Hillingdon SuDS Design & Evaluation Guide

9.4 Critical levels
Levels are important in any drainage system
and especially so for surface based SuDS.
The proposed surface levels should align with
the modified flow route analysis in providing
a flow path across the site and storage
volumes can be significantly affected by
inaccurate levels.
The following levels should be evaluated
when developing or reviewing a design:
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Areas contributing to a storage
component should not be situated below
the top level of storage as they may flood
prior to the storage being filled.
For storage components that are sloping,
such as permeable pavements or linear
basins, the ‘effective’ storage should be
determined rather than the entire volume
of the structure.
Detailed Design
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The flow control invert level relative to
storage - the flow control should not be
situated above the base level of the
storage component unless there is a
requirement for permanent or semipermanent
water.
The overflow level should demonstrate
that the required volume of storage is
contained between the flow control invert
level and the overflow level.
Facing: Accurate levels were critical at Bewdley
School Science Block.
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A review of site levels should not identify
any obvious obstructions along
exceedance flow paths.
Note :
The LLFA will carry out a high-level review
of levels only - Liability for design is
retained by the designer in all cases.
Grey to Green project, Sheffield.
The 3 flow control criteria: low flow, overflow
and exceedance are demonstrated
elegantly here.
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London Borough of Hillingdon SuDS D & E Guide
© 2018 McCloy Consulting & Robert Bray Associates