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August 2025

Mitigating Circumstances

SuDS ‘Best Practice’ focuses on ‘mimicking nature’ by managing rainfall where it lands using vegetative/green infrastructure, rather than below-ground manufactured products (grey infrastructure). These ‘green’ SuDS systems, can help capture and treat the accompanying pollutants in runoff from surfaces, preventing them passing through the drainage system where they may cause harm to the environment. However, where the pollutant loads from the runoff is high, vegetative SuDS used as a “first defence” may be unable to effectively capture, retain and treat the pollutants.

The case for ‘grey’ and ‘green’ infrastructure

‘Grey’ infrastructure treatment devices, placed upstream of ‘green’ vegetative SuDS, can provide a more effective “first defence” against incoming pollutants and protect the whole drainage system. This combined use of grey and green can lead to optimised designs that provide best hydraulic and water quality performance, together with lowest cost and environmental impact.

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Mitigation Indices (MIs) are metrics used to evaluate the effectiveness of stormwater treatment components in removing pollutants from surface water runoff. CIRIA's SuDS Manual C753 provides a simple method for dealing with pollutant risks by including hazard indices for each type of pollutant in a range of site types. Designers must then include SuDS components in the treatment train which have combined Mitigation Indices equal to or exceeding the hazard indices.

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The need for mitigation

Mitigation Indices are crucial for ensuring that SuDS intallations meet the required water quality standards and protect the environment from harmful contaminants. The British Water Code of Practice: Assessment of Manufactured Treatment Devices Designed to Treat Surface Water Runoff, the German Deutsches Institut für Bautechnik (DIBt), and the USA New Jersey Corporation for Advanced Technology (NJCAT) provide recognised methods for testing and validating the performance of manufactured treatment devices. By applying the British Water How to Guide, any of these these protocols can be used to validate the performance of proprietary systems and to ensure compliance with industry standards.

The British Water How to Guide emphasises the importance of controlled tests to generate MIs and provides a framework for assessing the effectiveness of proprietary stormwater treatment devices. With a hybrid approach, the guide uses results compliant with British Water, DIBt and NJCAT, adjusting the MI calculations to unify the test methods. A copy of the StormTech Isolator Row MI Datasheet based on the How to Guide can be downloaded below.

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Validating systems

Validating the performance of proprietary systems based on the above methodologies is important as it ensures that the systems are effective in removing pollutants and protecting water quality. This is essential for meeting regulatory requirements and safeguarding public health and the environment. It also helps designers and engineers, by ensuring that all manufacturers’ products are tested and evaluated to the same standards.

With Isolator Row, ADS StormTech is the only combined SuDS attenuation and water quality management system to have 3rd party verified MIs, with the capability to treat Medium Pollution Hazard applications without the need for other SuDS treatment components. The Performance Summary and SIA MI’s can be downloaded here

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ADS offers comprehensive technical support for design, installation and maintenance, 24/7, 365 days a year. And for further information, why not try our highly rated CPD programme – simply call 0203 442 0607 or email

ADVANCED DRAINAGE SYSTEMS

StormTech is manufactured by ADS, America's largest manufacturer of thermoplastic corrugated drainage pipes and a specialist in water management systems. StormTech has a long and successful track record with over 50,000 system installations using in excess of 3 million units.

To book a CPD seminar or to contact ADS UK call 0203 442 0607 or click here

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