Integrated Accommodation Services
December 2008
Prepared for:

Integrated Accommodation Services
NAP Benhall - Flood Risk Assessment
Level 2 Report
Level 2 Flood Risk Assessment
December 2008
Rev Date Details Prepared by
1 December Clare Postlethwaite
2008 Consultant
Reviewed by
Emily Blanco
Senior Consultant
Approved by
Jon Robinson
Associate Director
This document has been prepared in accordance with the scope of Scott Wilson's appointment with its client and is subject to the terms of that appointment. It is addressed to and for the sole and confidential use and reliance of Scott Wilson's client. Scott Wilson accepts no liability for any use of this document other than by its client and only for the purposes for which it was prepared and provided. No person other than the client may copy (in whole or in part) use or rely on the contents of this document, without the prior written permission of the Company Secretary of Scott Wilson Ltd. Any advice, opinions, or recommendations within this document should be read and relied upon only in the context of the document as a whole. The contents of this document do not provide legal or tax advice or opinion.
© Scott Wilson Ltd 2008
Scott Wilson
Scott House
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Basingstoke
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Tel +44 (0)1256 310 200
Fax +44 (0)1256 310 201 www.scottwilson.com

Integrated Accommodation Services
NAP Benhall - Flood Risk Assessment
Level 2 Report
1
1.1
1.2
1.3
1.4
2
2.1
2.2
2.3
2.4
3
3.1
4
4.1
4.2
4.3
4.4
4.5
4.6
4.7
6
6.1
6.2
6.3
6.4
5
5.1
5.2
5.3
5.4
7
7.1
Introduction ......................................................................................... 1
Commission ...................................................................................................................... 1
Background ...................................................................................................................... 1
Flood Risk Assessment Methodology .............................................................................. 2
Aim and Objectives .......................................................................................................... 3
Site Setting ......................................................................................... 4
Current Site ...................................................................................................................... 4
Surrounding Area ............................................................................................................. 4
Identification of Local Water Features .............................................................................. 4
Geology / Hydrogeology ................................................................................................... 5
Proposed Development ...................................................................... 6
PPS25 Sequential Test and Vulnerability Classification ................................................... 6
Flood Risk – To Development ............................................................ 8
Fluvial Flood Sources ....................................................................................................... 8
Tidal Flood Sources ......................................................................................................... 9
Drainage ........................................................................................................................... 9
Pluvial Flood Sources ....................................................................................................... 9
Groundwater ................................................................................................................... 10
Artificial Sources ............................................................................................................. 11
Summary of Flood Sources ............................................................................................ 11
Flood Risk – From Development ...................................................... 12
PPS25 Policy .................................................................................................................. 12
Site Runoff Modelling ..................................................................................................... 12
Storm Water Management Principles ............................................................................. 14
Proposed Drainage ........................................................................................................ 17
Flood Mitigation Measures ............................................................... 18
Mitigation of Flood Risk to the Development .................................................................. 18
Residual Risk to the development .................................................................................. 18
Mitigation of Flood Risk from the Development .............................................................. 18
Residual Risk from the development .............................................................................. 18
Conclusions ...................................................................................... 19
Flood Risk – To Development ........................................................................................ 19

Integrated Accommodation Services
NAP Benhall - Flood Risk Assessment
Level 2 Report
7.2
7.3
7.4
8
Flood Risk - From the Development ............................................................................... 19
Flood Mitigation Measures ............................................................................................. 19
PPS25 Classification ...................................................................................................... 19
References ....................................................................................... 20
9 Appendix A: Figures ........................................................................... 1
Figure 1: Site Location and Proposed Development Plan .............................................................. 1
Figure 2: Site photo ........................................................................................................................ 2
Figure 3: Flood extent map ............................................................................................................ 3
Figure 4: Drainage plan incorporating topographic survey ............................................................. 4
Figure 5: Proposed car park levels ................................................................................................. 5

Scott Wilson was commissioned by Integrated Accommodation Services (IAS), a consortium of
Carillion Building Ltd, Government Communication Headquarters (GCHQ), Global Solutions Ltd
(GSL) and British Telecom (BT), in August 2008 to prepare a Flood Risk Assessment (FRA) to be submitted with a detailed planning application for a proposed development at the GCHQ site
(the site) at Benhall in Cheltenham (known as NAP - New Accommodation Project).
This FRA has been prepared in accordance with the requirements of Planning Policy Statement
25: Development and Flood Risk (PPS25) and its associated Practice Guide (Reference 1) and the requirements of the Environment Agency’s (EA) Development Control officers in the West
Area of the Midlands region.
