Case study
A drainage strategy for a garden centre in Hemel Hempstead, Hertfordshire, 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 infiltration, a proprietary treatment system, 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 | 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 |
|---|---|---|
| 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. |
| Infiltration Systems | Yes | It is proposed that surface water runoff from the roof will utilise a crate type soakaway with all other surfaces being of permeable construction. |
| Proprietary Treatment Systems | Yes | An ACO StormSed Vortex unit is proposed to treat runoff from the roof area and improve the quality of water infiltrating into the ground. |
| 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 | The use of porous paving is considered economically viable on this site considering installation and operational costs. The use of soakaway crates will act as attenuation prior to discharge. |
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
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.
Browse the rest of our case studies, see what a drainage strategy involves, or find where we work.