Case study
A drainage strategy for a single dwelling in Chesham, 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 and permeable paving.
The numbers
| Climate change allowance, 1 in 30 year | 35% |
| Climate change allowance, 1 in 100 year | 40% |
| Management catchment | Colne |
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. Cellular Soakaway Systems have been proposed. |
| Proprietary Treatment Systems | No | The use of proprietary treatment systems is not considered required for this development due to the lack of hardstanding area that is subject to oil, or similar pollutants caused by vehicular. |
| Filter Drains | No | Filter Drains have not been proposed as they are not deemed required. |
| Swales | No | Swales have not been proposed as they are not deemed required. |
| Bioretention Systems | Yes | The use of rain gardens or over ground planters is recommended to help capture water at source and naturally reduce flows into the proposed soakaway. |
| Porous Pavements | Yes | It is recommended that all hard surfaces are to be of permeable construction. |
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 Chesham, browse the rest of our case studies, or see what a drainage strategy involves.