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Godalming Key Site

PPS25: Flood Risk Assessment

October 2009

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PPS25: Flood Risk Assessment

Godalming Key Site

Project: Godalming Key Site

Site: Godalming Key Site

For: Flambard Developments Ltd

Status: Final

Date: October 2009

Author:

Reviewer:

Keelan Serjeant, Senior Hydrologist

Matt Travis, Director

Disclaimer:

This report has been produced by Enzygo Limited within the terms of the contract with the client and taking account of the resources devoted to it by agreement with the client.

We disclaim any responsibility to the client and others in respect of any matters outside the scope of the above.

This report is confidential to the client and we accept no responsibility of whatsoever nature to third parties to whom this report, or any part thereof, is made known. Any such party relies on the report at their own risk.

Enzygo Limited Registered in England No. 6525159

Registered Office Stag House The Chipping Wotton-Under-Edge Gloucestershire GL12 7AD

Flambard Developments Ltd

© Enzygo Ltd 2009 i SHF.117.001

CONTENTS

TABLES, DRAWINGS & APPENDICES .............................................................................. III

1.0 INTRODUCTION...................................................................................................... 1

1.1 Background .............................................................................................................. 1

1.2 Project Scope ........................................................................................................... 2

1.3 Report Structure ....................................................................................................... 2

2.0 SOURCES OF INFORMATION ............................................................................... 3

2.1 Sources of Information ............................................................................................. 3

2.2 Discussion with Regulators ...................................................................................... 3

2.2.1 Environment Agency ................................................................................................ 3

2.2.2 Local Planning Authorities........................................................................................ 3

2.2.3 Local Planning Authorities........................................................................................ 3

3.0 DESCRIPTION OF THE APPLICATION AREA ...................................................... 4

3.1 Site Location ............................................................................................................ 4

3.2 Existing Development .............................................................................................. 4

3.3 Proposed Development............................................................................................ 5

3.4 Site Visit ................................................................................................................... 5

3.5 Topographic Survey ................................................................................................. 5

4.0 FLOOD RISK ........................................................................................................... 6

4.1 Potential Sources of Flooding – Level 1 Screening Study ....................................... 6

4.1.1 Fluvial Flooding Sources .......................................................................................... 6

4.1.2 Tidal Flooding Sources ............................................................................................ 7

4.1.3 Flooding from rising / high groundwater ................................................................... 7

4.1.4 Overland flow flooding.............................................................................................. 7

4.1.5 Flooding from Artifical Drainage Systems/Infrastructure Failure .............................. 8

4.2 Environment Agency Flood Map .............................................................................. 8

4.3 Historic Flooding .................................................................................................... 10

4.4 Existing and Planned Flood Defence Measures .................................................... 10

4.5 Current Flood Risk ................................................................................................. 10

5.0 SITE DRAINAGE ................................................................................................... 11

5.1 Surface Water Drainage......................................................................................... 11

5.2 Existing Drainage System ...................................................................................... 11

5.3 Current Runoff Rate ............................................................................................... 11

5.4 Post-Development Runoff Rate ............................................................................. 11

5.5 Developed Site Drainage ....................................................................................... 11

5.6 Sustainable Drainage Options (SUDS) .................................................................. 14

5.7 Green Roofs ........................................................................................................... 15

5.7.1 Extensive ............................................................................................................... 15

5.7.2 Intensive ................................................................................................................. 15

5.7.3 Benefits .................................................................................................................. 15

5.8 Sewer/Surface Water Flooding .............................................................................. 16

5.8.1 Flooding of Victoria Road ....................................................................................... 16

5.8.2 Victoria Road, Surface Water Drainage/Groundwater Water ................................. 16

5.8.4 Foul Sewage/Grampian Condition ......................................................................... 17

5.9 Geology and Groundwater ..................................................................................... 17

5.9.1 Geology .................................................................................................................. 17


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5.9.2 Groundwater .......................................................................................................... 17

6.0 SUMMARY AND CONCLUSION ........................................................................... 18

DRAWINGS ......................................................................................................................... 20

APPENDIX 1 ........................................................................................................................ 21

APPENDIX 2 ........................................................................................................................ 22

APPENDIX 3 ........................................................................................................................ 23

APPENDIX 4 ........................................................................................................................ 24

APPENDIX 5 ........................................................................................................................ 25


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TABLES, DRAWINGS & APPENDICES

