flood risk assessment in connection with a planning - Breckland ...

FLOOD RISK ASSESSMENT
IN CONNECTION WITH
A PLANNING APPLICATION
ON BEHALF OF
MILLNGATE SWAFFHAM LTD
FOR
A SUPERMARKET ON LAND AT
CASTLE ACRE ROAD, SWAFFHAM
Jubb Consulting Engineers Ltd
Plymouth City Airport
Crownhill
Plymouth
PL6 8BW
Tel: 01752 797000
Fax: 01752 797001
O.S. NATIONAL GRID REF. 5 81880, 3 09570
REPORT NO. P9632/G300/C
FEBRUARY 2012

Flood Risk Assessment Tesco Supermarket, Swaffham
REPORT CONTROL SHEET
Client/Applicant: Millngate Swaffham Ltd/Tesco Stores Ltd
Project: Tesco Supermarket, Castle Acre Road, Swaffham,
Norfolk
Job No: P9632
Title: Flood Risk Assessment
Report No.: P9632/G300C
Prepared by: A P Arscott
MEng (Hons)
P Webber
B.Sc (Hons), FGS.
Reviewed by: Ed Hoskins
Approved by: Gary Mitchell
Version Date Detail Author Reviewed Approved
A 30/11/11 Draft Issue
A .Arscott
P Webber
M Hadley G Mitchell
B 26/01/12
C 07/02/12
Updated
S/W Strategy
EA Draft
Issue
E Hoskins M Hadley G Mitchell
E Hoskins M Hadley G Mitchell
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Flood Risk Assessment Tesco Supermarket, Swaffham
CONTENTS
1.0 Introduction
2.0 The Site & Development Proposals
3.0 Planning Policy Context
4.0 Flood Risk Sources, Causes and Potential Extent of Flooding
5.0 Proposals for Flood Defence
6.0 Sustainable Drainage System (SUDS)
7.0 Offsite and Residual Impacts
8.0 Flood Risk Assessment Summary
APPENDICES
Appendix A - Site Location Plan and Aerial Photographs
Appendix B - Topographic Survey
Appendix C - Environment Agency Mapping
- Environment Agency Indicative Flood Map
- Indicative Groundwater Vulnerability Mapping
- EA Groundwater Source Protection Zones
Appendix D - National Planning Policy Summary
Appendix E - Site Investigation Results
Appendix F - Preliminary Drainage Calculations
Appendix G - Preliminary Drainage Strategy Diagram
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Flood Risk Assessment Tesco Supermarket, Swaffham
1 INTRODUCTION
1.1 Commission
This Flood Risk Assessment has been commissioned to support a
planning application in respect of the proposed Tesco supermarket at
Castle Acre Road, Swaffham, Norfolk.
This report is for the use of Millngate Swaffham Ltd/Tesco Stores Ltd
(to whom alone is owed a duty of care) and their professional advisors
and consultees; it may not be relied upon or reproduced by any third
party for any use without the prior written agreement of Jubb
Consulting Engineers Ltd.
1.2 Objectives
The objective of this Flood Risk Assessment Report is to be present a
viable flood defence and surface water drainage strategy addressing
the potential impacts of developing the existing site including an
assessment of flood risk as required by the Environment Agency as
consultees to the Planning Authority.
In summary this flood risk assessment assesses the following –
sources of flooding (including consideration of the potential
pathways which may result in potential flood risk) to and from
the site;
control of onsite surface water;
extent, location and adequacy of any existing defences;
the effect of climate change on all possible sources of flooding;
means of emergency escape and access;
offsite flood risk (the effect of the proposals on the downstream
environment);
residual risk.
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2 THE SITE & DEVELOPMENT PROPOSALS
2.1 Site Location
The subject site is located on Castle Acre Road, Swaffham, Norfolk.
The site is situated in a largely commercial area on the northern edge
of Swaffham, OS National Grid Reference 5 81880, 03 09570.
A site location plan and aerial photograph are reproduced in Appendix
A.
2.2 Site Description
The site can be broadly divided into two areas. The eastern side of the
site comprises a disused commercial unit with associated car park
area, and the western side is undeveloped land overgrown with scrub
and vegetation.
The disused commercial unit is a single storey portal frame style
building, with clad or masonry walls, and a number of access points
around the building.
The site is bordered to the north to by Brocks Road and to the south by
Bear‟s Lane. Castle Acre Road (A1065) forms the eastern site
boundary, and a number of commercial units boarder the west of the
site.
2.3 Topography
The site is rectangular in plan, being approximately 125m along its
east-west axis and 100m along its north-south axis. The topography
falls from the west to the east by approximately 3m over a length of
125m. The site is approximately 1.187 hectares in area.
The site topographic survey is reproduced in Appendix B.
2.4 Geology
The geological map for the site (BGS Digital Geology Map of Great
Britain, 1:50,000, Landmark Geological Mapping) indicates the site to
be underlain by superficial deposits of the Lowestoft Formation,
underlain by Undifferentiated Chalk Formations of Late Cretaceous
Age.
Previous site investigations in the area (May Gurney report no. 41001,
June 1996 – Appendix C) indicate that the Lowestoft Formation glacial
deposits consists of a mixture of cohesive and granular materials and
chalk fragments, to a depth of approximately 6.0m bgl. The Chalk
strata is encountered beneath this.
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2.5 Hydrogeology
The Environment Agency online indicative aquifer mapping and the
Landmark Aquifer Designation Mapping identifies that the solid geology
beneath the site is identified as a Principal Aquifer with the superficial
deposits classified as Unproductive Strata.
Principal Aquifers are classified as layers of rock or drift deposits that
have high intergranular and/or fracture permeability - meaning they
usually provide a high level of water storage. They may support water
supply and/or river base flow on a strategic scale. In most cases,
principal aquifers are aquifers previously designated as major aquifer.
Unproductive Strata is classified as rock layers or drift deposits with low
permeability that has negligible significance for water supply or river
base flow.
The subject site is located within an Environment Agency Source
Protection Zone 3 (SPZ3) „Source Catchment Protection Zone‟.
A Source Catchment Protection Zone is defined as the area around a
source within which all groundwater recharge is presumed to be
discharged at the source. In confined aquifers, the source catchment
may be displaced some distance from the source. For heavily exploited
aquifers, the final Source Catchment Protection Zone can be defined
as the whole aquifer recharge area where the ratio of groundwater
abstraction to aquifer recharge (average recharge multiplied by outcrop
area) is >0.75.
The Landmark Aquifer Designation Mapping is reproduced for
reference in Appendix D.
2.6 Hydrology
There are no watercourses within 1km of the site, although there are a
number of small surface water ponds, the closest being a natural pond
and several man made balancing/drainage ponds situated some 100m
to the north of the site. There are further natural surface water ponds to
the east of the site (including Northwell Pond approximately 250m to
the east).
The following hydrological catchment characteristics have been
obtained from Flood Estimation Handbook (FEH) CD –
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Flood Risk Assessment Tesco Supermarket, Swaffham
AREA 1.34
ALTBAR 74
ASPBAR 151
ASPVAR 0.42
BFIHOST 0.926
DPLBAR 1.03
DPSBAR 14.8
FARL 1
LDP 1.93
PROPWET 0.31
RMED-1H 11.2
RMED-1D 27.6
RMED-2D 36.9
SAAR 697
SAAR4170 698
SPRHOST 11.19
URBCONC1990 0.707
URBEXT1990 0.0494
URBLOC1990 0.327
C -0.022
D1 0.27284
D2 0.33836
D3 0.30622
E 0.31071
F 2.47456
C(1 km) -0.022
D1(1 km) 0.272
D2(1 km) 0.333
D3(1 km) 0.315
E(1 km) 0.311
F(1 km) 2.475
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2.7 Soils
According to the Wallingford Procedure Winter Rainfall Acceptance
Potential (WRAP) map, the site has a low acceptance potential (WRAP
Class 4), with an associated „SOIL‟ classification of 0.3. This SOIL
parameter indicates that, on an annual basis, approximately 30% of
rainfall does not infiltrate into the underlying ground and runoff as
surface water.
The standard percentage runoff derived using the hydrology of soils
types (SPRHOST) provides a measure of the volumetric characteristic
of the runoff response to rainfall. SPRHOST was obtained for the site
from the Flood Estimation Handbook (FEH) CD-ROM for the
catchment. The catchment was selected as it covers a small area
approximately 1.19km 2 . An SPRHOST value of 11.19 was obtained,
indicating that approximately 11% of rainfall runoff as surface water in
this catchment, An SPRHOST value of 11% corresponds with a SOIL
value of 0.45. This is the value for the whole catchment.
As discussed above the soil survey identifies the site to be underlain
wholly by the Denbigh 1 – 541j soil type.
Using the FEH volume 4, a specific assessment of the SPR for the site
can be calculated from the 29 index soil classes using the percentages
which make up the Denbigh 1 – 541j soil type. The SPR is estimated
from HOST soil class fractions, the table below showing how the soil
beneath the site is divided up and pro-rata‟d to give a site specific SPR.
Soil Type HOST HOST Class SPR value for Totals
Classes Percentage HOST Class
Denbigh 1 – 541j Class 1 50.00% 2% 1.0%
Class 5 30% 14.50% 4.35%
Class 16 20.00% 29.20% 5.84%
TOTAL
Site SPR =
11.19%
An SPR value of 11% corresponds with a SOIL value of 0.11. This is
the value for the site only. There is a significant difference in the WRAP
soil parameter, the SPRHOST and the SPR for the site. As the
SPRHOST and SPR values are derived using 29 soil classes as
opposed to the five defined on the WRAP map, it is generally
considered that SPR values provide a more accurate representation of
soil characteristics and variation in runoff between soil types. The most
conservative value, SPR, has therefore been adopted in the estimation
of greenfield runoff rates from the proposed site.
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2.8 Rainfall
The Average Annual Rainfall (also known as Standard-period Average
Annual Rainfall – SAAR) for the period 1961-1990 has been obtained
for the catchment from the FEH CD ROM. A catchment specific value
of 697mm for the catchment has also been obtained.
2.9 Existing Uses and Development Proposals
The site is currently a combination of unused overgrown greenfield land
and disused commercial brownfield land.
2.9.1 Existing Vulnerability Classification
The existing overgrown greenfield area of the site and the disused
commercial brownfield area is defined within national Planning Policy
Statement 25 (PPS 25) Table D.2. as „Less Vulnerable’ land use.
2.9.2 Flood Zone
In accordance with PPS 25 Table D.1, the majority of the site lies within
a Flood Zone 1 – Low Risk.
Flood Risk Mapping is reproduced for reference in Appendix D.
2.9.3 Proposed Development
The site is proposed for redevelopment as a supermarket including car
parking and service yard.
2.9.4 Proposed Vulnerability Classification
Residential development is defined within PPS 25 Table D.2 as ‘Less
Vulnerable’ development.
The site proposals are therefore considered to be “compatible” with
development in a Flood Zone 1.
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3 PLANNING POLICY CONTEXT
3.1 National Policy
In accordance with PPS25, the site proposals are classified as being
within Flood Zone 1 and have low probability of flooding (less that an
average 0.1% or 1-in-100 year annual probability of flooding from river
or tidal sources). All types of development are appropriate in this Flood
Zone; therefore the proposals pass the Sequential Test outlined within
PPS 25.
A full review of PPS 25 is reproduced for reference in Appendix E.
3.2 Local Planning Policy
3.2.1 Local Plan
The Breckland District Local Plan was adopted in September 1999. It
was originally prepared to plan for the period up to mid 2006.
Breckland Council is currently preparing a Local Development
Framework that has replaced the majority of the Breckland District
Local Plan. A number of policies have been saved from the Local Plan,
although none relate to Flood Risk.
3.2.2 Strategic Flood Risk Assessment
Breckland Council has commissioned a detailed Strategic Flood Risk
Assessment (SFRA) which was produced in October 2007 and updated
in February 2008.
This report sets out Breckland Council s‟ strategic planning in relation
to areas at risk of flooding as required by obligations under planning
guidance (PPS 25).
Various background information regarding flooding in the Swaffham
area is provided in the SFRA as summarised below:
“The town of Swaffham is at least 4 km from any Main River,
and well outside the Extreme Flood Outline. It is therefore
considered not to be at flood risk from this source. In addition,
there are no watercourses marked on the Ordnance Survey
maps in the Swaffham area.
“A site visit has shown that Swaffham is generally higher than
the area surrounding it and there are no watercourses in the
valleys surrounding the town, or surface water collecting in low
areas.
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“The town lies on a chalk basin, covered with poorly draining
clay soils. This means that flooding may occur due to the
collection of surface water from precipitation. At present there is
no positive drainage system for the town, with surface water
either draining to deep drainage boreholes direct to the chalk
aquifer or pumped to the River Wissey at North Pickenham.
“The Breckland District Council has reported surface water
flooding incidents in Northwell Road. The Lynn News has
reported regular sewer flooding in Sporle Road, New Sporle
Road and West Acre Road. The Highways Agency reported a
surface water incident on the A47 sliproad to the west of the
town.”
In addition, the SFRA states the following regarding the sewage works
in Swaffham:
“The only sewage works close to Swaffham is Swaffham STW
which discharges into the River Wissey to the south of
Swaffham. The STW is located approximately 3km from the river
and has a consented daily “dry weather” discharge of 1000 m 3 .
The SFRA assesses that the watercourse would flood in a 1%
event, but does not pose a significant flood risk to residential
areas immediately downstream of the discharge location.
“To assess the full impact of additional discharge downstream of
the STW a detailed assessment would be required, taking into
account the specific nature and size of the proposed
development.”
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4 FLOOD RISK, SOURCES & CAUSES
4.1 Level of Risk to Consider
The level of flood risk to consider (and the minimum level of flood
protection to provide) is that corresponding to an annual probability of
flood occurrence of 1% (i.e. a 100 year return period event) due to
Fluvial (river) and surface water flooding and that corresponding to an
annual probability of flood occurrence of 0.5% (i.e. 200 year return
