large River Trent
events). This is an improvement over the existing model as this flow route was
not captured in the 1D storage area floodplain representation.
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5 Proposed Development & Modelling
5.1 Summary of Proposed Development
The proposed development will comprise a mixed land use of residential and
commercial properties to the west of the railway line, and to the north and south
of Main Street. In broad terms, the residential area will be to the north, while the
commercial areas will be to the south. In between these areas (where the
Tatenhill Brook flows) will be an area of lower ground to act as both flood
storage, and to provide environmental enhancements. The two development
areas will be connected over this lower ground via a road embankment
incorporating an opening for the Tatenhill Brook and flood relief culverts.
The land to the east of the railway line will not be developed. However, re-
profiling of parts of the site will occur to provide additional floodplain
compensation, in addition to river channel, leisure and environmental
enhancements.
5.2 Vulnerability of Flooding
Before this FRA details the location and specification of the development
proposals, the vulnerability classification from PPS25 needs to be understood.
Table D2, Annex D, of PPS25 outlines classifications for the vulnerability to
flooding of a range of different land uses. With reference to this, Table 5-1 below
gives the vulnerability classification from flooding for the proposed land use
types that are applicable to this development proposal.
Table 5-1: Vulnerability Classification from PPS25 and Corresponding Appropriate Land Uses
Vulnerability classification from PPS25
Corresponding Land Uses
More Vulnerable Buildings used for dwelling houses
Less Vulnerable Buildings used for: shops; financial,
professional and other services; offices;
general industry; storage and
distribution; assembly and leisure.
Based on this information, the classification of land use for the proposed
development at Branston falls into two categories – More Vulnerable and Less
Vulnerable. PPS25 provides guidance about where these types of development
can be located in relation to Flood Risk. This is to be found in Table D3 of the
PPS25 document (reproduced in this FRA as Table 5-2 below).
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Table 5-2: PPS25 Table d3 – Flood Zones and Appropriate Land Use
To ensure that the development is located in areas of lowest flood risk, it is
proposed that:
• The land to the east of the railway line will not be developed as this area is
wholly within Flood Zone 3a
• Where development is planned on the land to the west of the railway line,
it is proposed that those areas which are currently in Flood Zone 2 are
raised to a minimum of 600mm above the 1% AEP (1 in 100 year) flood
level. For the areas of land to the west of the railway line which are
currently in Flood Zone 3, these areas will also be raised to a minimum of
600mm above the 1% AEP (1 in 100 year) flood level. All finished floor
levels will be set at 600mm above the 1% AEP (1 in 100 year) Climate
Change level
• To provide floodplain compensation for areas raised on the site to the west
of the railway line, the existing Tatenhill Brook channel running along side
the railway will be widened and lowered, in addition to creating a new
drainage / storage channel to the south of the site. Where the Tatenhill
Brook crosses the site between the A38 and the railway line, the
topography will be re-profiled to create a storage area and environmental
enhancements
• A River Trent backwater will be created to the north of the site, allowing
the River Trent to flow underneath the railway and fill an additional
storage area. This will provide a balance in pressure on either side of the
embankment (addressing the existing issue where there are two different
water levels either side of the railway line during flood events) in addition
to provide a refuge area for fish in times of flood. This backwater will not
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be connected to the widened Tatenhill Brook channel, and will be
separated by an area of high ground
• A flood embankment will be located around the existing properties on the
eastern side of the A38
• The ring banks to the east of the railway line will remain as they are in
existing conditions
• The existing Tatenhill Channel after flowing underneath the railway line
in the centre of the main site (OS NGR 422040, 320326) will be maintained /
improved (including de-silting) to ensure that the flows are conveyed to
the River Trent confluence
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Figure 5-1: Proposed Development Schematic (showing approximate zones of interest)
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6 Proposed Development and Flood Risk
6.1 Modelled Flood Events
Due to the proximity of the development site to the Tatenhill Brook and River Trent,
a combination of modelled flood events are required to enable a full understanding
of the associated fluvial flood risk. The following flood events have been approved
by the Environment Agency Development and Flood Risk team, as an appropriate set
of scenarios to inform this Flood Risk Assessment.
Table 6-1: Modelled Flood Events for the Flood Risk Assessment
Flood Event Scenari Tatenhill Brook
Return Period Event
River Trent
Return Period Event
1 100 Year 25 Year
2 20 Year 100 Year
3 100 Year + CC 25 Year + CC
4 20 Year + CC 100 Year +CC
5 20 Year 1000 Year
Note: +CC refers to Climate Change Scenario, with an increase of 20% on the inflows
A number of observation points were located throughout the study area, to assess the
impact of the proposed development, by allowing a direct comparison of results.
These were carefully selected to provide an indication of the changes in flood risk at
the proposed development site, in addition to upstream and downstream on the
River Trent and Tatenhill Brook.
Figure 6.1 details where these location points are located with Tables 6.2 and 6.3
detailing the results from the different return period combinations, for both the
existing conditions scenario and the proposed development scenario.
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Figure 6-1: Modelled Results Reference Points
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6.2 Proposed Development and Fluvial Flood Risk - Summary
The results of the proposed modelling has indicated that the proposed development
does not increase the risk of fluvial flooding either upstream, at the site itself, or
downstream of the proposed development site. The results have shown the proposed
development provides a betterment and reduces the risk of flooding in a number of
locations.
6.2.1 Dominant River Trent Scenario
For the dominant River Trent scenario, a comparison of the flood extents between the
existing conditions and the proposed development scenario are shown in Figure 6-2
(in addition to a larger version for clarity contained in Appendix B).
Figure 6-2: Comparison between existing and proposed development scenario – 100 year Trent & 20 year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario
This shows that the proposed design scenario results in a reduction of flood risk to
the area west and north-west of the A38. The flood risk to the industrial estate to the
west of the A38 has reduced, as also to the properties to the east of the A38. Flooding
on the A38 itself has significantly reduced – changing from being affected from an
approximate 1 in 40 year event to a 1 in 50 year event (based on River Trent flows).
