WESTROCK LTD.
11-13 The Boulevard, West Sussex, Crawley, RH10 1UX – DRAINAGE STRATEGY REPORT
Job No: 1640
Date: 16/12/16
Parmarbrook | 2nd Floor, 345 Old Street, London EC1V 9LL | tel 020 7839 3999 | www.parmarbrook.com
Contents
Abbreviations
1.0
Introduction
AOD - Above Ordnance Datum
2.0
Site Description and Context
EA -
Environment Agency
3.0
Development Proposals
FFL -
Finished Floor Level
4.0
Surface Water Management Strategy
NPPF - National Planning Policy Framework
5.0
Foul Water Strategy
SUDS - Sustainable Drainage Systems
6.0
Summary and Conclusions
SWMP - Surface Water Management Plan
APPENDICES
A
TOPOGRAPHICAL SURVEY DRAWING
B
DEVELOPMENT PLANS
C
THAMES WATER SEWER RECORDS
D
PRE-DEVELOPMENT DICHARGE RATES CALCULATIONS
E
THAMES WATER PRE-DEVELOPMENT RESPONSE
F
PROPOSED SURFACE WATER & FOUL WATER STRATEGY
G
MICRO-DRIANAGE MODEL OUTPUTS
Date 16/12/16
Prepared by
JD
Checked by
MG
Verified by
VB
Document ref
Z:\Parmarbrook\001 PB PROJECTS (NEW SYSTEM)\1640 - The
Boulevard, Crawley\5 - Design\5.6 - Drainage Design
Revision:- Rev 1 – Revised Site Plan
1.0 Introduction
2.0 Site Description and Context
Parmarbrook were commissioned to undertake a Drainage Strategy for the subject site known as “11-13 The Boulevard,,
West Sussex, Crawley, RH10 1XU”.
2.1
The Drainage Strategy is intended to support the associated planning application to the Local Authority for the
development. The information and strategies contained in this report aims to identify and outline the safe and controlled
management of surface and foul water from the site. Proposed mitigation measures are also presented to ensure the
effect of the development is not detrimental to existing drainage networks serving the site and area. The following nonexhaustive strategic and local documents were reviewed:
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National Planning Policy Framework (NPPF)
Crawley Borough Council Strategic Flood Risk Assessment (SFRA) incl interim report
West Sussex Local Flood Risk Management Plan
Thames Water incl Public Sewer Records
Site Description and Location
The Site is situated in Crawley, within the county of West Sussex. As shown in Figure 1, the site is approximately 100m
north of the town centre and approximately 550m north of Crawley Station. The site is bound by the Town Hall to the
east, residential properties and Territorial Army offices to the northwest, commercial properties to the west and fronts
onto ‘The Boulevard’.
The main site access is via The Boulevard at the south edge of the site, there is also a public footpath adjacent to the
Town Hall which provides access into the site.
The site is rectangular in shape and covers an approximate area of 0.273 ha. The sites national grid reference is
TQ271368.
The site is currently a hardstanding concrete deck car park and serves the general public.
Figure 1.0 Site Location Outlined in Red
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2.2
Topography
3.0 Development Proposals
A full topographical survey has been carried out on site and is attached within Appendix A of this report. The resultant
survey shows site topography generally falling from the south to the north of the car park at an approx. gradient of 1:125.
As per Figure 2, this is consistent with the site’s current use as a drained hardstanding car park.
3.1 Site
Site Plan
The proposed development will include the construction of circa. 91 no. residential units and all associated infrastructure
works including car parking, landscaping and site drainage. A proposed plan has been included within Appendix B.
The existing junction with The Boulevard will be upgraded to provide improved access to the subject site.
The proposed development will decrease the effective impermeable area on site due to the incorporation of soft
landscaping and a reduction in hard landscaping.
Figure 2.0 Site Topography
2.3
Site Geology and Hydrogeology
A site specific ground investigation study has not been carried out on the subject site at the time of writing of this report.
