Surface water flooding

Impact 5
risk indicator
Impact & Likelihood
Impact key
5 Catastrophic
4 Significant
3 Moderate
2 Limited
1 Minor
Likelihood key
5 >25%
4 5-25%
3 1-5%
2 0.2-1%
1 <0.2%


Surface water flooding occurs when rainfall overwhelms the capacity of drainage systems and surface water sewers, resulting in water flowing over the land instead of through drainage systems. This type of flooding can occur in a wide variety of locations, including towns or cities located far from the sea or rivers. It is also particularly difficult to forecast with accuracy and can happen at very short notice, with periods of short but intense rainfall likely to increase in the future due to the warming climate.


The reasonable worst-case scenario is based on a large flood event in a metropolitan area, resulting from a pocket of exceptionally high rainfall in the south east. The most severe impacts in any metropolitan area would lead to significant damage to homes and businesses. The evacuation of residents would be necessary, with short- to medium- term shelter being required. Depending upon the geological conditions, surface water flooding may lead to an increased likelihood of geological instability (for example sinkholes or landslides) in the impacted area. This could cause significant impacts to the local response, transport infrastructure, and other infrastructure in the impacted area.

Key assumptions

Areas most at risk would not be warned until 6-24 hours before the event and the exact location of the most intense rainfall would not be known in advance due to the unpredictability of where and when heavy thunderstorms will occur.


A high impact but less likely variation could involve surface water flooding in an urban area exacerbated by coastal or fluvial flooding. There would also be greater disruption from surface water flooding in areas with dense populations, with poor drainage creating greater levels of property damage and displacement of possibly thousands of people.

Response capability requirements

Lead Local Flood Authorities have responsibility for managing surface water flood risks, including assessing the risks, implementing a local flood risk management strategy and working in partnership with other involved agencies. The Department for Levelling Up, Housing and Communities has well-established multi-Local Resilience Forums mutual aid arrangements in place to support flood preparedness and a coordinated national response. Pre-prepared national resources are available, including sandbags, mobile flood barriers and mobile pumps, ready to be moved where needed.


There would be extensive damage to homes and businesses as people would be unprepared due to the lack of surface water specific warning systems. Surface water flooding would have major recovery impacts and long-term economic, environmental, infrastructure and humanitarian implications.