ALPHA HOUSE
WASSAGE WAY
HAMPTON LOVETT
DROITWICH SPA
WATER MANAGEMENT STATEMENT
PREECE CONSULTANTS LIMITED
AUGUST 2011
AIR LIQUIDE (HOMECARE) LTD
ADDITIONAL PARKING FACILITIES
ALPA HOUSE
HAMPTON LOVETT
PREECE CONSULTANTS LTD
ODDFELLOWS HALL
BRACKLEY ROAD
TOWCESTER
NORTHAMPTONSHIRE
NN12 6DH
Tel: 01327 358339
Fax: 01327 358345
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CONTENTS
Page No
1.0
Introduction
3-4
Site Location Plan
5
2.0
Existing Site Description
6
3.0
Proposed Development
7
4.0
Flood Zone Classification
7
Indicative Flood Plain Map
9
5.0
Sources of Flood Risk
10-11
6.0
Effects of Development on Flood Risk
11
7.0
Mitigation Measures
12-13
8.0
Conclusions and Recommendations
14
Appendices
Appendix A
Appendix B
Appendix C
Proposed Planning Layout
Catchment Greenfield Run-off Rates
Storm Water Storage Calculations
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1.0 INTRODUCTION.
This Water Management Statement has been based upon the requirements set out in
Planning Policy Statement 25 Paragraph E9 – Development and Flood Risk (PPS25)
published by Communities and Local Government 2006. The Water Management
Statement has been prepared on behalf of Air Liquide (Homecare) Ltd in support of a
proposed extension to existing parking facilities at Alpha House off Wassage Way,
Hampton Lovett, Droitwich. This assessment also gives due consideration to the
guidance provided in CIRIA publication Interim Code of Practice for Sustainable
Drainage Systems 2004 (ICPSDS) and compliant with Wychavon District Council
Water Management Supplementary Planning Document.
An assessment of the flood risk to the proposed development has been considered
on the basis of the best information available at the date of this report. The
calculation and modeling techniques herein are deemed appropriate to the
requirements of Wychavon District Council Water Management Advisory Notes, the
scale and nature of the development site, and the available data. The key elements of
this assessment are based on the following
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
Consultation with the relevant bodies
Review of surrounding topography based upon Ordnance Survey (OS)
Explorer series mapping
Review of proposed Parking Layout
Identification and consideration of data corresponding to the design flood
events
Consideration of climate change
Calculation of the impact of the development on surface water run-off
Recommendations on attenuation measures
PPS25 states that a Strategic Flood Risk Assessment should be carried out by the
local planning authority to inform the preparation of Local Development Documents,
having regard to catchment wide flooding issues which affect the area. The Strategic
Assessment will provide the information needed to apply the sequential approach.
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The application site lies within the north fringes of Hampton Lovett within the buildup industrial area of Wassage Industrial Estate, lying to the north of the town of
Droitwich Spa Worcestershire. The Industrial Estate lies to the west of the A442
major highway route of Kidderminster Road.
The planning application seeks to obtain planning consent to construct 12 additional
parking spaces to enhance the existing 18 parking spaces at Alpha House, consisting
of some 300 square meters of additional parking area. The overall land encompassing
Alpha House is approximately 0.16 Hectares.
This report concentrates solely on the proposed extension to the existing car park.
Air Liquide Ltd has commissioned Preece Consultants Ltd to prepare a compliant
Water Management Statement in support of the Full Planning Application to
Droitwich District Council Planning Department.
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SITE LOCATION PLAN
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2.0 EXISTING SITE DESCRIPTION.
The application site is located to the west of Kidderminster Road (A442) north of
Droitwich Spa within the northern Industrial suburb of Hampton Lovett. Hampton
Lovett old village lies off to the east of the site the other side of the A442. The
application site is with in the northern fringes of the Wassage Industrial Estate and
sides onto Doverdale Lane. To the north of Doverdale Lane lies open Agricultural
Land. To the south of the industrial estate lies Wassage Coppice woodland area. The
application site occupies a total of 0.16 Hectares with a National Grid Reference of
approximately 388518,265430. The land currently has a pitched roof building with a
footprint of approximately 240 sq.m together with existing parking facilities covering
approximately 445 sq.m. The remaining land is presently established as grassed
areas.
The site is predominantly open on all sides with no physical boundaries. There are
included with in the site area some semi-mature trees. There is no evidence of any
existing ditch lines/storm water routes within or adjacent to the site.
The closest main river is the Elmbridge Brook lying some 500m off to the south-east.
A tributary off the Elmbridge Brook runs back up into the Wassage Coppice located
some 300m off to the south of the site and this is the natural watercourse which
receives the present green field run-off from the application site.
Existing storm and foul sewers are located within the adjoining highway network
layout of Wassage North, Wassage Way and Hampton Park. Severn Trent Water has
been contacted to establish the status of these sewer networks.