IAS is designing a proposed extension at the NAP Benhall site, comprising:
• new office accommodation of 19,204 m2 floor area to provide space for 800 staff to be relocated from another site at Oakley (the existing main operations building at Benhall is approximately 104,000 m2);
• a total of 1,079 car parking spaces, incorporating 614 existing spaces being relocated within the site and 465 newly created spaces; and
• new landscape treatment to soften views from the A40 corridor to the south.
A review of the EA’s Flood Map (Figure 3, Appendix A), shows that the site resides in Flood
Zone 1, low probability. This Flood Zone is assessed as having less than 0.1% (<1 in 1000 year) annual probability of river or sea flooding in any year.
As the area of the proposed redevelopment is greater than 1ha (10,000m
2
) and despite the low fluvial flood risk posed to the site, in accordance with Planning Policy Statement 25:
Development and Flood Risk (PPS25), a FRA must be prepared and submitted with any planning application for the site. The FRA must demonstrate that the development will be safe for its proposed lifetime and will consider the risk to the site of flooding from sources other than fluvial. An assessment of the potential of the proposed development to increase flooding elsewhere through changes in the generation and management of surface water will also be made.
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1.3.1 Source Pathway Receptor Model
The aim of an FRA is to assess the risks of all forms of flooding to and from a development.
PPS25 emphasises the need for a risk-based approach to be adopted by planning authorities through the application of the Source-Pathway-Receptor model. Scott Wilson’s approach to
FRAs is based on the Source-Pathway-Receptor model and in accordance with the recommendations of Annex E of PPS25 (Reference 1).
The Source-Pathway-Receptor model firstly identifies the causes or ‘sources’ of flooding to and from a development. The identification is based on a review of local conditions and consideration of the effects of climate change. The nature and likely extent of flooding arising from any one source is considered, e.g. whether such flooding is likely to be localised or widespread.
The presence of a flood source does not always infer a risk. The exposure pathway or ‘flooding mechanism’ determines the risk to the receptor and the effective consequence of exposure. For example, sewer flooding does not necessarily increase the risk of flooding unless the sewer is local to the site and ground levels encourage surcharged water to accumulate.
The varying effect of flooding on the ‘receptors’ depends largely on the sensitivity of the target.
Receptors include any people or buildings within the range of the flood source, which are connected to the source by a pathway.
In order for there to be a flood risk all the elements of the model must be present. Furthermore, effective mitigation can be provided by removing one element of the model, for example by removing the pathway or receptor.
Nature and extent of flooding considered
Consideration of effects of climate change
Local conditions ‘FLOODING MECHANISM’
‘SOURCE’
No flooding
Determines risk to receptor and effective consequence of exposure
No flooding
‘RECEPTOR’
People or buildings within range of the flood source
Flooding dependent on sensitivity of target
1.3.2 FRA – Screening Study Approach
The Scott Wilson Level 2 FRA approach involves a desk-based review of available information and a site walkover survey to establish the likely flooding sources and mechanisms for the site.
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Once the flood risk has been established, mitigation measures are proposed (where necessary) and residual risks explained.
The scope of this commission excludes hydraulic and hydrological modelling and is therefore principally a qualitative assessment based on published data.
The aim of this report is to provide IAS with a FRA, to assess the flood risk to and from the proposed development. In order to achieve this, the following objectives will be met:
• consider the impacts of local flood levels (for the Hatherley Brook) for the lifetime of the proposed development, in accordance with Annex B of Planning Policy Statement 25;
• review the presence of local flood defences (official and unofficial) and determine their impact on local flooding mechanisms;
• where practical, use topographic data to determine areas and depths of inundation across the site and the flooding mechanisms and use this information to inform mitigation measures suitable for the proposed development; and
• where necessary, propose options to mitigate the flood risk to the site, including explanation of the residual risks of each mitigation option.
The drainage design and attenuation proposals have been commissioned separately from this
FRA, which will therefore not include determination of the storm water management requirements at the site in keeping with the principles of Paragraph F10 of Planning Policy
Statement 25.
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December 2008

The development site is located at National Grid Reference 391587, 222305 south-west of
Cheltenham, adjacent to the A40 and approximately 1 mile from the M5. The site is bordered to the south-east by the A40, to the east by Princess Elizabeth Way, to the north and south-west by residential properties and to the west by Fiddlers Green Lane (Figure 1, Appendix A). The total site area is 18.7 hectares. The site is currently partially developed with office buildings and associated hardstanding with an area of fallow land to the north east. See Figure 2, Appendix A for a photo of the current site; an artist’s impression of the proposed new office accommodation is superimposed and outlined in red. The area where it is proposed to build the new car park has not been developed for some nine years and is currently fallow.
