Page 83 - Updated SuDS Design & Evaluation Guide-Newham V3 Spreads

P. 83

Detailed Design 9.5 Designing for hydraulic requirements 9.5.3 Calculation processes There are a number of methods that can be Detailed Design

■
the long-term storage volume that needs
Expressing calculation outputs in an
Development causes an increase in runoff
Calculations used in SuDS design should
always be viewed as estimates of what is
understandable format allows for easy
to be managed (see 8.4.7)
which increases the risk of flooding on site
and elsewhere. Where runoff is temporarily
application within the design process as well
experienced in reality. Calculation outputs
■
flow velocities.
will vary depending upon how inputs are
as transparency for evaluation.
stored it allows for a controlled release either
selected and the calculation process used.
into the ground or into a watercourse or
sewer.
The calculations for SuDS design are used to
The storage volume required can be
manual calculations, spreadsheets, online
estimated using information such as the local
■
tools and a variety of hydraulic modelling
appropriate discharge rates via infiltration
rainfall characteristics and the rate at which assess: used to carry out the calculations including
or controlled discharge rates to a software packages.
flow is controlled to leaving the site.
watercourse or sewer
Calculation processes are summarised in the
■ the volume of runoff that requires storage following table:
to allow infiltration or attenuation to
9.5.1 Objectives of hydraulic calculations controlled discharge rates (see 9.6)
Hydraulic calculations can: ■ make allowance for unknown factors such Calculation process Purpose of calculation Main calculation inputs
as potential for runoff from off-site
■ inform and validate the SuDS design Runoff rates from Used to define flow control rate Local rainfall data; site area; soil
■ provide confidence that SuDS will
■ provide confidence that there is sufficient greenfield and characteristics.
capacity to cater for the additional runoff function hydraulically and will not be brownfield sites
prone to erosion.
77 generated by the development to desired estimate 78
design standards 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).
9.5.2 What calculations should demonstrate
Long term storage Determining the difference in the Local rainfall data; site area; existing
Designers should demonstrate through the estimate volume of runoff between pre- site impermeable area; proposed site
calculation process: development and post development impermeable area; infiltration rates;
scenarios soil characteristics; rain harvest
■ how the rates and volumes of runoff ■ that the correct calculation inputs and volume, losses provide by SuDS,

generated from development will not processes have been used proposed discharge rate(s).
pose a flood risk within site boundary or ■ Flow velocity check Flow velocity calculated to ensure: Component sectional geometry;
elsewhere where exceptional flows are experienced, component gradient; component
such as; design exceedance, instances of Conveyance along vegetated surface type (roughness); proposed
■ that future impacts to runoff such as blockage, or flows from offsite, they can channels do not cause erosion;
flow rates.
climate change and urban creep are be managed within flow routes without
Low flow velocities for 1 in 1 year
accounted for causing unreasonable risk to humans or rainfall to allow settlement of silt.
development.

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