The Civil Engineering Practice
11 Tungsten Building
George Street
Fishersgate
Sussex
BN41 1RA
T. 01273 424 424
F. 01273 424 425
E. design@civil.co.uk
www.civil.co.uk
Flood Risk Assessment
Proposed development at
Tilgate Business Park, Crawley
On behalf of
Helical Bar Developments (South East) Limited
May 2006
CONTENTS
1
1.1
1.2
2
2.1
2.2
2.3
3
3.1
4
4.1
4.2
4.3
5
5.1
5.2
6
Introduction ................................................................................................... 1
Overview....................................................................................................... 1
Objectives ..................................................................................................... 1
The Site and Surrounding Area .................................................................... 2
Site Description............................................................................................. 2
Proposed Development ................................................................................ 2
Site Description and Background .................................................................. 3
Flood Risk to Site .......................................................................................... 5
Flood Sources and Local Hydraulics ............................................................. 5
Flood Risk From Site..................................................................................... 6
Overview....................................................................................................... 6
Site General Runoff ...................................................................................... 6
Storm Water Runoff Attenuation and Disposal .............................................. 6
Conclusions and Recommendations ........................................................... 8
Conclusions .................................................................................................. 8
Recommendations ........................................................................................ 8
Appendices .................................................................................................... 9
1
Introduction
1.1
Overview
1.1.1 The Civil Engineering Practice has been commissioned by Helical Bar
Developments (South East) Limited to undertake a Flood Risk
Assessment for a commercial B1 industrial offices development located
on Tilgate Business Park on the southern outskirts of Crawley.
1.1.2 In keeping with PPG 25 and standard advice provided by the
Environment Agency to Local Planning Authorities a Flood Risk
Assessment for the proposed development is required to support the
Planning Application.
1.1.3 This report presents the findings of the Flood Risk Assessment and
addresses the points raised by Planning Guidance 25: Development and
Flood Risk (PPG25).
1.2
Objectives
1.2.1 The aims of the Flood Risk Assessment are to:
1.2.1.1
Assess the risk posed to the site from flood events.
1.2.1.2
Assess the storm water runoff potential for the site and
requirements for attenuation and disposal.
1.2.2 Report Synopsis
1.2.2.1
Section 2 describes the development site, the surrounding area
and the proposals for the site.
1.2.2.2
Section 3 covers the risk from flooding to the site.
1.2.2.3
Section 4 covers the risk from flooding from the site.
1.2.2.4
Section 5 provides the conclusions and the recommendations of
the report.
1
2
The Site and Surrounding Area
2.1
Site Description
2.1.1 The application site is located on Tilgate Business Park, Brighton Road
immediately to the south of the recently completed K2 Leisure Complex,
with Thomas Bennett Community College beyond to the north, on the
southern outskirts of Crawley.
2.1.2 An extant Planning Permission for a proposed B1 office development
was granted under reference CR/94/0633/FUL for the erection of a three
storey office development with 128 associated parking spaces on 16
February 1995 but was not implemented.
2.1.3 The main part of the development site is currently unoccupied land at the
northern end of the Tilgate Business Park which comprises 13,588m2 of
B1 industrial office use.
2.1.4 The proposed development site is located on land beyond the current
northern boundary of the Tilgate Business Park where two additional
buildings are proposed for commercial B1 industrial office space.
2.1.5 The site is adjacent to an existing watercourse and is subject to any
recorded flooding incidents recorded by the Environment Agency.
2.1.6 The underlying geology of the site is Tunbridge Wells Sandstones and
Clays which have very low permeability.
2.2
Proposed Development
2.2.1 The proposed development comprises two new three storey buildings of
B1 offices of 2,479m2 and 2,151m2 respectively. The proposed
development has 155 car park spaces.
2.2.2 The proposed development site is located on an area of land that was
the subject of Planning Permission CR/94/0633/FUL and was previously
used as a saw mill. This is currently unoccupied and hard surfaced by
concrete extending over an area of 0.7ha.
2.2.3 A site location plan drawing 1 is located in Appendix 1 and the proposed
site layout drawing 2 is located in Appendix 2 at the rear of this report.
2
2.3
Site Description and Background
2.3.1 Current Site
2.3.1.1
The proposed development site is located immediately to the north
of the Tilgate Business Park and to the south of K2 Leisure Centre
on the outskirts of Crawley.
2.3.1.2
The current Tilgate Business Park development comprises four
blocks of offices that are surrounded by areas of hard paved car
parking.
2.3.1.3