period event) due to sea or coastal flooding during the appropriately
determined lifetime of the development.
The above figures relate to flood risk from fluvial or tidal sources,
neither of which are relevant at the site – rather the design of the
sustainable drainage system to accommodate the required design
storms and climate change impacts are likely to be the key factor at this
site.
Whilst there is no specific planning policy relating to lifetime of
development, the EA usually accepts a design life time of development
of 60 years for commercial and industrial development and in line with
PPS 25 Practice Guide.
4.2 Sources & Potential Extent of Flooding
PPS 25 requires that all potential sources of flooding that could affect
the proposed development be considered.
The following potential sources of flood risk may be relevant (and need
to be considered) in this case:
Overland flow and surface water discharge;
Groundwater flood risk;
Incapacity of drains or sewer flooding;
Climate change impacts.
Other possible causes of flooding (which are not considered relevant
here) include:
Fluvial flood risk;
Tidal/ sea flooding;
Artificial flood risk (e.g. failure of impounding structures);
Flood plain displacement (e.g. by new development).
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4.2.1 Overland flow and Surface Water Discharge
At present the site is divided into two areas; an area of disused land
covered with scrub and vegetation that has no positive drainage, and
an unoccupied commercial building which is understood to be positively
drained. A small informal ditch runs along the northern side of the
disused site area.
For the unoccupied commercial area it is likely the roof water is dealt
with via local soakaways, and surface water from the car park is picked
up by a drainage channel and discharged to the adjacent ditch or
soakaways. Foul water appears to flow off site to the north, and is
presumably discharged to the existing foul sewer in Brocks Road.
Rainwater falling onto the disused scrub area of the site will soak to the
ground via natural drainage. During extended periods of rainfall which
may occur with any critical rainfall events, and during heavy rainfall,
water may not be able to soak into the ground. In these conditions it will
flow overland to the north in the form of ephemeral stream flow or
“sheet runoff”, to be picked up by the existing informal ditch.
Development of the site will create large areas of impervious surfacing,
and prevent natural soakage of rainfall to the ground. Surface water
runoff from impervious area will need to be managed by an
appropriately designed drainage system to prevent flooding
downstream of the site and designed to mimic existing flows and to
achieve a beneficial impact on the downstream areas.
Borehole soakaway tests were undertaken in 1995, the results are
discussed below.
4.2.1.1 Existing Peak Runoff rates – Baseline Conditions
As the site is currently occupied by both greenfield and brownfield land
uses, the baseline runoff baseline rates have been calculated for each
area and combined to provide an allowable surface water discharge
rate for the site.
The greenfield runoff rate is defined as the pre-development natural
runoff rate, as described in the SUDS Manual. The runoff is used to
estimate permissible discharge from the site. The greenfield runoff rate
has been calculated using two methods, the Institute of Hydrology
Report 124 Method, and the ADAS Reference Book 345 method. The
worst case figure from these two methods has been adopted.
In addition the brownfield area of the site has been assessed using the
Modified Rational Wallingford Procedure. These rates have been
added to the greenfield rates to give overall discharge rates for the site.
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The approximate required storage volumes (should off site attenuation
and discharge SUDS system be adopted) have been calculated using
the WINDES software package.
The calculations are reproduced for reference in Appendix F.
4.2.1.2 Infiltration rates
Borehole infiltration tests were conducted by May Gurney during a site
investigation for the adjacent commercial park in 1996 (report ref
41001, June 1996).
These give approximate soil infiltration rates into chalk at a depth of 6m
below ground level of approximately 8.4 x 10 -5 . Further porosity tests
are planned following planning approval to re-affirm these values.
4.2.2 Groundwater Flood Risk
Groundwater flooding can occur when groundwater levels rise above
ground levels at the site. The underlying solid geology (Chalk) is known
to be a factor in areas/cases of groundwater flooding, especially in low
lying Chalk valleys and areas were „winterbourne‟ type springs and
groundwater are known to cause flooding during the winter.
However, in this area the Chalk is known to be capped with a depth of
low permeability glacial deposits, and the Chalk groundwater beneath
is not known to be artesian. No records of previous groundwater
flooding in the area have been noted.
Groundwater flooding is therefore not considered to be a significant
risk.
4.2.3 Incapacity of drains or sewer flooding;
There are several references in the SFRA for the area to surface water
drainage incapacity and sewer flooding.
Providing that any off site surface water discharges are restricted to the
run-off rates set out in Appendix F and section 4.2.1 above, then
incapacity of drainage is unlikely to be an issue.
The proposed foul water discharge rates from the proposed
development will need to be established and the rates approved by the
local water authority/sewage undertaker.
The risk of flooding to the site from this source is considered low.
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4.2.4 Climate Change Impacts
Climate change impacts can increase flood risk or impact from all of the
sources of flooding noted above. PPS 25 provides precautionary
guidance in relation to potential adverse climate change effects on
these sources, and has been applied to the existing and proposed
estimates of surface water and fluvial discharges in this assessment
report.
Annex B of PPS 25 does provide specific figures to be employed when
making allowance for climate change. Table B.2 tentatively provides
“recommended national precautionary sensitivity ranges for peak
rainfall intensities, peak river flow, offshore wind speeds and wave
heights.”
For a 60 year “lifetime of development” (i.e. to in the period 2055 to
2085) the relevant recommended precautionary allowances are –
peak rainfall intensity + 30%
peak river flow + 20%
offshore wind speed + 10%
extreme wave height + 10%
above AD1990 figures.
4.2.5 Flood Risk Sources Summary
The principal source of potential flooding on site has been identified as
uncontrolled surface water runoff.
The proposals at the site are not located in an area where is it may be
at risk from fluvial, tidal, groundwater or infrastructure failure. The
principal concern is the potential flood impact of the development on
land downstream due to surface water runoff disposal from the subject
site.
There are therefore three matters to address –
collection and disposal of surface water due to rainfall runoff so
as to avoid flooding on-site;
control of surface water discharges from the site so as to meet
the criteria of PPS 25 ensuring that there is no increased risk of
flooding downstream discharges from the site, and that the
flooding protection benefits sought by the Strategic Flood Risk
Assessment for the area are obtained; and,
control / routing of offsite excess surface water runoff or overflow
discharges from drainage systems in residual risk condition so
as to ensure that existing flood escape routes are maintained or
improved with regard to offsite flood risk impacts and
consequences.
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5 PROPOSALS FOR FLOOD DEFENCE
5.1 Current Defences
The site is not currently defended from flooding other than by what is
provided by the site‟s existing ground levels and topography.
5.2 Proposed Scheme and Design Lifetime
The site is proposed for a new supermarket together with parking,
servicing and landscaping and highway improvements to provide new
access.
As noted above the proposed design life for the development will be 60
years.
5.3 Impacts of Climate Change
PPS 25 requires an „Integrated Approach‟ to climate change impacts
using the most up to date figures. In this instance the most up to date
information on climate change is provided in Annex B, Table B.2 of
PPS 25 (which adopts a very conservative “precautionary approach”).
Annex B of PPS 25 does provide specific figures to be employed when
making allowance for climate change. Table B.2 tentatively provides
“recommended national precautionary sensitivity ranges for peak
rainfall intensities, peak river flow, offshore wind speeds and wave
heights.”
For a 60 year development design lifetime up to AD 2115, rainfall
intensities are to be increased by +20% on the PPS tabulated figures.
5.4 Flood Mitigation Measures
The proposals at the subject site are not at risk from fluvial flooding and
therefore will not require to be defended from such an event. The “flood
defences” will be the appropriately designed sustainable drainage
systems.
In accordance with the EA guidance the proposed drainage system will
be designed to accommodate storm events up to those having a 100
year return period (plus due allowance for climate change during the
design life of the proposals). Care is needed to ensure that the
proposals do not increase flooding downstream.
The proposed development site will incorporate sustainable drainage
systems which will promote infiltration to ground as a first option of the
disposals of surface water.
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5.5 Downstream Flood Risk and Sustainable Drainage (SUDS)
The proposed means of preventing any increase in downstream flood
risk due to potentially increased discharges from the subject site is the
use of appropriate SUDS techniques to meet the criteria of PPS 25 by
restricting surface water discharges to current discharge rates.
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6.0 SUSTAINABLE DRAINAGE SYSTEMS (SUDS)
6.1 Existing Conditions
The site is currently brownfield site, partially developed on its southern
half. The site is located within a Flood Zone 1 – Low Risk.
6.2 Existing Drainage
The undeveloped area of the site currently drains via overland runoff
and natural soakage into the ground. The remainder of the site is
positively drained or drains to local soakaways.
During extreme rainfall events, the ground may not be able to absorb
all water falling on the site, in which case surface water runoff will be
displaced and flow as sheet runoff to the north of the site, to be drain
into the existing informal ditch on the northern boundary.
6.3 Sustainable Drainage Systems (SUDS)
The objectives of sustainable drainage systems are to –
recharge the groundwater resource as near to the source of
surface water runoff as practicable;
protect downstream watercourses and catchments from
flooding; and to
preserve or protect groundwater and surface water quality.
Accordingly, it is intended to employ SUDS techniques where
practicable to dispose of surface water on site.
The proposed SUDS aim at reducing the amount and rate of water flow
downstream by a combination of –
infiltration into the ground;
holding water in storage areas; and,
slowing down the movement of water.
Considerations which will influence the choice of SUDS are –
potential ecological, environmental and amenity benefits or
impacts;
permeability of the ground;
degree of hydraulic continuity with river water and reactivity to
changes in river levels;
degree and nature of any ground contamination found;
health and safety consequences.
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The primary objective of SUDS techniques in this instance is to avoid
increase flood risk downstream of the site. Another objective of SUDS
techniques is to provide groundwater recharge.
6.4 Proposed Conditions
The proposed surface water drainage scheme will be achieved by the
following means:
The percolation characteristics of the ground underlying the site are
anticipated to be poor at shallow depth. Historical ground investigation
result from the neighbouring site demonstrate that percolation at depth
via a borehole soakaway would provide a potential point for discharge.
During consultation over this development, the Environment Agency
have expressed concerns over the use of borehole soakaways due to
their capacity to provide a pathway for contaminants to reach the
aquifer, without effective control mechanisms. The Environment
Agency have indicated that the use of a borehole soakaway for roof
drainage is likely to be acceptable via a sealed system. Discharge of
surface water from hardstandings and carparks would be potentially
contaminated so would only be considered if no other means of
discharge to a watercourse or shallow soakaway is available. Such a
solution would require a permit from the Environment Agency and
would require stringent methods of removing contamination prior to
discharge.
A surface water sewer exists within Castleacre Road approximately
70m to the south of the site boundary. This occurs at a depth suitable
for a connection from the site and can be reached without the need to
cross third party land. Anglian Water have been consulted regarding
capacity within this sewer but will not consider any connection to it until
it has been demonstrated that all other means of disposal of surface
water have been investigated and demonstrated to be unsuitable. A
connection to the existing sewer would require the discharge rate to be
attenuated.
Given the above, the anticipated means of surface water disposal
would comprise discharge of hard paved areas via permeable paving
percolating at shallow depth and roof drainage to deeper borehole
soakaways. This is shown indicative on the diagram in Appendix G.
Further intrusive ground investigation works will be required to
ascertain percolation values in the shallow and deep underlying
material in order to finalise the surface water drainage strategy.
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7.0 OFFSITE AND RESIDUAL IMPACTS
7.1 Remaining on-site Risks
The site is currently defended from possible flooding by existing site
levels.
All surface water drainage systems will be designed to incorporate
climate change over the proposed lifetime of the development.
No significant on site risks remain.
7.2 Management of Residual Risks
There are no residual risks requiring specific management measures.
The management of the proposed surface water systems (those which
are unadoptable by the statutory drainage provider) will be by the
responsibly site owners or agreed designated management company.
Page 17
Jubb Consulting Engineers Limited February 2012
P9632-G300C-Rep-120209-MH