However, the general geology of the area is outlined in the historic borehole logs carried out within the adjacent site to
the west. The borehole logs show layers of mottled clay overlying shale at depth. The general bedrock of the area is
formed of Upper Tunbridge sand-sandstone and siltstone.
A site specific site investigation will be carried out within the subject site, including groundwater monitoring, to confirm
the exact geology and hydrogeology of the subject site.
The site was not located within a source protection zone and is not within a major aquifer, as confirmed by the EA Flood
Maps.
Figure 3.0 Development Plan
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4.0 Surface Water Management Strategy
4.2 Existing Surface Water RunRun-off
The development site is a Brownfield site and is effectively 100% impermeable. Therefore, using the MicroDrainage
Design Software and site specific parameters the pre-development Peak Discharge Rate for the site is calculated as (see
Appendix D for extract):
4.1 Existing Site Drainage
The site is currently drained by a series of drainage channels and gullies which cater for surface water run-off from the
existing hardstanding areas. It is assumed that these outfall to a 225mmø Thames Water Surface Water Sewer (MH ref:
1850) running along The Boulevard, a copy of Thames Water Sewer Records are included within Appendix C.
Q1
33.3l/s
Q20
79.7l/s
Q30
86.6l/s
Q100
109.9l/s
As this is an existing car park, it is anticipated that no foul sewers are present on site.
4.3 Proposed Surface Water Strategy
A full CCTV Survey will be carried out on site to ascertain the extent of existing drainage running within the site and
highlight existing connection points into the public sewers.
As outlined in Section 4.2 of this report the existing peak run-off rates from the site are calculated for the predevelopment scenario.
For new developments current policy advocates that Sustainable Drainage systems (SUDS) are incorporated in order to
control surface water run-off from a site at source. SUDS promotes the use of infiltration systems to allow surface water
to drain back into the ground, thereby mimicking natural pre-development conditions. The Crawley “SFRA” outlines the
requirements for SUDS systems within new developments and also states that for new sites:
“The developer should seek to reduce the rate of run-off from the site to Greenfield runoff rates.”
As the development area of the site is small (less than 1ha) the Greenfield Run-off rate would be too small to practically
and safely implement on site and would increase the risk of blockages and flooding. In this regard, the proposed surface
water network will aim to reduce the peak run-off rates from the site to 50% of existing pre-development rates. This rate
has been accepted by Thames Water as part of a “Pre Development Enquiry” during feasibility stages, see Appendix E.
To achieve this, a SUDS strategy will be incorporated within the development to manage the surface water run-off
generated on site. As mentioned previously, surface water infiltration systems are promoted to reduce peak run-off rates
from a site. An examination of the site specific soil investigation report shows that the site geology is not favourable to
surface water infiltration due to the presence of clay.
The SUDS strategy for the site will, therefore, focus on surface water attenuation within the site. The surface water
attenuation will incorporate flow control measures which will restrict the allowable outflows from the site to allowable
rates.
Surface water attenuation systems can take many forms and include
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Ponds
retention basins
underground modular tanks
tanked permeable paving systems
swales and wetlands
Figure 3.0 Adjacent Public Sewers,
Sewers, site boundary shown in red.
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4.5 Exceedance Events
Proposed drainage networks will be designed not to flood any part of the site in a 1:100year return period design storm.
During extreme storm periods, surface water sewers may be inadequate and surcharge causing surface water to escape
from manholes. This will cause floodwaters to flow to low points with the site. These low points are proposed within the
car park areas away from the buildings and so present a low risk of flooding to occupants.
4.6 Maintenance Strategy for SUDS
The successful implementation and operation of a SuDS system depends on a robust and clear maintenance strategy
being implemented. In this regard the following steps and measures should form an integral part of the site’s proposed
scheme.
It is envisaged that the site drainage will be maintained by the site’s Management Company and will form part of the
maintenance regime for the site.