TABLES

1. Potential Risk Posed by Flooding Sources

2. Environment Agency Flood Zones and Appropriate Land Use

3. Flood Risk Vulnerability and Flood Zone Compatibility

4. Changes in the Site Runoff Characteristics

5. Colebrook-White calculation for the conveyance of flow in the Phase 1 (Atrium)

culvert

DRAWINGS

1. Location Plan

2. Local Watercourses

3. BGS Groundwater Flooding

4. RMS Overland Flow Flooding

5. Environment Agency Flood Map

APPENDICES

1. Correspondence

2. Proposed Site Layout

3. Topographical Survey showing the Current Layout

4. Thames Water Plans and Sewer Impact Study

5. Surface Water Management Plan

FIGURES

1. The Proposed Development Site

2. The River Wey


© Enzygo Ltd 2009 1 SHF.117.001

1.0 INTRODUCTION

1.1 Background

At the request of Flambard Developments Ltd, a Flood Risk Assessment (FRA) has been

undertaken, in accordance with UK PPS251: Development and Flood Risk, for the proposed

development at the Godalming Key Site in Godalming, Surrey (Drawing 1).

It is recognised that developments that are designed without regard to flood risk may

endanger lives, damage property, cause disruption to the wider community, damage the

environment, be difficult to insure and require additional expense on remedial works.

Current guidance on development and flood risk2 identifies several key aims for a

development to ensure that it is sustainable in flood risk terms. These aims are as follows:

the development should not be at a significant risk of flooding and should not be

susceptible to damage due to flooding;

the development should not be exposed to flood risk such that the health, safety

and welfare of the users of the development, or the population elsewhere, is

threatened;

normal operation of the development should not be susceptible to disruption as a

result of flooding;

safe access to and from the development should be possible during flood events;

the development should not increase flood risk elsewhere;

the development should not prevent safe maintenance of watercourses or

maintenance and operation of flood defences;

the development should not be associated with an onerous or difficult operation and

maintenance regime to manage flood risk. The responsibility for any operation and

maintenance required should be clearly defined;

future users of the development should be made aware of any flood risk issues

relating to the development;

the development design should be such that future users will not have difficulty

obtaining insurance or mortgage finance, or in selling all or part of the development,

as a result of flood risk issues;

the development should not lead to degradation of the environment; and

the development should meet all of the above criteria for its entire lifetime, including

consideration of the potential effects of climate change.

1 Communities and Local Government (2006) Planning Policy Statement 25: Development and Flood Risk

(PPS25).

2 CIRIA (2004) Funders report CP/102 Development and Flood Risk – Guidance for the Construction Industry.


© Enzygo Ltd 2009 2 SHF.117.001

The FRA is undertaken with due consideration of these sustainability aims.

The key objectives of the FRA are:

To assess the flood risk to the proposed development and to demonstrate the

feasibility of appropriately designing the development such that any residual flood

risk to the development and its users would be acceptable;

To assess the potential impact of the proposed development on flood risk

elsewhere and to demonstrate the feasibility of appropriately designing the

development such that the development would not increase flood risk elsewhere;

and

To satisfy the requirements of national planning policy guidance which require FRAs

to be submitted in support of planning applications.

1.2 Project Scope

In order to achieve the aims outlined above, a staged approach has been adopted in

undertaking this FRA, in accordance with current best practice. A screening study has

initially been undertaken to identify whether there are any potential sources of flooding at

the site, which may warrant further consideration. Any potential flooding issues identified in

the screening study have subsequently been considered in a scoping study. The aim of the

scoping study is to review all available information and provide a qualitative assessment of

the flood risk to the site and the impact of the site on flood risk elsewhere.

1.3 Report Structure

This FRA has the following report structure:

Section 2 identifies the sources of information that have been consulted during the

FRA;

Section 3 describes the application area including the existing and proposed

development;

Section 4 outlines the flood risk to the existing and proposed development;

Section 5 details the site drainage and any potential impacts of the proposed

development on surface water drainage;

Section 6 presents a summary and conclusions.


© Enzygo Ltd 2009 3 SHF.117.001

2.0 SOURCES OF INFORMATION

2.1 Sources of Information

General information regarding the site setting and hydrology of the application site has

been obtained from OS Explorer Map 145: Guidford & Farnham. The National Grid

Reference of the site is SU 973 438.

Information regarding the current flood risk at the application site, local flood defences and

flood water levels has been obtained from the Environment Agency.

2.2 Discussion with Regulators

A wide range of regulators should be consulted when carrying out an FRA. These include

the Environment Agency, the Local Planning Authority, and Sewage/Water Companies.

