proposed solar park development at cave farm, nr pitney, somerset ...

PROPOSED SOLAR PARK
DEVELOPMENT AT CAVE FARM, NR
PITNEY, SOMERSET
FLOOD RISK ASSESSMENT
INCORPORATING SUSTAINABLE
URBAN DRAINAGE SYSTEM

PROPOSED SOLAR PARK
DEVELOPMENT AT CAVE FARM, NR
PITNEY, SOMERSET
FLOOD RISK ASSESSMENT
INCORPORATING SUSTAINABLE
DRAINAGE SYSTEM
For: AEE Renewables Plc. Job No: J-3999.7-FM
34 Brook Street Date: December 2012
London Edition: 02
W1K 5DN

PROPOSED SOLAR PARK DEVELOPMENT AT CAVE FARM, NR
PITNEY, SOMERSET
FLOOD RISK ASSESSMENT INCORPORATING SUSTAINABLE
DRAINAGE SYSTEM
Report No. Issue Detail Originator Date Checked By Date
02 AEE JCh 06/12/2012 JC 06/12/2012

CONTENTS
Item Content Page No.
1.0 Introduction 1
2.0 Site Location and Description 2
2.1 Site Location 2
2.2 Existing Usage 2
2.3 Proposed Usage 2
3.0 Existing Hydrology 3
4.0 Flooding Mechanisms 4
4.1 Groundwater Flooding 4
4.2 Overland Sheet Flow 4
4.3 Fluvial (River) Flooding 4
4.4 Tidal Flooding 5
4.5 Flooding as a Result of Development 5
5.0 Design Standards 6
5.1 The CIRIA SUDS Manual 6
5.2 Building Regulations Part H 6
5.3 The Wallingford Procedure 6
6.0 Proposed Sustainable Drainage Scheme (SUDS) 7
6.1 Overview 7
6.2 Percolation Testing 7
6.3 Surface Water Drainage 7
6.4 Maintenance Requirements 8
6.5 Summary 9
7.0 Affect on Adjacent Sites 10
8.0 Residual Risks After Development 11
9.0 Conclusions and Recommendations 12
APPENDICES
Appendix A
Appendix B
Appendix C
Appendix D
Site Location Plan
Proposed Site and Drainage Layout
Calculations
SFRA Extract

