Flood Risk Assessment, Near Horsham, West Sussex.
Feb 8, 2014

RAB Consultants has completed a flood risk assessment (FRA) and outline drainage strategy for a proposed large residential development at the site of an unused horticultural nursery.

Our assessment to meet the requirements of the National Planning Policy Framework, involved the following:

  • A site visit to assess flood risk at the site.
  • Assessment of the risk of flooding from all sources including surface water, groundwater and drainage related.
  • Desk based assessment of suitable mitigation measures to offset any risk of flooding.
  • Assessment of the local drainage infrastructure.
  • Liaison with Southern Water.
  • Defining the developments surface water management requirements.
  • Calculation of the predevelopment surface water runoff rates and volumes.
  • Outline surface water drainage strategy

The site is thought to have a moderate risk of surface water flooding.  Despite this we were able to demonstrate an effective mitigation strategy that will ensure the occupants remain safe from all sources of flooding for the lifetime of the proposed development.

As there are no public surface water sewers adjacent to the site and the local ground is composed predominantly of clay, we looked at surface water management measures that discharged directly to a watercourse at the existing greenfield runoff rate without any soaking into the ground (infiltration).  The proposed drainage strategy if will ensure that future surface water flood risk both at the site and in the surrounding area does not increase.

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