Consultation and discussions with the relevant regulators have been undertaken during this

FRA.

2.2.1 Environment Agency

A scoping opinion with regards to the Environmental Impact Assessment (EIA) was

received from John Woodhouse, Planning Liaison Officer at the Environment Agency. This

confirmed that an FRA to determine how surface water drainage mitigation measures can

be incorporated into the proposed development to sustainably manage the surface water

runoff from the site (see Appendix 1).

2.2.2 Local Planning Authorities

A scoping opinion with regards to the EIA was received from Elizabeth Sims, Development

Control Manager at the Waverley Borough Council. This confirmed that an FRA in

accordance with PPS25 will have to be undertaken for the proposed development at the

site (see Appendix 1).

The following planning guidance written by the Government Office for the East of England

included in the South East Plan - Regional Spatial Strategy for the South East (RSS), the

Regional Planning Guidance for the South East (RPG9), the Waverley Borough Council

Plan and the Waverley Borough Council Draft Core Strategy: Preferred Option and

Strategy was consulted to assess the mitigation policies in place.

2.2.3 Local Planning Authorities

A scoping opinion with regards to the EIA was received from Nicky McHugh, Development

Planner at Thames Water (see Appendix 1).


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3.0 DESCRIPTION OF THE APPLICATION AREA

3.1 Site Location

The Godalming Key Site is located within Godalming, in the Waverley district of Surrey,

7km south of Guildford (Drawing 1). The site is situated approximately 150m from the east

of Godalming Town Centre. The site is bounded to the north by Flambard Way/Wharf

Street, to the south by Catteshall Way and to the east by Victoria Road.

The proposed development site is located on a parcel of land covering an area of

approximately 1.11ha.

3.2 Existing Development

The site contains a mix of different light industrial uses and community facilities. Godalming

Police Station is situated at the western corner of the site with accommodation for offices

located on Flambard Way. A mix of light industrial business units are scattered across the

site including Dolphin Works, Wagstaff Works, Jordan’s Garage and G&S Valves Ltd. To

the east of the site is the Wharf Nursery School. The remaining areas of the site are

predominately hardstanding used for car parking by Godalming Police and WBC

employees. Therefore, the site consists of entirely impermeable surfaces such as

hardstanding and buildings.

Figure 1 – The Proposed Development Site


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3.3 Proposed Development

It is understood that the proposed development will consist of 182 residential units with

associated basement car parking, gardens and open spaces. Therefore, the amount of

impermeable surfaces such as hardstanding and buildings will be reduced.

Further details with regard to the proposed development can be found in the accompanying

Design and Access Statement submitted with the planning application.

3.4 Site Visit

A site visit was undertaken by an Enzygo Hydrologist and the Environment Agency on the

28th February 2008, this included a site walkover survey and a visual inspection of the

culvert running under Phase 1, which included lifting the manhole covers

3.5 Topographic Survey

A topographic survey of the site was carried out in April 2009 (Appendix 3). Surveyed

ground levels on the site range from approximately 38.20mAOD at the east of the site, to

approximately 43.90mAOD at the west of the site.


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4.0 FLOOD RISK

4.1 Potential Sources of Flooding – Level 1 Screening Study

All potential sources of flooding must be considered for any proposed development. A

summary of the potential sources of flooding and a review of the potential risk posed by

each source at the application site is presented in Table 1.

Table 1 - Potential Risk Posed by Flooding Sources

Potential Source Potential Flood Risk at

Application Site? Potential Source Data Source

Fluvial flooding No River Wey,

drainage channel

Environment

Agency

Tidal flooding No None reported Environment

Agency

Flooding from rising /

high groundwater No None reported BGS Map

Overland flow flooding No None reported RMS

Flooding from artificial

drainage systems No None reported Thames Water

Flooding due to

infrastructure failure No None reported Thames Water

4.1.1 Fluvial Flooding Sources

The River Wey is located approximately 200m to the north of the site and is designated as

a Main River by the Environment Agency (see Figure 2). A drainage channel is located

approximately 50m to the east of the site (see Drawing 2). However, due to the distance

and topography between the site, the River Wey and the drainage channel these sources

of fluvial flooding will not pose a flood risk to the site. Therefore, this source of flooding has

not been considered further within this FRA.


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Figure 2 – The River Wey

4.1.2 Tidal Flooding Sources

The site is located approximately 45km to the north of the English Channel is not at risk of

tidal flooding, therefore tidal flooding has not been considered further within this FRA.

