Hillingdon SuDS Design & Evaluation Guide

Detailed Design
9.5.4.5 Defining attenuation flow control rates
LPAs require that SuDS attenuate runoff from
all sites (Greenfield and Brownfield) to
equivalent greenfield runoff rates. There are 2
primary methods for controlling rates as
follows (see Section 6.4.3.5):
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Approach 1 - where the volume of runoff
is controlled, the rate of outflow is
controlled to the 1 in 1 year and 1 in 100
year greenfield runoff rate.
Approach 2 - where the volume of runoff
is not controlled the rate of outflow for all
rainfall events is controlled to Qbar/Qmed.
NSTS S2,S3 and S6
In most cases the value derived from IoH124
method is similar to FEH methods and due to
its common usage IoH124 values will be
accepted by the LLFA until FEH methods
become more commonplace.
Further notes on the application of the
different methods are listed below:
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FEH ReFHv2 – analysis should ensure that
there is no urbanised component within
the runoff estimate. The flow rate for any
return period can be derived using the
ReFHv2 software. The peak rate of
catchment runoff is factored back to the
site size to establish the greenfield runoff
for the site.
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Qmed / Qbar rates are anticipated to be in
the region of 2-7 litres per second per
hectare (l/s/ha) depending on local rainfall
and soil characteristics.
FEH methods are now preferred for
estimating Greenfield runoff rates. Care must
be taken when selecting the catchment to
define descriptors to ensure that a small
localised catchment is selected.
The IoH124 method has been superseded by
the FEH methods.
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FEH statistical method requires the
designer to establish Qmed (SuDS Manual
EQ.24.2) using FEH catchment
descriptors and then undertake a pooling
analysis to derive flow rates if 1 and 100
year flow rates are required.
Establishing Qbar using IoH124 (SuDS
Manual EQ.24.3) is based on 50ha area
input and then factored down to the size
of the site. Where Approach 1 is used, the
1 in 1 and 1 in 100 year Greenfield runoff
rates should be calculated by factoring
the Qbar rate using growth curve factors.
(SuDS Manual Table 24.2)
Design Note:
Regional maps may not be representative of site soil conditions and calculation inputs may
have to be adjusted accordingly.
London Borough of Hillingdon SuDS D & E Guide
© 2018 McCloy Consulting & Robert Bray Associates