SuDS Water quantity and Quality – LLFA Technical Assessment Proforma
Introduction
This proforma identifies the information required by Essex LLFA to enable technical assessment the Designers
approach to water quantity and water quality as part of SuDS design approach in compliance with Essex SuDS
Design Guide.
Completion of the proforma will also allow for technical assessment against Non-statutory technica
l
standards
(NSTS) for Sustainable Drainage.
The proforma will accompany the site specific Flood Risk
Assessment and Drainage Strategy submitted as part of the planning application.
Please complete this form in full for full applications and the coloured sections for outline appllications. This will
help us identify what information has been included and will assist with a smoother and quicker application.
Instructions for use
Use the units defined for input of figures.
Numbers in brackets refer to accompanying notes.
Where ………m
3
………m
3
/m
2
are noted both values should be filled in.
Site details
1.1 Planning application reference (if known)
1.2
Site name
1.3
Total application site area
(1)
ha
1.4
Predevelopment use
(4)
1.5
Post development use
1.6
Urban creep applicable
1.7
Proposed design life / planning application life
1.8
Method(s) of discharge:
(5)
Waterbody
Storm sewer
Combined sewer
Reuse
1.9
Is discharge direct to estuary / sea
1.10
Have agreements in principle (where applicable) for discharge been provided
Revision 1.1 Issued 2019
If other, please sepcify
Infiltration
Hybrid
if yes, factor applied:
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SuDS Water quantity and Quality LLFA Technical Assessment
Calculation inputs
2.1
Area within site which is drained by SuDS
(2)
m
2
2.2
Impermeable area drained pre development
(3)
m
2
2.3
Impermeable area drained post development
(3)
m
2
2.4
Additional impermeable area
(2.3 minus 2.2)
m
2
2.5
Method for assessing greenfield runoff rate
2.6
Method for assessing brownfield runoff rate
2.7
Coefficient of runoff (Cv)
(6)
2.8
Source of rainfall data (FEH Preferred)
2.9
Climate change factor applied
%
2.10
Drainage outlet at risk of drowning (tidal locking, elevated water levels in watercourse/sewer)
Note: Vortex controls require conditions of free discharge to operate as per manufacturers specification.
2.11
Invert level at final outlet mAOD
2.12
Design level used for surcharge water level at point of discharge
(16)
mAOD
Infiltration (Discharge to Ground)
2.13
Have infiltration tests been undertaken
2.15
Infiltration rate (where applicable)
m/s
2.16
Depth to highest known ground water table mAOD
2.18
Depth of infiltration feature mAOD
2.19
Factor of safety used for sizing infiltration storage
Revision 1.1 – Issued 2019
If yes, which method has been used
2.14
If there are multiple infiltration features please specify where they can be found in the FRA
2.17
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SuDS Water quantity and Quality LLFA Technical Assessment Proforma
Calculation outputs
Sections 3 and 4 refer to site where storage is provided by full attenuation or partial infiltration. Where all flows
are infiltrated to ground go straight to Section 6.
5.0
Proposed maximum r
ate of runoff f
rom
si
t
e
(incl.
Urban Creep)
(7)
5.1
1 in 1 year rainfall l/s/ha, l/s for the site
5.2
1 in 30 year rainfall l/s for the site
5.3
1 i
n 100 year
rainfall
+
CCA
l/s for the site
6.0
Attenuation storage to manage flow rates from site (incl. Climate Change Allowance (CCA) and Urban Creep)
4.3
m
3
/m
2
6.2
50% storage drain down time 1 in 30 years
m
3
7.0
Controlling volume of runoff from the site
(10)
5.1
Pre development runoff
volume
(12)
(development area)
7.2
Post development runoff volume (unmitigated)
(12)
7.3
Volume to be controlled (5.2 - 5.1)
m
3
for the site
m
3
for the site
m
3
for the site
Revision 1.1 – Issued 2019
hours
l/s/ha,
l/s/ha,
6.1
Stor
age - 1 i
n 100 year
+
CC
A
(9)
7.1
3.0
Greenfield runoff rates (incl. Urban Creep)
3.1
1 in 1 year rainfall l/s/ha, l/s for the site
3.2
1 in 30 year rainfall l/s for the site
3.3
1 in 100 year rainfall + CCA
l/s for the site
l/s/ha,
l/s/ha,
4.0
Brownfield runoff rates (incl. Urban Creep)
4.1
1 in 1 year rainfall l/s/ha, l/s for the site
4.2
1 in 30 year rainfall l/s for the site
4.3
1 in 100 year rainfall + CCA
l/s for the site
l/s/ha,
l/s/ha,
Revision 1.1 – Issued 2019
SuDS Water quantity and Quality LLFA Technical Assessment Proforma
Design Inputs
Proposed site use
Pollution hazard
category (see C753 Table 26.2)
High risk area defined as area storing fuels chemicals, refuelling area, washdown area, loading bay.
Design Outputs
List order of SuDS techniques proposed for treatment
Note that gully pots, pipes and tanks are not accepted by Essex LLFA as a form of treatment (for justification
see C753 Section 4.1, Table 26.15 and Box B.2)
Are very high pollution risk areas drained separate from SuDS to foul system
m
3
m
3
m
3
/m
2
(of developed impermeable area)
m
3
/m
m
2
7.4
Volume control provided by
:
m
3
m
3
m
3
-
Interception losses
(13)
- Rain harvesting
(14)
- Infiltration
- Attenuation
m
3
-
Separate volume designated as l
ong term storage
(15)
m
3
m
3
(17)
7.5
Total volume control (sum of inputs for 5.4)
8
.0 Site storage volumes (full infiltration only)
8.1
Storage - 1in 30 year + CCA
(8)
8.2
Storage - 1 in 100 year + CCA
(11)
Other
Please include any other information that is relevant to your application
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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
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