Flood Risk Assessment Tesco Supermarket, Swaffham
8.0 FLOOD RISK ASSESSMENT SUMMARY
8.1 Existing Conditions
The site is currently a brownfield area of land.
8.2 Requirements
A site specific FRA has been carried out for the site by Jubb Consulting
Engineers Ltd as reported herein.
In summary this Flood Risk Assessment assesses the following –
sources of flooding;
existing surface water disposal;
proposed surface water management (Including SUDS);
adequacy of the existing defences;
the effect of climate change;
means of emergency escape and access;
offsite flood risk and the effect of increased impermeable surface
area on the downstream environment;
residual risk.
8.3 Assessment
For the foregoing, the risk against each of these considerations has
been assessed as follows –
the primary source of potential flooding identified at the subject site
is from extreme onsite surface water flows;
current flood defences to the site are provided by existing ground
levels;
surface water discharges from the existing site currently soak into
the ground via natural infiltration;
the proposed development is to be defended against a minimum of
a 100 year return period event extreme surface water flood event,
plus an allowance for climate change over a design life (60 years for
residential);
the use of appropriate detention storage with capacity to infiltrate
discharges to ground during extreme events will ensure that the
proposed development will not increase the risk of flooding to any
off-site properties;
where possible drainage by means of sustainable drainage systems
(SUDS) will be employed, with the promotion of infiltration as the
primary option for surface water disposal.
8.4 Flood Risk to the Development
The development site is located within a Flood Zone 1 (Low annual
probability of fluvial and tidal flooding; i.e. land assessed as having less
Page 18
Jubb Consulting Engineers Limited February 2012
P9632-G300C-Rep-120209-MH

