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Detailed Design 9.5.4 Calculation inputs 9.5.4.3 Making allowances for interception losses Detailed Design
As a rule of thumb, where the total wetted
9.5.4.1 Rainfall data selection
area of SuDS components equates to at least
Rainfall depths and intensities for a range of
25% of the development area (all buildings
return periods and storm durations is one of
and hard surfaces) then it is acceptable to
the key calculation inputs.
make an allowance for interception losses.
For more detailed analysis methods
The choice of rainfall data can have a
This loss can be applied within storage
significant effect on the volume of storage
calculated.
by 5mm.
FEH 2013 rainfall data is considered the most calculations by reducing the rainfall depths see SuDS Manual Section 24.8
up-to-date data availabale and therefore
recommended for use. 9.5.4.4 Defining infiltration rates
FEH 2013 rainfall data can be sourced
Where FSR rainfall values are used the The specified infiltration test methodology For example, tests should not be undertaken
designer must demonstrate that rainfall online at fehweb.ceh.ac.uk should be representative of the proposed 1.5m below ground level when shallow
values are consistent with FEH 2013 data. design. infiltration is proposed from permeable
pavement, rain gardens or basins which will
The depth of water and depth of test trench
be located close to ground surface.
below ground level should seek to replicate
the attributes of the proposed infiltration
9.5.4.2 Defining runoff coefficients (Cv) system.
79 In extreme rainfall conditions the losses Some modelling software packages contain Bromsgrove Civic Centre re-development. 80
anticipated from hard development surfaces ‘Default’ Cv values (0.75 Summer, 0.84 Permeable block and slab paving with a central grass detention basin
such as roofs or paved areas are anticipated Winter) which assume that there will be 25% provide a fully integrated infiltrating SuDS scheme.
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 (Thames Water).
Newham Council SuDS D & E Guide © 2020 McCloy Consulting & Robert Bray Associates Newham Council SuDS D & E Guide © 2020 McCloy Consulting & Robert Bray Associates
