Sustainable Drainage Systems

Climate change appears to be increasing extremes of weather in the UK, periods of drought followed by periods of abnormally heavy rainfall. The problem with the heavy rain periods is that they lead to flash flooding often with devestating consequences (remember 20th July, 2007!).

To address this problem, the drainage systems of towns, cities and developments have been surveyed and a policy of Sustainable Urban Drainage Systems (SUDS) has been developed to counteract the problems being encountered.

SUDS addresses issues of the quantity of the water run off from sites with buidings and hard-standing by using stormwater attenuation systems (tanks that collect the water and allow discharge at a reduced rate) and/or rainwater harvesting systems (the collection of rainwater from roofs for storage and reuse).

Separators can also be employed to remove any oil contaminants from discharges from areas such as yards and car parks.

Stormwater Attenuation Systems

When the peak inflow rate in a storm exceeds the allowed discharge into the watercourse, the excess flow has to be ‘attenuated’ on the site for the duration of the storm. This is then released at, or less than, the allowed discharge rate after the storm.

To store the excess volume and allow the correct discharge rate to go to the watercourse, a flow regulator is installed to ‘bleed off’ the correct maximum flow rate. The flow regulator can be a vortex type system or a simple orifice plate system. The remainder of the water is stored either in-line or off-line, depending on the design.

Rainwater Harvesting Systems

Rainwater is collected from building roofs, filtered and then stored in above ground or underground tanks. It can be put to a variety of uses:

Cleaning of buildings, yards, vehicles, animals etc.
Irrigation.
Washing (further filtering and UV disinfection required).

Planning A Sustainable Drainage System

Sustainable Drainage Systems encompass much more than just installing a single product. The objective is to deal with the flow at source rather than traditional techniques that simply transfer the problem further down the drainage system.

Each site should be tackled with management and control measures designed to meet many of the following objectives:

Control and management of stormwater run-off
Reduce the impact of urbanisation
Ensure the protection and enhancement of local water quality
Enhancing the natural recharge of groundwater
Reusing stormwater to reduce load on local resources
Natural integration to the local environment

Initially the flow rate from the development is calculated from rainfall records and the run off rate for a particular type of surface, i.e. roadway, roofed area, grassed area, all of which allow surface water to run off at different rates. The design is normally based on the highest recorded rainfall in the previous 30 years, but this can vary.

Most authorities limit the amount of rainfall run off from a development to a level where the rate does not exceed the rate of discharge from a green field site (5 litres/sec/acre approx.) but this must be specified by the local authority in question.

A technical engineer should provide the design calculations for the surface water run off in order to size a system. This should also include the flow rate allowed for discharge to a watercourse, and the required storage volume.