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9.5.3 Calculation processes

Calculations used in SuDS design should ■ the long-term storage volume that needs

always be viewed as estimates of what is to be managed (see 8.4.7)
experienced in reality. Calculation outputs ■ flow velocities.
will vary depending upon how inputs are Detailed Design
selected and the calculation process used.
There are a number of methods that can be
The calculations for SuDS design are used to used to carry out the calculations including
assess: manual calculations, spreadsheets, online
■ appropriate discharge rates via infiltration tools and a variety of hydraulic modelling

or controlled discharge rates to a software packages.
watercourse or sewer
Calculation processes are summarised in the
■ the volume of runoff that requires storage following table:
to allow infiltration or attenuation to
controlled discharge rates (see 9.6)

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 78
Attenuation storage The runoff generated by the site is Local rainfall data; site area;
or infiltration storage balanced against the controlled rate proposed site impermeable area;
estimate. of outflow. 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 pre- site impermeable area; proposed site
development and post development 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;
component gradient; component
Conveyance along vegetated
surface type (roughness); proposed
channels do not cause erosion;
flow rates.
Low flow velocities for 1 in 1 year
rainfall to allow settlement of silt.

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