Water Cycle Study - East Devon District Council

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4.4 Impact of development on flood risk Ref EWCS April 2010 Exeter and East Devon Water Cycle Study Loss of existing floodplain due to encroaching development 4.4.1 The majority of the indentified growth areas and directions are partially or wholly located in the low risk flood zone. However, most areas are also in close proximity to areas of greater flood risk and often border flood zones 2 and 3. In these circumstances development must avoid future encroachment into the floodplain by establishing a distinct and significant buffer zone between the edge of development and zones of higher risk. The scale and extent of buffers should be determined in consultation with the Environment Agency, relative to the topography and elevation of each site. Flood buffer zones present the opportunity for multi-functional Green Infrastructure that may be used for biodiversity enhancement and recreational uses. Surface water run-off from new development 4.4.2 Future development in Flood Zone 1 should realise opportunities to reduce the overall level of flood risk in the area and beyond through the layout and form of the development. All new development must ensure that flood risk is not increased elsewhere by achieving greenfield runoff rates for surface water. 4.4.3 The proposed Cranbrook and Science Park developments already have drainage strategies in place that indicate that greenfield run-off rates can be achieved. Additional flows from new development into existing sewer networks, exceeding their capacity 4.4.4 Provided that all new development is served by separated sewer systems1 and that South West Water have adequate notice of development in order to design and implement new or improved foul sewers of adequate capacity, then the risk of foul sewer flooding from new development should be negligible. The risk of sewer flooding occurring is slightly greater within Exeter’s existing urban area due to the in-combination affect of infil developments utilising existing combined sewer systems that have limited residual capacity. Local authorities should Supporting the South West Water ‘Sewers for Sewage’ initiative to reduce foul sewer flooding due to surcharging by surface water and blockage by grease, oils and waste. Increased discharge from WwTW 4.4.5 Increased discharges from WwTW due to development may adversely affect flood risk downstream of the works. PPS25 requires that there is no increase in flood risk due to development. Mitigation measures may be required where either (a) there is a quantifiable increase in the frequency of spill from storm storage tanks due to additional foul flows, or (b) the receiving watercourse and associated flood risk area is particularly sensitive to changes in flows. 4.4.6 Consultation with South West Water has not identified any locations within the study area where there is a known problem associated with discharge from a WwTW increasing flood risk in the receiving waters. Typically, the discharge consent determination process assesses downstream flood risk as part of the determination process. We have assumed that the existing discharge 1 A separate system comprises a foul system which conveys wastewater or foul drainage only to the wastewater treatment works, and a separate surface water system that collects roof and highway runoff and discharges the clean runoff into rivers and coastal waters. 30
Exeter and East Devon Water Cycle Study consent granted for each WwTW included an assessment of flood risk impact and that the outcome was considered to be acceptable. Some of the older consents may not have been assessed for Flood Risk Impact at the time of issue but will have been considered as part of the more recent Catchment Flood Management Plans (CFMP). 4.4.7 In granting any future consents, the Environment Agency will review the flood risk impact to ensure that there is no adverse affect. Applications for future flow increases at WwTW are assessed for potential impact upon flood risk, and where there is considered to be a potential problem, compensatory measures will be introduced. 4.4.8 South West Water should consult with the Environment Agency to identify an appropriate policy for identifying locations where mitigation measures would be required and agree suitable methods for mitigation where required. For example, one option for mitigating the increase in flows could be to provide additional storage volume in the floodplain or in any flood attenuation facilities in the vicinity of the WwTW to compensate for the increase in flows. 31 Ref EWCS April 2010
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Water Cycle Study - East Devon District Council

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