Page 109 - Updated SuDS Design & Evaluation Guide-Newham V3 Spreads

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Detailed Design 9.7.2 What water quality design should demonstrate For low to medium risk sites, the indices Detailed Design
approach for discharge to surface waters is
Effective treatment is provided through
For effective treatment of runoff SuDS
reasonably simplistic to apply.
provision of source controls and a
should be designed to:
A level of understanding of the site’s soil and
management train.
■
reduce the frequency of runoff by
underlying geology is required to undertake
incorporating interception losses
the infiltration risk screening assessment. The
screening assessment will determine whether
■
maximise travel time along the
it will be permissible to infiltrate and the
management train
indices approach is applied to define the level
■
trap a range of contaminates
of treatment required prior to the point of
■ minimise impacts from accidental spillage. infiltration.
Discussion will be required with EA where the
SPZ areas identified on the EA website:
site overlies Source Protection Zones 1 or 2 or http://apps.environment-agency.gov.uk/
where contamination is identified on wiyby/37833.aspx
9.7.3 Hazard and mitigation risk assessment brownfield sites.

Prior to 2015, SuDS water quality design ■ On low to medium risk sites where
adopted the ‘treatment train’ approach. This discharge is to surface water – apply
inferred that treatment was provided by ‘Hazard and Mitigation’ Indices approach

allowing runoff to pass through a series of to identify the number of SuDS
suitable SuDS components prior to components required (CIRIA SuDS Manual
103 discharge. This method remains robust if Section 26.7.1). 104
applied correctly, but has been refined by the
■ For medium risk sites where discharge is
2015 CIRIA SuDS Manual which adopts a
via infiltration, undertake risk screening to
‘Source-Pathway-Receptor’ approach, with
establish whether infiltration will be
the extent of analysis required associated
permitted and apply the Indices approach
with the level of risk.
to identify the number of SuDS
The varying levels of assessment are components required prior to infiltration
identified as follows: (CIRIA SuDS Manual Section 26.7.2).

■ For High Risk sites, there is likely to be a
requirement for a discharge licence. The
Environment Agency will outline level of
assessment required and discharge water Design Notes:
quality parameter compliance limits. On freely draining sites where insufficient treatment is provided at the first stage of treatment
source control, initial SuDS components may require lining to prevent direct infiltration
carrying pollutants into underlying geology.
Design Note:

Table 26.15 of the 2015 SuDS Manual denotes that conventional gully and pipe drainage On low to medium risk sites permeable pavement will provide sufficient treatment prior to
provide zero treatment. infiltration into the ground via the pavement subbase.

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