Page 44 - Updated SuDS Design & Evaluation Guide-Newham V3 Spreads

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Concept Design 7.4.8 Method of discharge – how rainfall leaves the site 7.4.9 Preliminary flow and volume calculations ’ allows Concept Design

It is convenient to consider flow and volume
1.
re-use on site
comparison of storage values between similar
Rainfall should not discharge into the foul
sites.
sewer.
requirements at this stage in the design
infiltration into the ground
2.
process to ensure that natural losses are
Expressing storage as ‘volume per m 2
The way that rainfall leaves a development
3.
a natural watercourse
replicated and sufficient volumes of runoff
the designer to allocate storage throughout a
should follow the preferred hierarchy:
can be temporarily accommodated to allow
site in discrete sub-catchments, and provides
4.
surface water sewer
for discharge from site via a flow control
a straightforward way for the evaluation team
and/or infiltration.
combined sewer.
5.
acceptable.
In some circumstances, for example where
development is speculative, it may be to check that calculated storage volumes are
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
The final swale at Bewdley School is a colourful discharge location. storage within the subcatchment, with all
outfall into the existing watercourse.
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.
39 Flow control with 40

controlled discharge
C1
1 in 2 yr from one catchment to
the next

C2 Residual flows
1 in 10 yr
C3
1 in 30 yr

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).

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