Last edited 10 Dec 2020

Surface water

Very broadly, surface water refers to water that is present on the surface of the Earth. The term covers a host of water-body types, including oceans, seas, lakes, rivers, streams, tarns, ponds and so on. It is water that has not penetrated below the ground.

Surface water may also refer to water that, for whatever reason, is present on a building’s roof, in a park, road, car park and so on, which has not drained away. The process by which this surface water is drained is known as ‘surface water drainage’. Surface water drainage may involve conveying water into the public sewer system, storing it, draining it into a soakaway and so on.

Surface water that flows from the land, roofs, hard-landscaped and other surfaces into the various drainage systems is called ‘surface water run-off’ or just simply ‘run-off’.

[edit] Surface water flooding

Surface water flooding occurs when there is an excessive quantity of surface water which has not drained away. This may be due to the sheer quantity of rain in a short time period or it may be due to an inefficient or outdated drainage system, impermeability or saturation of the ground, a defect such as a burst water main and so on.

High-intensity downpours can cause havoc in urban areas resulting in overwhelmed drainage systems, flooded underpasses, impassable roads, flooded tunnels, power cuts and catastrophic damage to property.

The 2008 Pitt Review concluded that much of the flooding which occurred the previous summer and which caused 13 deaths and £3bn of damage was primarily the result of water pouring off the land (surface water run-off). As a result, maps were created that highlighted areas with higher risk of surface flooding (Risk of Flooding from Surface Water maps, 2013) and there were record levels of investment in flood defence construction projects.

Surface water flooding has become more common in the UK, and this is thought to be due to:

Climate change and the increasing prevalence of extreme rainfall events.
The increasing use of hard-surfaced areas in dense urban areas, where large quantities of water cannot penetrate into the ground and instead are channelled into drainage systems that were not designed to cater for such large volumes.
The removal of water attenuation features in rural areas, resulting in surface water reaching watercourses and drainage systems faster that it might have done previously.

The challenge now is to think about how surface water flood risk can be managed and then designing rural and urban areas and infrastructure to mitigate the risk. This might include changing the design of new housing developments, better surface-water flood forecasting, increasing adoption of sustainable urban drainage systems (SUDS) and so on.

Sustainable urban drainage systems can provide an alternative to, or addition to, traditional drainage systems where surface water is drained directly and quickly into underground, piped drainage. They adopt techniques to deal with surface water runoff locally, through collection, storage, and cleaning before allowing it to be released slowly back into the environment. For more information see: Sustainable urban drainage systems.

NB The SuDS Manual (C753), published by CIRIA in 2015 defines surface water as: 'Water bodies or flows that appear as a result or rainfall.' It defines a surface water body as: 'Permanent flows or bodies of water on the surface, ssuch as lakes, rivers, streams, standing water or ponds.'

The London Plan, published by the Mayor of London in March 2016, suggests surface water is: ‘Rainwater lying on the surface or within surface water drains/sewers.’