4.1.3 Flooding from rising / high groundwater

The site is located over the Lower Greensand major aquifer with soils of high (HU) leaching

potential. The British Geological Survey (BGS) Groundwater Flooding Susceptibility Map

shows that areas of the site moderate to high susceptibility to groundwater flooding (see

Drawing 3).

This map is based on geological and hydrogeological information and shows that the area

has characteristics which would make it prone to groundwater flooding due to presence of

gravels in the floodplain. The flood risk posed by this source would be of a minor nature.

The data set is a hazard data set, not a risk data set, meaning that it does not provide any

information about the likelihood of a groundwater flooding event occurring. It is noted that

the BGS flood map is to be used as a screening tool, and should not be used to inform

planning decisions.

4.1.4 Overland flow flooding

Surface water runoff from adjacent land usually due to poor soil permeability i.e. water runs

off over hard surfaces such as tarmac and concrete. The site is surrounded by a number of

areas with poor permeability which may cause localised overland flow flooding. The Risk

Management Solutions (RMS) map shows that that site is not at risk of overland flow

flooding, with no areas of the site being located within the 1 in 75 year, 1 in 100 year and 1

in 1000 year flood zones (see Drawing 4).


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These areas of poor permeability would have their own drainage/sewage network built to

industry standards (e.g. capacity to convey the 1 in 30 year event). Therefore flooding from

this source has not been considered further within this FRA.

4.1.5 Flooding from Artifical Drainage Systems/Infrastructure Failure

The Thames Water Sewer Plans (see Appendix 4) indicate a 225 mm trunk surface water

sewer running from west to east along Catteshall Lane, and continuing north along Victoria

Road, and a 375-600 mm trunk surface water sewer running north east along Flambard

Way and then along Woolsack Way.

There is a 225 mm trunk foul sewer along Catteshall Lane, north up Victoria Road and

across the site to meet a second 150 mm trunk foul sewer at the junction between

Flambard Way and Woolsack Way.

A sewer impact study has been undertaken by Thames Water to determine the capacity of

the sewerage network and its ability to accept the developments sewage flows. The

assessment consisted of a preliminary desktop study based on the available information

and an existing validated hydraulic model of the sewerage network.

The model indicated flooding across the sewerage network catchment, but no flooding was

experienced downstream of the development connection point, which was taken as the

manhole at the junction of Woolsack Way and the Wharf.

It was noted that flooding occurs along a separate foul sewer to the east of the

development site, therefore the option of connecting development flows to this was not

considered.

The modelling indicated that the foul sewer had sufficient capacity to accept flows from the

new development and that the increased flows would not cause any surcharging or flooding

at the connection point (see Appendix 4).

4.2 Environment Agency Flood Map

A review of the Environment Agency’s flood maps indicates that the site lies within Flood

Zone 1 and therefore has a ‘low probability’ of fluvial and/or tidal flooding (see Drawing 5)

as defined in Table D1 of PPS25.The annual probability of flooding is less than 1 in 1000

year (<0.1% annual probability) of river or sea flooding in any year.

The Environment Agency Flood Zones and acceptable development types are explained in

Table 2.

In PPS25 (Table D2 and 3) appropriate uses have been identified for the Flood Risk Zones.

Applying the Flood Risk Vulnerability Classification in Table D2 of PPS25, the proposed

development is classified as ‘more vulnerable’.

Table 3 of this report and Table D1 of PPS25 states that ‘more vulnerable’ uses are

appropriate within Flood Zone 1 after the completion of a satisfactory FRA.


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Table 2 - Environment Agency Flood Zones and Appropriate Land Use

Flood

ZoneProbability Explanation

Appropriate

Land use

Zone

1Low

Less than 1 in 1000 annual probability of river

or sea flooding in any year (<0.1%)

All development

types generally

acceptable

Zone

2 Medium

Between a 1 in 100 and 1 in 1000 annual

probability of river flooding (1% - 0.1%) or

between a 1 in 200 and 1 in 1000 annual

probability of sea flooding (0.5% 0.1%) in any

year

Most

development

type are

generally

acceptable

Zone

3aHigh

A 1 in 100 or greater annual probability of river

flooding (>1%) or a 1 in 200 or greater annual

probability of flooding from the sea (>0.5%) in

any year

Some

development

types not

acceptable

Zone

3b

‘Functional

Floodplain’

Land where water has to be flow or be stored in

times of flood. SFRAs should identify this zone

(land which would flood with an annual

probability of 1 in 20 (5%) or greater in any year

or is designed to flood in an extreme (0.1%

flood, or at another probability to be agreed

between the LPA and the Environment Agency,

including water conveyance routes)

Some

development

types not

acceptable

Note:

The Flood Zones are the current best information on the extent of the extreme flood from rivers or the sea that would occur without the presence of flood defences, because these can be breached, overtopped and may not be in existence for the lifetime of the development.


