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The weather in 2024 has given us a stark reminder of the challenges of heavy rainfall. Frequent storms have caused flooding across the UK, with the effect exacerbated by dense, urban development. On impermeable surfaces like roads, roofs and driveways, 95% of the rainfall can flow off the catchment as surface runoff, overwhelming the outfalls and causing flooding.
Attenuation of surface water in vegetative SuDS, such as swales, detention basins and ponds, can be a good solution, particularly for developments with lots of space and low pollutant loads. However, vegetative or ‘green’ SuDS is not always the answer. Despite that, many policy documents from SuDS adopting bodies often mandate ‘green’ SuDS, thereby prohibiting the use of proprietary or ‘grey’ SuDS.
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Water Quality
Legislation across the UK is ramping-up in terms of water quality from surface runoff discharges, habitat creation and environmental protection. Government policies such as the Plan for Water and the anticipated introduction of Schedule 3 of the Flood and Water Management Act 2010 in England, suggest that SuDS designs must consider both water quantity and quality together. However, a vegetative solution may not always address these concerns in the most cost-effective way.
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If there’s a risk that surface runoff will carry pollutants, it must be treated. Where pollutant loads are low, vegetative SuDS may be sufficient. However, where SuDS applications have pollution loadings that will harm plant and animal life, a ‘green’ infrastructure solution deployed as the first-line defence may be inadvisable, with a manufactured treatment device a prudent addition. This more flexible approach to SuDS allows a combination of ‘green’ and ‘grey’ elements to better meet local needs.
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It is worth remembering that ADS StormTech is the only combined SuDS attenuation and water quality management system to have 3rd party verified Mitigation Indices, with the capability to treat Medium Pollution Hazard applications without the need for other SuDS treatment components.
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Maintenance
SuDS operational and maintenance costs are often overlooked, but the proposed implementation of Schedule 3 of the Flood and Water Management Act 2010 in England will bring this to the fore. SuDS Approval Bodies (SABs), will require robust information about inspection and maintenance regimes and expected costs over the lifetime of a development.
There is a common misconception that vegetative ‘green’ SuDS can be left to nature, but this is not the case. Vegetation needs to be maintained, litter must be picked, and for ponds, sediment build-up needs to be removed to maintain capacity. A poorly thought-out or ‘value-engineered’ SuDS design focusing on attenuation storage volume can lead to compromised structural integrity, shorter service life, failure of the treatment train to meet water quality objectives and higher operational costs.
A good SuDS design needs to address many factors (water quantity, water quality, biodiversity, amenity) including the cost and ease of maintenance of the system. ADS StormTech’s Isolator Row removes sediments and other surface water pollutants and can mitigate against the need to install upstream pre-treatment devices. This results in considerable cost savings, particularly as StormTech is so easy to maintain. Only Isolator Row needs cleaning, typically every 3 – 5 years using standard sewer cleaning equipment.
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To restrict designs to vegetative SuDS is to limit the choice of tools available to provide the optimum solution. By balancing the benefits of ‘grey’ and ‘green’ SuDS elements, a greater opportunity exists to create solutions that are technically, environmentally and economically viable over the lifetime of the development.
For a full analysis of the benefits of ‘grey’ and ‘green’ infrastructure, click below
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