Strategic Flood Risk Assessment (SFRA)

Regional Spatial Strategies and Local Development Frameworks form a key part of the planning process that local authorities use to identify development allocations. These must take into account flood risk as well as other factors such as long term sustainability. All new development allocations and proposals in England must be tested against PPS25 with the aim of steering potential development away from highest flood risk zones while taking into consideration development pressures. To undertake these tests, local authorities are required to develop a Strategic Flood Risk Assessment (SFRA) in which all sources of flood risk are assessed.

Flooding is a natural process that generally results from capacity exceedence in a natural or engineered drainage system. Flooding can occur from a variety of sources and each should be considered and their relative impacts and consequences assessed.

SFRAs are undertaken to provide a detailed and robust assessment of the extent and nature of the risk of flooding and its implications for land use planning. These are a key tool within the planning process and the SFRA sets the criteria for the submission of planning applications in the future and for guiding subsequent development control decisions. The primary consideration being a preference to direct development to the areas of lowest flood risk.

An SFRA can also be used as part of a local authority’s emergency planning strategy for dealing with extreme flooding, evacuation and contingency planning. An understanding of flood risk extent and severity can help local authorities in the implementation of the Pitt Review recommendations in terms of strategic planning, flood resilience measures, communication and emergency planning

 

Approach

 

Clear is able to provide local authorities with appropriate and expert advice regarding the scope and detail of works required for a SFRA. Flooding data and assessments can be derived from historical data, existing modelled data and from new hydraulic modelling studies depending on the availability of existing studies.

While fluvial and tidal flooding are usually considered to be the primary sources of flooding, the SFRA must also assess potential flooding from groundwater, overland flows, surface water runoff sources individually and in combination.

Development often imposes new land uses on an area, which in turn can increase the surface water runoff during a storm event. Therefore, the assessment and limitation of surface water runoff is key aspect of flooding mitigation for new developments. Reductions in runoff from brownfield sites provide betterment and reduces the ‘fast response’ urban runoff into watercourses.

 

Expertise

 

Clear provides niche services within the water industry and has an excellent reputation for quality and delivery. Clear has a team of engineers and specialists in river and sewer hydraulics capable of providing clients with expert technical advice on all levels of flood risk assessment. This includes the production of flood envelope maps, hydraulic modelling, breach analysis, overland flood routeing and 2-D flood modelling.

It is important to take account of any flood risks early in the development process as planning policy and EA advice can become significant constraints on the potential use of available land packages.

Where river modelling is required, the extents must be appropriate and sufficient to avoid unnecessary cost, particularly in survey requirements. The use of LiDAR data and other digital terrain data can reduce survey costs and timescales provided that there is a good degree of confidence in the data.

Since 2004 Clear has undertaken over 300 flood risk assessments of varying size and complexity. Clear also undertakes a large number of Drainage Area Studies for UK Water Companies, which also include significant data collection, modelling, analysis and flooding assessment.

These skills and knowledge are complimentary to FRAs and SFRAs and enhance our capability in this discipline.

Clear is also highly experienced in the design and management of SUDS and has designed schemes for a diverse range of projects. The implementation of SUDS is implicit in the requirements of PPS25.

Clear provides the following capabilities to support the completion of strategic flood risk assessments:

  • Desktop investigations to utilise existing and historic data
  • Site Surveys and topographical surveys
  • Hydrological studies
  • Sewer Hydraulic modelling
  • River Hydraulic modelling
  • 2-D overland flow modelling
  • Drainage impact assessments