This reduction in flood risk is due to the increased floodplain storage (from the
drainage channel, the widened Tatenhill Brook channel, and the embankment joining
the two development areas on the west of the railway line (removing the former
River Trent flow path over the A38).
There are no increases in flood level and flood risk either upstream or downstream of
the River Trent , or in the northern area where the existing Tatenhill Brook outfall is
situated, as detailed in the modelling results shown in Table 6.2 below.
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Table 6-2: 100 year River Trent & 20 year Tatenhill Brook Results Comparison
Comparison Point Location
Ordnance Survey National Grid Reference
Existing Water Max Level (m)
(A)
Design Water Max Level (m)
(B)
B-A (m)
316126934 (River
Trent Channel)
424,492.7,
321,070.8
46.649 46.570 -0.079
Tatenhill Brook
outfall
(Hollyhock Way)
422,526.41,
321,001.21
47.561 47.492 -0.069
A38 421,807.96,
320,755.20
47.579 No Flooding N/A
Properties (west)
adjacent to A38
421,640.83,
320,601.25
47.752 No Flooding N/A
Properties (east)
adjacent to A38
421,532.45,
320,030.97
48.235 No Flooding N/A
Trent Floodplain
left
422,104.14,
319765.96
48.104 48.065 --0.039
Trent Floodplain
right
422,923.15,
320,476.12
47.292 47.216 -0.076
Trent Floodplain
downstream
424,001.80,
320,883.80
46.833 46.748 -0.085
6.2.2 Dominant Tatenhill Brook Scenario
A comparison of the flood extents between the existing conditions and the proposed
development scenario when there is a dominant event on the Tatenhill Brook is
shown in Figure 6.3.
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Figure 6-3: Comparison between existing and proposed development scenario – 25 year Trent & 100 year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario
This shows that proposed design scenario results in slight reduction of flood risk to
the Branston Water Park area, and to the north of here. This reduction in flood risk is
due to the increased floodplain storage of the Tatenhill Brook (from the drainage
channel and the widened Tatenhill Brook channel).
There are no increases in flood level and flood risk either upstream or downstream on
the Tatenhill Brook or River Trent as a result of the proposed development. Table 6.3
details the maximum water level results for both the existing conditions and
proposed development scenarios, at the selected comparison points.
Table 6-3: 25 year River Trent & 100 year Tatenhill Brook Results Comparison
Comparison Point Location
Ordnance Survey National Grid Reference
Existing Water Max Level (m)
(A)
Design Water Max Level (m)
(B)
B-A (m)
316126934 (River
Trent Channel)
424,492.7,
321,070.8
45.468 45.448 -0.020
A38 422,526.41,
321,001.21
No Flooding No Flooding N/A
Properties (west)
adjacent to A38
421,807.96,
320,755.20
No Flooding No Flooding N/A
Properties (east)
adjacent to A38
421,640.83,
320,601.25
No Flooding No Flooding N/A
Trent Floodplain
left
421,532.45,
320,030.97
47.793 47.788 -0.005
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Comparison Point Location
Ordnance Survey National Grid Reference
Existing Water Max Level (m)
(A)
Design Water Max Level (m)
(B)
B-A (m)
Trent Floodplain
right
422,104.14,
319765.96
46.967 46.947 -0.020
Trent Floodplain
downstream
422,923.15,
320,476.12
46.464 46.443 -0.021
For the scenario where the Tatenhill Brook is dominant, again the results show that
there is no increased flood risk. Note that for the existing lower Tatenhill Outfall near
Hollyhock Way, a comparison has not been included in this table as the 100 Year
event on the Tattenhill Brook does not reach this location – due to there being an area
of high ground between the Tatenhill channel and River Trent Backwater. The water
level adjacent to Hollyhock Way will be the backwater from the existing conditions
1% AEP (1 in 25 year) River Trent event.
6.2.3 Dominant River Trent Scenario – Climate Change
The PPS25 guidance requires that climate change is taken into account when
assessing flood risk. Figure 6-4 details the Climate Change dominant River Trent
scenario, with Table 6-4 showing the results at the comparison locations.
Figure 6-4: Comparison between existing and proposed development scenario – 100 year + CC Trent & 20 + CC year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario
This shows that whilst the extent of flooding has increased for both of the scenarios,
the preferred design option considerably reduces flood risk to the surround area, in
particular the A38.
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There are no increases in flood level and flood risk either upstream or downstream of
the River Trent , or in the northern area where the existing Tatenhill Brook outfall is
situated, as detailed in the modelling results shown in Table 6-4 below.
Table 6-4; 100 year + CC River Trent & 2- year + CC Tatenhill Brook Results Comparison
Comparison Point Location
Ordnance Survey National Grid Reference
Existing Water Max Level (m)
(A)
Design Water Max Level (m)
(B)
B-A (m)
316126934 (River
Trent Channel)
424,492.7,
321,070.8
46.829 46.748 -0.081
Tatenhill Brook
outfall
(Hollyhock Way)
422,526.41,
321,001.21
47.739 47.523 -0.216
A38 421,807.96,
320,755.20
48.144 47.887 -0.257
Properties (west)
adjacent to A38
421,640.83,
320,601.25
48.144 47.891 -0.253
Properties (east)
adjacent to A38
421,532.45,
320,030.97
48.400 No flooding N/A
Trent Floodplain
left
422,104.14,
319765.96
48.304 48.263 -0.041
Trent Floodplain
right
422,923.15,
320,476.12
47.512 47.410 -0.102
Trent Floodplain
downstream
424,001.80,
320,883.80
47.042 46.955 -0.087
6.2.4 Dominant Tatenhill Brook Scenario – Climate Change
A comparison of the flood extents between the existing conditions Climate Change
scenario and the proposed development Climate Change scenario when there is a
dominant event on the Tatenhill Brook is shown in Figure 6-5.
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Figure 6-5 Comparison between existing and proposed development scenario – 25 year + CC Trent & 100 year + Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario
This shows that again whilst the extent of flooding has increased for both of the
scenarios, the preferred design option considerably reduces flood risk to the
surround area, in particular the A38.
There are no increases in flood level and flood risk either upstream or downstream on
the Tatenhill Brook or River Trent as a result of the proposed development. Table 6-5
details the maximum water level results for both the existing conditions and
proposed development scenarios, at the selected comparison points.
Table 6-5: 25 year + CC River Trent & 100 year + CC Tatenhill Brook Results Comparison
Comparison Point Location
Ordnance Survey National Grid Reference
Existing Water Max Level (m)
(A)
Design Water Max Level (m)
(B)
B-A (m)
316126934 (River
Trent Channel)
424,492.7,
321,070.8
46.842 46.542 -0.300
Tatenhill Brook
outfall
(Hollyhock Way)
422,526.41,
321,001.21
47.755 47.289 -0.466
A38 421,807.96,
320,755.20
48.145 No flooding N/A
Properties (west)
adjacent to A38
421,640.83,
320,601.25
48.145 No flooding N/A
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Comparison Point Location
Ordnance Survey National Grid Reference
Existing Water Max Level (m)
(A)
Design Water Max Level (m)
(B)
B-A (m)
Properties (east)
adjacent to A38
421,532.45,
320,030.97
48.405 No flooding N/A
Trent Floodplain
left
422,104.14,
319765.96
48.299 48.028 -0.271
Trent Floodplain
right
422,923.15,
320,476.12
47.502 47.114 -0.388
Trent Floodplain
downstream
424,001.80,
320,883.80
47.058 46.613 -0.445
6.2.5 UK Flood Hazard
The UK Flood Hazard is calculated by using the following equation from Defra’s
Flood Risks to People – Phase Two Document (FD2321/ TR2) (2006). Hazard is
calculated as follows:
• Hazard = d x (v + 0.5) + DF
Where:
- d = depth (m)
- V = velocity (m/s)
- DF = debris factor
Based on the value of the hazard for a given area, a Hazard Classification is then
assigned. The Flood Hazard classifications are divided into four classes of risk:
Table 6-6: Flood Hazard Rating and associated category
Flood Hazard Rating
Category Category Colour
0.0 – 0.75 Low Low
0.75 – 1.25 Moderate Moderate
1.25 – 2.5 Significant Significant
2.5 + Extreme Extreme
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These classes of risk then translate into the following Flood Hazard classification
(Figure 6-6):
• Class 1: Danger for some – Flood zone with deep or fast flowing water that
presents a hazard for some people (i.e. children)
• Class 2: Danger for most – Flood zone with deep or fast flowing water that
presents a hazard for most people
• Class 3: Danger for all – Flood zone with deep or fast flowing water that
presents a hazard for all people
Figure 6-6: Flood Hazard Classification
For example, if peak water depths are 1.0 m, for velocities less than 1.0 m/s, the
flooding is considered to present ‘Danger for some’. For velocities between 1.0 m/s
and 2.0 m/s the flooding is considered to present ‘Danger for most’. For velocities
greater than 2.0 m/s the flooding is considered to present ‘Danger for all’.
6.2.6 Flood Hazard at the Proposed Branston Development
Figures 6-7 and 6-8 show the flood hazard maps for both the existing and proposed
design scenarios for the dominant River Trent scenarios (selected for the report as
these have the greatest hazard due to the large River Trent event).
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Figure 6-: Hazard Map – 20 year Tatenhill Brook & 100 year River Trent – Existing Conditions
Figure 6-7: Hazard Map – 20 year Tatenhill Brook & 100 year River Trent – Design Conditions
The results of the Hazard Mapping comparison shows that the hazard rating to the
surrounding area has not increased as a result of the development proposals. Within
the site boundary, hazard has increased along the Tatenhill channel between the two
development areas of Branston West – however this is expected as the depth of
flooding has increased as a result of the creation of a flood storage area.
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6.2.7 Impounded Water Bodies
The Trent and Mersey Canal lies approximately 500m to the west of the development,
and is perched in a number of locations. Therefore a breach could potentially affect
the site. However, the risk to the development site from a breach occurring on this
canal has been assessed and judged to be minimal. Two breaches were considered at
locations judged to be most significant. The first is in the southern area where the
canal is perched. If a breach occurred here, due to the eastward sloping land,
flooding would occur towards the A38. However, this road is higher than the
surrounding land and therefore, flood water from the canal would pond up behind
here and not affect the site.
A second scenario has been considered where the breach occurs near the Tatehhill
Brook channel. Any increased flows in the Tatnehill Brook as a result of a breach
would be restricted by the A38 culvert and accommodated in the storage areas
created on site as a result of the proposed development.
The Branston Water Park is situated to the west of the development (a series of water
filled sand and gravel pits). However under normal flow conditions, these lakes do
no interact with the Tatenhill Brook and are therefore considered as no significant
risk to the proposed development site. During the November 2000 flood event, the
Tatenhill Brook did overtop into the Branston Water Park lakes1, however, this has
been assumed to be from a combination of both high flows on the Tatenhill Brook,
and the high water levels further downstream in the River Trent, backing up and
locally raising flood levels on the Tatenhill Brook. Despite this occurring, there
would be no increased risk to the site, or additional flood risk created due to the
proposed development scheme.
6.2.8 Groundwater
Ground water flood risk has not been assessed in detail as part of this study, however
it is not anticipated that groundwater is a significant risk at this location. There is one
recorded incidence of groundwater flooding in this location, recorded in the Level 1
SFRA, which the Environment Agency’s groundwater team were consulted on as part
of the production of that document. They confirmed that there have been very few
recorded incidences of groundwater flooding within East Staffordshire, and that the
only occurrences have been the result of ending of quarrying of gravel and sand in
the area – therefore resulting in an end to the abstraction of water from the pits. Once
these abstraction machines were removed, the groundwater levels rose locally
(resulting in Branston Water Park for example) and as a result there was a report of
minor cellar flooding.
1 East Staffordshire Level 1 SFRA, Feb 2008, Haskoning UK Ltd
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In addition, the underlying surface of the proposed development area is PFA which is
highly impermeable. Furthermore the scheme will not exasperate any existing
ground water flooding.
