Hillingdon SuDS Design & Evaluation Guide

9.5.3 Calculation processes
Calculations used in SuDS design should
always be viewed as estimates of what is
experienced in reality. Calculation outputs
will vary depending upon how inputs are
selected and the calculation process used.
The calculations for SuDS design are used to
assess:
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■■
appropriate discharge rates via infiltration
or controlled discharge rates to a
watercourse or sewer
the volume of runoff that requires storage
to allow infiltration or attenuation to
controlled discharge rates (see 9.6)
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the long-term storage volume that needs
to be managed (see 8.4.7)
flow velocities.
There are a number of methods that can be
used to carry out the calculations including
manual calculations, spreadsheets, online
tools and a variety of hydraulic modelling
software packages.
Calculation processes are summarised in the
following table:
Detailed Design
Calculation process Purpose of calculation Main calculation inputs
Runoff rates from
Used to define flow control rate
Local rainfall data; site area; soil
greenfield and
characteristics.
brownfield sites
estimate
Attenuation storage
The runoff generated by the site is
Local rainfall data; site area;
78
or infiltration storage
estimate.
balanced against the controlled rate
of outflow.
proposed site impermeable area;
climate and creep adjustments;
infiltration rates; soil characteristics;
discharge rate(s).
Long term storage
Determining the difference in the
Local rainfall data; site area; existing
estimate
volume of runoff between predevelopment
and post development
site impermeable area; proposed site
impermeable area; infiltration rates;
scenarios
soil characteristics; rain harvest
volume, losses provide by SuDS,
proposed discharge rate(s).
Flow velocity check
Flow velocity calculated to ensure:
Component sectional geometry;
Conveyance along vegetated
channels do not cause erosion;
component gradient; component
surface type (roughness); proposed
flow rates.
Low flow velocities for 1 in 1 year
rainfall to allow settlement of silt.
London Borough of Hillingdon SuDS D & E Guide
© 2018 McCloy Consulting & Robert Bray Associates