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Critical Drainage Area (CDA) – May 2015

Catchment Drainage / Flooding Issues

Cornwall Council’s Local Flood Risk Management Strategy (Strategy) identifies St Austell as a Priority Community. A strategy covering St Austell, Par and will be undertaken to investigate opportunities to improve existing flood alleviation schemes. It will also research impacts of mine and mineral workings on flood risk as well as the link between land use practice and run-off.

This may show that in addition to controlling run-off from new development there is the need for infrastructure changes such as construction of new sewers. Detailed actions to address this will be developed in partnership through the Strategy. Therefore as well as following this guidance, applicants should refer to on the development of the Strategy.

The St Austell area spreads across the Sandy River, St Austell River and Tregrehan Stream catchments. The mining history means that through the town the watercourses are highly modified. Historic mining in the catchment has resulted in perched and diverted channels as well as disused adits. These watercourses are designated Main River and require ongoing maintenance. Flood Alleviation Schemes have been constructed in the Sandy Bottom, Holmbush, Par Moor and .

Flooding problems exist across the catchments associated with small constrained channels and culverts and natural floodplain areas which are obstructed and developed.

Continued ......

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Catchment Drainage / Flooding Issues (continued)

St Austell has been identified as having potential for significant growth. Many of the areas identified with potential for development drain through the existing urban area.

The flooding problems combined with the development pressures, particularly in the headwaters of the catchment. Therefore surface water runoff must be robustly controlled for all new development, in-fill development and redevelopment.

In this catchment SuDS design should include features to manage water quality to protect bathing water quality.

Minimum Drainage Standards Required

All new developments will have to play their part in reducing current rainfall runoff rates. This requirement also applies to brownfield sites that will have to match the same standards. The surface water drainage hierarchy should be followed by using infiltration as far as is practicable. Further guidance on such systems can be found in the CIRIA SuDS Manual and in Lead Local Flood Authority guidance.

All off-site surface water discharges from developments should mimic greenfield performance up to a maximum 1 in 10 year discharge rate. On site all surface water should be safely managed up to the 1 in 100 plus climate change conditions. This will require additional water storage areas to be created thereby contributing to a reduction in flooding downstream.

Additional Guidance – Mining Surveys

There are a number of historic underground mining features in this area. Therefore additional ground surveys may be necessary to ensure that infiltration and attenuation drainage avoid these features.

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