Case study
A drainage strategy for 3 dwellings in Irby, 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
Surface water was discharged to the public sewer at a restricted rate agreed with the water company.
The scheme uses raingardens and attenuation storage.
The numbers
| Discharge rate | 0.78l/s |
| Climate change allowance, 1 in 30 year | 35% |
| Climate change allowance, 1 in 100 year | 40% |
| Management catchment | Lower Mersey |
| Site classification | Greenfield |
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 | 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. |
| 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 | Yes | Bioretention Systems in the form of rain gardens have been incorporated into the proposed drainage strategy. |
| Porous Pavements | No | The use of porous paving is not considered economically viable on this site considering installation and operational costs. |
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, including the rate of discharge to the public sewer and written confirmation of the sewerage undertaker’s agreement to that rate, has been submitted to and approved in writing by the Local Planning Authority.
The restricted discharge rate and the storage sized to hold the difference 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 Irby, browse the rest of our case studies, or see what a drainage strategy involves.