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SUDS explained

Implicit to the correct design and operation of a good SUD System is an understanding of the following:

• the SUDS philosophy
• the SUDS triangle
• the SUDS treatment train

The SUDS philosophy is simply to mimic natural drainage. When construction takes place the urbanisation of catchments and sub-catchments occurs, and the natural hydrology is affected. This results in reduced supply of rainfall to groundwater by the interception of run-off on areas of hardstanding (eg roads, roofs and paved surfaces). This in turn means less supply to springs and river network resulting in lower flows and increased likelihood of drought during dry periods.

In addition, by intercepting run-off and routing it through smooth conduits and piped networks, we accelerate the passage of flow to the receiving watercourses resulting in higher peak flows and flooding. Another effect of this is greater erosion and damage to river banks and beds.

The SUDS triangle encourages us to take account of:

By considering all three functions we should be able to provide adequate and well designed systems that offer water quality treatment through natural process inherent in the system, encourage infiltration where appropriate and attenuate peak flows in addition to providing habitat and function for those using the area, including the local community and wildlife.

The SUDS treatment train uses a logical sequence of SUDS facilities in series thus allowing run-off to pass through several different SUDS before reaching the receiving watercourse or water bodies. By using the treatment train, run-off will encounter different passive treatment processes that are active in different types of facilities.

The treatment train comprises four stages:

1. Good houskeeping    
2. Source control   
3. Site control  
4. Regional control

Good housekeeping

Uses best practice to reduce the prospects of pollutants reaching the environment by improving techniques that may cause pollution, or produce or use less polluting materials. Eg use of more environmentally friendly materials such as phosphate free detergents.

Good housekeeping also includes measures that reduce the prospects of flooding by encouraging more natural run-off paths.
Eg canopies over factory or shop loading bays that have their flow routed through a SUD system, rather than routing this high-risk area to the sewage works.

Source controls

These facilities allow control of run-off at or adjacent to the source (ie where rainfall lands on a surface). Eg permeable, or porous surfaces such as permeable paving, grasscrete, filter trenches, or swales.

Site controls

Local facilities that will receive run-off from upstream locations, often with several inlets and only one controlled outlet. Eg detention basin, small pond.

Regional control

These include ponds and wetlands and are larger features that will collect run-off from upstream controls. They should not receive significant pollutants; these should have been managed by the upstream provisions. Where possible regional controls should be used as landscape features and provide final 'polishing' of the treatment train prior to discharge.

The schematic diagram below shows the treatment train concept and samples taken from a treatment train serving a timber yard near Lockerbie. Note the importance of source control.