2.1.1 Topography
An Environmental Impact Assessment (EIA) was carried out in 1997 by Land Use Consultants
(Reference 2), which indicated that the site lies between 42 and 58 metres Above Ordnance
Datum (m AOD). This was supplemented by a topographic survey carried as part of the drainage strategy design (Reference 2) in 1999 and a site walkover, carried out on the 10 th
October 2008.
2.2.1 Appearance
The land use of the immediate surrounding area mainly comprises hardstanding/car parking and residential properties.
2.2.2 Topography
The Hatherley Brook is located 200 metres to the south-west of the site boundary. Land falls away from the site to the Brook from a maximum height of 52.65 m AOD immediately north of the main ring building to a minimum of 42 m AOD at the southwest of the site, adjacent to the
Hatherley Brook.
The A40 forms the southern boundary of the site and is slightly raised from the southern end of the site, at 44 m AOD compared to 43 m AOD of the Estate Road to the north of the A40. See
Figure 4, Appendix A for a previous site drainage plan incorporating site levels.
The Hatherley Brook is the major water feature in the area, flowing in a westerly direction 200 m to the southwest of the site. The Hatherley Brook is a tributary of the River Severn. There is a culverted watercourse flowing under the southern area of the site, this takes surface water run-
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4

off from the site and discharges to the Hatherley Brook at the western boundary of the site. The site is located outside the area benefiting directly from the protection offered by flood defences.
The geological map for the area (Reference 3) indicates that the underlying geology of the site is
Lower Lias Clay, with calcareous mudstone and clay shales up to 100 m thick. There are also some Cheltenham gravels to the east of the site. During the site walkover on the 9 th
October
2008, it was noted that the undeveloped area of the site, known as ‘A2 retained’ (marked as New
Car Park on Figure 1, Appendix A) appears to comprise fill and demolition waste, although it was not apparent how deep this layer was.
The groundwater vulnerability map for the site (Reference 4) shows that the geology beneath the site has been classified as a non-aquifer (Negligibly Permeable). It does not lie on a Source
Protection Zone (SPZ), as designated by the EA around potable groundwater abstraction points with the aim of limiting potentially polluting activities.
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December 2008

The proposal is for new office accommodation of 19,204 m
2
gross floor area, which will provide space for 800 staff to be relocated from another site. This new accommodation will cover a part of the area currently used for car parking spaces, as shown on Figure 1, Appendix A, and it is proposed to relocate 614 existing spaces to a new three deck car park, to be constructed in the currently undeveloped area marked ‘A2 retained’, which allows for the creation of 1,079 new car parking spaces. The proposal also incorporates site landscaping to soften views from the A40 corridor to the south.
The site is partially developed brownfield land, although the car parking in the ‘A2 retained’ area will be constructed on land which is currently vacant and has not been occupied by any buildings or structures since 2001. Although this area is classed as brownfield land due to previous development, reference is made in this report to it being greenfield. This refers to the application of greenfield run-off rates to the area and not the overall status of the land.
In order to achieve the desired Building Research Establishment's Environmental Assessment
Method (BREEAM) rating for the new building, it is proposed to incorporate greywater and rainwater recycling techniques to the new building. However, any recommendations for the design of this are beyond the scope of this FRA.
Figure 1 indicates the proposed layout and car park building placement on site. Access to the new areas of the site will be via the current entrances and exits, in order to maintain the high level of security required at the site.
Planning Policy Statement (PPS) 25: Development and Flood Risk (ODPM, 2005) provides guidance and policy on development and flood risk.
Based on the Environment Agency’s flood map the site is entirely located in Flood Zone 1
(<0.1% annual probability) of the Hatherley Brook (Reference 9, Figure 3).
PPS 25 indicates that Flood Zone 1 is suitable for the location of any development types.
However, as PPS25 outlines, all developments in this zone should still be considered in relation to:
• their vulnerability to flooding from other sources as well as from river and sea flooding;
• their potential to increase flood risk elsewhere through the addition of hard surfaces
• and the effect of the new development on surface water run-off.
In addition, PPS25 also outlines that in this zone, developers and local authorities should seek opportunities to:
• reduce the overall level of flood risk in the area and beyond through the layout and form of the development; and
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• mitigate the potential to increase flood risk elsewhere through the appropriate application of sustainable drainage techniques.
As detailed in PPS25, Local Planning Authorities (LPAs) are required to undertake Strategic
Flood Risk Assessments (SFRA) of their administrative areas as part of the evidence base for producing their Local Development Framework (LDF). As part of the SFRA, the LPA are required to define the extent of flood zones for their administrative area including definition of the functional floodplain (Flood Zone 3b) which is not currently included on the published EA maps.
