Hillingdon SuDS Design & Evaluation Guide

Detailed Design
9.5 Designing for hydraulic requirements
Development causes an increase in runoff Expressing calculation outputs in an
which increases the risk of flooding on site understandable format allows for easy
and elsewhere. Where runoff is temporarily application within the design process as well
stored it allows for a controlled release either as transparency for evaluation.
into the ground or into a watercourse or
sewer.
The storage volume required can be
estimated using information such as the local
rainfall characteristics and the rate at which
flow is controlled to leaving the site.
9.5.1 Objectives of hydraulic calculations
Hydraulic calculations can:
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make allowance for unknown factors such
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inform and validate the SuDS design
as potential for runoff from off-site
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provide confidence that there is sufficient
capacity to cater for the additional runoff
generated by the development to desired
design standards
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provide confidence that SuDS will
function hydraulically and will not be
prone to erosion.
9.5.2 What calculations should demonstrate
Designers should demonstrate through the
calculation process:
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how the rates and volumes of runoff
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that the correct calculation inputs and
generated from development will not
processes have been used
pose a flood risk within site boundary or
elsewhere
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where exceptional flows are experienced,
such as; design exceedance, instances of
■■
that future impacts to runoff such as
blockage, or flows from offsite, they can
climate change and urban creep are
be managed within flow routes without
accounted for
causing unreasonable risk to humans or
development.
London Borough of Hillingdon SuDS D & E Guide
© 2018 McCloy Consulting & Robert Bray Associates