Sustainable Urban Drainage
Systems (SUDS)
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 and
quality of the water run off from sites.
Attenuation systems (tanks and rainwater
harvesting systems) and separators (to remove
oil contaminants from discharges) are required.
- To size the systems required, 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 authorities.
- 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.
How Does An Attenuation
System Work?
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.
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