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Detailed Design 9.5.4 Calculation inputs

9.5.4.1 Rainfall data selection

Rainfall depths and intensities for a range of
return periods and storm durations is one of
the key calculation inputs.

The choice of rainfall data can have a
significant effect on the volume of storage
calculated.

FEH 2013 rainfall data is considered the most
up-to-date data availabale and therefore
recommended for use.
FEH 2013 rainfall data can be sourced
Where FSR rainfall values are used the
designer must demonstrate that rainfall online at fehweb.ceh.ac.uk
values are consistent with FEH 2013 data.

9.5.4.2 Defining runoff coefficients (Cv)

79 In extreme rainfall conditions the losses Some modelling software packages contain
anticipated from hard development surfaces ‘Default’ Cv values (0.75 Summer, 0.84
such as roofs or paved areas are anticipated Winter) which assume that there will be 25%
to be minimal. summer and 16% winter losses from hard
surfaces.
The designer must evaluate the runoff
coefficient (Cv) for the types of surfaces These default values should not be used for
contributing runoff to the storage location. storage estimation calculations.
Sewers for adoption (Section C5.1)
The designer must justify where a Cv of less
recommends assuming 100% runoff from
than 0.9 is used for calculations.
impermeable areas which equates to a Cv of
1.0. Where a reasonable amount of permeable
surface contribution to SuDS storage, then
Runoff coefficients of 0.95 for roofs and 0.9
this should be considered within calculations.
for paved areas would be considered
The ‘UKSuDS’ website was recently updated
acceptable by the LLFA where drainage is
to allow input for permeable surface runoff
not being adopted by a Water and Sewerage
contribution within attenuation calculations.
Company (WaSC).

The Royal Borough of Kensington & Chelsea SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates