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Detailed Design 9.5.10 Calculation checklist Information for technical

Key calculation inputs and outputs should be
presented in the ‘Flows and Volumes

checklist’ (see appendix). The following
checklist identifies useful calculation checks:

Parameter
Guidance on design/calculation input
Rainfall
Confirm the rainfall source and
data is used, conversion factors should be applied to
any conversions applied to
data. FEH 2013 rainfall data preferred. Where FSR rainfall assessment
bring in line with FEH rainfall data. data.
Areas All area of contributing runoff should be represented Provide a drawing clearly
generating within the storage calculation. identifying the areas of surface
runoff The designer must justify where a Cv of less than 0.9 runoff contribution within each
for impermeable area is used for calculations. subcatchment.
Designer to state Cvs used and
justify use of Cv less than 0.9.
Maximum Statutory authorities e.g. LLFA, sewerage undertaker, The flow control rate should be
flow control IDB or EA, might place restrictions on the outfall flow identified along with the
rate rates based on the available capacity of receiving method for defining the rate.
infrastructure.
Climate CCA has been applied within calculations based on Designer to justify selection of
91 change design life of development and any applied sensitivity CCA based on development
allowance assessment. type and design life.
Urban creep Urban creep allowance applied to non-adoptable Designer to justify selection of
impermeable areas on developments where permitted Urban Creep percentage
development is likely to occur.
Initial As a rule of thumb, where the area of development is Designer to confirm whether
interception no greater than 4 times the SuDS wetted area, a 5mm 5mm interception losses have
losses allowance may be made for interception losses for been applied in calculation.
2
each m of development.
Critical A range of rainfall durations must be considered when Designer to demonstrate that
duration calculating attenuation storage. sufficient rainfall durations have
been considered to achieve
worst case scenario.
Control of Where the designer demonstrates that water can be Designer to confirm how
runoff ‘lost’ or stored separately Approach 1 can be applied volume of runoff has been
volume for the control of flow being discharge from the site. controlled.

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