Page 97 - Bexley SuDS DESIGN & EVALUATION

Information for technical
Parameter Guidance on design/calculation input
assessment
Modelling It is not anticipated that SuDS design will require The designer is to justify where
of the SuDS modelling of extensive piped systems. In some no hydraulic modelling is
layout. instances where the scheme is relatively small and not undertaken. Calculations/model Detailed Design
hydraulically complex standard calculations will be outputs should be provided to
accepted in lieu of a hydraulic model. Layout support the Flows and Volumes
drawings should be clearly labelled with the proforma
numbering convention used by models.
Outfall Outfalls into receiving sewers or watercourses can be Designer is to indicate whether
design at risk of surcharge and lack of free discharge due to SuDS storage calculation is
elevated water levels. This can result in additional likely to be influenced by high
storage being required. Free discharge should not be water levels at the point of
assumed. The risk of surcharge should be assessed discharge.
and accounted for within calculations as appropriate.
Long Long sections will allow detailed consideration of Long section showing peak
section levels across the site. water levels.
Erosion Flows along swales (or other vegetated surfaces) are Designer to demonstrate that
check at risk from erosion. Peak flow velocities should be they have considered risk of
less than 1 - 2 l/s. erosion and taken measures to
Concentrated inlet points are also prone to erosion. safeguard scheme. Peak flow
velocity calculations to be
provided as appropriate.
Designing The design should incorporate overflows at each Locations of overflows should 92
for SuDS component. Hydraulic calculations should be identified on the layout
exceedance demonstrate that overflows have sufficient capacity to drawing along with proposed
deal with anticipated flow rates. SuDS layout drawing exceedance flow route.
should identify the anticipated flow route for
exceedance events.
Managing The FRA should identify the potential for flows from The designer should
flows from offsite. These flows can be unpredictable and difficult demonstrate how anticipated
off site. to quantify. Management of flows through the site flows from off site will be
should not increase flood risk elsewhere. managed through the site using
Detailed modelling to establish the rates of flow the layout drawing and design
anticipated would not be considered compulsory (but statement.
may be required on a case by case basis).
Consistency Detailed design of SuDS components should reflect Drawings should clearly identify
of hydraulic calculations / hydraulic models, taking into site levels, storage locations and
calculations account slopes and low lying levels. flow controls with cross
and design. The LLFA will consider design drawings to ensure that sections and long sections. The
flow control sizing and storage provision is as per design statement should
calculations. confirm that drawings deliver
calculated volumes.

London Borough of Bexley SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates