Flood Risk Assessment and Drainage Strategy · Flood Risk Assessment and Drainage Strategy . 5...

Flood Risk Assessment and Drainage Strategy

Land South of Branston, Burton-upon-Trent

Proposed Development at Branston

St Modwen Developments

August 2011

lisa.roberts

Text Box

P/2011/01243 Received 25/10/11

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




August 2011


Document history




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


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


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


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


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.


V:\PJF116 SMD Land South of Branston\21.0 Hydrology and Meteorology\21.1 Storm Drainage\2011-08-24 Final FRA

Report\24-08-11 Final FRA and Drainage Strategy Branston.doc

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.




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.




1

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)




3

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%




4

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




5

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




6

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.




7

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.




8

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.




9

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




10

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.




11

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.




12

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.




13

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.




14

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.




15

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).




16

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




17

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


V:\PJF116 SMD Land South of Branston\21.0 Hydrology and Meteorology\21.1 Storm Drainage\2011-08-24 Final FRA Report\24-08-11 Final FRA and Drainage Strategy Branston.doc

18

Figure 5-1: Proposed Development Schematic (showing approximate zones of interest)


V:\PJF116 SMD Land South of Branston\21.0 Hydrology and Meteorology\21.1 Storm Drainage\2011-08-24 Final FRA Report\24-

08-11 Final FRA and Drainage Strategy Branston.doc

19

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.


V:\PJF116 SMD Land South of Branston\21.0 Hydrology and Meteorology\21.1 Storm Drainage\2011-08-24 Final FRA Report\24-08-11 Final FRA and Drainage Strategy Branston.doc

20

Figure 6-1: Modelled Results Reference Points




21

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.




22

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


Properties (east)

adjacent to A38

421,532.45,

320,030.97


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.




23

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




(A)


(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


Properties (east)

adjacent to A38

421,640.83,

320,601.25


Trent Floodplain

left

421,532.45,

320,030.97

47.793 47.788 -0.005




24




(A)


(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.




25

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




(A)


(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.




26

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




(A)


(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


Properties (west)

adjacent to A38

421,640.83,

320,601.25





27




(A)


(B)

B-A (m)

Properties (east)

adjacent to A38

421,532.45,

320,030.97


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




28

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).




29

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.




30

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




31

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.




32

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.




33

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.




34

• 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




35

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.




36

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.




37

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.




38

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.




39

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).




40

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).




41

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.




42

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]




43

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




44

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


year Trent & 100 year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed

Design Scenario ..................................................................................................................... 23


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




45

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


Appendix A

Proposed Development Outline Plan


Appendix A Proposed Development Outline Plan


Appendix A Figure 1: Site Locations – Development site demonstrated by red polygons


Proposed Development Schematic (showing approximate zones of interest)


Appendix B

Modelling


Appendix B Modelling & Flood Outlines


Appendix B Figure 1-0: Additional Channel and Cross Section Locations


Comparison between existing and proposed development scenario – 100 year Trent & 20 year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario


Comparison between existing and proposed development scenario – 25 year Trent & 100 year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario


Comparison between existing and proposed development scenario – 100 year + CC Trent & 20 + CC year Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario


Comparison between existing and proposed development scenario – 25 year + CC Trent & 100 year + Tatenhill Brook (Magenta = Existing Scenario / Blue = Proposed Design Scenario


Hazard Map – 20 year Tatenhill Brook & 100 year River Trent – Existing Conditions


Hazard Map – 20 year Tatenhill Brook & 100 year River Trent – Design Conditions


B.1 Hydrology Technical Note


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.


Appendix C

Surface Water Drainage Details


Appendix C Surface Water Drainage Details

For details of your nearest Halcrow office, visit our website halcrow.com

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


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


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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