Critical Drainage Area reviews

Critical drainage areas can cover wide areas within both rural and urban environments:

  • villages
  • towns (such this example from Barnstaple in North Nevon)
  • cities (for example London has many including much of Chelsea)

They are where man made drainage infrastrure has been identified as at critical risk of failure, resulting in flooding. Such areas can be completely different or similar, to the areas identified by the Environment Agency as at risk of natural watercourse, river and sea flooding.

critical drainage area


So how is a Critical Drainage Area defined?

A critical drainage area is defined in the Town and Country Planning (General Development Procedure) (Amendment) (No. 2) (England) Order 2006 a Critical Drainage Area is “an area within Flood Zone 1 which has critical drainage problems and which has been notified… [to]…the local planning authority by the Environment Agency”. 

There can be a strong link to a Water and Sewerage Company's (eg Thames Water) DG5 register.

This is a formal register of properties which have experienced sewer flooding (either internal or external flooding) due to hydraulic overload, or properties which are ‘at risk’ of sewer flooding more frequently than once in 20 years.

This relates to DAP (District Area Plan) which is a comprehensive assessment of a sewer system’s performance and condition made by the WaSC.  It generally includes a hydraulic model of the foul, combined and some surface water sewers. It also proposes sewerage improvements or repairs to achieve desired levels of service (e.g. the alleviation of DG5 sewer flooding properties) 


Where do Critical Drainage Areas occur?

Critical drainage areas can occur right across the UK, from the far end of Cornwall, Newcastle to large areas of Central London.

The Council says I am in a critical drainage area and I need a report to address their concerns

It is standard practice for a Council to require a developer to obtain the services of a Civil Engineering Consultant to write an independent professional flood risk assessment in an area of high natural flood risk. Likewise they require a Civil Engineering Consultant to produce Critical Drainage Area assessments in areas of critical drainage risk. This report can often contain a mini-flood risk assessment, combined with a sustainable drainage assessment. A key element is an understanding of the specific flood risk associated with the developments location and appropriate consideration of the practical mitigation measures applicable. Even more than normal, there is a focus on the use of sustainable drainage practices, where discharge to the network can be minimised in total and not just attenuated like in the past.


Can you produce a critical drainage report for my development and how much does it cost?

Typically we provide fixed fees, where a fixed scope is clear. Every proposal and location is unique so a proposal will be considered. To enable this initial review and proposal, you should email the following (where available):

  • Development plans
  • Correspondence from Local Planning Authority
  • Cross sections
  • Soil reports
  • Public sewer plans

Realistically, you should expect all professional critical drainage reports to cost between £1500 and £3000, given the time and experience required.

Please email these documents with your client and location details to:

How long would a critical drainage report take to produce approximately?

It can often take between 2 and 4 weeks from formal instruction, but can take considerably longer depending on the availability of evidence.