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1.0 INTRODUCTION
AEE Renewables Plc are currently investigating the development of Solar PV farm sites around
the UK. They are currently investigating the possibility of developing fields at Cave Farm, near
Taunton. The current proposal is to accommodate a photovoltaic solar park within 6 fields,
totalling around 11.5 hectares.
As the development site is over 1 hectare (ha) in area, the application for planning permission
must be accompanied by a Flood Risk Assessment (FRA). The EA indicative mapping shows
the site to be located within Flood Zone 1 (Low Risk), therefore the primary aim of the FRA is
to ensure the development does not increase flood risk to properties downstream of the site
through increasing the surface water runoff rate. The FRA will therefore make
recommendations in keeping with SUDS best practise principles, in order to mitigate any
potential issues created by the development.
In order to address this requirement AEE Renewables has commissioned H 2 OK Systems Ltd.
The objective of this appointment is to prepare a Flood Risk Assessment (FRA) and surface
water drainage strategy for the proposed development, in accordance with the best practice
principles of the Sustainable Drainage Scheme (SUDS) and the National Planning Policy
Framework (with Associated Technical Guidance). The report details the findings of this study.
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2.0 SITE LOCATION AND DESCRIPTION
2.1 Site Location
The proposed solar park development is to take place within 6no. fields associated with Cave
Farm near Pitney. The approximate Ordnance Survey Grid Reference for the site is ST 45312
28980. A site location plan is included in Appendix A.
The subject site covers an area of 11.5ha over 6 no. fields. The fields are located
approximately 600m northeast of the village of Pitney. The B3153 is found around 1.22Km to
the south, which connects with the town of Somerton around 3.25km east of the site. The site
is bordered to the north, south, east and west by agricultural land. The Stowey Road runs
adjacent to the north-east boundary and intersects with the Somerton field road, which runs
adjacent to the northern boundary.
Assessment of Ordinance Survey mapping shows that topographically the site falls from a
level of around 50m AOD at the north east boundary, to a level of 35m AOD near the south
east boundary. This gives a typical fall gradient of around 1 in 10 to 1 in 20 across the site.
2.2 Existing Usage
The site is currently used for agricultural purposes and consists of 6 arable fields.
2.3 Proposed Usage
It is proposed to install a solar PV farm consisting of approximately 20,000 modules which are
supported in racks which typically hold 44 or 22 modules. The 44 unit rack will be supported
by 12 piles, the 22 unit rack will be supported by 8 piles. The modules will be served by 4
inverter units and a sub-station as indicated in Appendix B. These will be accessed by
roadways constructed using permeable methods. Further landscaping works are also put
forward within the design proposal, such as hedging and planting arrangements.
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3.0 EXISTING HYDROLOGY
The development site is located high within the catchment of a tributary that serves the River
Parrett. According to FEH CD-ROM 3 the catchment area upstream of the site is less than
1km 2 . The tributary then joins the River Parrett close to the town of Langport, around 4.2km
southwest of the site. The River Parrett discharges into the Bristol Channel at Burnham On-
Sea around 25km north of the site.
The greenfield surface water runoff rate for the site has been assessed using the ICP SUDS
method and WinDes Microdrainage software. The rates for several return periods are
included in Appendix C. A 1 year return period event produces a runoff rate of 39.4 l/s. A 1
in 100 year event is expected to produce a runoff rate of around of 122.3 l/s, including the
effects of climate change (+20% to SAAR values) this rises to 48.8 l/s and 151.3 l/s
respectively. Assessment of the design rainfall for the site; for a 1 in 100 year event, applying
a 6 hour storm duration, gives a design rainfall depth of 66.7mm.
Analysis of the Environment Agency flood map shows the subject site is entirely located within
Flood Zone 1. The relevant extract from the Environment Agency indicative flood map is
shown in Figure 3.1 below.
Approximate Site Location
No areas indicated to
be flood zone 2 or 3
Figure 3.1
Environment Agency Indicative Flood Zone Map
and FEH Catchment Area
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4.0 FLOODING MECHANISMS
A number of possible flooding mechanisms have been considered at the site, and are
discussed below.
4.1 Groundwater Flooding
Investigation of ground water flooding has been undertaken within the South Somerset SFRA.
Springs showing the emergence of ground water at the surface have been identified and are
shown in Appendix D. Several springs are located north of the site near Bramwell, however,
no springs are located within the region of the site. A large area to the north of the River
Cary is low-lying (around 5m AOD) and requires significant land drainage in the form of
rhynes, and ditches etc. The site is located between 35m AOD and 50m AOD, which suggests
it should not suffer from similar ground water issues
The SFRA does report on suspected ground water flooding events around Langport, however,
this was recorded in 1954. Ground water as a source of flooding is particularly difficult to
identify. Areas are typically assessed with respect to their geological properties and historic
events , The SFRA continues with:
“The available records appear to indicate emerging groundwater (i.e. in excess of the norm)
in response to extreme rainfall. These appear to be localised and may in part relate to poor
“land” drainage. None of the recorded occurrences appear to be widespread and do not
therefore appear to be a reflection of regional rises in groundwater level.”
Given that the development is non-residential, the site topography is high/steep relative to the
surrounding area, and, that the solar panels are supported on frames which give limited
resistance to the flow of water, ground water flooding will not be considered further within
this report.
4.2 Overland Sheet Flow
The site is set in a rural environment with fields surrounding the entire site. Due to the sites
elevation relative to the surrounding topography, and the location of the nearby watercourses
which can intercept overland flows, the risk of any significant overland flows being generated
that can effect the site are considered to be small and will not be considered further within
this report.
Surface water runoff generated by the development of the site is considered further in section
6 of this report.
4.3 Fluvial (River) Flooding
The EA’s Indicative Flood Map illustrates that the entire site is located within Fluvial Flood
Zone 1. This implies the site has greater than a 1 in 1000 year chance of being affected by
fluvial flooding.
As discussed in Section 3.0, the closest watercourse/drainage ditch to the site has a very