The proposed development site is in the form of two plateaux. The
major area plateau to the west of the site upon which the proposed
development will be constructed ranges between 97.9m AOD in the
west to 97.5m AOD in the east. There is a significantly smaller
plateau that is also surfaced with concrete falling from 97.5m at the
western boundary to 95.0m AOD on the eastern boundary.
2.3.1.4
A stream is located beyond the eastern boundary of the site with a
bed gradient of approximately 1:60. The bed level is 90.31m AOD
and top of bank level is 91.1m AOD, the stream is approximately
4m wide. With these physical characteristics, the stream is steeply
graded and will flow in the rapid shooting range of velocities.
2.3.2
The Surrounding Area
2.3.2.1
The site is located on the southern outskirts of Crawley where a
series of steeply graded streams flow northwards. These streams
are intercepted by the Broadfield Brook that flows from east to west
crossing the Brighton Road (A23) and discharges into a significant
lake that acts as storage on the river system.
2.3.2.2
The Environment Agency Flood Map obtained from their website,
located in Appendix 3 at the rear of this report, identifies an area to
the west of Brighton Road (A23) north of the Southgate Avenue
Roundabout close to the larger surface pond as lying within the
0.1% annual probability floodplain.
3
2.3.2.3
The geological map for the West Sussex shows the site and
surrounding area are underlain by Tunbridge Wells Sandstones
and Clays.
2.4
Existing Drainage
2.4.1 A study of the existing drainage drawings for the Business Park
indicates that surface water is collected by a network of drains which
discharge into the balancing pond at the bottom western end of the site.
The balancing pond was constructed in the 1980s as part of the
development and acts as attenuation for surface water runoff. Although
no correspondence with the Environment Agency (National Rivers
Authority) have be seen from that time it is believed that approval to the
system was granted.
4
3
Flood Risk to Site
3.1
Flood Sources and Local Hydraulics
3.1.1 The Environment Agency Flood Map located in Appendix 3 at the rear of
this report identifies the site and majority of the surrounding area as lying
outside the 0.1% annual probability floodplain.
3.1.2 The site is located within an area that is extremely unlikely to flood.
3.1.3 A stream is located approximately 30m from the boundary of the
proposed development site. The top of the banks of the stream are
nearly four metres lower than the eastern end of the proposed
development site that in turn is a further 2.5 metres lower than the
development site boundary.
3.1.4 The steeply graded nature of the stream adjacent to the site also means
that a significant increase in flow in the steam is required in order to
create a situation whereby the proposed development site will flood.
3.1.5 Therefore, the site is not liable to flooding from the adjacent stream.
5
4
Flood Risk From Site
4.1
Overview
4.1.1 Flood Risk in any area is controlled by a number of contributing factors
throughout the catchment. At the local scale, when developing or
redeveloping a site, it is usual to determine the contribution to the flood
risk.
4.2
Site General Runoff
4.2.1 Developments have been encouraged by the Environment Agency to
attenuate runoff to manageable levels/rate prior to discharge. This
approach is designed to ensure storm water runoff is managed as
close to its source as possible and not transferred downstream where it
could exacerbate flooding.
4.2.2 The 0.1% annual probability rainfall event is the surface water design
standard for this site. The site generated peak rate of runoff for the 0.1%
annual probability based on our previous experience has been
estimated as 7l/s/ha so the runoff from the site should be restricted to
5l/s.
4.2.3 Whilst the main part of the development site is currently hard paved by
concrete with the exception of some permeable soiled areas at the
western and eastern ends, there are no visible gulleys that would drain
the area. It must be assumed therefore that the surface water discharge
from the site is currently uncontrolled.
4.3
Storm Water Runoff Attenuation and Disposal
4.3.1 Attenuation, reduction in discharge and disposal of runoff can be
achieved through the introduction of a Sustainable Urban Drainage
System (SUDS) and other allied technologies such as:

Soakaways

Green roofs

Underground tanks

Rainfall harvesting

Swales

Porous pavements/hardstanding

Ponds
6
4.4
Proposed Drainage
4.4.1 The development area will drain to an existing 300mm diameter pipe
which has been constructed under the site access road up to the site
boundaries. The flow from the site for a 1 in 30 year event will not exceed
the capacity of this pipe.
7
5
Conclusions and Recommendations
5.1
Conclusions
5.1.1 The Environment Agency flood map indicates that the site is located
outside any 0.1% annual probability (1 in 100 year) floodplain.
5.1.2 The Environment Agency has advised that the 0.1% annual probability (1
in 100 year) flood event should be taken as the design standard for this
site.
5.1.3 The Environment Agency has stated that the proposed development on
the site is not precluded.
5.1.4 The peak rate of site runoff generated from site should be restricted to
5l/s in order to achieve a controlled discharge.
5.2
Recommendations
5.2.1 As there is no floodplain requirement there are no restrictions regarding
finished floor levels.
8
6
Appendices
Appendix 1
Site Location Plan
Appendix 2
Site Layout Plan
Appendix 3
Environment Agency Flood Map
9