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

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J-3999.7-FM Flood Risk Assessment & SUDS Design, Cave Farm, Nr Pitney, Somerset V2
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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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