small catchment of less than 1km 2 and will therefore, have limited capacity to generate
significant flows.
As such the fluvial flooding will not be considered further within this report.
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4.4 Tidal Flooding
The site is well outside of any areas of tidal influence, located at a minimum elevation of
around 35m AOD. Given the nature of the site, and surrounding topography, tidal flooding has
not been considered further within this report.
4.5 Flooding as a Result of Development
The proposed development will introduce impermeable area around the site to areas that
were previously permeable. This could have the potential to increase the runoff rates across
the site which could increase the flood risk to adjacent sites.
Following further inspection of the development layout (shown in Appendix B) it can be seen
that the proposed solar park infrastructure only introduces a small area of impermeable
surfaces through the foundations of the solar panel arrays, inverters and the central control
station. It is thought that rain falling on each solar panel table will runoff the panels and
flow/infiltrate in the sheltered rain shadow area underneath the down-slope modules.
All access and maintenance roads are proposed to be constructed from permeable methods
and therefore, will not contribute to increased runoff rates from the site. This impermeable
area totals 350m 2 , which is only around 0.3% of the total site area. Therefore, the drainage
system need only contend with the volume of runoff from this area to ensure flood risk is not
increased, the potential for a sustainable drainage system to be installed within the
development will be outlined further in Section 5.0 of this report.
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5.0 DESIGN STANDARDS
Design of the site drainage infrastructure and Sustainable Urban Drainage System (SUDS) is
to be carried out in line with best practice, and to industry standard design procedures. A
number of publications, including design guidance and best practice guidance will be applied
to different components of the final infrastructure. The sections below provide an overview of
the design standards to be used on this project for various aspects of the infrastructure
design.
5.1 The CIRIA SUDS Manual
This document is a comprehensive publication covering design, construction, operation and
maintenance of SUDS. The advice and best practice outlined in this document has been
utilised in the design of the site SUDS features, which have been detailed in this report.
5.2 Building Regulations Part H
Building Regulations Part H ‘Drainage and Waste Disposal’ covers the design and installation
of surface water and foul water systems. All private drainage including pipes, manholes, down
pipes, and other drainage infrastructure on the site should be designed and installed in
accordance with this document.
5.3 The Wallingford Procedure
Developed by HR Wallingford this publication covers the design of urban drainage systems. In
addition, the document includes regional rainfall data for use in design for varying return
period events.
Basic sizing calculations for the swales and the estimation of the runoff volumes have been
made using this method. Further modification of these calculations, to enhance the design,
was carried out using WinDes software which is also based in part on the Wallingford
Procedure
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6.0 PROPOSED SUSTAINABLE DRAINAGE SCHEME (SUDS)
6.1 Overview
SUDS is a concept that incorporates long term environmental and social factors in order to
design surface water drainage systems, in accordance with the ideals of sustainable
development. SUDS takes into account the quantity and quality of surface water runoff, and
the value of surface water to the urban and rural environments. Many existing urban drainage
systems can cause problems of flooding, pollution or damage to the environment, so it is the
aim of the SUDS to avoid this in the future.
Most proposed urbanisation creates impermeable surfaces which will need drainage solutions
to remove surface water runoff. Traditionally it is only quantity of flow that has been
accounted for in drainage solutions, preventing floods locally by conveying the water away
from the site swiftly in underground pipes. These traditional methods frequently alter natural
flow patterns which can lead to problems elsewhere in the catchment area. More recently,
water quality issues must be accounted for, in order to avoid pollutants from urban areas
being transported into rivers or groundwater.
Other aspects, such as water resources, community facilities, landscaping and provision of
wildlife habitats have been largely ignored; a well designed and well managed SUDS can offer
the following benefits:
• management of runoff flow rates, reducing the environmental impact of urbanisation
• maintenance or enhancement of water quality
• consideration to the requirements of the local community
• enhancement of biodiversity in urban watercourses
• maintain the natural groundwater level
6.2 Percolation Testing
In view of the relatively small areas of impermeable surface being introduced across the site,
there will be a negligible impact in the runoff rates, resulting from the development.
Therefore percolation testing has not been conducted.
In addition, during the design of the SUDS across the site, it has been assumed that the
primary function of the SUDS will be the interception, storage and redistribution of the runoff
from the site with infiltration into the system a secondary benefit.
6.3 Surface Water Drainage
Environmental Considerations
The nature of the development means that runoff could originate from the solar panels
arrays, solar panel foundations, inverters and sub-station hut. The runoff from the panels
poses a low environmental risk. It has been assumed that any additional foul/industrial waste
from the maintenance and operation of the park will be disposed of elsewhere.
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SUDS Design
The impermeable areas across the site are small, therefore no formal drainage is required. As
such a pragmatic approach has been taken to promote infiltration and create storage across
the site. This involves the installation of swale and scrape features running parallel to the site
contours within downstream areas of the site. These features will intercept flows, create
storage, attenuate runoff and promote infiltration across the site.
It is thought that rain falling on each solar panel table will runoff the panels and flow/infiltrate
in the sheltered rain shadow area underneath the down-slope modules. However where the
panels are aligned at variance to the contours then intensification of the runoff into rivulets or
small channels could occur beneath the lower end of the panels. Conceivably this could result
in a slight increase in the amount of runoff when compared to the undeveloped condition. The
swales and scrapes discussed below have been put in place to deal with this effect.
In addition the SUDS design will not consider the runoff from the access and maintenance
