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7.4.9 Preliminary flow and volume calculations
It is convenient to consider flow and volume comparison of storage values between similar
requirements at this stage in the design sites.
process to ensure that natural losses are
Expressing storage as ‘volume per m 2 ’ allows
replicated and sufficient volumes of runoff the designer to allocate storage throughout a Concept Design
can be temporarily accommodated to allow
site in discrete sub-catchments, and provides
for discharge from site via a flow control a straightforward way for the evaluation team
and/or infiltration.
to check that calculated storage volumes are
In some circumstances, for example where acceptable.
development is speculative, it may be Ideally each sub-catchment will manage its
acceptable for the Concept Stage to omit own runoff up to the 1 in 100 year return
flow and volume calculations, but a Modified period rainfall event. Where this is not viable,
Flow Route analysis will be required to show part of the storage volume will be provided
that runoff can be effectively conveyed to a depending upon the opportunities for
discharge location. storage within the subcatchment, with all
Storage volumes are usually presented as a residual flows cascaded into an adjacent
single volume. sub-catchment or ‘site control’.
This form of expression encourages the ‘pipe This approach maximises the opportunity for
to pond’ practice and prevents simple storage throughout the development.
40
Flow control with
controlled discharge
C1
1 in 2 from one catchment to
the next
C2
1 in 10 Residual flows
C3
1 in 30
C4
1 in 100 yr (+CCA)
+ residual flows from
C1, C2 & C3 upto
1 in 100 yr (+CCA)
In this example the first three catchments
(C1, C2 & C3) only partially attenuate their
Each catchment may only control and attenuate own runoff, with residual flows passing into
runoff up to lesser rainfall events (eg. 1 in 2 catchment C4 where these residual flows must
years, 1 in 10 year, 1 in 30 years) with residual be attenuated, along with C4’s own runoff, to
flows passing into the next subcatchment. the maixmum design storm (eg. 1 in 100 + CCA).
London Borough of Bexley SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates
