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suitable sediment capture component such as a vortex grit separator or oil/water separator (road gullies and catch pits are not recommended) which removes most of the sediment before transferring flow to growing plants that can remove more of the phosphorous. More guidance can be found in CIRIA publication C808 Using SuDS to reduce phosphorous in surface water runoff.
Low risk Case Study 2: School Pollution hazard level
Total suspended solids (TSS) Metals
Hydrocarbons
Low 0.5 0.4 0.4
Table 3: Pollution hazard indices for school (source: CIRIA C753 Table 26.2)
Type of component* TSS Metals Hydrocarbons Swale
Permeable pavement Detention basin
0.5 0.7 0.5
Bioretention system 0.8
A bioremediation zone in the middle of Sheffield city.
Consider a new housing development of around 50 homes. According to the CIRIA SuDS Manual, there are likely to be low levels of pollutants in any surface water runoff which means that they can be dealt with using natural SUDS.
Surface water from roads around the development could be collected in swales – shallow drainage channels which run parallel to the roads. These could convey water into a detention basin which would fill up during a heavy rainfall event and then dry out over time, so that pollutants left on the surface are degraded. Run-off from roofs and footpaths would be directed into bioretention zones or rain gardens with engineered media and planting to store and attenuate water.
Residential parking areas may have permeable paving to allow the rainwater to return to the ground where it falls. Permeable concrete block paving is often cited as a permeable paving media, but alternatives include appropriately graded bituminous and concrete pavements, grass reinforcement and bound or unbound gravels.
In areas where planning requirements call for phosphorus neutrality, the treatment train should start by maximising the opportunity for infiltration of stormwater to the ground, since the soil will capture phosphorous in the runoff. A bioretention zone or rain garden would be a good way to achieve this.
Water that cannot be infiltrated may need to pass through a 6
0.6 0.6 0.5 0.8
0.6 0.7 0.6 0.8
Table 4: 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.
When designing SuDS for schools, there should be a strong focus on amenity. There is a fantastic opportunity to tell the story of the water cycle to the children who attend the school through the choice of components selected for the SuDS management train.
Water from the roof of the building can be carried in leaping gutters from the edge of the building into a bioremediation zone, which is an area of vegetation with layers of gravel and sand below them, designed to channel and filter surface water. Or the water could run down rain chains into planters, which the children would be able to plant up each year.
As for the housing development, school parking could utilise a permeable pavement. Water from the access road could run off into swales or filter strips. If the water can be managed at the surface, so that the children can see it moving around the site, this can help them to see how precious rainfall is.
For some schools, it may be possible to include a pond or permanent wetland, subject to a risk assessment which should take into account factors such as the ages and abilities of children at the school, whether they can access the water body when unsupervised and the depth of the water. Where they can safely be included, ponds or wetlands in the management train creates habitats for wildlife and provides new learning opportunities.
| February 2024 |
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