MAIN FEATURE
instance directing water to a soakaway or infiltration device and then onto an attenuation pond or tank. Finally, regional control would see a system that could handle run-off fromseveral sites, perhaps resulting in a detention basin or wetland.
The SuDSManual, C753, published by CIRIA, prescribes a risk-based approach to designing SuDS for water quality. If pollution risks are deemed to be low, then SuDS designers can prioritise water quantity, amenity and biodiversity. If they aremedium, then all considerationsmust be balanced. And if they are high, water quality evaluation must take precedence, for instance on haulage yards, industrial sites, trunk roads andmotorways. It should however be noted that in all cases, includingmediumand low pollution
risk sites, appropriatemitigation should be put in place to reduce the risk of pollution.
There are a range of water quality treatmentmeasures that can be included in a SuDS train. Sometimes it is possible to use a completely natural SuDS train to deal with both water quantity and quality issues. At other times, the best solution combines natural and engineered SuDS elements, ormay require proprietary manufactured elements only. This could be due to high levels of pollution loading or the space available.
There are fourmain types of pollutant that can be found in stormwater run-off: sediments,metals, hydrocarbons and nutrients. Sediments, often referred to in pollutionmitigation as total suspended solids (TSS), is particulate
matter. It includes tiny particles of soil, such as silt and clay, which have been dislodged by rainwater as it passes over the run-off surfaces.
Metals andmetal compounds can be dissolved in run-off or attached to silts and sediments in the water. Copper and zinc aremost commonly found in surface water in the UK but there can be cadmiumand other toxicmetals too. Although plants require very small amounts of copper and zinc to grow, higher concentrations can be damaging to them.
Hydrocarbon pollution comes with run- off fromroads, car parks and areas wheremachines operate and are maintained, due to oil and fuel spills, tyre and brake wear. Changing climate means that rainfall events can be further apart butmore intense, which
A Bioswale at a development in Hampshire taken in March 2023, showing how good SuDS can be. Photo courtesy of Stormwater Shepherds UK.
FOLLOW US June 2023 | 5
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