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9.6.6 The importance of protected openings
There are no minimum thresholds for SuDS components such as permeable
attenuated flow rates in SuDS design. pavements, bioretention or filter drains are
Previously the drainage industry has applied pre-filtered, and assuming collection through
a minimum flow rate of 5 l/s but this does not perforated pipes or similar, the flow control Detailed Design
take into account the need in SuDS for low opening requires little additional protection.
flow rate controls and the design of
Open SuDS components such as swales,
protected openings.
ponds and basins, require additional
Small sites and sub-catchments of larger protection. One way to provide this
sites may need to meet minimal outflow flow protection is to use a stainless steel basket
rates. Flows can be controlled down to 0.5 filled with 80-150mm stone with the
– 2 l/s using small openings (15-20mm connecting pipe opening set within the stone
diameter) with shallow depth of storage. to prevent floating debris reaching the flow
control.
A stainless steel mesh basket filled with 80-
150mm aggregate forms an effective
protection for pipe openings. Note the pipe
opening has a mesh guard to stop stone
migrating through the pipe.
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Key points to be considered when designing pass over the outlet as water rises in the
protected openings: SuDS component.
■ Protection to the opening should be of a ■ Openings in the protective screen should
reasonable surface area to allow for be smaller than the orifice opening size,
accumulation of litter and vegetation thus any residual silt passing through
across the surface of the protection. protective screen will pass through the
orifice opening.
■ Outlets in open structures should be
located on a slope to encourage debris to
Oxfordshire County Council SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates
