MAIN FEATURE
Rretrofit SuDS in the Sheffield Grey-to-Green
scheme, photographed inMay 2022. Photo courtesy of Stormwater Shepherds UK..
systemsmust demonstrate that they can address each of the contaminant types to acceptable levels.
Themitigation indices for the natural SuDS components were compiled by a teamof experts, drawing information froma selection of published papers.
More recently, in 2016, BritishWater published a Code of Practice for the Assessment ofManufactured Treatment Devices Designed to Treat Surface Run- off. It combines the rainfall time-series data for the UK to determine an appropriate treatment flow rate based on first-flush principles and uses a combination of two test protocols – the German Deutsches Institut für Bautechnik (DIBt) and the New Jersey Comprehensive Assessment Tool (NJCAT) fromthe US – to create the BritishWater testmethods.
In 2022 BritishWater has also published a HowTo Guide, Applying The CIRIA
10
SuDSManual (C753) Simple Index Approach To Proprietary/Manufactured Stormwater Treatment Devices, which provides a calculationmethodology to derivemitigation indices for TSS,metals and hydrocarbons based on the Code of Practice, or the DIBt or the NJCAT, test results. This allowsmanufactured treatment products to have their mitigation indices published so that they can be considered as part of a SuDS Management Train, often alongside vegetative treatment components.
The value of water quality
One of the challenges in the delivery of well-designed SuDS treatment trains is that important elements of the train can be removed during‘value engineering’ exercises. For instance, a design or specificationmay include guidance to say that sediment should be removed upstream, but this is then considered
| June 2023 |
www.draintraderltd.com
unnecessary during a ‘value engineering’exercise and removed or compromised.
Unfortunately, decisions like this are about short-termcapital cost rather than whole life cost. They don’t consider the important issues of how maintenance should be carried out, its frequency and its cost – in terms of both cash and carbon. Itmay also result in failure of the systemto continuously provide the required performance according to the original design, throughout the life of the development.
Should Schedule 3 of the Flood and WaterManagement Act come into legislation,maintenance and longevity issues will be brought to the fore. In Wales, where Schedule 3 has already been adopted, developers are expected to create amaintenance plan and the adopting authority will be required to carry out themaintenance for the
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