What is a Critical Drainage Area?

A Critical Drainage Area (CDA) is defined by the Drain London Project as “an area where there are known flooding problems. Will be a discrete geographic area (usually a hydrological catchment) where multiple and interlinked sources of flood risk (surface water, groundwater, sewer, main river and/or tidal) cause flooding in one or more Local Flood Risk Zones during severe weather thereby affecting people, property or local infrastructure.”

A Critical Drainage Area encompasses the upstream catchment contributing to flooding in the LFRZ, and can include downstream areas that could influence flood risk within the LFRZ.

Common Misconceptions

Misconception 1: CDAs indicate a site is at risk of flooding. CDAs denote areas where mitigation measures could reduce flooding in the LFRZ. Changes to a site within a CDA could either improve or worsen flood risk in the LFRZ.

Misconception 2: All development in CDAs requires a Flood Risk Assessment. FRA requirements depend on Local Planning Authority policies. For example, the London Borough of Southwark requires FRAs only for “Major applications in Critical Drainage Areas for surface water flooding.”

Areas with Critical Drainage Problems (ACDPs)

Paragraph 167 of the NPPF and Footnote 55 reference “land which has been identified by the Environment Agency as having critical drainage problems,” which differs from CDAs. The Environment Agency confirmed that CDAs are not the same as critical drainage problems.

ACDPs are identified separately by the Environment Agency and fall only within the south-west river basin district (primarily Cornwall, South West England). Sites within ACDPs must prepare a Flood Risk Assessment per NPPF Footnote 55.

Conclusion

Development proposals in CDAs or ACDPs should be assessed against local planning requirements. Aegaea offers guidance on whether sites require FRAs and can provide preliminary site assessments.