EnvGeoGeo delivered a Sustainable Urban Drainage (SUDs) Assessment suitable for submission to the local authority in support of a planning application for development of 41 apartments on land in central Preston, Lancashire.

SUDS control surface water runoff with the intention of replicating natural site drainage as a strategy to control and reduce the impacts of flood risk downstream under current and probable future climate scenarios. SuDS can also remove pollutants from urban runoff at source.

Options appraisal for SUDs is now expected for all new developments, and government policy regarding such development states an order of preference for surface water runoff management as follows:

1. Discharge to the ground.
2. Discharge to a surface water body.
3. Discharge to a surface water sewer or local highway drain.
4. Discharge to a combined sewer.

The EnvGeoGeo assessment included:

  • Assessment of potential for infiltration drainage.
  • LiDAR assessment of surface topography of the site and to within 50 m of the sites comparing average ground levels on the Site with ground levels in the surrounding area.
  • A review of surface water features within 250 m which may be suitable for receiving surface water drainage from the site.
  • Review of risk of flooding from rivers and the sea.
  • Review of risk of flooding from other surface waters.
  • Review of risk of flooding from groundwater.
  • Review of Environment Agency groundwater vulnerability maps and groundwater Source Protection Zones.
  • A preliminary assessment of ground conditions and how these may impact water quality of surface runoff and groundwater drainage.
  • Assessment of the range of SuDS options available for the site.

The preferred SUDs option of discharge to the ground has only a MODERATE suitability due to the classification of the underlying Principal Aquifer as having HIGH groundwater vulnerability. This option is also impacted by the possibility of ground contamination from Made Ground soils on site, and by the presence of a culverted water course beneath the site. These drawbacks may potentially be mitigated through discussion with the Environment Agency, site investigation and surface water runoff pretreatment as part of SUDs design. These conversations are ongoing.

No option is available for discharge to a surface water course or to a surface water sewer or highway drainage, as these do not exist within 50 m of the site boundary. The final option of discharge to combined sewer has a MODERATE suitability for the site and is recommended should discharge to ground not be possible.

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