The SFRA covering the Cheltenham area and the Hatherley Brook, the Gloucestershire SFRA, was produced in September 2008 (Reference 13), and covers the following local authorities’ areas:
•
Forest of Dean District Council;
•
Tewkesbury Borough Council;
•
Gloucester City Council;
•
Cheltenham Borough Council;
•
Cotswold District Council; and
•
Stroud District Council.
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December 2008

Fluvial sources include rivers, streams and ditches. The main fluvial flood source located within the proximity of the site is the Hatherley Brook, designated as Main River by the EA downstream of the A40. A small tributary of the Hatherley Brook (the Stream) flows through a culvert under the site, from the pond at the southeastern corner to a culvert on the western side of the Link
Road.
A site walkover was carried out on the 9th October 2008, to establish whether the flood extents published on the EA website are realistic given the local topography and the location of the watercourses. The walkover survey identified that the topography local to the Hatherley Brook slopes upwards to the northeast, away from the river, such that an out of bank flood event associated with the Brook would be expected to extend outwards into the floodplain in a narrow strip, as reflected in the flood extents given in Figure 3, Appendix A. The Stream is culverted under the majority of the site and therefore could only be expected to flood from the pond at the southeast or the culvert to the southwest where it joins the Hatherley Brook. As previously discussed the site is elevated from the Hatherley Brook such that it would not be expected to flood; similarly the site is elevated from the level of the pond.
The EA does not hold data on flood levels for the nearby watercourses and no flood levels have therefore been obtained, but the Gloucestershire SFRA (Reference 13) does not highlight the
Hatherley Brook as a known source of flooding problems. Therefore, as site levels are significantly higher than the levels of both the Stream and the Hatherley Brook, the site is located within Flood Zone 1 and there are no records of it having flooded in the past, the risk to the site from fluvial flood sources has been assessed as low.
PPS25 (Reference 1) requires developments to consider the potential impacts of climate change on flood risk for the lifetime of proposed developments.
The EA has confirmed that they would expect a drainage strategy to look at a 20% increase in peak rainfall intensity due to climate change (parameter to between 2055 and 2085 in Table B2 of PPS25).
The EA floodmap indicates that there are no flood defences in the locality of the site; and consequently the site is not considered to be defended.
The EA operates a flood warning service in all areas at risk of flooding and flood warning services have been in operation on the River Severn and its tributaries as a means of reducing
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flood risk for a number of years. However, as the site it outside of the area considered to be at risk from flooding it is not covered by the flood warning service.
Tidal flood sources include the sea and estuaries. The site is not located in an area which is affected by tidal sources and consequently there is negligible risk from tidal flooding.
Drainage includes those parts of the urban environment dedicated to surface and foul water management. There are numerous foul and surface water drains across the site, providing services to the existing office accommodation.
Surface water currently drains to a holding tank under the visitors’ car park to the south of the site, where flows are attenuated before discharging to the Hatherley Brook via the culvert on the western side of Link Road. Foul water is collected from the various existing buildings on the site before draining via gravity to the main Severn Trent Water (STW) foul water sewer at the north west of the site, at the junction of Link Road and Fiddler’s Green Lane. However, there are manholes associated with the sewers which could potentially surcharge should the sewer become blocked or overloaded. Sewerage systems generally have a design standard of 1 in 30 year flood; therefore, if there is a storm greater than this there is a possibility of surcharge from the sewers.
STW were contacted regarding sewer flooding, but as they are unable to provide any information on known problems for commercial properties (Reference 6) no information from STW has been included within this FRA. The SFRA (Reference 13) states that in general the level of flood risk from artificial drainage systems within the Borough is medium to low, although t he areas at highest risk are located towards the south west of the Borough by Hatherley, Tivoli and Lansdown.
However, as conversation with the NAP Benhall site manager did not indicate known sewer flooding problems the risk of sewer flooding is considered to be low .
Pluvial (or surface water) flooding can occur when rainfall is unable to infiltrate the ground surface resulting in ponding of water in flat areas or rapid routing of surface water across steep sloping land. Lack of infiltration can occur for a number of reasons, including:
• impermeable or shallow soils;
• impermeable base geology;
• high groundwater levels; or
• a high percentage of impermeable urban surfaces.