6.2.9 Pluvial Flooding / Overland Flows
The risk of pluvial flooding (overland flooding) to the site is judged to be minimal.
Water draining from the Tatenhill catchment will enter the brook west of the Trent
and Mersey canal. There is an extensive field drainage network in this area, which
flows into the Tatenhill Brook. During large events, any additional overland flows
could possibly enter the surrounding lakes, or the Branston Water Park. West of
here, if any large overland flows were to occur over the fields, they would be
impounded by the raised canal in many locations.
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7 Surface Water Drainage
7.1 General Introduction
The effect of development is generally to reduce the permeability of a site. The
consequence of this, if no measures are put in place, is to increase the volume of
water and the peak flow rate from the developed site during and after a rainfall
event. Increases in the volume of water and the peak flow rate can cause flooding to
occur both within a development site, and can increase flood risk downstream of the
development, unless mitigated by a suitable drainage system.
7.2 Geological Environment
The underlying geology of the study area has an influence on the nature of SUDS
available for adoption and their performance. In determining the suitability of SUDS,
bedrock and superficial geology, and the location of designated aquifers are
considered. It is assumed that areas with predominately permeable bedrock and
superficial geology that are designated as aquifers are potentially suitable for design
of an infiltration system. Infiltration systems are generally preferred in reducing and
attenuating run-off in areas where there are suitable ground and groundwater
conditions.
An analysis of the bedrock geology underlying the site, indicates that the entire site is
underlain by Triassic mudstones including Keuper Marl, Dolomitic Conglomerate
and Rhaetic. This is impermeable bedrock which has low permeability. The
superficial geology is predominantly River Terrace Deposits to the west of the
railway line, and predominantly Alluvium to the east of the railway line. The
Groundwater Vulnerability maps indicate the entire site is within a minor aquifer
(the site includes minor aquifer classes HU, H1 and H2). The SPRHOST2 value is
36%, which is relatively high.
Given that the underlying geology is relatively impermeable and the SPRHOST is
high it is unlikely that infiltration of surface water would be applicable across the site.
Given also the underlying surface of PFA, this impermeability factor would be
compounded.
2 SPRHOST is the standard percentage runoff associated with each HOST soil class. The HOST
(Hydrology of Soil Types) soil class is a delineation of UK soil types according to their
hydrological properties. A low SPRHOST indicates the soil is highly permeable, whereas a high
SPRHOST indicates the soil is less permeable.
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7.3 Surface Water Runoff Rates
Using the methodology set out in ‘Preliminary rainfall runoff management for
developments’ R&D Technical Report W5-074/A/TR/1 Revision D (Defra /
Environment Agency Flood and Coastal Defence R&D Programme), runoff rates
were calculated for each of the sub-areas outlined in Appendix C.
Following development it is anticipated that proposals will give rise to a volume of
19,077 m3 during a 1% AEP (1 in 100 year) storm event. The predevelopment site is
likely to generate a similar volume of surface water due to the impermeable nature of
the geology and PFA as discussed in Section 7.2 above. The enhanced Tatenhill Brook
and the proposed flood compensation channel have been sized to accommodate the
surface water arising from the site during the 1% AEP (1 in 100 year) event, as well as
the volumes of water to be compensated for fluvial flooding during the 1% AEP (1 in
100) in 100 year flood event. The storage capacity of drainage channels and re-
profiled Tatenhill Brook has been calculated to be approximately 36,000m3, ensuring
that the site is able to drain under all flooding and rainfall events.
It is proposed that the enhanced Tatenhill Brook and the proposed flood
compensation channel will form the major part of the SUDS element of the site and
will be implemented in conjunction with other SUDS measures as detailed in the
following section.
7.4 Sustainable Urban Drainage Systems
The ethos of sustainable surface water drainage is to mimic, as far as possible, the
surface water flows (volume and peak flow rate) from the site prior to development.
The effect of development is generally to reduce the permeability of a site, however,
for the larger site (Branston East and Branston West) the existing site is largely
impermeable due to the PFA ground conditions.
This design of drainage infrastructure has become commonly known as Sustainable
Drainage Systems (SUDS). In addition to reducing the peak flow rate and volume of
water from a development site that is directly discharged to a river, SUDS techniques
can be used to improve the quality of surface water runoff and provide amenity and
biodiversity benefits.
This FRA therefore proposes the adoption of a SUDS management train, which will
ensure surface water drainage is managed in a sustainable manner and that the
natural catchment processes are mimicked as closely as possible. As a general rule,
surface water should be managed as close to source as is practicable.
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• Prevention - The use of good site design and site housekeeping measures to
prevent runoff and pollution (e.g. sweeping to remove surface dust and
detritus from car parks), and rainwater harvesting. Prevention policies should
generally be included within the site management plan
• Source and Site control - Control of runoff at or very near its source, and the
management of water in a local area. For the proposed development,
appropriate measures such as swales and localised basins will be incorporated
as appropriate into the overall site design. Where appropriate grassed
depressions will lead surface water overland to the discharge system, and will
be built into the design of open spaces and roadside margins.
The SUDS which have been considered for the site are discussed in the following
sections.
7.4.1 Infiltration techniques
Infiltration systems include trenches, basins or soakaways. The practicality of these
as a means of draining the site depends upon ground conditions which as discussed
above have low levels of permeability and as such are unlikely to be suitable for the
use of infiltration techniques. Ground investigation and geo-environmental desk
studies of the site (see Section 2.6 of this report) have indicated that sources of ground
contamination may exist. Therefore it is likely that infiltration techniques will not be
suitable for use on the site (or will only be suitable for limited areas of the site) due to
increased risk of leaching potential contaminants. This will be considered during
detailed design of the drainage for the individual plots and such techniques will be
implemented if and where appropriate. Other forms of infiltration not reliant on
ground conditions will be incorporated into the overall design of the surface water
drainage systems for the site.