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Table 3 - Flood Risk Vulnerability and Flood Zone ‘Compatibility’ as identified in

Table D.3 of PPS25

Flood Risk Vulnerability classification(see Table D2

of PPS25)

EssentialInfrastructure

WaterCompatible

Highly Vulnerable

MoreVulnerable

LessVulnerable

Zone 1 Yes Yes Yes Yes Yes

Zone 2 Yes Yes Exception

testrequired

Yes Yes

Zone 3a Exception test

requiredYes No

Exceptiontest

requiredYes

Zone 3b ‘FunctionalFloodplain’

Exception test required

Yes No No No

Key:

Yes: Development is appropriate, No: Development should not be permitted.

4.3 Historic Flooding

In autumn 2000 the flooding in the district was the worst since September 1968 however,

the site did not flood and there are no historic records of the site flooding.

4.4 Existing and Planned Flood Defence Measures

There are no existing and planned flood defence measures that protect the site from

flooding.

4.5 Current Flood Risk


a Main River by the Environment Agency. A drainage channel is located approximately

50m to the east of the site.

As noted in Section 4.2, the site is located within Flood Zone 1 and therefore has a ‘low

probability’ of fluvial and/or tidal flooding. The annual probability of flooding is less than 1 in

1000 year (<0.1% annual probability) of river or sea flooding in any year.

No other secondary sources of flooding have been identified within this FRA, there are no

historic records of the site flooding and there are no existing and planned flood defence

measures that protect the site from flooding.


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5.0 SITE DRAINAGE

5.1 Surface Water Drainage

It is recognised that consideration of flood issues should not be confined to the floodplain.

The alteration of natural surface water flow patterns through developments can lead to

problems elsewhere in the catchment, particularly flooding downstream. For example,

replacing vegetated areas with roofs, roads and other paved areas can increase both the

total and the peak flow of surface water runoff from the development site. Changes of land

use on previously developed land can also have significant downstream impacts where the

existing drainage system may not have sufficient capacity for the additional drainage. This

section considers the existing drainage system at the application site and potential impacts

resulting from the proposed re-development.

5.2 Existing Drainage System

The site is almost entirely impermeable. The Thames Water Sewer Plans (see Appendix 4)

indicate a 225 mm trunk surface water sewer running from west to east along Catteshall

Lane, and continuing north along Victoria Road, and a 375-600 mm trunk surface water

sewer running north east along Flambard Way and then along Woolsack Way.

There is a 225 mm trunk foul sewer along Catteshall Lane, north up Victoria Road and

across the site to meet a second 150 mm trunk foul sewer at the junction between

Flambard Way and Woolsack Way.

5.3 Current Runoff Rate

As noted above the site is almost entirely impermeable therefore, currently the majority of

surface water will runoff from the site into the sewer network and/or surrounding areas.

5.4 Post-Development Runoff Rate

The development of the site will result in an increase in the amount of permeable surfaces.

Soft landscaping, gardens and green roofs will be incorporated into the final design which

will allow a larger proportion of the surface water runoff to infiltrate into the soil substrate

than currently occurs.

5.5 Developed Site Drainage

An assessment of the surface water runoff rates has been undertaken, in order to

determine the surface water attenuation requirements for the site. The assessment

considers the impact of the site compared to current conditions. Therefore, the surface

water attenuation requirement for the developed site can be determined and reviewed

against existing arrangements.

A surface water management strategy has been developed for the site (see Appendix 5).

Due to the ground conditions infiltration mitigation measures (e.g.soakaways, swales) have


© Enzygo Ltd 2009 12 SHF.117.001

not been considered suitable for the site. The use of the following mitigation measures

have been incorporated into the site layout:

Pipes;

Landscaped areas;

Green roofs; and

Attenuation storage tanks

In order to quantify any potential increase in surface water runoff, the existing and pre-

development runoff rate from the site must initially be determined. The rates of runoff have

been determined using the current ‘industry best practice’ guidelines as outlined in the

Interim Code of Practice for SUDS3. The recommended methodology for sites up to 50

hectares in area is the Institute of Hydrology Report 124 method (IoH124)4. The surface

water runoff rates have been calculated using the Micro Drainage WinDes software suite.