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The majority of the land surrounding the site is developed for residential use, although this does incorporate landscaping and there is also an area of parkland to the northeast of the site. It could therefore be expected that significant rates of surface water flow may be produced in areas surrounding the site. However, there are no available records of this having occurred and neither GCC nor the EA hold any information regarding flooding from surface water runoff. The
SFRA (Reference 13) highlights low-lying areas as susceptible to surface water flooding, which would therefore not include the NAP Benhall site.
Observations made during the site walkover on the 9th of October 2008 indicate that it is unlikely that there will be surface water flooding to the new building, due to its elevated position above the surrounding land. This building will be constructed on an area which is currently hard standing used for car parking and there are no records of it having previously flooded from pluvial sources. There are numerous surface water drains across the car parking area, which were installed in 1999 when the current building was constructed and are thought to be adequately sized. As the new building will not result in any change to the area of impermeable surface there will be no change to the current low flood risk from overland flow.
The new three deck car park on area ‘A2 retained’ will be built into an existing earth bank; in order to minimise the visual intrusion to the adjacent residential properties the majority of the car park will be below existing ground levels. Drainage design and attenuation proposals will be produced separately to this FRA, but as the new building will not increase the area of impermeable surface there will be no flood risk from overland flow it is thought unlikely that the carpark will be subject to flooding from fluvial sources. The surrounding area has adequate provision to manage surface water run-off and even the lower levels of the car park will be higher than the majority of the surrounding area (Figures 4 and 5, Appendix A).
The flood risk posed from pluvial flood sources to the site is therefore considered to be low .
Groundwater flooding can occur when groundwater levels rise above ground levels at the site.
The groundwater vulnerability map of the area (Reference 4) indicates that the geology beneath the site has been classed as a non-aquifer with negligible leaching potential. The underlying geology is Lower Lias Clay, with calcareous mudstone and clay shales up to 100 m thick. There are also some Cheltenham gravels to the east of the site. It is thought that the clay underlying the site will prevent groundwater levels increasing and causing flooding to the site during times of heavy rainfall.
Data received from the EA regarding groundwater levels at three boreholes located within 10 km of the site, at Alney Island, south west of the site (SO824191). The EA does not hold records of groundwater levels closer to the site. Although these records are from boreholes at some distance from the site it is felt that they are applicable to the site. Alney Island lies on alluvium next to the river Severn, which is cut into the clay layer on which the site is located. It is therefore thought likely that groundwater levels will be similar at both the site and Alney Island.
These records show groundwater levels to be between 9 and 10 m AOD, recorded from
09/12/05 to 13/02/07. The lowest point on site will be level one of the proposed new carpark,
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which at 52 m AOD is 42 m above the highest recorded groundwater level at Alney Island
(Figure 5, Appendix A). Consequently even during periods of heavy rainfall, when groundwater levels are at their highest, there is unlikely to be a risk to the site from groundwater flooding. The
SFRA (Reference 13) states that t here are no records of groundwater flooding in the Borough.
The flood risk posed to the site from groundwater flooding toward the site is considered to be low.
Artificial flood sources include storage features such as ponds and reservoirs or raised channels such as canals. The SFRA (Reference 13) states that there are no canals within the Cheltenham area and no records of breaching or overtopping or reservoirs within, or adjacent to, the
Cheltenham area. There is a pond to the southeast of the site, created during the construction of the current office accommodation, to replace an existing pond located in an area that is now used for car parking. This pond is connected to the culverted stream which runs under the site, from east to west. As discussed above in section 3.1, the pond lies well below the level of the proposed new office building and car park.
Consequently the risk of flooding to the site from artificial sources has been assessed as low .
•
The EA flood map shows the site to be located within the undefended Flood Zone 1 and the site is therefore at low risk from fluvial flood sources.
•
The site is at negligible risk from tidal flood sources.
•
The site is at low risk from sewer flood sources.
•
The site is at low risk from pluvial flood sources.
•
The site is at low risk from groundwater flood sources.
•
The site is at low risk from artificial flood sources.
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December 2008

PPS25 states that as well as assessing risk to a development, an FRA should consider the risk of flooding arising from a development.
Greenfield and undeveloped sites rely on natural drainage to convey or absorb rainfall. In general, site development reduces the permeability of the site, increasing the volume and rate of water running off the site to nearby watercourses, potentially increasing flood risk to downstream areas. Therefore appropriate drainage arrangements are required for new developments to ensure that flood risk to others is not increased.
Annex F of PPS25 requires the surface water arrangements for any development site to 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 which result in the same net effect.
Localised flooding is possible if the capacity of any local storm water system is exceeded or if there is a failure in the storm water system; this is the residual risk associated with surface water systems. Storm water drainage systems are typically constructed to accommodate storm events with a return period of 30 years or less. As such, events with a greater return period would be expected to overwhelm the system, resulting in localised flooding. The design of the site and storm water drainage system should take into account extreme events and the influence climate change will have on the intensity and frequency of storms.