Flood Risk Assessment Tesco Supermarket, Swaffham
than a 0.1% annual probability of occurrence or 1000 return period).
The primary source of flooding to the site is from overland flow and
surface water runoff.
8.5 Flood Risk from the Development
The main source of potential flooding from the new development is
currently from overland flow and surface water drainage.
The proposed development site will not increase offsite discharges
from the proposed increase in impermeable surfacing. Rainwater falling
onto the site will be managed by a combination of sustainable drainage
techniques including soakaways and permeable paving. Overflows for
these will be linked to an infiltration basin which will have capacity for
extreme events (up to 1-in-100 years) to facilitate soakage to ground.
8.6 Flood Mitigation Measures
Infiltration systems have been assessed for their possible use at the
site. The soakaway testing during the site investigation works has
identified that the permeability of the ground in good.
It is proposed to service the subject site with a new positive drainage
system employing SUDS techniques including a combination of an
infiltration drainage, soakaways and permeable paving. Diagram SK03
_A illustrates the drainage strategy as evolved with discussion to date
with the Environment Agency.
The exact attenuation volumes will be verified at the detailed design
stage. The final drainage strategy will need to be agreed with the EA.
8.7 SUDS
Excess surface water produced by the site will be managed by
infiltration. This is consistent with the sustainability objectives set out in
the planning policy. The subject site‟s discharge will not increase runoff
from the site.
8.8 Conclusion
The scheme proposed adequately meets the standard criteria for
acceptable flood risk. The proposed development is located within a
Flood Zone 1 (Low Risk), all uses proposed uses in the flood zone are
therefore acceptable. No further sequential test assessment is
required.
Page 19
Jubb Consulting Engineers Limited February 2012
P9632-G300C-Rep-120209-MH

