Page 94 - Hillingdon SuDS DESIGN & EVALUATION

Detailed Design 9.5.7 Designing for exceedance

The designer must demonstrate that extreme
flows, beyond design parameters, can be

managed in a safe and predictable manner.
Site levels should be designed to allow
exceedance flows to flow from one storage
location to the next along a defined
management train/conveyance route.

9.5.8 Managing off-site flows

flows which are generated from off site. This
Many sites are at risk of significant surface
advice may be revised in exceptional
runoff from offsite with indicative flow routes
circumstances which will be determined on a
identified by Surface Water flood maps.
case-by-case basis.
SuDS design should demonstrate how offsite
flows are intercepted and managed through
the site without causing flood risk to the site EA Flood maps - www.flood-warning-
or increasing flood risk elsewhere. Unless information.service.gov.uk/long-term-
specifically required by LPA / LLFA flood-risk/
developers are not required to attenuate
89

9.5.9 Flow velocities

Peak flows should be retained to less than
1m/s velocity to avoid risk of erosion of
vegetated surfaces such as swale channels.

Where velocities are less than 0.3m/s this will
encourage silts to drop out of flow along the
Management Train.

The Manning’s Equation (SuDS Manual
EQ.24.12) is used to estimate open channel
flow velocities. The depth of flow will affect
how much ‘roughness’ is applied by the
channel. The SuDS Manual Figure 17.7 details
the manning’s roughness values which should
be adopted for SuDS calculations.

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