Climate change is expected to result in more frequent short duration, high intensity rainfall and more frequent periods of long duration rainfall (PPS25), of the type responsible for recent flooding. According to FCDPAG3 (Reference 3), peak rainfall intensity will increase from 5% between 1990-2025, to 30% between 2085-2115.
5.2.1 Calculating Greenfield runoff
A study undertaken by Defra and the EA (Reference 10) identified the Institute of Hydrology
Methodology Report 124 (Reference 12) to be the preferred method for calculation of greenfield runoff rates.
To derive the potential flood risk from the development the amount of rainfall that does not penetrate through the ground, but contributes to runoff from the site, is calculated.
The Institute of Hydrology 124 Method uses the following formula:
Q
BAR ( rural )
=
0 .
00108
(
AREA
) (
SAAR
) (
SOIL
)
2 .
17
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12

Where:
Q
BAR(rural)
B
AREA
SAAR
SOIL is the mean annual flood (a return period in the region of 2.3 years) is the area of the catchment in km² is the standard average annual rainfall for the period 1941 to 1970 in mm is the soil index, which is an index found in the Flood Studies Report soil maps or the WRAP map of the Wallingford Procedure.
5.2.2 Calculating Proposed Development runoff
The Rational Method (Reference 10 and 11) has been used to estimate the runoff that will occur from the development. Runoff has been calculated for the existing site and the proposed development. The Rational Method uses the following formula:
Q p
( m
3 s
−
1
)
=
0 .
278 Ci ( mmh
−
1
) A ( km
2
)
Where:
C i
A is the coefficient of runoff that is dependent on the characteristics of the ground surface. This varies from 0.05 for flat sandy areas to 0.95 for impervious urban surfaces (tarmac/concrete).
is the intensity of rainfall for the time of concentration? The time to concentration
(Tc) for this site was calculated to be 30 minutes using the Bransby-Williams equation. From this, the amount of rainfall in this time can be calculated using the
Depth Duration Frequency module in the Flood Estimation Handbook (FEH) CD-
ROM (Reference 13). is the area of the catchment in km².
5.2.3 Runoff Rates for Varying Development Scenarios
For the purposes of this FRA it is assumed that the proposed building will not alter the current runoff rates from the site, as it will effectively be replacing the impermeable surface of the carpark with the impermeable surface of the new building. It has therefore not been considered further in the calculations of runoff from varying development scenarios. Only the proposed new carpark in area ‘A2 retained’ is included in the calculations for varying development scenarios given below.
The IH124 method was applied to the car park area, assuming the entire area is currently undeveloped. The Rational Method was applied to two different scenarios:
• runoff from the site under the undeveloped status; and
• runoff from the proposed new carpark (all impervious surfaces).
For each scenario the runoff from the site was calculated for the 1% annual probability storm event. The runoff coefficients (C) that were used for the different scenarios were:
• current covering – 0.1; and
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• proposed impermeable covering – 0.9.
The runoff rates for the two different scenarios are displayed in Table 5-1 below. There is an increase in runoff rates between the current and proposed scenarios. This change in runoff rates is due to the replacement of the area of currently unused land with impermeable materials, which have little, if any, infiltration capacity.
Table 5-1: Summary of Modelled Runoff from the Different Development Scenarios for car park
Scenario
Current (greenfield runoff rate)
Proposed Development
(un-attenuated)
1% annual probability storm event
Runoff l/s
1% annual probability storm event plus 20% climate change
7.15 9.21
167.8 177.9
5.3.1 Sustainable Drainage Systems
The sustainable management of rainfall from a development is an essential element in reducing the risk of flooding to and from a development site in the future. Annex F of PPS25 promotes the use of Sustainable Drainage Systems (SuDS) in new developments and the EA policy position on drainage design for new developments is to incorporate and maximise the use of SuDS over and above the traditional method of engineered piped drainage systems connecting to an existing system or local watercourses. The term SuDS covers the whole range of sustainable approaches to the management of surface water drainage systems. The primary objectives of
SuDS are to minimise the impact from urban runoff in terms of water quality and quantity, as well as to maximize amenity and biodiversity opportunities wherever possible. The objectives all carry equal weight and should be considered in all SuDS schemes to achieve benefits in all three categories of:
• control of runoff water quantity;
• improvement of runoff water quality; and
• enhancement of the amenity value of the site and promotion of biodiversity
The philosophy behind SuDS is that surface water should be dealt with locally, as close to the source as possible. If designed, constructed and maintained correctly, SuDS are more sustainable than the conventional drainage methods since they can provide a wide range of benefits. This is in contrast to the traditional approach to drainage design whereby the main objective is to deal with the quantity of water runoff as efficiently as possible. The main benefits of using SuDS are:
• reducing runoff rates and volumes, thereby reducing the downstream flood risk;
• encourages natural ground water recharge where appropriate;
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• reducing pollutant concentrations and may act as a buffer for accidental spills; and
• providing habitats for wildlife and opportunities for biodiversity, thereby contributing towards enhancing amenity values for developed areas.