An initial investigation undertaken identifies the soils in the vicinity as comprising of
Red Boulder Clays. Wychavon District Council Building Control Section (Malvern)
have been contacted to advise on the likelihood of water infiltration into the
surrounding sub-soils in the area. They consider it is highly unlikely that soakaways
would be suitable for the disposal of surface water drainage.
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3.0 PROPOSED DEVELOPMENT.
The current proposal is for an extension to the existing car parking facilities at Alpha
House to include 12 additional spaces. There are currently 18 car parking spaces, 4
existing spaces of which will be reoriented.
The layout arrangements will allow for 300 sq.m of additional impermeable area
within the existing 0.10 Hectares of grassed area. The proposed development layout
is contained in Appendix A.
The vehicle site access will remain as it presently does unaltered off the turning head
to Hampton Park being served off Wassage Way.
4.0 FLOOD ZONE CLASSIFICATION.
Following brief discussions with Andrew Fell in Wychavon District Council’s Drainage
Engineers the specific requirements for the Water Management Statement were
agreed for the application site, together with the management of the future surface
water from the development. The Environment Agency is responsible for the
provision of information pertaining to flood risk from tidal and main rivers
throughout England and Wales. The Agency provides an information service through
Indicative Flood Plain Maps data. This data is not intended to provide detailed flood
information for individual properties, but provide a useful resource at scoping stage.
Indicative flood mapping identifies the extent of present flood zones. This site has
been identified and confirmed with the Agency as located within Zone 1, comprising
of land assessed as having less than 1 in 1000 annual probability of river or sea
flooding (<0.1%) in accordance with Planning Policy Statement 25. Flood Plains Zones
2 & 3 are located approximately 500m south-east of the development site associated
with the Elmbridge Brook.
General Industrial and Non-Residential developments are classified as being “Less
Vulnerable” in accordance with Table D2 of PPS25 and the proposed land use would
be considered to be appropriate development within Zone 1. As the site is less than
1.0 Hectare in area, the effect of the proposed car park extension on surface water
run-off is the primary consideration with regard to flood risk.
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The topography information from OS Mapping has established that the lowest part
of the development site is located at approximately 42.0m AOD. From the
information gained within the O.S Explorer Maps and Flood Plain Maps, the highest
adjacent Elmbridge Brook 1 in 100 flood plain level is sighted at approximately 35.0m
AOD, some 7m below the lowest site levels.
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INDICATIVE FLOOD PLAIN MAP
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5.0 SOURCES OF FLOOD RISK.
To understand the risk of flooding to a site, it is imperative that potential sources of
flooding be clearly defined. The likelihood and severity of flooding depends on the
characteristics of the flood sources and the degree to which the site is currently, or
can potentially, be protected against flooding from these sources. PPS25 identifies
flood source items as Rivers, Sea, Overland Flow, Groundwater, Sewers, Reservoirs
and Canals.
Risk of Flooding for the Proposed Development.
5.1 Fluvial Flooding – source Elmbridge Brook and its Tributaries.
The Environment Agency Flood Plain Maps indicate the flood risk posed to the site
from the Elmbridge Brook. The application site is confirmed as falling within Zone 1,
having less than 1 in 1000 annual probability of river or sea flooding (<0.1%). The site
can be considered not to be at significant risk of fluvial flooding from the river given
the level difference from the site any the present river levels.
5.2 Pluvial Flooding – source rainwater shedding onto site
Doverdale Road to the north has a cambered carriageway with associated highway
gullies and storm drainage. Any pluvial flooding within this highway will result in
excess storm waters gravitating along Doverdale Lane in an easterly direction
towards the Kidderminster Road. Similarly Wassage North carriageway will take
excessive storm water within existing channels away from the development site. The
remaining highway network of the Wassage Industrial Estate predominantly falls
away from the application site.
5.2 Existing Public Storm Sewers
Existing storm sewers are located within Wassage Way. The main sewer manhole
covers will not pose a potential flood risk as they are located below the development
site and outfall towards the southern part of the industrial estate.
The installation of the proposed storm sewers to serve the proposed parking area
may be considered as a potential source of flood risk. This assessment of these
development sewers are considered under paragraph ‘Development Drainage
Strategy’ in this document.
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5.3 Ground Waters
With soil characteristics given by Wychavon District Council Building Control, the
elevation of the site at the higher end of the industrial area and the extent of the
proposed works to only include new parking facilities, any ground water levels
should not pose an apparent threat to the site. Intrusive site investigations will be
undertaken during construction to confirm any potential issues arising from high
water tables.
6.0 EFFECTS OF THE DEVELOPMENT ON GENERAL FLOOD RISK.
Due to the nature of the existing site, the proposed scheme will result in the
introduction of an additional impermeable area and consequently an increase in the
surface water run-off rate and an additional storm water volume. There will therefore
be a potential for flood flows from the development drainage system to have an
adverse effect on flood risk elsewhere in the surrounding storm catchment.
The proposed storm water from the developed site will be collected, stored and
discharged within the drainage infrastructure being constructed as part of a
development storm water mitigation scheme, implementing Sustainable Urban
Drainage Techniques, and therefore will not exacerbate any existing flooding issues
within the surrounding catchment area.