7.4.2 Source Control techniques
Methods of source control which will be considered include: green roofs, rainwater
harvesting and permeable paving where appropriate. Intensive green roofs have the
potential to store run-off, but require maintenance, irrigation and due to the required
thickness impose additional loading requirements. They do however have other
environmental benefits including improvement of water quality, biodiversity and
habitat creation and improvements to the visual impact of the development.
Rainwater harvesting will also be considered where appropriate for the resulting
benefit as a means of conserving water resources. Rainwater harvesting can provide
some water supplies required to irrigate green roofs as well as for uses such as toilet
flushing.
Permeable paving can provide significant surface water storage on sites, there will be
water quality improvements as a result of filtration through the medium. As
discussed with the infiltration techniques above, permeable paving will be
considered where appropriate and where ground conditions permit
7.4.3 Pollution Prevention
Suitable pollution control measures will be incorporated to ensure that there is no
adverse impact elsewhere, in addition, oil interceptors will be installed to ensure that
contaminants associated with highway runoff and car parking areas are removed and
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water quality is at an optimum before discharging to the Tatenhill Brook and the
flood compensation channel.
As well as facilitating the required flood compensation and accommodating surface
water runoff for the whole site the enlarged Tatenhill Brook and the flood
compensation channel will also provide water quality improvements (described
below) and wider benefits such as bio-diversity and public amenity. The
compensation channel will act as an attenuation area due to the enlarged volume and
storage capacity and will offer water quality improvements through the
incorporation of vegetative systems.
The most important treatment processes in vegetative treatment systems are:
i) settlement and filtering of particulate constituents;
ii) adsorption of heavy metals and organic compounds to vegetation and soils;
iii) microbial degradation and assimilation of organic compounds;
iv) uptake of nutrients and metals by higher plants.
These processes, as applicable to the proposed flood compensation channel are set
out in the Table 7-1 below.
Table 7-1 Treatment of Highway Runoff: Principal processes in proposed compensation channel
Runoff Constituent Potential Water Quality Improvement process
Solids Filtering
Settlement
Heavy Metals (particulate and soluble) Filtering
Settlement
Adsorption
Plant uptake
Organic Compounds (particulate and
volatile)
Filtering
Settlement
Adsorption
Biodegradation
Volatilisation
Nutrients Plant uptake
Oil & Grease (particulate) Filtering
Settlement
Adsorption
Biodegradation
The compensation channel will be designed in accordance with the CIRIA SuDS
Manual (C697) and will require easements of 3metres from top of bank to allow
access for maintenance purposes.
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7.5 Site Drainage
The design of the site will incorporate drainage channels which the surface water
runoff from proposed development will drain to. The first proposed drainage
channel is to the south of the site (south the of the Tatenhill Brook elbow where it
turns and flows northwards at the railway line). The second is the combined storage
and Tatenhill Brook channel. The drainage channel locations are shown in Figure 5-1,
with a detailed volumetric analysis contained within Appendix C
By discharging the surface water to these storage areas, they will be attenuated on
site prior to discharging into the River Trent. A modelling exercise has been
undertaken where the 1% AEP (1 in 100 year) event surface water drainage is
discharged into the drainage ditch and storage areas, which has shown that the
effects are negligible with no increase in flood risk or water level.
The proposed development areas to the North and South of Main St will be drained
into the existing surface water system and fed to the pump discussed in Section 3.3 of
this report. This pump will be improved if necessary (to accommodate additional
flows), with a new downstream culvert being located across the northern section of
Branston West development area (as indicated in Figure 7-1). This would enable the
Environment Agency to abandon the existing falling main – which is currently
believed to be in a poor condition.
Figure 7-1: Approximate line of Proposed Surface Water culvert – to discharge into the River Trent Backwater (shown in green)
To minimise the additional land take required by the use of any of these measures,
the detailed drainage strategy will be developed in conjunction with the master
planning process whilst further exploring the suitability of sustainable drainage
techniques.
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7.6 Section 7.7 Ground condition considerations
As detailed in Section 2.6 and in the three Geo-environmental reports, which have
been submitted in support of this planning application, there are areas of the site
where possible sources of contamination may be present. Where contamination is
identified to be present, the design of surface water drainage features will need to
ensure that there is no risk of creating pathways for any potential contaminants to
cause pollution, either to the groundwater or the surface water exiting the site.
During construction appropriate site controls will be put in place in order to prevent
any earthworks and general construction activity from creating potential pathways
for any potential contaminants or causing pollution.
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8 Proposed Foul Water System
It is anticipated that the development will generate the foul flows detailed below
based on the recommended rates in Sewers for Adoption 6th Edition: 4000 litres per
residential dwelling over 24 hours and 0.5 l/s per hectare from industrial plots with
an allowance of 0.6 l/s per hectare for trade effluent from industrial plots.
• Land North of Main Street 2.78 l/s
• Residential Development (Branston West) 27.78 l/s
• Employment Development (including the Mixed Use plot) 20.22 l/s
• Total Foul flow of 50.78 l/s
A developer enquiry has been submitted to Sever Trent Water in order to establish
the most suitable point of connection into the public network. It is anticipated that the
connection point may be made into the existing foul sewer on Main Street for the
Land North of Main Street. It is anticipated that the connection point for the main
Branston site will either be made into the existing public sewer along the A38 or into
the existing public sewer to the south of the development, or a combination of both.
The foul sewer strategy for the site will be developed in liaison with Severn Trent
Water as the site design is developed. Phasing plans and anticipated connection
schedules will be submitted to Severn Trent Water in order that any required offsite
improvements can be made in due course.
Therefore it is proposed that the foul systems will drain by gravity to onsite pumping
stations and will be pumped to the existing public sewer systems.
The foul system (including the pumping stations and rising mains) will be designed
and constructed in accordance with “Sewers for Adoption 6th Edition”, relevant
British Standards and regulations. This would facilitate adoption by STW at a later
date if required. The necessary easements required easements for the gravity and
rising mains within the site boundary will be provided as required by Severn Trent
Water in order to facilitate their adoption.
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9 The Sequential Test
9.1 The Sequential Approach
The Sequential Approach is a simple decision-making tool designed to ensure that
areas at little or no risk of flooding are developed in preference to areas at higher risk.