5.3.2 Motivation for using SUDS
A number of drivers have contributed towards a shift to sustainable drainage, in particular the impact of urbanisation with reference to the effect of runoff and water quality. More recently the suggested influence of climate change (based on predictions published by the UK Climate
Impact Programme) suggests that Britain is likely to experience wetter winters with more extreme weather events. This will increase the load on the existing sewer systems, (which due to the increased levels of urbanisation in many areas are already operating at maximum capacity) and thereby also increase the risk of sewer overflow and input of pollutants to the water environment. SuDS are therefore promoted as the preferred drainage option by all national planning polices governing development and flooding within the UK, as well as the majority of local and regional planning policy and guidance.
According to PPS 25, sites which have been identified to be either at risk from surface water flooding, or likely to increase the net surface water runoff generated onsite, will need to consider
SuDS as part of the drainage scheme.
5.3.3 SUDS Design
The management train is a concept that is fundamental to designing a successful SuDS scheme and involves the management of runoff in stages as it drains from a site. This is achieved by linking drainage techniques in series; incrementally reducing pollution, flow rates and volumes of runoff. The hierarchy of techniques in the management train are explained below in Table 5-2:
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Table 5-2: Summary of Modelled Runoff from the Different Development Scenarios
Technique
Prevention
Source control
Site control
Description
Rainwater is prevented from generating excess runoff by using techniques such as rainwater reuse and harvesting, or through improved site design and housekeeping.
The runoff is controlled at or very near its source through techniques such as soakaways, green roof and pervious pavements
Runoff from within a local area is managed and routed to large soakaways, infiltration or detention basins
This refers to the management of runoff from one or several larger sites by using for example ponds or wetlands.
Regional control
The preference of techniques in the above table is hierarchical in ascending order. As such, prevention and control of water at source should always be considered first before site or regional controls.
In general, hard engineering based drainage solutions such as underground attenuation storage should be considered last, unless there are obvious advantages in terms of availability of underground storage space, water quality improvements to be gained or the underlying geology precludes other options. Maximising the use of SuDS will also contribute to the Code for
Sustainable Homes (Reference 8) scoring (replacing the EcoHomes scoring) which is required for the proposed development.
The underlying impermeable geology of the site presents an obstacle to infiltration SuDS as the underlying clay will have very low permeability. A key component of the drainage design and attenuation proposals will be to develop a strategy which incorporates storage techniques to attenuate surface water run-off from the site while adhering to the principles of the Interim Code of Practice for Sustainable Drainage Systems (Reference 7).
IAS has expressed a strong preference for rainwater recycling at the NAP Benhall site, which will manage and reduce the run-off from the large areas of hardstanding, car parking and roofs and improve the Code for Sustainable Homes scoring for the building. Rainwater can be used for flushing toilets, washing machines and landscape maintenance without further treatment.
Table 5-3 below (Reference 9, Figure 3) gives an indication of the volumes of water different surface areas and rainfall intensities may yield, assuming 60 per cent retention rate to allow for times when the storm water storage tank is already at capacity and is therefore unable to collect additional rainwater.
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Table 5.3: Approximate yield of rainwater (m3/yr) for a range of surface areas with varying rainfalls
Surface area m
2
P
Annual rainfall (mm) 50 75 100 125 150
500
1000
15
30
22.5
45
30
60
37.5
75
45
90
As discussed in section 1.4 above, the drainage design and attenuation proposals have been commissioned separately from this FRA, which will therefore not include determination of the storm water management requirements at the site in keeping with the principles of Paragraph
F10 of Planning Policy Statement 25. No stormwater management proposals will therefore be put forward as part of this FRA.
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December 2008

6.1.1 Finished Floor Levels
The EA flood map of the site (Reference 9, Figure 3) indicates that the entire site is located within Flood Zone 1. It is therefore not proposed that site levels or floor levels be raised.
6.1.2 Egress and Access
Access to the site is via the security office to the southwest of the site. This area is shown on the
EA’s flood maps (Reference 9, Figure 3) to lie within Flood Zone 1 and will not be impacted by flood waters. Access and egress will therefore be possible from the south by pedestrians and emergency vehicles in a flood event.
