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Medium risk Case Study 3: Retail car park Pollution hazard level
Total suspended solids (TSS) Metals
Hydrocarbons
Medium 0.7 0.6 0.7
Table 5: Pollution hazard indices for retail car park (source: CIRIA C753 Table 26.2)
Type of component* TSS Metals Hydrocarbons Vortex separator Detention basin
0.8 0.5
0.5 0.5
0.7 0.6
Table 6: Mitigation indices for SuDS components selected (source: CIRIA C753 Table 26.3)
*Remember that after the first SuDS component in the treatment train, only half of the mitigation indices for other downstream components can be used in the calculation.
For a site where there will be a larger car park, such as a retail development, SuDS management trains must be designed to cope with anthropogenic pollutants in the surface water: tyre wear particles, brake dust and hydrocarbons. Permeable paving may not be appropriate as an infiltration system as the risk of pollutants getting into the groundwater could be too high.
In this imagined development, there is sufficient space around the parking bays for a detention basin which will hold surface water in the case of heavy rainfall events. However, before the water goes to the basin, it must be pre-retreated to remove as much of the sediment as possible, and with it the pollutants that are attached to the sediment particles. A vortex separator, upstream of the basin, could perform this role.
From a biodiversity perspective, note that just because it is 8
possible to create a new habitat doesn’t mean it is the right thing to do. Creating a small green space in a very dense urban area with no connectivity to other green spaces could be detrimental, rather than beneficial, to any wildlife that finds its way there.
Medium risk Case Study 4: Car park, constrained space
Pollution hazard level
Total suspended solids (TSS) Metals
Hydrocarbons
Medium 0.7 0.6 0.7
Table 7: Pollution hazard indices for retail car park (source: CIRIA C753 Table 26.2)
Type of component TSS Metals Hydrocarbons 0.8
StormTech with Isolator Row
0.6 0.7
Table 8: Mitigation indices for SuDS components selected (source: British Water Web Site – List of Assessed Surface Water Treatment Devices)
*Remember that after the first SuDS component in the treatment train, only half of the mitigation indices for other downstream components can be used in the calculation.
For car parks and other areas with a similar pollution risk, where the space for vegetative SuDS is limited, underground attenuation and treatment could make sense. The constraint on space could be due the limited land availability, which can be the case with smaller brownfield plots, or because losing too many parking spaces would make a development unviable.
In this situation, below-ground attenuation and treatment devices could work well. While components such as geocellular
| February 2024 |
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