Page 92 - Hillingdon SuDS DESIGN & EVALUATION

Detailed Design 9.5.6 Managing runoff rates from

Brownfield sites

On Brownfield sites (also known as Previously
Developed Land or PDL), if infiltration of the 1
in 100 year rainfall event is not possible, the
rate of discharge should be reduced to
greenfield runoff rates. Where greenfield
rates cannot be achieved, the designer must

demonstrate why reduction in rate is not
achievable. The designer will be required to
demonstrate that they have explored all
options for storage including the use of
storage on roofs (e.g. blue-green roofs),
permeable pavements, and the use of
appropriately designed underground storage.
(NSTS S3 and S6.)

Not all planning applications comprise a
complete redevelopment of the site, and only
a small parcel of the overall site may be
planned for re-development. On such
occasions LLFA will not expect the entire
87
development to be returned to greenfield
runoff status.

In these circumstances LLFA will not accept

the combining of the greenfield runoff rate
for the development parcel with the existing
impermeable runoff rate from the remainder
of the site when the designer is undertaking
storage calculations.

The existing development remaining intact
and the parcel of land proposed for
development should be treated separately in
terms of calculations and drainage strategy.

London Borough of Hillingdon SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates