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9.4 Critical levels
Levels are important in any drainage system ■ Areas contributing to a storage
and especially so for surface based SuDS. component should not be situated below
The proposed surface levels should align with the top level of storage as they may flood
the modified flow route analysis in providing prior to the storage being filled. Detailed Design
a flow path across the site and storage ■ For storage components that are sloping,
volumes can be significantly affected by
such as permeable pavements or linear
inaccurate levels.
basins, the ‘effective’ storage should be
The following levels should be evaluated determined rather than the entire volume
when developing or reviewing a design: of the structure.
■ The flow control invert level relative to ■ A review of site levels should not identify
storage - the flow control should not be any obvious obstructions along
situated above the base level of the exceedance flow paths.
storage component unless there is a
Note :
requirement for permanent or semi-
permanent water. The LLFA will carry out a high-level review
of levels only - Liability for design is
■ The overflow level should demonstrate
retained by the designer in all cases.
that the required volume of storage is
contained between the flow control invert
level and the overflow level. Grey to Green project, Sheffield.
The 3 flow control criteria: low flow, overflow 76
Facing: Accurate levels were critical at Bewdley and exceedance are demonstrated
School Science Block. elegantly here.
London Borough of Hillingdon SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates
