Case study
A drainage strategy for an extension in Walsall, 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 a detention basin.
Attenuation storage of 18.9m³ was provided for the 1-in-100-year event including a 40% climate change allowance.
The numbers
| Discharge rate | 0.6l/s |
| Attenuation storage | 18.9m³ |
| Climate change allowance, 1 in 30 year | 35% |
| Climate change allowance, 1 in 100 year | 40% |
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 |
|---|---|---|
| Rainwater Harvesting | No | The use of rainwater harvesting is not considered the most effective or economical way capturing water at source on this site considering installation and operational costs. |
| Green Roofs | No | Green roofs have not been proposed for this site as they are not considered the most effective or economical way of capturing water at source for the proposed development. |
| Infiltration Systems | No | Systems such as soakaways are not considered viable for this site due to underlaying ground conditions and the lack of available space to site the soakaway whilst ensuring compliance with Building Regulations. |
| Proprietary Treatment Systems | No | The use of proprietary treatment systems is not considered economically viable or deemed required on this site. |
| 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. |
| Swales | No | Swales are not suitable for this scheme due to available space and proposed land use. |
| Bioretention Systems | No | Bioretention Systems have not been considered the most effective proposal for this site due to the lack of available landscape areas. |
| Porous Pavements | No | The use of porous paving is not considered suitable or viable for the proposed development Attenuation storage has been included within the proposed. |
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
Hydraulic simulation showed no flooding in the 1-, 30- or 100-year storm events, including the climate change allowance. 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 Walsall, browse the rest of our case studies, or see what a drainage strategy involves.