Sustainable Drainage (SUDS)
Infiltration and run-off
Naturally vegetated, rural ground is permeable and has the capacity to absorb high levels of rainfall through its surface (‘infiltration’) and to release the infiltrated water gradually into natural water courses.
Development of this ground can greatly reduce permeability by the introduction of hard surfaces — asphalt, paving, concrete, roofs — and the compaction caused by plant or vehicular movement. Less water infiltrates the ground and more water
runs off the surface, more rapidly. Traditionally, to avoid local flooding, surface run-off has been drained into underground pipe systems and delivered elsewhere into the water catchment area.
As development increases, so do run-off speeds and volumes. Their rapid delivery into the local water courses can cause flooding downstream, resulting in danger to life and damage to property, or erosion and destruction of natural habitats.
Traffic, hard surfaces and increased run-off can increase the risk of pollutants, which are literally washed off the streets into the water catchment, contaminating the downstream water supply.
It is generally accepted that the UK’s climate is changing and that our weather is becoming more extreme. Paradoxically, in the short term, it is not the overall levels of rainfall that influence the need for sustainable drainage but the ferocity and frequency of the single rainfall events that may cause flooding. These, combined with on-going development, add to the necessity for SUDS solutions.
The Key SUDS Principles
1. Source Control
Sustainable drainage systems control surface water run-off as close to its origin as possible. This is termed ‘source control’.
This minimises changes in the volume and rate of surface run-off from developed sites and thereby minimises flood risk and other environmental damage. Additionally, SUDS may offer an element of pollution control as well as ‘attenuation’ — where infiltrated water is stored before its controlled release.
2. Management train
These levels of control and treatment result from a series of techniques (‘management train’) that integrate with one another to gradually reduce levels of pollution,
3. Water Quality
Removing pollution from run-off is an important part of SUDS design. The use of SUDS to control pollution and replace oil separators is highlighted in the most recent edition of Pollution Prevention Guideline PPG3 published by the Environment Agency 2006.
4. Amenity and Biodiversity
Trees (the urban forest) are an important element in the biodiversity of urban areas and improve amenity by providing enhanced aesthetics, cooling and shading. SUDS can be designed to protect and enhance the health of existing and new trees by protecting the roots systems and providing irrigation and aeration to the soil.
5. Rainwater Harvesting
Rainwater harvesting involves collecting rainwater in storage tanks and reusing it for non-drinking purposes such as watering gardens or flushing toilets. Rainwater harvesting can be incorporated into SUDS. The storage required for SUDS would be additional to that provided for the harvesting system.