Flood Risk Assessment Tesco Supermarket, Swaffham
Jubb Consulting Engineers Limited
Appendix A
Site Location Plan and Aerial Photographs

Project: By APA Checked APA
Castle Arce Road, Swaffham
Title:
Date
Job Number
02/08/2011 Date 02/08/2011
Figure
Aerial Photograph
P9632 Appendix A

Project: By PW Checked PW
Castle Arce Road, Swaffham
Title:
Date
Job Number
25/01/2012 Date 25/01/2012
Figure
Site Location Plan
P9632 Appendix A

Flood Risk Assessment Tesco Supermarket, Swaffham
Jubb Consulting Engineers Limited
Appendix B
Topographic Survey

Flood Risk Assessment Tesco Supermarket, Swaffham
Jubb Consulting Engineers Limited
Appendix C
Site Investigation Results
- May Gurney report no. 41001, June 1996

Flood Risk Assessment Tesco Supermarket, Swaffham
Jubb Consulting Engineers Limited
Appendix D
Environment Agency Mapping
- Environment Agency Indicative Flood Map
- Indicative Groundwater Mapping
- EA Groundwater Source Protection Zones

Project: By PW Checked PW
Castle Arce Road, Swaffham
Title:
Date
Job Number
25/01/2012 Date 25/01/2012
Figure
EA Indicative Flood Mapping
P9632

Project: By PW Checked PW
Castle Arce Road, Swaffham
Title:
Date
Job Number
25/01/2012 Date 25/01/2012
Figure
EA Aquifer Designation - Bedrock Deposits
P9632

Project: By PW Checked PW
Castle Arce Road, Swaffham
Title:
Date
Job Number
25/01/2012 Date 25/01/2012
Figure
EA Aquifer Designation - Superfical Deposits
P9632

Flood Risk Assessment Tesco Supermarket, Swaffham
Jubb Consulting Engineers Limited
Appendix E
National Planning Policy Summary

Flood Risk Assessment Tesco Supermarket, Swaffham
NATIONAL PLANNING SUMMARY
The Government‟s Planning Policy Statement 25 (PPS 25): “Development
and Flood Risk” (first issues December 2006 - revised March 2010) sets out
current planning policy in relation to flood risk and provides requirements for
flood risk assessment. PPS 25 replaced Planning Policy Guidance Note 25,
published in 2001.
Responsibility for Protecting Land and Property against Flooding
The primary responsibility for safeguarding land and other property against
flooding remains with the owner and/or developer. Within this context, those
proposing development are responsible for:
demonstrating that it is consistent with the policies in the PPS
and those on flood risk in the appropriate Local Development
Documents;
providing a FRA demonstrating:
o whether any proposed development is likely to be
affected by current or future flooding from any source;
o to the satisfaction of the local planning authority (LPA)
that the development is safe and where possible reduces
flood risk overall;
o whether it will increase flood risk elsewhere; and
o the measures proposed to deal with these effects and
risks;
ensuring that any necessary flood risk management measures
will be sufficiently funded to ensure that the site can be
developed and occupied safely throughout its proposed lifetime;
designs which reduce risk to the development and elsewhere,
by incorporating sustainable drainage systems and where
necessary, flood resilience measures; and,
identifying opportunities to reduce flood risk, enhance
biodiversity and amenity, protect the historic environment and
seek collective solutions to managing flood risk.
It is a purpose of this Flood Risk Assessment (FRA) to provide appropriate
demonstration of the points listed above.
Flood Risk Assessment
Annex E of PPS 25 provides guidance on undertaking a FRA: at all stages of
the planning process, the minimum requirements for flood risk assessments
include that they should:
be proportionate to the risk and appropriate to the scale, nature
and location of the development;
consider the risk of flooding arising from the development in
addition to the risk of flooding to the development;
Jubb Consulting Engineers Limited

Flood Risk Assessment Tesco Supermarket, Swaffham
take the impacts of climate change into account;
consider both the potential adverse and beneficial effects of
flood risk management infrastructure including raised defences,
flow channels, flood storage areas and other artificial features
together with the consequences of their failure;
consider the vulnerability of those that could occupy and use the
development, taking account of the Sequential and Exception
Tests and the vulnerability classification including arrangements
for safe access;
consider and quantify different types of flooding (whether from
natural and human sources including joint and cumulative
effects) and identify flood risk reduction measures, so that
assessments are fit for the purpose of the decisions being
made;
consider the effects of a range of flooding events including
extreme events on people, property, the natural and historic
environment and river and coastal processes;
include the assessment of the remaining (known as „residual‟)
risk after risk reduction measures have been take into account
and demonstrate that this is acceptable for the particular
development or land use;
consider how the ability of water to soak into the ground may
change with development, along with how the proposed layout
of development may affect drainage systems; and,
be supported by appropriate data and information, including
historical information on previous events.
For the purposes of risk assessment, this FRA report relies upon the survey
data provided by the client; it employs methods of assessment given in PPS
25 and relevant British Standards, CIRIA and Environment Agency R&D
Guidance.
This report conforms to the relevant guidance provided by PPS 25.
Flood Risk Zone and Appropriate Planning Response
As set out in PPS 25 Table D1, the site is defined by the Environment Agency
indicative Flood Mapping as wholly within an area classified as Flood Zone 1
- an area of Low Flood Risk (refer to Appendix C).
The Environment Agency (EA) indicative flood plain mapping takes no
account of existing flood defences and is often based on preliminary data,
very limited surveys and non site specific assessment.
The proposed development is classified as „More Vulnerable‟ in Table D.2.
PPS 25, and as the whole of the site is in a Flood Zone 1 the Exception Test
(see 3.1.6 below) will not be required as set out in paras 18-20 and Annex D
of PPS 25.
Jubb Consulting Engineers Limited

