Water Cycle Study - East Devon District Council

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Ref EWCS April 2010 Exeter and East Devon Water Cycle Study Table 4.6 – Key impacts of source protection zones on surface water management Groundwater source protection zone description Zone 1 (Inner protection zone) Any pollution that can travel to the borehole within 50 days from any point within the zone is classified as being inside zone 1. This applies at and below the water table. This zone also has a minimum 50 metre protection radius around the borehole. These criteria are designed to protect against the transmission of toxic chemicals and water-borne disease. Zone 2 (Outer protection zone) The outer zone covers pollution that takes up to 400 days to travel to the borehole, or 25% of the total catchment area – whichever area is the biggest. This travel time is the minimum amount of time that the EA thinks pollutants need to be diluted, reduced in strength or delayed by the time they reach the borehole. Zone 3 (Total catchment) The total catchment is the total area needed to support removal of water from the borehole, and to support any discharge from the borehole. Key impacts on surface water management The EA will object to: • any new trade effluent, storm sewage or other significantly contaminated discharges to ground. • the use of deep soakaways (including boreholes or other structures that bypass the soil layers) for surface water disposal unless the developer can show that there is no viable alternative, that there is no direct discharge of pollutants to groundwater, that risk assessment demonstrates an acceptable risk to groundwater and that pollution control measures are in place. The discharge of clean roof water to ground is acceptable provided that all roof water down-pipes are sealed against pollutants entering the system from surface run-off, effluent disposal or other forms of discharge. The discharge of clean roof water to ground is acceptable provided that all roof water down-pipes are sealed against pollutants entering the system from surface run-off, effluent disposal or other forms of discharge. This must not create new pathways for pollutants to groundwater. The EA will object to the use of deep soakaways (including boreholes or other structures that bypass the soil layers) for surface water disposal unless the developer can show: • there is no viable alternative, and • that there is no direct discharge of pollutants to groundwater, and • that risk assessment demonstrates an acceptable risk to groundwater and • that pollution control measures are in place. Outfall availability 4.5.21 If discharge to ground is not feasible for surface water management, there will be a need to identify a suitable point of discharge to either a watercourse or a South West Water adopted sewer. South West Water have stated that the connection of a surface water discharge to a public foul sewer will not be permitted under any circumstances, in line with the company’s initiative ‘Sewers for Sewage’ which seeks to combat sewer flooding through reducing surcharging by surface water and blockage by debris. 38
Exeter and East Devon Water Cycle Study 4.5.22 This assessment has identified the nearest available points of discharge. Whether or not there is available capacity within the watercourse or sewer has not been assessed. 4.5.23 All of the identified growth areas have access to a sewer or watercourse within their site extents or less than 500m from their site boundary except for the Science Park site. Any new outfall for surface water from this development could potentially connect into the highways drainage if necessary along the old A30, but this would need to consider that that sewer is only likely to be designed to accommodate up to the 5yr return period storm. Urbanised areas 4.5.24 In highly urbanised areas the potential for surface SuDS – such as ponds and swales - is constrained by the lack of undeveloped land available. Furthermore the high percentage of existing impermeable surfaces in urbanised areas increases the risk of overland flows both towards and from new developments. 4.5.25 The majority of the identified development areas in the study area are located on undeveloped land on the periphery of existing urban areas. The only development area significantly affected by this criteria is C4 (Exeter City) due to the existing urban nature of the City restricting the potential for surface SuDS solutions such as ponds and swales. In urbanised areas such as this the focus on controlling surface water runoff should be at the prevention and source control level with green roofs, green walls, and rainwater harvesting, as well as more engineered solutions such as cellular storage tanks. Recommendations 4.5.26 The following actions are recommended in order to ensure that the surface water management of new developments is planned and implemented appropriately and in line with the draft Floods and Water Management Bill: • Adopt a policy of ‘No surface water connection to foul sewer’. • Prepare a SuDS policy and adoption guide to assist planners and developers. This should become a Supplementary Planning Document (SPD) and be closely linked to the Green Infrastructure Study recommendations and the emerging Green Infrastructure strategy. • When determining land allocations within the Local Development Framework, the land take required for SuDS should be estimated prior to applying the development densities to the land parcels. This will ensure sufficient land is set-aside to implement successful SuDS solutions, which will also perform as multi-functional green infrastructure. • Prepare Surface Water Management Plans for areas of widespread development i.e. Science Park, Skypark, Intermodal Freight Terminal, and Cranbrook to ensure a coherent approach is taken to surface water management and ensure delivery of strategic SuDS as opposed to multiple developments generating their own isolated approach to SuDS. • Work collaboratively with key stakeholders and the County Council to reflect their proposed new responsibilities for local flood risk management when generating policy documents for surface water management to ensure that the links necessary for partnership working are already established. • Ensure through the planning consent process that SuDS solutions are technically feasible - developers should demonstrate a thorough understanding of site conditions and provide evidence to substantiate that sustainable urban drainage solutions are deliverable. 39 Ref EWCS April 2010
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