roads as these will be constructed of unbound crushed stones / gravel or similar permeable
materials, which will allow infiltration of water on these areas. The access roads will therefore
not increase surface water runoff rates from the site.
Over the whole site there are approximately 384 full size solar tables with ‘potentially’ 12
foundation posts each. Based on 0.1m diameter support piles, this equates to an impermeable
area of 36m 2 . The 56 half size tables with 8 posts each will create a further 4m 2 . Including
the 4 new inverter stations with a plan area of 50m 2 each and the substation of 50 m 2 , the
total impermeable area is around 290m 2 . Applying a 20% increase for inconsistencies gives
350m 2 . Therefore, the volume of runoff from the total impermeable area would be 28m 3 for a
6 hour duration 1 in 100 year rainfall event.
In order to adopt a pragmatic approach and promote infiltration across the site a swale
system is proposed to deal with any runoff. It is therefore proposed to install swales of 0.15m
base width and 0.15m depth with side slopes of 1 in 4 at the down slope edges of the site.
The total length of these swales is around 1,300m providing a storage volume of 130m 3 . The
layout of these swales is shown in more detail in Appendix B. In order to ensure interception
of flows an oversized system has been implemented which will serve to reduce the runoff rate
to less than the pre developed rates. Thus reducing the potential flood risk created by the
site. Furthermore check dams will be installed at set intervals along the swales to reduce flow
rates and ensure storage capacity is maintained over steeper gradients.
It is proposed to install scrape features at specific locations on the site to limit the erosion risk
posed by the intensification of rainwater as a result of the solar arrays, in line with advice
given by the Environment Agency. The scrapes will also improve storage and infiltration
across the site. The indicative layout in shown in Appendix B
Provided the swale structures outlined above are installed prior to other on site construction
works it is likely that the runoff from the construction phase could be suitably managed. The
swales will also serve to improve the water quality of runoff discharged from the site
For more detail on calculations of runoff refer to Appendix C.
6.4 Maintenance Requirements
Maintenance of the drainage network is essential to ensure optimal performance of the
drainage elements. As such maintenance requirements for the drainage system will include
but not be limited to:
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• Inspection and cleaning of the swales to ensure that the capacity and infiltration rates
are maintained.
The drainage system will remain in private ownership; the site operator would therefore be
responsible for the maintenance of the drainage features within the site. The developer of the
site should make this responsibility clear to the site operator by providing a maintenance plan
for the development.
6.5 Summary
Percolation testing has not been conducted at the proposed development site, due to the
small area of impermeable surface introduced by the development. It is anticipated that there
will be only a slight increase in runoff from the development which can be adequately
managed on site by the swales outlined in Section 6.3.
The conceptual design of a surface water drainage scheme has been carried out based on the
current proposed development layout. This has demonstrated that the surface water runoff
from the development can be managed entirely on site so as not to unduly affect the flood
risk to neighbouring areas. Furthermore the proposed system serves to reduce runoff rates to
less than the undeveloped rate.
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7.0 AFFECT ON ADJACENT SITES
As outlined in Section 6.0 the extent of impermeable areas introduced across the site by the
development is extremely small. Therefore any additional runoff from the impermeable areas
will be small and more than adequately managed by the swale system outlined in Section
6.0. As such there will be no impact on the nearby watercourses and neighbouring sites as a
result of the proposed development. In addition, the pragmatic approach to the design of the
swales will provide an improved storage and interception capacity and will reduce any risks to
adjacent sites from runoff, when compared to the pre-development situation.
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8.0 RESIDUAL RISKS AFTER DEVELOPMENT
The capacity of the swales has been oversized to manage the small additional runoff and to
promote infiltration across the site. As such the swales have ample capacity to contend with
runoff from the impermeable areas of the site under rainfall events in excess of the 1 in 100
year storm. Despite the relatively large capacity of the swales there is still a risk that shallow
surface water pooling could occur across the site in events in excess of the 1 in 100 year
storm.
The drainage system proposed has been specifically designed for the volume of surface runoff
resulting from the solar panel foundations and inverter stations. Therefore, any unauthorised
future connections into the site drainage network will potentially overload the system.
Therefore any future development on the site, beyond the current proposal, should be
suitably planned and considered, especially if new impermeable areas are to be introduced to
the site.
As long as these factors are considered in the design of the development there is the capacity
to manage any associated residual risks.
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9.0 CONCLUSIONS AND RECOMMENDATIONS
The study has investigated the mechanisms for flooding at the proposed Cave Farm Solar
Park site and has identified that the development is not at any direct risk of flooding.
This study has investigated the impact that the development will have on runoff rates from
the site. It has been shown that the impermeable area introduced across the site is very small
relative to the size of the site and as such will have limited impact on the runoff rates from
the site. A swale system has been proposed to allow the interception and infiltration of the
flows from these areas. A pragmatic approach has been used to design the swale system to
promote infiltration across the site.
If the recommendations made in this report are followed during the development of the site,
there will be a reduction in the discharge rate to any adjacent watercourse, and no extra risk
of flooding to properties located in the area.
On the basis of this report and by reference to EA flood maps, it is concluded that the site is
located within Flood Zone 1 and is therefore not at any direct risk of flooding. The National
Planning Policy Frame work and associated Technical Guidance therefore states ‘All uses of
land are appropriate in this zone’ from a flooding perspective. Consequently, with regard to
flood risk, the proposed development is entirely appropriate in this area.
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APPENDIX A
SITE LOCATION PLAN

APPENDIX B
PROPOSED SITE AND
DRAINAGE LAYOUT

Sewage Treatment
Industrial Effluent Treatment
AD/Biogas
Flood Risk Management/SUDS
Infrastructure
Service and Maintenance

APPENDIX C
CALCULATIONS

Volume of Runoff Caculations
Client: AEE
Engineer: H2OK Consultants
Cave Farm, Nr Pitney,
Location:
Somerset
Site:
Date:
Cave Farm
06/12/2012
SITE INFORMATION
Pre-development total area
Pre-development permeable area
Pre-development impermeable area
Post-development total area
Post-development permeable area
Post-development impermeable area
RAINFALL EVENT INFORMATION
Return period
Whole area greenfield runoff rate inclusive of 20% increase due to climate change (see attached MicroDrainage
calcs (SAAR values multiplied by 1.2))
IoH124 post-development runoff rate inclusive of 20% increase due to climate change (see attached
MicroDrainage calcs (SAAR values multiplied by 1.2))
Duration of rainfall event
Depth of Rainfall (calculated using Wallingford Procedure including 20% increase for climate change)
115000 m²
115000 m²
0 m²
115000 m²
114650 m²
350 m²
100 year
151.3 l/s
151.3 l/s
6 hours
80.04 mm
RUNOFF CALCULATION
Pre-development permeable area runoff
3268080 litres
Pre-development impermeable area runoff
0 litres
Total pre-development runoff
3268.1 m³
Post-development permeable area runoff
3268080 litres
Post-development impermeable area runoff
28014 litres
Total post-development runoff (without mitigation)
3296.1 m³
Difference in runoff 28 m³

H2OK Systems Limited Page 1
Nanjerrick Court
Allet, Truro
TR4 9DJ
Date 03/12/2012 11:28 Designed by jchapman
File
Checked by
Micro Drainage
Source Control W.12.6.1
ICP SUDS Mean Annual Flood
Input
Return Period (years) 100 Soil 0.450
Area (ha) 11.500 Urban 0.000
SAAR (mm) 840 Region Number Region 8
Results
l/s
QBAR Rural 62.5
QBAR Urban 62.5
Q100 years 151.3
Q1 year 48.8
Q30 years 119.2
Q100 years 151.3
©1982-2011 Micro Drainage Ltd

H2OK Systems Limited Page 1
Nanjerrick Court
Allet, Truro
TR4 9DJ
Date 03/12/2012 11:28 Designed by jchapman
File
Checked by
Micro Drainage
Source Control W.12.6.1
ICP SUDS Mean Annual Flood
Input
Return Period (years) 100 Soil 0.450
Area (ha) 11.500 Urban 0.000
SAAR (mm) 700 Region Number Region 8
Results
l/s
QBAR Rural 50.5
QBAR Urban 50.5
Q100 years 122.3
Q1 year 39.4
Q30 years 96.3
Q100 years 122.3
©1982-2011 Micro Drainage Ltd

APPENDIX D
CALCULATIONS

13 July 2012 10:39:13