The proposed development will have no impact on existing flood flows or flood
plains in the catchment.
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7.0 MITIGATION MEASURES.
The development will seek to reduce the level of flood risk in the area through the
form of an appropriate storm water limitation run off system and the use of
sustainable urban drainage techniques by the use of underground storm water
storage crates. PPS 25 states that the surface water arising from a development site
should, as far as practical, be managed in a suitable manner to mimic the surface
water flows arising from the original site. As this scheme is of a small nature there
will be accommodated within the proposed drainage scheme the storm water
storage facilities to cater for up to the 1 in 100 year rainfall volumes plus a climate
changes allowance of 30%.
8.1 Development Drainage Arrangements.
The development project sets out proposals for an additional 300 sq.m of
impermeable parking area.
The desk study suggests that soil infiltration rates will be very poor through the
existing conditions of heavy clays and that soakaways are not considered to be a
viable option for the discharging of storm water.
The storm water run-off from the prospective parking area will be restricted to the
appropriate green field run-off rate from the associated element of the site and
attenuated within an underground storm water balancing facility. It is proposed for
the storage facility will take the form of specialised cellular block crates, encased in a
geotextile material aiding any soil infiltration that may exist in the sub-soils. The
outflow from the attenuation system will discharge into the existing storm water
drainage system on site and ultimately through to the sewer network within
Wassage Way. The necessary discharge consent(s) into the recipient main sewers will
be required from the sewerage undertakers at detailed designed stage prior to
commencement of construction.
8.2 Development Storm Water Run-off Rates and Volumes.
Preliminary calculations have been undertaken in order to provide an assessment of
the acceptable storm run-off rates for the drainage system from the future
developed site. The Institute of Hydrology Report 124 (IoH 124) has been used to
estimate the existing green field run-off rates from the equivalent net developable
catchment area of 0.03 Ha. This was calculated for various return periods.
The primary results are presented in the table below with the IoH 124 Mean Annual
Flood Hydrology Microdrainage printout contained in APPENDIX B.
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Return Period
1
30
100
Greenfield Run-off Rate
0.20 l/s
0.40 l/s
0.60 l/s
Reference is made to PPS25 (Annex F10) and ICPSDS (Item 6.2.8), advices that the
volumes and peak flow rates of surface water leaving a developed site are to be no
greater than the rates prior to the development. The practicalities in obtaining a
control device to restrict the future storm water flows from the attenuation facility
to these minimal green field figures proved extremely difficult. A simple orifice outlet
installation, allowing storm water control to mimic the above minimal green field
rates would be unacceptably small and create maintenance and flooding problems,
resulting in precisely what this exercise and the installation proposals aim at
preventing. The preferred option is to install a specialised Hydrobrake Unit to provide
a much abler maintenance installation, which would greatly reduce the risk of failure
and flooding to control the storm water outlet. However, this type of installation
would hold a minimum operating flow of 1 lit/sec.
WinDes software has established the storm water balancing volumes required for up
to the 1 in 100 year return period inclusive of a 30% allowance for future climate
change. This climate change allowance provides for up to year 2115, giving the lifespan of the development scheme a 104 year expectancy. The hydraulic storage
calculations for the maximum 1 in 100 year return periods are contained in Appendix
C.
The main storm water drainage to serve the development site will serve a single
occupancy and remain in private ownership. The system will be designed in
accordance with Building Regulations Part H.
9. CONCLUSIONS & RECOMMENDATIONS.
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Based upon the available information, there is not considered to be a significant risk
from fluvial or pluvial storm water run-off from the surrounding roads or land, which
will affect the proposals.
The existing watercourses in the area, lying to the south and east, have proven to be
at a much lower level than the lowest part of the site with indicative flood plains
some 7m below site levels.
The development will generate an impermeable area on the site, resulting in an
increased storm water run-off, which will require mitigation. The mitigation system
will ensure that the development’s storm water flows and outfall volumes will be
limited closely to existing green field rates.
Storm water run-off from the proposed car parking extension will require
attenuation following the reduction of outflow prior to any discharge to the existing
sewers, watercourse(s) and ultimately the Elmbridge Brook. The balanced volumes
will be placed in specialist on-line storm water modular units (crates) located beneath
the proposed car park. Appropriate storm water discharge consents will be required
from the sewerage undertaker for the development storm water outfall connections.
Soakaway testing to BRE 365 may need to be undertaken to confirm the Desk Top
Statement on the exclusion of soakaways as disposal for storm waters.
In accordance with PPS25 Development & Flood Risk, subject to the mitigation
measures recommended, the proposed development is not considered to be at risk
from flooding and will not have an adverse impact on flood risk elsewhere.
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APPENDIX A
Proposed Layout
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APPENDIX B
Catchment Storm Water Greenfield Run-off Rates
established through
Institute of Hydrology Report 124
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APPENDIX C
Storm Water Storage Calculations
1 in 100 year (+30%)
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