Sustainable Urban Drainage Systems - SUDS

What is SUDS ?

It is an alternative approach to conventional drainage design and implementation that replicates natural drainage and deals with runoff where it occurs.

Why consider SUDS?

• It can save money
• It can reduce pollution
• It can introduce improved methods of surface water design
• It can improve the environment
• It can reduce the risk of flooding
• It can infiltrate water back into the ground locally

 

How does SUDS do this?

By introducing into a surface water system the following:

• Porous pavements to roads and car parks
• Infiltration trenches to roads, car parks and existing drainage
• Infiltration basins as storage tanks
• Permeable conveyance systems or french drains
• Swales to slow surface water flows
• Passive treatment systems such as filter strips, detention basins, retention ponds and wetlands

 

What are the advantages of introducing SUDS?

The traditional approach to draining developed areas is having a damaging impact on our environment and is not sustainable. SUDS offers a wide range of techniques which can be adopted for most new and redeveloped sites to give a reduced environmental impact from surface water drainage. Widespread adoption of these techniques will see a long-term improvement in the quality of our urban rivers, contributing to a more varied and attractive urban environment built on a sustainable basis.

In addition this approach to surface water drainage is now being promoted in the UK by the Environment Agency and the Scottish Environment Protection Agency (SEPA), and will become their preferred option in the future.

Any proposed new surface water drainage systems at a site should be designed to not only deal with the problem of surface water run off, but it should also enhance the site considerably for children and nearby residents.

These SUDS systems offer the following advantages:

• They reflect the natural drainage of the site and give sustainable management of the water environment
• They offer considerable visible safety advantages for children and adults alike
• They are very easily and economically maintained
• They create natural wildlife habitat areas which can be readily used for education purposes
• They offer cost effective and economic infrastructure and drainage systems

 

The following sections describe and show photographs of some of the techniques used in these systems.

 

Swales

An example of a swale adjacent to a main road

Swales are grassland depressions, which lead water overland from the drained surface to a storage or discharge system. When compared to a conventional ditch, a swale is shallow and relatively wide, providing temporary storage for storm water and reducing peak flows. During dry weather a swale will be dry.

Swales:

• Undertake retardation of surface water flows
• Undertake balancing of surface water flows
• Increase the loss of surface water through infiltration, evaporation and transpiration
• Undertake filtering and improving the quality of water in the grass / vegetation / soil.
• Reduce the need for expensive roadside drainage and gullies.
• Reduce the risk to amphibians such as toads and newts, which are often trapped in gully pots.

 

Balancing Ponds

The first part of a Balancing Pond is usually the Detention Basin, which is designed to hold back storm runoff for a short while to allow the settlement of the solids. Detention basins are dry outside of storm periods. All Balancing Ponds are designed to retain flood events, reducing peak flows and limiting the risk of flooding. Solids removal is one of the main features of detention basins and high removal rates are possible. Nutrient and trace metals removal is more modest. Extended detention basins incorporate a small permanent pond or wetland, which can enhance the appearance of the basin.

Balancing Ponds retain a certain volume of water at all times. This can avoid possibly unsightly exposure of banks of collected sediment and enhance performance in removing nutrients, trace metals, coliforms and organic matter. Allowance for a considerable variation in water level during storms will be incorporated in the design, so that a significant storage volume can still be provided. The permanent water will usually be visually more attractive, and elevated nutrient concentrations, which result in algal blooms, are avoided.

Wetlands are a further enhancement of Balancing Ponds, and incorporate shallow areas planted with marsh or wetland vegetation. These provide a much greater degree of filtering and removal of nutrients by algae and, to a lesser extent, by incorporation into plant material. Only specially constructed wetlands should be used to treat surface water.

In many developed countries constructed wetlands have been proven to be effective, providing moderate to high levels of pollutant removal throughout the year as well as having excellent surface water balancing capacity.

NOTE - A conventional Balancing Pond will have considerably steeper side slopes and will need fencing at least 1.2m (4 ft) high at the top of the embankments.

 

Porous Pavements

At a motorway service area the parking bay areas are built with porous blocks and rainfall from the tarmac access roads runs into these bays.

A Porous Pavement is an alternative to conventional paving in which water permeates through the paved structure rather than draining off it. The surface water will be held in a reservoir structure under the paving for subsequent infiltration or delayed discharge. The porous paving can be materials such as gravel, grasscrete, porous no fines concrete, or porous asphalt. Pollutant removal rates have been shown to be high, as the majority of the removal occurs as a result of the infiltration of the water and the subsequent absorption and filtering within the subsoil.

Removal of up to 80% of sediment, 60% of phosphorus and 80% of nitrogen has been measured, as well as high removal rates for trace metals and organic matter.

 

 

With many thanks to SEPA for photographs (The Environment Agency only covers England and Wales.)