Surface water (pluvial) flooding

Drainage systems for surface water include traditional drainage, sustainable drainage systems and designing for exceedance.

Traditional drainage

Traditionally the design of drainage systems has been to remove rainwater as quickly as possible from where it falls. Surface water from urban areas is predominantly directed into public (highway and public surface water) sewers. In some areas the public sewers take foul (waste) water as well as surface water and are then referred to as 'combined sewer systems'. 

This addition of surface water into the foul system places a significant burden on wastewater treatment works in addition to the piped network.

To alleviate this burden during extreme events, combined sewers often have overflows – 'combined sewer overflows' (CSOs) – through which excess water can be discharged directly to watercourses. These are essentially designed to spill diluted sewage into watercourses and are regulated by the Environment Agency. Excess surface water and sewage may overflow from sewage networks in other locations, such as manholes, in extreme events.

The expansion of impermeable surfaces in the urban environment increases the surface water flows in sewers, and climate change is likely to exacerbate this problem. This increases the risk of systems overloading in sustained rainfall events and potentially causes pollution of watercourses when CSOs discharge. By transferring water rapidly away from where it falls there is also the possibility of a receiving watercourse being inundated and causing flooding.

Traditional sewer systems were not designed with sustainability in mind and most were not designed to take into account:

  • effective flood control
  • water quality management
  • water resources amenity
  • biodiversity requirements.

Sustainable drainage systems

Sustainable drainage systems (SuDS) are designed to manage stormwater locally, as close to its source as possible, to mimic natural drainage and encourage infiltration, attenuation and passive treatment.

In addition to managing flood risk, SuDS can be used to both manage pollution risks resulting from urban runoff and contribute wherever possible to environmental enhancement and place making in local communities.

SuDS can be used in new build, re-development and retrofit situations, although each development site should be considered on its own merits to ensure that the correct SuDS measures are used. 

Recent changes in legislation mean that SuDS should become commonplace in new and re-developments. The Flood and Water Management Act (FWMA), calls for SuDS to be used to manage all surface water, where possible, in new and re-developments, and these systems, if they meet certain standards, will become adopted by the lead local flood authority (LLFA).

In order to approve SuDS systems, LLFAs must set up SuDS Approving Bodies (SABs) which will have the responsibility of reviewing drainage plans against a set of new National Standards for Sustainable Drainage. If a drainage plan is approved by the SAB and constructed correctly, then it must be adopted by the SAB.

To find out more, see the section on SuDS andSABs.

Flood and Water Management Act – on the Defra website

More about SuDS – on the CIRIA website 

Designing for Exceedance

Flooding can occur on a local scale when surface water volumes exceed the capacity of the sewer system during an extreme rainfall event. Properties should be protected against such flooding through the identification of flood pathways. This is known as designing for exceedance.

Successful design and management of exceedance pathways can help to reduce the likelihood and consequence of flooding during extreme events, and understanding where extreme flood flows will travel during flood events can also help to inform emergency management plans.

Successful communication of where exceedance pathways exist will be essential to ensure that everyone involved in a flooding event are aware that flood waters will be deliberately allowed to occur on the surface.

For more information on designing for exceedance, visit the Susdrain website.