SuDS Water quantity and Quality – LLFA Technical Assessment Proforma
Notes
1. All area with the proposed application site boundary to be included.
2. The site area which is positively drained includes all green areas which drain to the SuDS system and
area of surface SuDS features. It excludes large open green spaces which do not drain to the SuDS
system.
3. Impermeable area should be measured pre and post development. Impermeable surfaces include,
roofs, pavements, driveways and paths where runoff is conveyed to the drainage system.
4. Predevelopment use may impact on the allowable discharge rate. The LLFA will seek for reduction in
flow rates to GF (Essex SuDS Design Guide).
5. Runoff may be discharge via one or more methods.
6. Sewers for Adoption 6
th
Edition recommends a Cv of 100% when designing drainage for impermeable
area (assumes no loss of runoff from impermeable surfaces) and 0% for permeable areas. Where
lower Cv’s are used the applicant should justify the selection of Cv.
7. It is Essex County Council’s preference that discharge rates for all events up to the 1 in 100 year event
plus climate change are limited to the 1 in 1 greenfield rate. This is also considered to mitigate the
increased runoff volumes that occur with the introduction of impermeable surfaces. If discharge rates
are limited to a range of matched greenfield flows then it is necessary to provide additional mitigation of
increased runoff volumes by the provision of Long-term Storage.
8. Storage for the 1 in 30 year must be fully contained within the SuDS components. Note that standing
water within SuDS components such as ponds, basins and swales is not classified as flooding.
Storage should be calculated for the critical duration rainfall event.
9. Runoff generated from rainfall events up to the 1 in 100 year will not be allowed to leave the site in an
uncontrolled way. Temporary flooding of designated areas to shallow depths and velocities may be
acceptable.
10. The following information should only be provided if increased runoff volumes are not mitigated by
limiting all discharge rates back to the greenfield 1 in 1 year rate.
11. Climate change is specified as 40% increase to rainfall intensity, unless otherwise agreed with the
LLFA / EA.
12. To be determined using the 100 year return period 6 hour duration winter rainfall event.
13. Where Source Control is provided Interception losses will occur. An allowance of 5mm rainfall depth
can be subtracted from the net inflow to the storage calculation where interception losses are
demonstrated. The Applicant should demonstrate use of subcatchments and source control
techniques. Further information is available in the SuDS Design Guide.
14. Please refer to Rain harvesting BS for guidance on available storage.
15. Flows within long term storage areas should be infiltrated to the ground or discharged at low flow rate
of maximum 2 l/s/ha.
16. Careful consideration should be used for calculations where flow control / storage is likely to be
influenced by surcharged sewer or peak levels within a watercourse. Outlets can be tidally locked
where discharge is direct to estuary or sea. Calculations should demonstrate that risk of downed outlet
has been taken into consideration. Vortex controls require conditions of free discharge to operate as
per specification.
17. In controlling the volume of runoff the total volume from mitigation measures should be greater than or
equal to the additional volume generated.
Revision 1.1 – Issued 2019
