Page 82 - Bexley SuDS DESIGN & EVALUATION

Detailed Design 9.5 Designing for hydraulic requirements

Development causes an increase in runoff
Expressing calculation outputs in an
understandable format allows for easy
which increases the risk of flooding on site
application within the design process as well
and elsewhere. Where runoff is temporarily
as transparency for evaluation.
stored it allows for a controlled release either
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: ■ make allowance for unknown factors such
as potential for runoff from off-site
■ inform and validate the SuDS design
■ provide confidence that SuDS will
■ provide confidence that there is sufficient
capacity to cater for the additional runoff function hydraulically and will not be
prone to erosion.
77 generated by the development to desired
design standards

9.5.2 What calculations should demonstrate

Designers should demonstrate through the
calculation process:

■ how the rates and volumes of runoff ■ that the correct calculation inputs and

generated from development will not processes have been used
pose a flood risk within site boundary or ■
elsewhere 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 Bexley SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates