Case study
A drainage strategy for an extension in New Malden, prepared for planning and approved.
The constraint
The first question on any site is whether the ground will take water, because that decides whether the scheme is a soakaway or attenuation with a restricted discharge.
What we did
The scheme uses rainwater harvesting, raingardens, permeable paving and attenuation storage.
The numbers
| Climate change allowance, 1 in 30 year | 35% |
| Climate change allowance, 1 in 100 year | 40% |
| Management catchment | London |
Every SuDS option, and why
Each option in the SuDS hierarchy was considered for this site. This is the assessment as it went into the report:
| Option | Used | Why |
|---|---|---|
| Green Roofs | No | Green roofs have not been proposed for this site as there is insufficient access to roof areas for maintenance and as such the system could not be effectively maintained to ensure long term performance. Ground conditions are not deemed suitable for the use of. |
| Filter Strips | No | Filter strips have not been considered the most effective proposal for this site due to the proposed site layout. |
| Filter Drains | No | Filter Drains have not been considered the most effective proposal for this site due to the proposed site layout. August 24. |
| Swales | No | Swales are not suitable for this scheme due to available space and proposed land use. |
| Bioretention Systems | Yes | A rain garden has been proposed to help reduce flow of run-off into proposed network. |
| Porous Pavements | Yes | Although ground conditions are not suitable for the use of infiltration, all surfaces should be of permeable construction. |
| Attenuation Storage | Yes | Tanked permeable sub-base has been proposed to effectively capture surface water at source and provide aDention to the system. |
The kind of condition this discharges
Wording differs between officers, but a scheme like this one is normally answering a condition along these lines:
No development shall commence until a detailed surface water drainage scheme, demonstrating that surface water will be restricted to greenfield run-off rates for all storm events up to and including the 1 in 100 year event plus an allowance for climate change, has been submitted to and approved in writing by the Local Planning Authority.
The greenfield run-off calculation and the attenuation volume are what answer this one. There is a fuller guide to discharging drainage planning conditions if you want to read up on the process.
Outcome
The scheme was approved at planning.
Every scheme in these case studies was approved — that is the point of the exercise. If you have a drainage condition to discharge, send us the wording and the site address and we will tell you exactly what is needed.
More on drainage design in New Malden, browse the rest of our case studies, or see what a drainage strategy involves.