PPS25 (paragraphs 14-15) sets out the requirement to apply the Sequential Approach.
The aim of the Sequential Approach should be to direct all new development to Flood
Zone 1 and away from locations affected by other sources of flooding. Opportunities
to locate new developments in reasonably available areas of little or no flood risk
should be explored, prior to any decision to locate them in areas of higher risk.
9.2 The Sequential Test Background Information
The Sequential Test is a key component of the hierarchical approach to avoiding and
managing flood risk. It is a decision making tool designed to ensure that sites at little
or no risk of flooding are developed in preference to areas at higher risk. The
Sequential Test can be applied at a number of levels – from Local Authority Planning
decisions to site specific flood risk assessments:
• Local Authority Level – the Sequential Test will assist in the defining of
development zones, seeking to locate all new development to Flood Zone 1. If
a development zone was selected that was in a higher flood risk zone, there
would be a requirement to demonstrate that there are no less vulnerable sites
available to accommodate the development, and that the development
provides wider sustainability benefits which outweigh the risk from flooding
(the Exception Test).
• Site Specific – A Sequential approach should also be applied on a site specific
basis, providing a tool to ensure the correct placement of development.
Consideration of flood risk at the earliest opportunity in the master-planning
process will enable the location, layout and design of the development to
deliver maximum reductions in flood risk.
9.3 Proposed Branston Site Sequential Test & Exception Test
The modelled Flood Zones indicate that the sites are affected by Flood Zones 3 and 2.
Through the application of the Sequential Test approach, built development will be
directed towards the western part of the site (Branston West) located fully in Flood
Zone 2.
The land to the east of the railway line (Branston East) is shown to be in Flood Zone 3,
therefore this will be kept as open space.
Areas of the site of lower flood risk, and the parts of the site affected by the 1% AEP
(1 in 100 year) climate change event will be left as open space, thereby ensuring that
surface water flow routes are not obstructed.
The location of the proposed development units will also be sequentially appraised,
with areas of development being located in areas of lower flood risk. For this site,
this means that areas of open spaces such as amenity or car parking, will be located
closest to the watercourses and drainage channels (but will remain in Flood Zone 2).
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The location of the proposed development units will also be sequentially appraised,
with areas of development being located in areas of lower flood risk. For this site,
this means that areas of open spaces such as amenity or car parking, will be located
closest to the watercourses and drainage channels (but will remain in Flood Zone 2).
10 Conclusions
10.1 Conclusions
Halcrow Group Ltd were commissioned by St Modwen to produce a Flood Risk
Assessment (FRA) for a proposed development at Branston, south of Burton-upon-
Trent.
The FRA has been produced in accordance with Planning Policy Statement 25
(PPS25), and will be used to support an outline planning application for the
development of a mixed use site.
A range of data has been gathered for this commission including the purchasing of
the River Trent Strategy hydraulic model from the Environment Agency, in addition
to commissioning topographic, channel and structural survey. Additional
information has been supplied by Environment Agency which included survey data
and information regarding the Main Street surface water pumping station.
The proposed development site comprises three sites of undeveloped land and has
been sequentially tested and demonstrated to be appropriate for this location. The
River Trent flows to the east of the larger site, whilst the Tatenhill Brook flows
through the site, entering from the west.
The existing River Trent Strategy model was improved with 2D floodplain
representation and a new model of the Tatenhill Brook was combined. The linked
1D2D ISIS-TUFLOW hydraulic model of the study area was used to assess the
existing and proposed design flood risk, at the site and in the wider area upstream
and downstream.
During the existing conditions, some areas of the large site to the west of the railway
line were shown to be within fluvial Flood Zone 3. As part of the development
proposals, some of these areas were simulated to be raised 600mm above the 1%
Annual Exceedance Probability (AEP) (1 in 100 year) flood event level. To provide
compensation, the existing Tatenhill Brook channel was widened and lowered, in
addition to creating a new storage / drainage area. A River Trent backwater has also
been incorporated into the design, providing a balance in water levels either side of
the railway line to the north and a refuge area for fish in times of flood. A flood
embankment has also been located around the existing properties on the eastern side
of the A38.
The proposed design scenario incorporates a raised embankment across the line of
the Tatenhill Brook channel. This removes the flood flow route of the River Trent and
significantly reduces flooding to the A38 (in addition to the frequency, with the
approximate onset of flooding reducing from a 1 in 40 year event to a 1 in 50 year
event).
The increased storage and drainage ditches have been designed to act as a
Sustainable Urban Drainage System, accommodating surface run-off volumes (in
addition to the fluvial flow).
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Property finished flood levels are recommended to be set at 600mm above the 1%
AEP (1 in 100 year) + Climate Change levels.
Through the hydraulic modelling, the proposed development has been demonstrated
to cause no additional increase in flooding at the site, or upstream and downstream
for both a dominant River Trent scenario and a dominant Tatenhill Brook scenario
(including with allowances made for the possible effects of Climate Change). A
Hazard analysis and mapping study has also been undertaken to recommended
published guideline standards, and has shown not to increase risk to the surrounding
area.
