Hillingdon SuDS Design & Evaluation Guide

Parameter
Modelling
of the SuDS
layout.
Outfall
Guidance on design/calculation input
It is not anticipated that SuDS design will require
modelling of extensive piped systems. In some
instances where the scheme is relatively small and not
hydraulically complex standard calculations will be
accepted in lieu of a hydraulic model. Layout
drawings should be clearly labelled with the
numbering convention used by models.
Outfalls into receiving sewers or watercourses can be
Information for technical
assessment
The designer is to justify where
no hydraulic modelling is
undertaken. Calculations/model
outputs should be provided to
support the Flows and Volumes
proforma
Designer is to indicate whether
Detailed Design
design
at risk of surcharge and lack of free discharge due to
SuDS storage calculation is
elevated water levels. This can result in additional
likely to be influenced by high
storage being required. Free discharge should not be
water levels at the point of
assumed. The risk of surcharge should be assessed
discharge.
and accounted for within calculations as appropriate.
Long
Long sections will allow detailed consideration of
Long section showing peak
section
levels across the site.
water levels.
Erosion
Flows along swales (or other vegetated surfaces) are
Designer to demonstrate that
check
at risk from erosion. Peak flow velocities should be
they have considered risk of
less than 1 - 2 l/s.
erosion and taken measures to
Concentrated inlet points are also prone to erosion.
safeguard scheme. Peak flow
velocity calculations to be
provided as appropriate.
Designing
for
exceedance
The design should incorporate overflows at each
SuDS component. Hydraulic calculations should
demonstrate that overflows have sufficient capacity to
deal with anticipated flow rates. SuDS layout drawing
Locations of overflows should
be identified on the layout
drawing along with proposed
exceedance flow route.
92
should identify the anticipated flow route for
exceedance events.
Managing
The FRA should identify the potential for flows from
The designer should
flows from
offsite. These flows can be unpredictable and difficult
demonstrate how anticipated
off site.
to quantify. Management of flows through the site
should not increase flood risk elsewhere.
flows from off site will be
managed through the site using
Detailed modelling to establish the rates of flow
anticipated would not be considered compulsory (but
the layout drawing and design
statement.
may be required on a case by case basis).
Consistency
Detailed design of SuDS components should reflect
Drawings should clearly identify
of
hydraulic calculations / hydraulic models, taking into
site levels, storage locations and
calculations
and design.
account slopes and low lying levels.
The LLFA will consider design drawings to ensure that
flow control sizing and storage provision is as per
calculations.
flow controls with cross
sections and long sections. The
design statement should
confirm that drawings deliver
calculated volumes.
London Borough of Hillingdon SuDS D & E Guide
© 2018 McCloy Consulting & Robert Bray Associates