Flood Risk Assessment Tesco Supermarket, Swaffham
Previously Developed Land
Paragraph F6. of PPS 25 deals with previously developed land and states that
–
“surface water arising from a developed site should, as far as practicable, be
managed in a sustainable manner to mimic the surface water flows arising
from the site prior to the proposed development, while reducing the flood risk
to the site itself and elsewhere, taking climate change into account; this
should be demonstrated as part of the flood risk assessment.”
The subject site is currently greenfield. Accordingly, this area is not identified
as „redevelopment‟ but as „development‟ for the purposes of this FRA.
The Sequential Test and the Exception Test
Paragraphs 16. and 17. of PPS 25 require a "Sequential Test" to be applied
by the local planning authority (LPA) when allocating land for development
in flood risk areas and when considering any application for new
development.
Essentially, the Sequential Test, where applicable, should demonstrate that
there are no reasonably available sites in areas with a lower probability of
flooding that would be appropriate to the type of development being proposed
unless there is reasonable justification for such development in such a
location.
If, following the application of the Sequential Test, it is not possible (consistent
with wider sustainability objectives) for the development to be located in
zones of lower probability of flooding, an “Exception Test” should be applied
by the LPA, depending on the vulnerability classification of the proposed
development at the subject site.
Paragraphs D9. – D14. of PPS 25 detail the criteria to be met under the
Exception Test once the Sequential Test has been carried out.
Managing Surface Water and Sustainable Drainage Systems (SUDS)
PPS 25 recommends the use of Sustainable Drainage Systems (SUDS)
where practicable. Annex F of PPS 25 deals with Managing Surface Water
and also Sustainable Drainage Systems.
As noted above, Paragraph F6. states that “surface water arising from a
developed site should, as far as practicable, be managed in a sustainable
manner to mimic the surface water flows arising from the site prior to the
proposed development…”
Jubb Consulting Engineers Limited

Flood Risk Assessment Tesco Supermarket, Swaffham
Paragraph F7. states, “the term Sustainable Drainage Systems (SUDS) is
frequently used and taken in this PPS to cover the whole range of sustainable
approaches to surface water drainage management including:
Jubb Consulting Engineers Limited
source control measures including rainwater recycling
and drainage;
infiltration devices to allow water to soak into the ground,
that can include individual soakaways and communal
facilities;
filter strips and swales, which are vegetated features that
hold and drain water downhill mimicking natural drainage
patterns;
filter drains and porous pavements to allow rainwater and
runoff to infiltrate permeable material below ground and
provide storage if needed; and
basins and ponds to hold excess water after rain and
allow controlled discharge that avoids flooding.”
Paragraph F8. states that “local planning authorities should promote the use
of SUDS for management of runoff”.
Paragraph F11. states, “for new development, it may be necessary to provide
surface water storage and infiltration to limit and reduce both peak rate
discharge from the site and the total volume discharged by the site.”
Paragraph F13. continues, “LPAs should work closely with the Environment
Agency, Internal Drainage Boards, sewerage undertakers, navigation
authorities and prospective developers to enable surface water runoff to be
controlled as near to the source as possible”.
Minimum Standards of Flood Defence
PPS 25 does not give any specific standards for flood defence beyond the
requirements listed in its Annex F addressing surface water disposal only.
Paragraph F6. has been quoted above; paragraph F10. states that –
“the surface water drainage arrangements for any development site should be
such that the volumes of peak flow rates of surface water leaving a developed
area are no greater than the rates prior to the proposed development, unless
specific off-site arrangements are made and result in the same effect”.
Other than that, PPS 25 requires explicitly only that the provision for flood
defence should render the development “safe”.
Under PPS 25 and also referred to in the accompanying “Practice Guide” and
the DEFRA / Environment Agency‟s report FD2320/TR2, it is required that the
planning authority provide an acceptable risk level to which the site defence
proposals should be designed.

Flood Risk Assessment Tesco Supermarket, Swaffham
In regard to flood risk to the subject site, in the absence of planning authority
policy on flood defence this flood risk assessment is based on the guidance
contained in the previous Planning Policy Guidance Note, PPG 25,
“Development and Flood Risk”, Paragraph 31 (as modified below*), which
states –
“flood risk defences for most new housing developments should be designed
and constructed to protect against the flood with an annual probability of 1%
for river flooding and 0.5% for coastal flooding for a period of 50 years,
taking into account the allowances for climate change….; commercial and
industrial development should aim to achieve the same minimum standard of
defence”.
This approach is consistent with both the safety requirements of PPS 25 and
the standards for flood defence implied by the flood risk zoning criteria set out
in Table D.1 of PPS 25.
Table D.1 of PPS 25 highlights the Flood Zoning for all land uses. In this table
it states that flooding with an annual probability of 1% for river flooding and
0.5% for coastal flooding is “high risk”. This will be the standard to which the
site will be defended in the absence of any specific planning authority policy
and taking account of climate change.
Climate Change
Annex B of PPS 25 deals with Climate Change, and requires an “integrated
approach” to climate change when dealing with flood risk over the lifetime of a
new development, and the use of the most up to date guidance on climate
change and flooding should be reflected in FRAs.
The lifetime of the development should be designed to reflect the current
trend in redevelopment and new development. Whereas PPG 25
recommended a design lifetime of 50 years generally (see above), PPS 25
provides no recommendations for lifetime of the redevelopment, and in line
with unofficial advice from the EA, a figure of 100 years for residential (more
vulnerable development) has been adopted for the purposes of this FRA.
Annex B of PPS 25 does provide specific figures to be employed when
making allowance for climate change. Table B.2 tentatively provides
“recommended national precautionary sensitivity ranges for peak rainfall
intensities, peak river flow, offshore wind speeds and wave heights.”
For a 60 year and 100 year “lifetime of development” (i.e. within the period AD
2085 to AD2115) the relevant recommended allowances are –
100 year
off shore wind speed + 10%
extreme wave height + 10%
peak rainfall intensity + 30%
peak river flow + 20%
Jubb Consulting Engineers Limited

Flood Risk Assessment Tesco Supermarket, Swaffham
above AD 1990 figures.
Residual Risk
Paragraph 8 of PPS 25 states that “LPAs should, in determining planning
applications…. ensure that all new developments in flood risk areas is
appropriately flood resilient and resistant, including safe access and escape
routes where required, and that any residual risk can be safely managed”.
Annex G of PPS 25 sets out the policy and principles relating to the
management of the Residual Flood Risk.
Jubb Consulting Engineers Limited

Flood Risk Assessment Tesco Supermarket, Swaffham
Jubb Consulting Engineers Limited
Appendix F
Preliminary Drainage Calculations

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
Downstream Flood Risk and Watercourse Protection
The existing watercourse regime protection will be achieved by limiting the sites surface water discharges to greenfield runoff rates for return periods of 1, 30 and 100
years which therefore requires attenuation storage to enable storm water discharges to meet this criteria. This is best evaluated using a simulation model to calculate
this volume by using the estimated greenfield runoff rates as fixed throttle rates for these three return periods.
Institution of Hydrology Report 124 method - Calculating greenfield discharge rates
Total Development Area (ha): 1.19
SAAR (mm):
697
IoH 124 Equation:
QBAR = 0.00108 A
SPRHOST (Catchment): 11.19
SPR (Site): 11.19
A = Area
SOIL: 0.45
SAAR = Average Annual Rainfall (mm) - (From FEH CD-ROM Catchment Descriptors)
Hydrological Region: 8
SOIL = Derived from SPRHOST (from FEH catchment descriptors) as recommended by FEH
0.89 SAAR1.17 SOIL2.17 Q BAR (l/s/ha): 0.2
N.B. When A is Less Than 0.5km2, a value of 0.5km2 is used in the above equation and the result
Q BAR (l/s): 0.3
pro-rated to determine site-specific value
Return Period
(Years)
1
2.33
5
10
25
30
50
100
200
500
Growth Factors
Unit Area
(from IoH124 and based on corresponding
Runoff Rate
Hydro Region)
(l/s/ha)
0.88
0.2
1.00
0.2
1.23
0.3
1.49
0.3
1.84
0.4
1.89
0.4
2.12 0.5
2.42
0.5
2.77
0.6
3.41
0.7
Page 1
Site Specific
Runoff Rate
(l/s)
0.2
0.3
0.3
0.4
0.5
0.5
0.5
0.6
0.7
0.9

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
UK Growth Curves Uk Growth Curve Factors
Page 2
Hydrometric
Area 1 2.33 5 10
Return Period
25 30 50 100 200 500
1 0.90 1.00 1.20 1.45 1.81
2.12 2.48
3.25
2 0.91 1.00 1.11 1.42 1.81
2.17 2.63
3.45
3 0.94 1.00 1.25 1.45 1.70
1.90 2.08
2.73
4 0.89 1.00 1.23 0.49 1.87
2.20 2.57 3.62
5 0.89 1.00 1.29 1.65 2.25 2.83 3.56
5.02
6 / 7 0.88 1.00 1.28 1.62 2.14 2.62 3.19
4.49
8 0.88 1.00 1.23 1.49 1.84 1.89 2.12 2.42
3.41
9 0.93 1.00 1.21 1.42 1.71
1.94 2.18
2.86
10 0.93 1.00 1.19 1.38 1.64 1.85 2.08
2.73
Ireland 0.95 1.00 1.20 1.37 1.60
1.77 1.96
2.40
Figure 1: Hydrological Areas Map

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
ADAS Reference Book 345 "Design of Field Drainage systems" - Calculating greenfield discharge rates
Introduction
This guidance indicates an acceptable method for the assessment of greenfield runoff rates from small sites in the Cornwall Area of the Environment Agency. The method
is based on the ADAS reference book 345 ‘The design of field drainage systems’ combined with the regional growth curves developed by South West Water Authority as
well as area data on soil and rainfall statistics. Reviews of small catchments in Cornwall have been carried out that demonstrate that this method produces results that,
when extrapolated to a larger area, give a reasonable degree of agreement with the Flood Estimation Handbook estimates.
The method for assessing runoff rates in Cornwall has been adapted to cover sites outside this region. This assessment specifically assesses the runoff rates for Swaffham.
Methodology
The ADAS method first assess the Mean Annual Flood, which is used as an index runoff rate. The index runoff rate is then multiplied by a range of multipliers , which
indicate increasing runoff rates with increasing severity of storm event.
The steps involved in this process are given below.
Site area and slope
The ADAS method was designed for calculating allowable discharges from field systems. As such, where the proposed development site is of a large size, of a complicated
shape, has complicated topography or even a watercourse running through it, it may necessary to break down the area into smaller more uniform areas and derive index
runoff rates from each of these individually.
The site area is input in hectares; the maximum length of the site area, or the length across the typical slope of the area; the maximum height difference across the site area
or typical slope respectively.
Site/Catchment characteristic flood flow intensity relationship
Figure 1 below relates the site topography via the Average Annual Rainfall rate to a Flood Flow Factor F.
The slope and length of the site are used to determine a site/catchment characteristic C. The Average Annual Rainfall (AAR) can either be taken the Wallingford maps,
which gives a broad region wide indication of this value, or in more accurately, local values can be obtained from the Agency , the Met Office or the Flood Estimation
Handbook CD.
In the case of a site in Plymouth, the AAR has been taken from Wallingford Procedure mapping (as tabulated at the end of the document).
Page 3

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
Soil Type HOST Classes
HOST Class
Percentage
SPR value for
HOST Class
Denbigh 1 – 541j Class 1 50.00% 2%
Class 5
Class 16
30%
20.00%
14.50%
Totals
1.00%
4.35%
29.20% 5.84%
TOTAL
Site SPR =
TOTAL 11.19%
An SPR value of 11% corresponds with a SOIL value of 0.11. This is the value for the site only. There is a significant difference in the WRAP soil parameter, the
SPRHOST and the SPR for the site. The SPRHOST and SPR values are derived using 29 soil classes as opposed to the five defined on the WRAP map, it is generally
considered that SPR values provide a more accurate representation of soil characteristics and variation in run-off between soil types. The most conservative value, SPR,
has therefore been adopted in the estimation of greenfield runoff rates from the proposed site.
Greenfield runoff rates
The Mean Annual Flood (MAF) is estimated in Table 1 part 9. For this the Soil Types factor is multiplied by the Flood Flow Factor F and the site area to give the MAF
Q o.
Where appropriate it may be necessary to consider the runoff rates that would occur from the site during more intense storms. To estimate these more extreme
conditions the UK Growth Curve multipliers are applied to the Mean Annual Flood.
Page 4

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
Determination of mean Annual Flood Q0 1. Locate a suitable map of the area and determine the catchment area A in hectares. A =
General
1.19 Ha
2. Determine the maximum length of catchment L in meters L = 153 m
3. Determine the average slope of the catchment S =
ht (m)
(Highest point) S =
S = ht/L
L (m) (Lowest point)
4. Determine the catchment characteristic C C = 0.0001 x L/S C =
5. Determine the dominant crop type Grass / Arable / Horticultural =
6. Determine the average annual rainfall AAR in mm AAR = 697 mm
7. Determine the soil type factor S T
Permeability
Class Range (m/day) WRAP Class Soil Index ST ----
----
5
0.50
1.3
Very Slow < 0.01 - 0.1 4
0.45
1.0
Slow - Mod 0.1 - 0.3
3
0.40
0.8
Moderate 0.3 - 1.0
2
0.30
0.5
Mod - Rapid 1.0 - 10.0
1
0.15
0.1
Rapid
>10
1
0.1
Page 5
S T = 0.5
69.31 73.07
153
0.025
0.62
G

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
Determination of mean Annual Flood Q 0 Cont….
8. At figure 4 below, enter the graph at C. Move across (left) to crop type, down to average
annual rainfall (AAR), across (right) to the standard line and up to F number. F =
Figure 1. Flood Flow Intensity Relationship
Catchment Characteristics
Flood Flow Intensity
Relationship
Page 6
7.8
General