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11 References
1. Planning Policy Statement 25
2. Office of the Deputy Prime Minister (2002). Planning Policy Guidance
Note 25: Development and Flood Risk.
www.planning.opdm.gov.uk/ppg25/appendf.htm
3. Chow, V.T. (1959). Open channel Hydraulics. McGraw-Hill, USA [ISBN
07-010776-9]
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Glossary
AEP: Annual Exceedance Probability
EA: Environment Agency
FEH: Flood Estimation Handbook
FRA: Flood Risk Assessment
HoMR: Head of Main River
OS NGR: Ordnance Survey National Grid Reference
PFA: Pulverised Fuel Ash
SUDS: Sustainable Urban Drainage Systems
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List of Figures
Figure 2-1: Site Location – Development site demonstrated by red polygons ............... 4
Figure 3-1: River Trent shown in dark blue, Tatenhill Brook channels shown in light
blue............................................................................................................................................ 7
Figure 3-2: Environment Agency Flood Zone Maps – Blue = Flood Zone 2, Cyan =
Flood Zone 3 (hatched area shows areas benefitting from existing formal
Environment Agency defences) ............................................................................................ 8
Figure 3-3: Area of known flowpath from the River Trent across the railway line
(shown as blue hatched area) ................................................................................................ 9
Figure 3-4: Location of Local Authority Pump (green star) and Environment Agency
Maintained Culvert (green line).......................................................................................... 10
Figure 5-1: Proposed Development Schematic (showing approximate zones of
interest) ................................................................................................................................... 18
Figure 6-1: Modelled Results Reference Points................................................................. 20
Figure 6-2: Comparison between existing and proposed development scenario – 100
year Trent & 20 year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed
Design Scenario ..................................................................................................................... 21
Figure 6-3: Comparison between existing and proposed development scenario – 25
year Trent & 100 year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed
Design Scenario ..................................................................................................................... 23
Figure 6-4: Comparison between existing and proposed development scenario – 100
year + CC Trent & 20 + CC year Tatenhill Brook (Magenta = Existing Scenario / Blue =
Proposed Design Scenario ................................................................................................... 24
Figure 6-5 Comparison between existing and proposed development scenario – 25
year + CC Trent & 100 year + Tatenhill Brook (Magenta = Existing Scenario / Blue =
Proposed Design Scenario ................................................................................................... 26
Figure 6-6: Flood Hazard Classification............................................................................. 28
Figure 6-7: Hazard Map – 20 year Tatenhill Brook & 100 year River Trent – Design
Conditions.............................................................................................................................. 29
Figure 7-1: Approximate line of Proposed Surface Water culvert – to discharge into
the River Trent Backwater (shown in green)..................................................................... 36
Figure 11-1: Hazard Map – 20 year Tatenhill Brook & 100 year River Trent – Design
Conditions.............................................................................................................................. 58
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List of Tables
Table 1-1: Reproduction of Table B2 PPS25, Annex B – giving precautionary
sensitivity ranges for taking climate change into account................................................. 3
Table 4-1: Preferred Peak Flow Estimates for Tatenhill Brook ....................................... 14
Table 5-1: Vulnerability Classification from PPS25 and Corresponding Appropriate
Land Uses............................................................................................................................... 15
Table 5-2: PPS25 Table d3 – Flood Zones and Appropriate Land Use .......................... 16
Table 6-1: Modelled Flood Events for the Flood Risk Assessment ................................ 19
Table 6-2: 100 year River Trent & 20 year Tatenhill Brook Results Comparison ......... 22
Table 6-3: 25 year River Trent & 100 year Tatenhill Brook Results Comparison ......... 23
Table 6-4; 100 year + CC River Trent & 2- year + CC Tatenhill Brook Results
Comparison............................................................................................................................ 25
Table 6-5: 25 year + CC River Trent & 100 year + CC Tatenhill Brook Results
Comparison............................................................................................................................ 26
Table 6-6: Flood Hazard Rating and associated category................................................ 27
Table 7-1 Treatment of Highway Runoff: Principal processes in proposed
compensation channel .......................................................................................................... 35
Flood Risk Assessment and Drainage Strategy
Appendix A
Proposed Development Outline Plan
Flood Risk Assessment and Drainage Strategy
Appendix A Proposed Development Outline Plan
Flood Risk Assessment and Drainage Strategy
Appendix A Figure 1: Site Locations – Development site demonstrated by red polygons
Flood Risk Assessment and Drainage Strategy
Proposed Development Schematic (showing approximate zones of interest)
Flood Risk Assessment and Drainage Strategy
Appendix B
Modelling
Flood Risk Assessment and Drainage Strategy
Appendix B Modelling & Flood Outlines
Flood Risk Assessment and Drainage Strategy
Appendix B Figure 1-0: Additional Channel and Cross Section Locations
Flood Risk Assessment and Drainage Strategy
Comparison between existing and proposed development scenario – 100 year Trent & 20 year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario
Flood Risk Assessment and Drainage Strategy
Comparison between existing and proposed development scenario – 25 year Trent & 100 year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario
Flood Risk Assessment and Drainage Strategy
Comparison between existing and proposed development scenario – 100 year + CC Trent & 20 + CC year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario
Flood Risk Assessment and Drainage Strategy
Comparison between existing and proposed development scenario – 25 year + CC Trent & 100 year + Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario
Flood Risk Assessment and Drainage Strategy
Hazard Map – 20 year Tatenhill Brook & 100 year River Trent – Existing Conditions
Flood Risk Assessment and Drainage Strategy
Hazard Map – 20 year Tatenhill Brook & 100 year River Trent – Design Conditions
Flood Risk Assessment and Drainage Strategy
B.1 Hydrology Technical Note
Halcrow Group Limited
Lyndon House 62 Hagley Road Edgbaston Birmingham B16 8PE
Tel +44 (0)121 456 2345 Fax +44 (0)121 456 1569
www.halcrow.com
Technical note
Project Branston FRA Date 7 June 2011
Note Hydrological Modelling Appproach Ver 1
Authors Rebecca Bailey & John Parkin
Introduction
This Technical Note provides information on the hydrological analysis undertaken for the
Branston Flood Risk Assessment.
Peak flow justification
Various methods have been considered. All are a standard approach for an un-gauged
catchment.
FEH-Rainfall Runoff
This method has received criticism for overestimating flows, although it is still considered
the best approach in some scenarios. For this catchment, the peak flows are thought to be
overestimated.
FEH statistical pooled analysis
Analysis relies on a representative pooled group of stations that are hydrologically similar to
the subject site. Due to the small catchment area of Tatenhill Brook (15 sq km) the pooling
group was significantly heterogeneous, this reduces confidence in the peak flow estimates.
ReFH
The hydrograph has been derived using catchment descriptors only as no gauged data exists.
The peak flows have been estimated using a storm duration of 5.15 hours which is the
calculated critical storm duration for peak levels.
