Appendix 8.1 Flood Risk Assessment - Peel

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Peel Environmental Ltd Flood Risk Assessment – South Clyde Energy Centre, Glasgow SEPA have confirmed 7 that they hold no information regarding the risk of flooding from the unnamed watercourse. Glasgow CC have also not raised any concerns regarding flooding from this watercourse. The catchment area of the unnamed watercourse has been investigated on the FEH CD v3 produced by the Centre for Ecology and Hydrology. The minimum catchment size on the FEH CD is 0.5 km 2 . The catchment area for this watercourse is not shown, inferring that it is less than 0.5 km 2 . 3.2.1 Risk of Flooding from Unnamed Watercourse In order for the site to be at risk of flooding from the unnamed watercourse, floodwaters would need to overtop the channel and the railway line. The most likely location for overtopping would be upstream of the 450mm culvert under the railway line (see Figure 6). The 450mm culvert is smaller than the 675mm culvert upstream of the open channel and may well restrict flows leaving the open channel. The culvert entrance may also be prone to blockage, especially given the amount of debris in the open channel upstream (see Figure 5). Should water from the open channel be unable to discharge through the culvert under to the site due to blockage, floodwaters would build up within the channel. The catchment of the unnamed watercourse is believed to be less than 0.5km 2 in area and highly urbanised. The urbanisation means that the catchment is likely to have a rapid response time. However, it also means that the catchment will probably drain mainly via the public sewer system. In the vicinity of the site, the public surface water sewer system includes a 1,200mm public surface water sewer running through the site, and the combined sewers under Bogmoor Road (Appendix E). One of these combined sewers measures 900mm x 1,350mm in size. The capacity of the surface water drainage system in the vicinity of the site is therefore much larger than the capacity of the culverts associated with the unnamed watercourse. Given this, the watercourse catchment is believed to drain primarily via the public sewer system, with only minor flows reaching the open channel near the site. Observations during the site visit support this conclusion. Despite the fact that it was raining during the site visit, water within the open channel was generally stagnant with no significant rates of flow observed. In addition, the open channel has a relatively large capacity in relation to the observed flow. It is therefore not considered that the site is at risk of flooding from the unnamed watercourse. In the unlikely event that the open channel filled to capacity (e.g. if the 450mm outflow culvert became completely blocked) water would need to overtop the railway line before reaching the site. LiDAR data has been obtained for the site and surrounding area. The cross sections in Figure 7 overleaf show that the depth of water within the open channel would need to exceed 1.5m before overtopping the railway line. Floodwaters would then spread out to cover the large area of low-lying land available in the vicinity of the site rather than building up to a depth sufficient to pose a risk to users of the site. 7 See footnote 2 ©Weetwood www.weetwood.net 8 1727 SCEC/FRA_Final v2.1 14 May 2012
Peel Environmental Ltd Flood Risk Assessment – South Clyde Energy Centre, Glasgow Recognising the above, it is not considered that the site is at significant risk of flooding from the unnamed watercourse. Open Channel Railway Line Site Open Channel Railway Line Site Cross Section 1 Cross Section 2 Open Channel Railway Line Site Cross Section 3 Figure 7: LiDAR Data 3.3 GROUNDWATER FLOODING Groundwater flooding generally occurs during intense, long-duration rainfall events, when infiltration of rainwater into the ground raises the level of the water table until it exceeds ground levels. It is most common in low-lying areas overlain by permeable soils and permeable geology, or in areas with a naturally high water table. Paragraph 21 of PAN69 states that ‘water tables are not rising generally in Scotland, but there can be localised problems in some areas’. SEPA’s “Local Plan Districts and Potentially Vulnerable Areas” report 8 states that there is potentially a “low to moderate” risk of groundwater flooding within part of the catchment area, but does not provide any details of potential problems in the vicinity of the site. 8 “Local Plan Districts and Potentially Vulnerable Areas – Flood Risk Management (Scotland) Act 2009 – 11 Clyde and Loch Lomond” SEPA, 2011 ©Weetwood www.weetwood.net 9 1727 SCEC/FRA_Final v2.1 14 May 2012
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Appendix 8.1 Flood Risk Assessment - Peel

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