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Flood Risk Assessment

  • Flooding of Infrastructure

Flood Risk Assessment & Drainage Design

When is a Flood Risk Assessment (FRA) Required?

Current Planning Legislation requires that a ‘Flood Risk Assessment’ report be prepared for all development sites located in EA flood Zone 2/3 and all developments sites larger than 1 hectare located in Flood Zone 1 [see www.environment-agency.gov.uk for flood zone maps].

‘The Code for Sustainable Homes’, the national Standard for the sustainable design and construction of new homes, includes the category of Surface Water Runoff (flooding and flood prevention), for which a flood risk assessment report is required.

What does a Flood Risk Assessment Report Comprise?

For purposes of Planning, the FRA report is required to assess the potential for the existing site, and immediate surrounding area, to be affected by flooding in accordance with Planning Policy Statement 25: Development and Flood Risk (PPS25), and address any additional flood risk concerns raised by relevant statutory bodies i.e. Local Authority, the Environment Agency and the United Utilities.

In preparation of such a report Atkinson Peck consider the recommendations of the Strategic Flood Risk Assessment (SFRA) and assess all potential sources of flooding including: Rivers (Fluvial), Coastal Waters (Tidal), Surface Water (Pluvial), elevated Groundwater, overloaded Public Sewers, and Canal overtopping/embankment failure etc. An assessment is also carried out for various storm events of the surface water run-off rates and volumes of both the pre-development and post development sites, utilizing the industry standard Micro Drainage Software. Where required a preliminary drainage layout plan can also be produced, which can also aid in pricing by tendering contractors.

For purposes of the Code for Sustainable Homes, the mandatory element of Sur 1 (Management of Surface Water Run-off) requires that a FRA be prepared to confirm the risk of flooding from all sources. For Sur 2 (Flood Risk) additional credits can be achieved for production of a more detailed site specific FRA.

Whether produced for purposes of planning or to comply with the Code for Sustainable Homes, the impact of future climate change also needs to be considered as effects of climate change are likely to mean an increase in peak river flows and rainfall intensities.

How much will a Flood Risk Assessment Cost?

Senior members of staff at Atkinson Peck are experienced in producing Flood Risk Assessment Reports for sites located in Flood Zones 1 and 2 and in certain circumstances sites located in Flood Zone 3. As the cost of a Flood Risk Assessment will vary depending on the size of the development, its flood zone allocation and the primary purpose for production of the report, please contact the office for further advice or a quotation.

How can Surface Water Run-off be Managed?

In both of the above instances management of surface water run-off rates and volumes is often required to limit Greenfield sites to existing rates/volumes and Brownfield sites to either existing or reduced rates/volumes, whilst taking into account climate change. As a result, the incorporation of Sustainable Drainage Systems (SuDS) and Rainwater Harvesting has become a common practice, often needing designer input at an early stage followed by detailed analysis using storm simulation software.

Where existing geological conditions are conducive to infiltration; Soakaways or Permeable paving will always be the first option considered, however if this is not the case attenuation systems and flow control (hydro-brakes, orifice plates, throttle pipes etc) can be incorporated within the design of the below ground drainage system. Such systems can include permeable paving, oversized pipes/manholes and cellular storage. If required Atkinson Peck can assist in arranging in-situ testing to determine the infiltration potential of the site.

Adopted Drainage Systems

Atkinson Peck Ltd are often engaged to implement the adoption of both storm-water and foul-water drainage systems under S104 of the Water Industry Act. We find that entering into a constructive dialogue with either United Utilities, Yorkshire Water & Welsh Water etc at an early stage aids in the production of construction drawings, drainage calculations and the subsequent all important Technical Approval.