This has been compared in Table 4, to the estimated rate of surface water runoff from the

developed site, also calculated using the IoH124 method. The following parameters have

been incorporated into the runoff calculations:

Catchment Area: 1.11ha (from site development plans);

Average Annual Rainfall (SAAR): 877mm/year;

Soil: 0.15

Impermeable Areas:

Greenfield = 0%;

Pre-development = 100%;

Post-development = 81% (does not include green roofs)

Region No.: 7

In order to represent the change in runoff at the site as a result of the proposed

development, and to ascertain the required attenuation volumes, the urban function within

the IoH 124 method has been altered. It is preferable that the developed runoff rate is

returned to the pre-development runoff rate, or Greenfield runoff rate (i.e. natural runoff

rate). However, paragraph F10 of PPS25 Appendix F states that:

“The surface water drainage arrangements for any development site should be such that

the volumes and peak flow rates of surface water leaving a developed site are no greater

than the rates prior to the proposed development, unless specific off-site arrangements are

made and result in the same net effect”.

3 Office of the Deputy Prime Minister, National SUDS Working Group, July 2004, Interim Code of Practice for

sustainable drainage systems. 4 Institute of Hydrology (1994) Flood estimation for small catchments. Report no 124.


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Table 4 below shows a comparison of surface water runoff rates pre- and post-application,

without attenuation. Table 4 indicates that the proposed development of the application site

would result in an decrease in surface water runoff from the site. The 1 in 100 year

(including climate change) runoff for the site will decrease by 0.6l/s from 5.8l/s to 5.2l/s.

As noted above, it is proposed that the surface water runoff would be dealt with through a

series of pipes, landscaped areas, green roofs and attenuation storage tanks to further

reduce the rate of surface water runoff from the site. The mitigation measures shown above

will be used to attenuate surface water runoff from the site and will manage the flood risk

from surface water runoff. The surface water runoff from the site will be restricted to the 1 in

2 year pre application runoff rate of 2.4l/s.

Table 4 - Changes in the Site Runoff Characteristics (Without Attenuation)

Annual Probability

(Return Period, years)

Greenfield

Runoff (l/s)

Pre application

Runoff (l/s)

Post application

Runoff (l/s)

Difference in

Runoff (l/s)

100% (1) 0.5 2.0 1.7 - 0.3

50% (2) 0.5 2.4 2.0 - 0.4

3.33% (30) 1.3 4.1 3.7 - 0.4

1% (100) 1.8 4.8 4.3 - 0.5

1% + Climate Change 2.2 5.8 5.2 - 0.6

Note:

20% added to rainfall data to account for long-term climate change as stated in PPS25. The 2 year, 30 year and 100 year annual probability events are of importance to the Water Companies and the Environment Agency when looking at sewage discharge and flood risk.

The total storage volume that can be accommodated on the proposed site layout within the

attenuation storage tanks is 1147m3. There will be 4 storage tanks which will collect the

surface water runoff from 4 catchment areas using pipes. Further attenuation and storage

of the surface water runoff from the site will be provided within the pipes, landscaped areas

and green roofs.

The maximum volume required to attenuate the surface water runoff to the pre-application

1 in 2 year runoff rate of 2.4l/s using 1 storage tank is 340m3 as calculated using the

WinDes software suite. The system was modelled within WinDes as a storage tank with

controlled discharge via an orifice outflow control. However, due the sequence of the

storage tanks cascading the runoff from one to another extra storage will be needed. A

maximum volume of 440m3 will be required.

Therefore, there is adequate storage within the storage tanks to attenuate the surface

water runoff to the pre-application 1 in 2 year runoff rate of 2.4l/s. This drainage network

will provide attenuation and reduce the risk of a pollution incident.


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5.6 Sustainable Drainage Options (SUDS)

Current guidance promotes sustainable water management through the use of SUDS. A

range of SUDS options is described in Annex F of PPS25 and includes:

Green roofs

Water butts

Porous and pervious paving

Rainwater harvesting

Filter strips

Wetland Areas

Infiltration basins

Detention basins

Retention ponds

Swales

A hierarchy of techniques is identified5:

1. Prevention – the use of good site design and housekeeping measures on individual

sites to prevent runoff and pollution (e.g. minimise areas of hard standing).

2. Source Control – control of runoff at or very near its source (such as the use of

rainwater harvesting).

