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The IoH124 method has been superseded by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