Hillingdon SuDS Design & Evaluation Guide
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Detailed <strong>Design</strong><br />
9.5.6 Managing runoff rates from<br />
Brownfield sites<br />
On Brownfield sites (also known as Previously<br />
Developed Land or PDL), if infiltration of the 1<br />
in 100 year rainfall event is not possible, the<br />
rate of discharge should be reduced to<br />
greenfield runoff rates. Where greenfield<br />
rates cannot be achieved, the designer must<br />
demonstrate why reduction in rate is not<br />
achievable. The designer will be required to<br />
demonstrate that they have explored all<br />
options for storage including the use of<br />
storage on roofs (e.g. blue-green roofs),<br />
permeable pavements, and the use of<br />
appropriately designed underground storage.<br />
(NSTS S3 and S6.)<br />
87<br />
Not all planning applications comprise a<br />
complete redevelopment of the site, and only<br />
a small parcel of the overall site may be<br />
planned for re-development. On such<br />
occasions LLFA will not expect the entire<br />
development to be returned to greenfield<br />
runoff status.<br />
In these circumstances LLFA will not accept<br />
the combining of the greenfield runoff rate<br />
for the development parcel with the existing<br />
impermeable runoff rate from the remainder<br />
of the site when the designer is undertaking<br />
storage calculations.<br />
The existing development remaining intact<br />
and the parcel of land proposed for<br />
development should be treated separately in<br />
terms of calculations and drainage strategy.<br />
London Borough of <strong>Hillingdon</strong> <strong>SuDS</strong> D & E <strong>Guide</strong><br />
© 2018 McCloy Consulting & Robert Bray Associates
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