3. Site Control – management of water from several sub-catchments (including

routing water from roofs and car parks to one/several large soakaways for the whole

site).

4. Regional Control – management of runoff from several sites, typically in a

detention pond or wetland.

It is generally accepted that the implementation of SUDS as opposed to conventional

drainage systems, provides several benefits by:

reducing peak flows to watercourses or sewers and potentially reducing the risk of

flooding downstream;

reducing the volumes and frequency of water flowing directly to watercourses or

sewers from developed sites;

improving water quality over conventional surface water sewers by removing

pollutants from diffuse pollutant sources;

reducing potable water demand through rainwater harvesting;

improving amenity through the provision of public open spaces and wildlife habitat;

and

replicating natural drainage patterns, including the recharge of groundwater so that

base flows are maintained.

It is proposed that the detailed design of the final scheme would be agreed with the

Environment Agency and Local Planning Authority prior to works commencing.

5 CIRIA (2004) Report C609, Sustainable Drainage Systems – Hydraulic, Structural and Water Quality advice.


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5.7 Green Roofs

The development of the site will result in a decrease in the amount of impermeable

surfaces due to the incorporation of sedum grass green roofs within the proposed

development. Green roofs comprise a multi-layered system that covers the roof of a

building or podium structure with vegetation cover/landscaping/permeable car parking, over

a drainage layer. They are designed to intercept and retain precipitation, reducing the

volume of runoff and attenuating peak flows6. There are two types of green roof systems:

Extensive; and

Intensive.

5.7.1 Extensive

Extensive green roofs are lightweight, free draining and shallow, typically to a maximum

depth of 10cm. They are generally planted with low growing, drought resistant vegetation

that requires little maintenance. Because they are lightweight they require little structural

support which means they are suitable for most building types and can be fitted

retrospectively.

5.7.2 Intensive

Intensive green roofs have deeper substrates that are able to carry larger types of

vegetation such as trees and shrubs. This deeper substrate makes the green roof system

much heavier than the extensive system and as such requires substantial structural

support. The nature of the planted vegetation means that the green roof will also required

regular maintenance such as pruning and clipping.

5.7.3 Benefits

Hard construction materials, like roof tiles, discharge water very quickly reducing effective

evaporation and loading the drainage systems, which in turn can contribute to flooding.

Incorporating a green roof into a proposed development has the advantage of mimicking

the pre-development state of the site. This will reduce the volume and rate of run-off,

improve aesthetics and remove pollutants, such as heavy metals.

Typically an intensive green roof with a deep soil layer can absorb up to 90% of the

average rainfall, but even the thinnest of extensive green roofs will still absorb around 50%

of average rainfall with the remaining 50% draining off at a much slower rate, therefore

allowing more time for the drainage system to manage the runoff.

At the moment we are awaiting confirmation from the Environment Agency however, it is

assumed that the proposed 100mm thick substrate of the green roof system would only

provide attenuation levels enough to show as a ‘bolt on’ gain.

6 The SUDS Manual, 2007.


© Enzygo Ltd 2009 16 SHF.117.001

Table 7.1 of CIRIA C644 shows examples of the attenuation properties green roof systems

with substrates of 250mm and 350mm thick. These are used as a guide as to the rates

which the Environment Agency will accept for green roof attenuation.

The green roofs proposed at this site will provide some attenuation of surface water runoff.

German Guidelines on Green-roof sites7 (at the time of writing there is no UK guidance)

state that multi-course layered extensive green roof systems have the capacity to attenuate

35%, by volume, of the area of the green roof.

Using these guidelines, the water storage capacity for the combined green roof can be

calculated and may reduce the required volume of storage tanks that would be needed on

the proposed development site and provides a ‘bolt on gain’ or may be used to further

enhance the sustainability of the proposed development.

5.8 Sewer/Surface Water Flooding

Previously a concern was raised by local residents with regards to surface water and sewer

flooding. Included in Appendix 4 is a Thames Water Sewer Impact Study, this

demonstrates that flows from the new development will not increase the risk of flooding.

5.8.1 Flooding of Victoria Road

It is recognised that land levels at the bottom of Victoria Road are below the majority of the

land levels in the Phase II development.

The surface water drainage from all areas of the Phase II development will be discharged

as per the surface water management strategy.

Therefore, water from the Phase II development will not flow on to Victoria Road, as may

occur currently.

