SAB Evaluation sheet – SuDS Flows and Volumes
This form identifies the information required by Hackney SuDS Approval Body (SAB) to enable evaluation of SuDS flows
and volumes. The applicant may supplement their submission with further information (software outputs for example).
Hackney SAB will not review any volume submission unless calculation inputs and outputs are provided in this standard
format.
Note : * means delete as appropriate; Numbers in brackets refer to accompanying notes.
SITE DETAILS
1.1
Planning application reference
…………………………………………………………………
1.2
Site name
…………………………………………………………………
1.3
Total application site area (1)
…………………………m2
VOLUME AND FLOW DESIGN INPUTS
2.1
Site area which is positively drained by SuDS (2) …………………………m2
2.2
Impermeable area proposed for development (3) …………………………m2
2.3
Predevelopment use (4)
Greenfield / Brownfield / Mixed*
2.4
Method of discharge
Infiltration / waterbody / storm sewer / combined sewer*
2.5
Infiltration rate (where applicable)
…………………………m/hr
2.6
Depth to maximum ground water table
…………………………
2.7
Coefficient of runoff (Cv) (5)
…………………………
2.8
Rainfall data used for calculation
FEH / FSR*
2.9
Method(s) of source control
2.10
Treatment train (name stages) ………………………………………………………………………
………………………………………………………………………
CALCULATION OUTPUTS
Sections 3 and 4 refer to site where storage is provided by attenuation and / or partial infiltration. Where all flows are
infiltrated to ground please ignore Section 3 and 4 and complete Section 5.
3.0
Controlling flow discharging from the site
3.0(a) Qbar rate used in calculations ………………..
3.1
Approach used to control flows (6)
Approach 1 / Approach 2/Agreed betterment rate*
3.2
Max. discharge for 1 in 1 year rainfall event
……………l/s/ha, ……………l/s for the site
Revision 1.1 – Issued June 2014
SAB Evaluation sheet – SuDS Flows and Volumes
3.2
Max. discharge for 1 in 2 year / Qbar rainfall event ……………l/s/ha, ……………l/s for the site
3.3
Max. discharge for 1 in 30 year rainfall event
……………l/s/ha, ……………l/s for the site
3.4
Max. discharge for 1 in 100 year rainfall event
……………l/s/ha, ……………l/s for the site
4.0
Site storage volumes (full attenuation / partial infiltration & partial attenuation)
4.1
Source control provided (7)
Yes/No*
4.2
Storage - 1 in 1 year
………m3 ………m3/m2 (of developed impermeable area)
4.3
Storage - 1in 30 year (8)
………m3 ………m3/m2
4.4
Storage - 1 in 100 year plus CC (9)
………m3 ………m3/m2
4.5
Long term storage (10)
………m3 ………m3/m2
4.6
Volume discharged via infiltration
………m3 ………m3/m2
4.7
Total site storage (11)
………m3
5.0
Site storage volumes (full infiltration)
5.1
Source control provided (7)
Yes/No*
5.2
Storage - 1 in 1 year
………m3 ………m3/m2 (of developed impermeable area)
5.3
Storage - 1in 30 year
5.4
Storage - 1 in 100 year plus CC (9)
………m3 ………m3/m2
5.5
Total site storage (11)
………m3
(8)
………m3 ………m3/m2
DESIGN CHECKS
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
6.12
Have flow routes through the site been identified?
Has the site been split up into a series of subcatchments and SuDS techniques provided in each?
Have critical durations been determined for storage calculations?
Has infiltration been used as a means of discharging flow (large areas of Hackney are suitable for infiltration)?
How has runoff from green areas been accounted for within calculations?
Is SuDS storage located outside Q100 floodplain?
Is the point of discharge invert level below the predicted watercourse flood level or sewer surcharge level?
Does the storage volumes calculated match the provision made on drawings?
Does the depth of storage in open water features meet health and safety requirements?
Time taken for 50% of attenuation storage to drain down less than 24 hours?
Have flow controls been adequately protected from blockage?
Has provision been made for blockage / design exceedance within the design?
Revision 1.1 – Issued June 2014
SAB Evaluation sheet – SuDS Flows and Volumes
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 the area
within which the vegetated SuDS reside. It excludes large open green spaces which do not drain to the SuDS
system.
3. The developed area is identified as all areas which are developed as impermeable surfaces, including, roofs,
pavements, driveways and paths. All vegetated areas (landscaped areas, gardens, public open space and
vegetated SuDS features) and surface waterbody areas can be excluded from the storage calculation.
4. Discharge from the site by means other than infiltration, means that the site cannot be fully drained by infiltration.
The present use of the site (Greenfield / Brownfield) and the way that storage is calculated (Approach 1 /
Approach 2/PDL) will dictate the allowable discharge flow rate from the site. Where the site is mixed use,
calculations should be developed for the respective sections of the site.
5. Sewers for Adoption 6th Edition recommends a runoff coefficient of 100% when designing drainage for
impermeable area. In relation to the Modified Rational Method default Cv factors of 0.75 (summer) and 0.84
(winter) assume that at least 50% of the drainage area is permeable and are therefore not considered applicable
whenever a calculation considers impermeable area only.
6. There are two general approaches to for providing storage, with a brief description is provided below;
a. Approach 1- Allows maximum flow rate leaving the site to increase in relation to rainfall return period. For
this approach Long Term Storage must be provided.
b. Approach 2- Requires a maximum flow rate leaving the site for the 1 in 100 year rainfall event to be
restricted to the Greenfield Qbar flow rate. Long Term Storage is not provided with Approach 2.
c. Previously Developed Land (PDL) – To increase flood resilience in the catchment, Hackney suggest that
runoff from PDL be restricted to greenfield rates. Where restriction to greenfield rates cannot be achieved
Hackney will seek that the first 30mm of rainfall for all return periods is controlled to Greenfield runoff
rates. The remainder of flows will be allowed to discharge un-attenuated.
7. 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. To make the allowance the Applicant must demonstrate;
a. The site has been split up into sub-catchments.
b. A SuDS component acting as a Source Control technique is located within each sub-catchment.
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.
9. Runoff generated from rainfall events up to the 1 in 100 year (+CC) will not be allowed to leave the site in an
uncontrolled way. Temporary flooding of specified areas to shallow depths (150mm) may be permitted in
agreement with Hackney.
10. All attenuation storage requirements should be calculated for the critical duration.
11. Flow diverted to Long term storage areas should be infiltrated to the ground, or where this is not possible,
discharged to the receiving water at slow flow rates (maximum 2 l/s/ha). LT storage would not be allowed to
empty directly back into attenuation storage and would be expected to drain away over 5-10 days. Typically LT
storage may be provided on multi-functional open space or sacrificial car parking areas.
12. The total site storage is the full storage provided by the SuDS on site. Care should be taken not to duplicate
storage within the calculation. The following examples are provided.
a. Approach 1 Site storage = 1 in 100 year plus CC storage + LT Storage
b. Approach 2 Site storage = 1 in 100 year plus CC storage
Revision 1.1 – Issued June 2014