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 ___________________________________________________________________________________________ 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: P:\Job Files 3900-3999\J-3999-FM AEE Flood Risk Assessments\J-3999.7-FM Cave Farm\Report V2 8
____________________________________________________________________________________________ J-3999.7-FM Flood Risk Assessment & SUDS Design, Cave Farm, Nr Pitney, Somerset V2 ___________________________________________________________________________________________ • 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. P:\Job Files 3900-3999\J-3999-FM AEE Flood Risk Assessments\J-3999.7-FM Cave Farm\Report V2 9
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Page 17: APPENDIX A SITE LOCATION PLAN
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