5.8.2 Victoria Road, Surface Water Drainage/Groundwater Water

It is noted that comments have been made regarding a separate surface water drainage

system exists underneath the properties of Victoria Road. This drain is not within the

planning application boundary of the Phase II site, and there are no proposals to connect

into or amend the drain. Therefore this is not relevant to this planning application.

Reference has also been made to a number of springs under the Phase II site and an 18”

concrete drain, constructed in 1954, which collects groundwater and drains this into the

culvert underneath the Phase I site.

Services plans obtained for the site do not show the presence of this drain. However,

should such a drain be present, and in a serviceable condition, it could be incorporated into

7 German Forschungsgellschaft Landschaftsentwicklung Landschaftsbau, Guidelines for the Planning,

Execution and Upkeep of Green-roof sites, 2002.


© Enzygo Ltd 2009 17 SHF.117.001

the detailed design of the scheme. It is also noteworthy that no surface water would flow

into this drain following the development due to the onsite attenuation proposed. Therefore

any flooding of this drain would be reduced.

5.8.4 Foul Sewage/Grampian Condition

As stated earlier the Thames Water sewerage impact assessment results demonstrate that

there is spare capacity within the sewerage network, and that including the Phase II

development would not increase sewage flooding.

With regard to the current sewer which transects the site from Victoria Road to Woolsack

Way. This sewer lies outside of the development footprint, and as such would not be

impacted by this development.

5.9 Geology and Groundwater

The following information should be taken into account when the detailed design of the

surface water drainage system is finalised for the proposed development site.

5.9.1 Geology

There is a variable thickness (0 to 7 m) of made ground on the site, which is generally

greater beneath Area 5b (police station) in the vicinity of the former gasholders. Superficial

alluvium and River Terrace Deposits are located on the eastern site area whilst head

deposits are mapped on the western extent of the site. The superficial deposits directly

overlie the Lower Greensand Hythe Formation.

5.9.2 Groundwater

The site is located over the Lower Greensand major aquifer with soils of high (HU) leaching

potential. The groundwater in the immediate vicinity of the site is not used for potable

supply. The site is not within a Source Protection Zone and there are no licensed

abstractions nearby.

Further more detailed information on Geology can be found within the Environmental

Statement chapter.


© Enzygo Ltd 2009 18 SHF.117.001

6.0 SUMMARY AND CONCLUSION

This report presents an FRA, in accordance with UK PPS25: Development and Flood Risk,

to support a planning application for the proposed development at the Godalming Key Site

in Godalming, Surrey.


a Main River by the Environment Agency. A drainage channel is located approximately

50m to the east of the site.

As noted in Section 4.2, the site is located within Flood Zone 1 and therefore has a ‘low

probability’ of fluvial and/or tidal flooding. The annual probability of flooding is less than 1 in

1000 year (<0.1% annual probability) of river or sea flooding in any year.

No other secondary sources of flooding have been identified within this FRA, there are no

historic records of the site flooding and there are no existing and planned flood defence

measures that protect the site from flooding.

In addition, the assessment has considered the potential impact of the proposed

development on surface water runoff rates. Appropriate mitigation measures to attenuate

surface water have been presented. Current guidance promotes sustainable water

management through the use of SUDS.

A surface water management strategy has been developed for the site. Due to the ground

conditions infiltration mitigation measures have not been considered suitable for the site.

The use of the following mitigation measures have been incorporated into the site layout:

Pipes;

Landscaped Areas;

Green Roofs; and

Attenuation Storage Tanks

It is proposed that the surface water runoff would be dealt with through a series of pipes,

landscaped areas, green roofs and attenuation storage tanks to further reduce the rate of

surface water runoff from the site. The mitigation measures shown above will be used to

attenuate surface water runoff from the site and will manage the flood risk from surface

water runoff. The surface water runoff from the site will be restricted to the 1 in 2 year pre

application runoff rate of 2.4l/s.

The total storage volume that can be accommodated on the proposed site layout within the

attenuation storage tanks is 1147m3. There will be 4 storage tanks which will collect the

surface water runoff from 4 catchment areas using pipes. Further attenuation and storage

of the surface water runoff from the site will be provided within the pipes, landscaped areas

and green roofs.


© Enzygo Ltd 2009 19 SHF.117.001

Therefore this FRA demonstrates that the proposed development would be operated with

minimal risk from flooding, would not increase flood risk elsewhere and is compliant with

the requirements of PPS25. The development should not therefore be precluded on the

grounds of flood risk.

godalming key site pps25: flood risk assessment€¦ · matt travis, director disclaimer: this...

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