Preferred Method
The choice of method is a subjective one. Figure 1-1 shows the Flood Frequency Curves
for the methods outlined above. In the absence of any data with which to verify flows, we
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Technical note
recommend adopting the peak flow estimates from the ReFH method (as detailed in Table
1-1). This is a slightly more conservative approach than the statistical, but within the
confidence limits and appropriate for Flood Risk Assessment. In is from the hydrologists
experience that for a rural catchment of this size, the magnitude of the flows are deemed to
be realistic.
FFC Estimate Comparison for Branston
20 25 100 10000
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
Logistic reduced variate
Peak flow (m
3s-1)
Return period axis
Lumped FEHBDY
Lumped ReFHBDY
Statistical Estimate
Statistical 95% Lower Confidence Interval
Statistical 95% Upper Confidence Interval
Figure 1-1 Flood Frequency Curves
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Technical note
Table 1-1 Peak Flow Estimates
Return Period FEH-RR ReFH Statistical
(pooled)
20 11.4 9.1 4.3
25 12.1 9.6 4.5
100 17.2 13.0 6.1
1000 31.2 23.4 9.6
Table 1-2 Preferred Final Design Flows
Return Period ReFH
20 9.1
25 9.6
100 13.0
1000 23.4
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Technical note
Storm duration sensitivity
A B C D E F G H I
Method Scenario Peak flow Hydrograph
duration
Hydrograph
volume
Extended
baseflow
Total
volume
Return
period
(years)
Storm
duration
(hours)
Resulting
rainfall depth
(mm)
(m3s-1) (hours) m
3
To be calculated as
necessary m
3
ReFH 20 5.15 26.8 9.1 16.3 241813 N/A N/A
ReFH 20 24 41.6 7.8 35.2 446960 N/A N/A
ReFH 100 5.15 41.1 13 16.3 337800 N/A N/A
ReFH 100 24 59.5 10.6 35.2 595460 N/A N/A
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Technical note
Storm durations
5.15 hours has been calculated as being the critical duration for the Tatenhill catchment (gives the largest peak flow for the catchment).
A longer duration was necessary to assess total volumes when assessing volumes of storage.
24 hours was considered an appropriate estimate. It is less than the Trent hydrograph, but considered realistic.
If the option progresses to more detailed design, then it may be pertinent to consider different durations and optimise the required storage providing a balance
between probability of flood risk and potential storage available. The volumes quoted in column G are for the hydrograph duration shown in column F. If the
outlet closure is for longer than this duration then baseflow needs to be added at a constant value of 0.679 m3s-1 for the extended time. This will be
automatically done in the model, but will be required for quoting volumes. For the ReFH method, this hand calc is a good approximation of the total volumes
required but modelled results will be more representative because of how the baseflow tapers back down after the event from its elevated value.
Flood Risk Assessment and Drainage Strategy
Appendix C
Surface Water Drainage Details
Flood Risk Assessment and Drainage Strategy
Appendix C Surface Water Drainage Details
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Appendix C – Figure 1-0: Proposed plans for the main development site, showing separate site allocations.
For details of your nearest Halcrow office, visit our website halcrow.com
Development site
Summary of results R1 R2 R3 R4 R5 M1 E1 E2
UPDATE MANUALLY EACH TIME
Development size 1.5 ha 6.02 ha 3.97 ha 4.28 ha 2.43 ha 0.35 7.66 ha 2.92
Development Mean annual Peak flow (QBAR) 3.8 l/s 15.24 l/s 10.05 l/s 10.83 l/s 6.15 l/s 0.89 l/s 19.39 l/s 7.39
(1.08 (A/100)0.89 SAAR1.17 SPR2.17)x1000
Mean annual peak flow per unit area (QBAR/A) 2.531 l/s/ha 2.531 l/s/ha 2.531 l/s/ha 2.531 l/s/ha 2.531 l/s/ha 2.531 l/s/ha 2.531 l/s/ha 2.531
Discharge
Q1yr 2.15 l/s/ha 2.15 l/s/ha 2.15 l/s/ha 2.15 l/s/ha 2.15 l/s/ha 2.15 l/s/ha 2.15 l/s/ha 2.15
89.5 l/s for site 13 l/s for site 8.5 l/s for site 9.2 l/s for site 5.2 l/s for site 0.8 l/s for site 16.5 l/s for site 6.3
Q30yr 4.68 l/s/ha 4.68 l/s/ha 4.68 l/s/ha 4.68 l/s/ha 4.68 l/s/ha 4.68 l/s/ha 4.68 l/s/ha 4.68
194.9 l/s for site 28.2 l/s for site 18.6 l/s for site 20 l/s for site 11.4 l/s for site 1.6 l/s for site 35.9 l/s for site 13.7
Q100yr 6.53 l/s/ha 6.53 l/s/ha 6.53 l/s/ha 6.53 l/s/ha 6.53 l/s/ha 6.53 l/s/ha 6.53 l/s/ha 6.53
9.795 l/s for site 39.3 l/s for site 25.9 l/s for site 27.9 l/s for site 15.9 l/s for site 2.3 l/s for site 50 l/s for site 19.1
Final estimated Attenuation Storage
(At. Vol1yr) 237 m3 952 m
3 628 m3 677 m
3 384 m3 55 m
3 1212 m3 462
(At. Vol30yr) 515 m3 2068 m
3 1364 m3 1470 m
3 835 m3 120 m
3 2632 m3 1003
(At. Vol100yr) 741 m3 2974 m3 1961 m3 2114 m3 1200 m3 173 m3 3784 m3 1442
Long term storage volume
(LTVol100yr 6hr) 213 m3 853 m3 563 m3 607 m3 344 m3 50 m3 1086 m3 414
Assumptions
No assumptions made outside of spreadsheet's own calculations.
Preliminary calcs for high level review.
Calculations have been carried out for the development area as a whole for the part to be developed (excluding open space and landscaping).
75% runoff has been assumed.
Development size includes infrastructure.
Appendix C – Table 1-0: Development Sites – Discharge and Attenuation Required for Greenfield Run-off Rates
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