Flood Risk Assessments (FRA)

Under current planning law, developers are required to carry out site-specific Flood Risk Assessments to accompany planning applications in accordance with National Planning Policy Framework (NPPF). The requirement for submission of a FRA and the level of detail required are typically determined based on the following criteria:

  • Is any part of the site shown to be within the Environment Agency’s Flood Zones 2 or 3? Check your risk of flooding
    All new developments in Flood Zones 2 and 3 require a flood risk assessment, regardless of the size of the site or scope of the development. Flood zones are determined based on the risk of fluvial and/or tidal flooding to the area around the site. Flood Risk Assessments focus on the risk of flooding from a rivers, the sea, groundwater and overland flow, in addition to consideration of the general drainage strategy and use of Sustainable Urban Drainage Systems (SUDS) on the site.
  • Is the site area greater than 1 hectare? Measure your site area
    All sites greater than 1 hectare in size require a flood risk assessment, even if the site is shown to be located wholly in Flood Zone 1. The focus of this type of FRA is on surface water; the risk of surface water flooding to the proposed sites and the impacts of runoff rates and volumes from the development.
  • Is the development considered to be ‘major’?
    From 6th April 2015, the Lead Local Flood Authority (typically the county council or borough council in London) became a statutory authority with regard to surface water. Many are now requesting assessment for ‘major’ development sites to consider the risks of surface water flooding and encourage the incorporation of sustainable drainage. These are defined as 10 or more residential units or 1,000m² of non-residential floor space. Assessments range in scope but typically include; consideration of sustainable drainage and how these measures can be incorporated into the site design, completion of SUDS Pro-Forma templates required by the council, and development of drainage strategies.

Climate Change

New climate change allowances were published and adopted in February 2016. The new allowances are dependent on development vulnerability, expected lifetime, flood zone and river basin, and are applied to:

  • Rainfall intensity ranging between 10% and 40% (previously 30% applied nationally)
  • Flood flows in rivers, ranging between 10% and 105% (previously 20% applied nationally)
  • Static sea levels ranging between 2.5mm/year and 15mm/year (cumulative rise of 0.99m to 1.21m by 2115)

The Environment Agency Flood Zones do not take into account any increase in flood depth or extent due to climate change. All flood risk assessments regardless of the level of detail needed are required to consider the implications of Climate Change on all sources of flooding over the lifetime of the development.