The development is not located in the operational floodplain and consequently there is no requirement for provision of compensatory storage on the site.
The residual risk to the development is low as there will be no inundation of the building, car park or exit and entry points to the site.
6.3.1 Storm Water Management Principles
The sustainable management of rainfall from a development is an essential element in reducing the risk of flooding to and from a development site in the future. Annex F of PPS25 promotes the use of Sustainable Drainage Systems (SuDS) in new developments. SuDS aims to mimic natural systems, whereby water is held close to the source, then released slowly over time. This acts to both reduce peak discharge and to promote the settlement of sediment thereby improving the water quality of any resulting discharge.
Separate drainage design and attenuation proposals will be produce for the site, which will give indications for attenuating stormwater generated on the site.
The residual risk of flooding from the development will be assessed in the separate drainage design and attenuation proposals.
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Following the completion of this Flood Risk Assessment, in line with PPS25, the following conclusions and recommendations have been made:
•
The site would not be inundated by the 0.1% annual probability flood event between 2055 and 2085.
•
The site is at low risk from fluvial flood sources.
•
The site is at negligible risk from tidal flood sources as it does not lie within a tidal area.
•
The site is at low flood risk from artificial flood sources.
•
The site is at low flood risk from groundwater flood sources as it lies on a non-aquifer with impermeable underlying geology.
•
The site is at low flood risk from sewer flood sources.
•
The site is at low risk from flooding from pluvial flood sources as it is raised above the surrounding area.
•
Greenfield runoff rate from the site in 2085 has been calculated as 9.21 l/s.
•
Runoff rate from the current site arrangement (pre-development) in 2085 has been calculated as 9.21 l/s.
•
The report concludes that if no surface water attenuation measures were applied to the redevelopment of the site, runoff rates would increase by 168.69 l/s to 177.9 l/s.
•
Surface water attenuation will be assessed in the separate drainage design and attenuation proposals.
•
Access to the site is via the security office to the southwest of the site. This area lies within
Flood Zone 1 and will not be impacted by flood waters. Access and egress will therefore be possible from the south in a flood event.
•
The development is not located in the operational floodplain and there is therefore no requirement for provision of compensatory storage on the site.
•
The EA Flood Map shows the site to be entirely located within the undefended Flood Zone 1
(< 0.1% annual probability floodplain) for the Hatherley Brook.
•
In accordance with the Sequential Test, in Table D1 of PPS25, all development types are permissible within Flood Zone 1 and are therefore suitable on this site.
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•
•
•
•
•
•
•
•
•
•
•
•
•
Reference 1: Planning Policy Statement 25: Development and Flood Risk (PPS25), 2006,
Communities and Local Government.
Reference 2: GCHQ Redevelopment Options Environmental Assessment, Land Use
Consultants, 1995.
Reference 3: Solid and Drift Geology Map, Sheet 216, Tewkesbury, 1:50,000, British Geological
Survey.
Reference 4: Groundwater Vulnerability 1:100,000 Map Series, Sheet 29, Worcestershire,
National Rivers Authority (NRA).
Reference 5: River Severn Catchment Flood Management Plan, Consultation Draft Plan, the
Environment Agency, May 2008.
Reference 6: Helen Myrie, Customer Services Advisor, Personal communication, 3rd November
2008.
Reference 7: Interim Code of Practice for Sustainable Drainage Systems, National SuDS
Working Group, Office of Deputy Prime Minister, July 2004.
Reference 8: The Code for Sustainable Homes: Setting the standard in sustainability for new homes, Department of Communities and Local Government, 2007.
Reference 9: The Environment Agency, http:// www.environment-agency.gov.uk/
Reference 10: Defra & Environment Agency, (2004), Preliminary Rainfall Runoff Management for Developments, Technical Report W5-074A/TR/1 Revision B.
Reference 11: CIRIA X108, Drainage of Development Sites – A Guide, 2004, Kellagher, R. HR
Wallingford.
Reference 12: CEH Institute of Hydrology (1999) ‘FEH CD-ROM’.
Reference 13: Gloucestershire County Council Strategic Flood Risk Assessment Level 1,
Halcrow Group Limited, September 2008.
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Legend
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© Environment Agency copyright and / or database rights 2008. All rights reserved. © Crown Copyright. All rights reserved. Environment Agency, 100026380, 2008.

D121932 December 2008

D121932 December 2008

Scott Wilson
Scott House
Alençon Link
Basingstoke
Hampshire
RG21 7PP