Water Cycle Study - East Devon District Council

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Site Ref. Ref EWCS April 2010 E5 E6 E7 E8 E9 E10 E11 E12 Underlying geology Source protection zones (SPZ) Discussion of assessment criteria and results Exeter and East Devon Water Cycle Study Outfall availability Urbanised area Overall assessment Permeablility of underlying geology 4.5.13 The underlying geology is of great importance to the management of surface water because it determines the viability of infiltration techniques, which are viewed as the primary means of surface water disposal (source control). The presence of underlying geology of low permeability will not rule out the use of SuDS techniques, but will reduce the options for disposal at source. It is also important to note that some types of highly permeable underlying geology, such as chalk, may be at risk of dissolving if surface water is encouraged to soak away through the rock. It is recommended that a thorough ground investigation is commissioned by the developer to determine the viability of any proposed SuDS schemes. 4.5.14 The geology underlying each of the identified development areas was determined from British Geological Society maps (Sheets 325, 326, and 339). The geological classification assigned to each site is recorded in the table in Appendix B. 4.5.15 The majority of the development areas were assessed as notable, because the large size of the sites results in variation in the underlying geology - and therefore the permeability - across a single site. 4.5.16 The development areas that were assessed as critical (i.e. poor potential for drainage by infiltration) were those which overly geology that primarily consists of materials that are either impermeable or have such slow rates of infiltration that they are considered practically impermeable. 4.5.17 In locations where underlying chalk is located (notably Seaton, Axminster and Beer) caution must be exercised when considering infiltration techniques, as introducing concentrated volumes of surface water into chalk formations can result in dissolution (dissolving of the rock) and destabilisation of ground conditions. Groundwater source protection zones (SPZ) 4.5.18 The Environment Agency has established Source Protection Zones (SPZ) to protect drinking water abstraction boreholes. The zones are described in Table 4.6 (extracted from the 36
Exeter and East Devon Water Cycle Study Environment Agency web site) together with an explanation as to the constraints posed by each zone on developments; 4.5.19 Surface water drainage by infiltration to ground could potentially contaminate the groundwater and therefore a thorough understanding of the underlying hydrogeology would need to be demonstrated by a developer proposing infiltration techniques to confirm that their proposals would not lead to contamination of ground water. 4.5.20 Ottery St Mary and Newton Poppleford are both located within source protection zones and as a result any discharge of surface water to ground will need to be discussed with the Environment Agency at an early stage of development. Both Budleigh Salterton and Beer have all three zones present but on a much smaller scale affecting less of the area that may be developed in the future. Sidmouth and Honiton are shown as not being affected by SPZ’s although it is possible that extensive development could extend into SPZ3. 37 Ref EWCS April 2010
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Water Cycle Study - East Devon District Council

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