Case study
A drainage strategy for a single dwelling in Surbiton, prepared for planning and approved.
The constraint
Infiltration testing on site confirmed the ground would not take water, so a soakaway was not an option. That ruled out the top of the drainage hierarchy and set the shape of everything that followed.
What we did
The scheme uses a green roof, a proprietary treatment system, filter drains, permeable paving and a detention basin.
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 |
|---|---|---|
| Rainwater Harvesting | No | The use of rainwater harvesting is not considered economically viable on this site considering installation and operational costs. |
| Green Roofs | Yes | A green roof has been proposed. |
| Infiltration Systems | No | Ground conditions are not suitable for the use of infiltration. |
| Proprietary Treatment Systems | Yes | An ACO V Septor 1000 has been proposed to help capture any sediment and contaminates. |
| Filter Strips | No | Filter strips have not been considered the most effective proposal for this site due to the proposed site layout. |
| Filter Drains | Yes | A filter drain is proposed. |
| 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 | Yes | It is recommended that all hard surfacing is of permeable construction however due to the cohesive nature of the ground our model takes into account all hardstanding catchments entering the proposed system Due to tight constraints onsite, the attenuation is provided in. |
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 Surbiton, browse the rest of our case studies, or see what a drainage strategy involves.