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
Determination of mean Annual Flood Q 0 Cont….
9. Peak flood Flow Q o = S T x F x A Q 0 =
10. Mean Annual Estimated peak flow (+5% for climate change from 1990 figures) Q 0 + 5% =
11. Greenfield runoff Rates -
SITE AREAS
Return Period (Years)
South West Region
Multiplier
Mean Annual
Probability
Greenfield Discharge
Rates Average (l/s) Greenfield - General
Discharge Rate
(l/s/ha)
Q2
0.93
63%
4.5
3.8
MAF (2011)
Q2.33
1
MAF (2011)
4.9
4.1
Q5
1.28
18%
6.2
5.2
Q10
1.58
10%
7.7
Q30
2.14
3.30%
10.4
6.5 8.8
Q50
2.45
2%
11.9
10.0
4.6
4.9
Q100
2.93
1%
14.3
12.0
*(using the Neil Whiter 1981 multiplyers)
The proposed site storage assessment is based on the site greenfield runoff rates calculated for the 1-in-1 year, 1-in-30 year and the 1-in-100 year return period
storm event, plus an allowance for climate change and the brownfield runoff from the vets to the exisitng positive surface water sewer offsite.
The site areas are divided up as follows:
- Total site area = 11,852m 2
- Existimg Impermeable Area = 3,570 m 2 - 30%
- Existing Permeable Area = 8,282 m 2 - 70%
- Proposed Impermeable Area = 10,597m 2 - 90%
- Proposed Impermeable Area = 1,255m 2 - 10%
Page 7

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
MicroDrainage Quick Storage Estimate (Using the FEH Quick Storage Estimate)
Catchment Plan
AREA = 1.34 C = -0.022
ALTBAR = 74 D1 = 0.27284
ASPBAR = 151 D2 = 0.33836
ASPVAR = 0.42 D3 = 0.30622
BFIHOST = 0.926 E = 0.31071
DPLBAR = 1.03 F = 2.47456
DPSBAR = 14.8 C(1 km) = -0.022
FARL = 1 D1(1 km) = 0.272
LDP = 1.93 D2(1 km) = 0.333
PROPWET = 0.31 D3(1 km) = 0.315
RMED-1H = 11.2 E(1 km) = 0.311
RMED-1D = 27.6 F(1 km) = 2.475
RMED-2D = 36.9
SAAR = 697
SAAR4170 = 698
SPRHOST = 11.19
URBCONC1990 = 0.707
URBEXT1990 = 0.0494
URBLOC1990 = 0.327
Page 8

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
Existing Discharge Rates from the brownfield element of the site (Using the Wallingford Procedure)
The existing discharge rate from the site will be determinded using the Rational
Method from the Wallingford Procedure.
Q = C i A
Where - Q = Peak Discharge
C = Runoff Coefficient
i = Rainfall Intensity
A = Drainage
C = Cv x Cr
Where - Cv = Volumetric Runoff Coefficient
Cr = Routing Coefficient (Value of 1.3 is recommended in the Wallingford Procedure,
when designing surface water systems to ensure adequate
capacity, but 1.0 cnan be moreappropriate for long duration
storms (t>>tc) or when assessing realistic net discharge rates).
Cv = PR / 100
PR = Percentage runoff from the total catchment area
(not from the impervious area alone)
PR = 0.829 PIMP + 25.0 SOIL + 0.078 UCWI - 20.7
PIMP = 30%
SOIL = 0.4
UCWI = 68
Page 9

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
PR = (0.829 x 30)+ (25.0 x 0.4) + (0.078 x 68) - 20.7
PR = 19.47
Cv = 19.47/ 100 = 0.19
C = 0.19 x 1.0 = 0.19 (using Cr = 1.0 in this case) C =
tc - Time of Concentration
tc = te + td
te = Time of Entry
td = Time to Drain
te = 0.744 x LENGTH^0.133 x SLOPE^-0.274 (Vol. Equation 7.23)
LENGTH = sub catchment overland flow length (m) LENGTH = 64m
SLOPE = sub catchment slope (%) SLOPE = 0.025 %
te = 3.6mins
td = 64m / 1m/s = 1.1 mins
tc = 4.7 mins
Page 10
0.19

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
The time of concentration given by this method is within the range given by
Wallingford and realsitic for the site considering its slope and roughness etc..
i 1 = 55.16 mm/hr
i 2 = 70.29 mm/hr
i 5 = 90.76 mm/hr
i 10 = 104.05 mm/hr
i 30 = 128.90 mm/hr
i 50 = 143.39 mm/hr
i 100 = 163 mm/hr
Existing Discharge Rates - (Q = C i A)
Q1 = 0.19 * ( 55.16 / 3600 ) x 1300 = 3.78 l/s
Q2 = 0.19 * ( 70.29 / 3600 ) x 1300 = 4.82 l/s
Q5 = 0.19 * ( 90.76 / 3600 ) x 1300 = 6.23 l/s
Q10 = 0.19 * ( 104.05 / 3600 ) x 1300 = 7.14 l/s
Q30 = 0.19 * ( 128.9 / 3600 ) x 1300 = 8.84 l/s
Q50 = 0.19 * ( 143.39 / 3600 ) x 1300 = 9.84 l/s
Q100 = 0.19 * ( 163 / 3600 ) x 1300 = 11.18 l/s
Page 11

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
The following Quick Storage estimates, carried out using MicroDrainage Windes software, demonstrate the necessary storage volumes required for attenuation of the
site impermeable surface are runoff at the calculated greenfield runoff rates. (The variation in storage is dependent on the type of structures constructed and the flow
controls used.)
At the detailed design stage a check that the system has the capacity for the following will be carried out -
1) no external flooding except where specifically planned (30 year high intensity rainfall event);
2) no internal flooding (100 year high intensity rainfall event);
3) no internal flooding (100 watercourse event and critical duration for site storage);
4) no flood routing offsite expect where specifically planned (100 year high intensity rainfall event).
Where long term flood storage is to be provided by diverting flows from the attenuation storage system, this needs to be checked by running the proposed storage
arrangement in Windes with a range of events to check how frequently and to what extent the long term storage comes into effect.
The current proposals supported by this FRA require the use of soakaways to be investigated prior to detailed design. Soakaway tests giving percolation rates should be
carried out to determine the feasibility of their used before other options for surface water drainage are sought. The site is assumed to have generally good natural
drainage characteristics, and may offer good soakaway potential. For the purposes of this assessment an assumed soakaway rate of 0.3m/hr (8.3 x 10 -5 m/s) has been
adopted.
Due to the proposals to utilise Soakaways on site there is no requirement to provide volumetric runoff storage on site. Excess runoff volumes will be infiltrated to
ground.
Water quality measures proposed include the use of infiltration techniques which will provide filtration of surface water flows removing potential contaminants before
reaching the aquifer below the site. This proposal will dilute initial storm discharges to reduce the impact of positive drainage within the catchment.
Page 12

CALCULATION SHEET
Details: Preliminary Surface Water Drainage Calculations
Total Application Site Area
Project: Castle Acre Road, Swaffham
Date: 28/11/2011
Project No: P9632 Prepared By: S. Hurdwell
Checked By:
Site Peak Runoff Attenuation Volumes
Residential Area - 1 year storage estimate
Maximum Allowable Discharge = l/s
Required Storage Volume = m 3
3.8
130
Residential Area - 30 year storage estimate
Maximum Allowable Discharge = l/s
Required Storage Volume = m 3
8.8
345
Residential Area - 100 year storage estimate
Maximum Allowable Discharge = l/s
Required Storage Volume = m 3
11.2
494
Residential Area Attenuation Summary
1 year = m 3
30 year = m 3
100 year = m 3
Total Required Volume = m 3
130
215
149
494
Page 13

Flood Risk Assessment Tesco Supermarket, Swaffham
Jubb Consulting Engineers Limited
Appendix G
Preliminary Drainage Strategy Drawing