Case study
A drainage strategy for a single dwelling in Spilsby, Lincolnshire, prepared for planning and approved.
The constraint
Infiltration testing confirmed the ground would take water, so the scheme could discharge at the top of the drainage hierarchy rather than relying on a restricted discharge.
What we did
The scheme uses rainwater harvesting, infiltration, 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 | Witham |
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 | Yes | Rain water bus have been proposed to help promote water reuse. |
| 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. |
| Infiltration Systems | Yes | Soakaways and permeable surfaces proposed. |
| Proprietary Treatment Systems | No | The use of proprietary treatment systems is not considered economically viable or required on this site considering installation and operational costs. |
| 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 | Yes | Access and parking areas are proposed to be of permeable construction. Soakaway crates to act as attenuation prior to discharge into the. |
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, based on sustainable drainage principles and an assessment of the hydrological and hydrogeological context of the development, including infiltration testing carried out in accordance with BRE Digest 365, has been submitted to and approved in writing by the Local Planning Authority.
The infiltration testing and the soakaway sizing 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 Spilsby, Lincolnshire, browse the rest of our case studies, or see what a drainage strategy involves.