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A basin serving a residential development in Cheshire.


surface water runoff, such as small or medium-sized housing developments, a focus on natural SuDS works well. However, where there is a higher risk of the presence pollutants such as sediments, metals, hydrocarbons and nutrients, a more balanced approach to designing SuDS management trains is needed, with an understanding of the expected concentrations of such pollutants and how to deal with them.


The CIRIA SuDS Manual C753 provides a method for dealing with water pollution risks by determining pollution hazard indices for a particular site and then matching SuDS components in the treatment train which have combined pollution mitigation indices equal to or exceeding the pollution hazard indices. The manual provides pollution hazard indices for a variety of land uses and for three types of pollution: total suspended solids (TSS), metals and hydrocarbons. It also provides mitigation indices for natural SuDS components.


For manufactured components, British Water’s 2022 How To Guide, Applying The CIRIA SuDS Manual (C753) Simple Index Approach To Proprietary/Manufactured Stormwater Treatment Devices, provides a method for calculating mitigation indices for TSS, metals and hydrocarbons.


Note that maintenance considerations are a key part of the design process. All SuDS components, including pretreatment and storage elements, whether natural or manufactured, must be properly maintained so that the system operates as designed. Failure to do so could mean that pollutants are washed on into water bodies and ground water.


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More background on these issues can be found in our previous article in the June 2023 issue of Drain Trader.


Low risk Case Study 1: Medium sized residential development


Pollution hazard level


Total suspended solids (TSS) Metals


Hydrocarbons


Low 0.5 0.4 0.4


Table 1: Pollution hazard indices for medium-sized housing development (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


0.6 0.6 0.5 0.8


0.6 0.7 0.6 0.8


Table 2: 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.


February 2024 | 5


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