Figure 5.0
5.0 – Indicative Attenuation Tank (Source: Stormbloc)
Stormbloc)
Due to site constraints and the urban setting of the development site, the inclusion of ponds and detention basins is not
suitable for the subject site. For this reason, modular underground attenuation tanks will be incorporated to provide the
storage volume required. These will be positioned in public open areas as highlighted on the Proposed Drainage
Strategy contained in Appendix F.
Annual inspections of the proposed catchpit manholes and attenuation tanks will need to be carried out to ensure any
blockages are cleared and full storage volume is being provided.
Any sediment within the proposed oil/petrol interceptor will need to be cleared and removed to ensure the efficient
operation of the unit.
The proposed modular attenuation tank is sized to provide storage to the surface water network and to ensure that peak
run-off rates of the 1in20 year return are reduced by 50% when compared to pre-development rates (circa 39.9l/s) as
requested by Thames Water. The storage volume requirement is circa 50m3.
The proposed surface water attenuation tank is modelled using the MicroDrainage Design Software and shows a 50m3
storage requirement for the 1in100year storm event, this also includes a 30% allowance for climate change as required by
Crawley Borough Council. A Hydrobrake flow control device has been incorporated within the network to restrict the
surface water flow discharge rates. This will be contained in the final manhole before the outlet from the tank. Please
refer to Appendix F for the proposed drainage strategy and Appendix G of this report for model outputs.
The proposed network will discharge into the adjacent site knows as ’11 The boulevard’, the existing sewer on the
adjacent site connects to a 300mm ø sewer within ‘The Boulevard’ as shown within the proposed drainage strategy.
All proposed drainage networks will be designed to Adoptable Standards as outlined in “Sewers for Adoption – 7th
Edition”.
4.4 Pollution Control
The proposed Surface Water Network will include pollution control measures in accordance with measures stipulated
within “PPG3 – Use and Design of Oil Separators in Surface Water Drainage Systems”. These measures will include:
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Large trapped gullies within proposed car park
Catchpit manholes at required locations
Oil/Petrol by-pass separator before outfall to receiving Sewers
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5.0 Foul Water Strategy
6.0 Conclusions and Recommendations
5.1 Existing Foul Water Network
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Site is a Brownfield site with existing surface water drainage
As the site is an existing car park, it is assumed that there are no existing Foul Water Public Sewers serving the
development site.
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Existing surface water flows from the site outfall uncontrolled to the exiting Public Sewers
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Surface water infiltration not applicable within site due to clay subsoil.
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The proposed surface water network will incorporate a SUDS strategy which will control surface water run-off
from the site at source using a modular attenuation tanks
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The peak run-off rates from the site will be reduced to 50% of pre-development peak 1in20 year rates, as
confirmed by Thames Water.
5.2 Proposed Foul Water Network
The proposed development will include a new foul water network to serve the development. This network will run
through the adjacent site to the connection point shown on the Drainage Plan contained in Appendix E. The closest ideal
connection point to the site is located to the north of No. 11, The Boulevard. This is 225mm ø Thames Water sewer
(manhole ref: 0801) the records show that the Cover Level as 73.30m AOD and Invert Level as 70.34m AOD.
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SUDS Scheme will incorporate all relevant pollution control measures as required.
A pre-development enquiry was submitted to Thames Water and they have confirmed that there are no capacity issues
within the existing system. This means that the proposed development can connect to the Public Sewers without the
requirement for any attenuation or off-site works.
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A new foul water network is proposed for the development. Connection to the Public Sewer via. TW MH ref
0801.
All drainage networks will be constructed to “Sewers for Adoption – 7th Edition” standards.
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Thames Water has confirmed that there are no capacity issues following a pre-development enquiry.
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All drainage will be designed and constructed in accordance with “Sewers for Adoption”, Building Regulations
Part H and West Sussex County Council Requirements.
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