In the second of a two-part article, Stuart Crisp, Advanced Drainage Systems and Jo Bradley, Stormwater Shepherds, considers the use of mitigation indices (MIs) in managing risk, and the possibility of Schedule 3 being implemented.
Last month we focussed on the issue of water quality, the types of pollutants likely to be encountered and the importance of capturing sediments, aka total suspended solids (TSS). This month we look at the use of the mitigation indices in The SuDS Manual, C753, published by CIRIA, and the possibility of the implementation Schedule 3 of the Flood and Water Management Act 2010 in England.
The SuDS Manual sets out a simple method for dealing with water pollution risks, requiring the determination of pollution hazard indices for the area under consideration and then matching a SuDS management train with appropriate MIs.
In Table 26.2, The SuDS Manual provides pollution hazard indices for a range of applications and for three types of pollution: TSS, metals and hydrocarbons. So, for example, for a busy public car park such as a supermarket or hospital, the hazard index for TSS is 0.7, for metals is 0.6 and for hydrocarbons 0.7.
The next step is to identify a SuDS treatment train that can provide the necessary MIs, either as a single treatment stage or using a combination of SuDS components. The manual has a table for that too, Table 26.3.
However, this table only provides MIs for natural SuDS components, stating that proprietary treatment systems must demonstrate that they can address each of the contaminant types to acceptable levels.
The MIs for the natural SuDS components were compiled by a team of experts, drawing information from a selection of published data.
In 2022 British Water published a ‘how to’ guide, Applying The CIRIA SuDS Manual (C753) Simple Index Approach To Proprietary/Manufactured Stormwater Treatment Devices, which provides a calculation methodology to derive MIs for TSS, metals and hydrocarbons based on the British Water Code of Practice: Assessment of Manufactured Treatment Devices Designed to Treat Surface Water Runoff, or the “equivalent” DIBt (Germany) or NJCAT (USA) test results. This allows manufactured treatment products to have their MIs published so that they can be considered as part of a SuDS management train, often alongside vegetative treatment components.
The last Government’s Plan for Water referenced plans to finally implement Schedule 3 of the Flood and Water Management Act 2010 in England, which covers SuDS approval and adoption– subject to consultation. Initially announced in January 2023, this could make the inclusion of SuDS mandatory in the design, construction and adoption of nearly all new developments.
Should Schedule 3 of the Flood and Water Management Act come into legislation, maintenance and longevity issues will be brought to the fore. In Wales, where Schedule 3 has already been implemented, developers are expected to create a maintenance plan and the adopting authority will be required to carry out the maintenance for the life of a development.
SuDS Approval Bodies (SABs) within county and unitary authorities will be responsible for securing the means to maintain the SuDS they adopt, and it could be that the regulatory framework in England is similar to that used in Wales. As currently understood, funding will be provided by the developer in the form of a commuted sum to the SAB at the point of handover.
The timeframe for the enactment of Schedule 3, and the many other pieces of proposed legislation for the Plan for Water, remains uncertain. However, the urgent need to address water quality issues will only move up the political agenda.
Incorporating SuDS trains that manage both water quality and quantity into new developments does not necessarily have to increase capital costs. Good SuDS design can reduce costs over the lifetime of the development.
