The Need for Flow Controls for SuDS
Essentials of Good Design
The use of Sustainable Drainage Systems or SuDS is set to become mandatory for developments throughout England during 2024. Then, the government will expect nature-based solutions using components near the surface, combined as ‘management trains’ to manage rainfall. SuDS should deliver multiple benefits including pollutant removal, amenity and biodiversity, in addition to managing both everyday rainfall and flood mitigation.
Conventional Piped Drainage
Current conventional drainage regimes use gullies and pipes to transfer polluted runoff straight to expensive underground tanks or large perimeter storage ponds. This approach cannot meet the new requirements, particularly with capacity limited by pipe size or localised blockages.
The SuDS Approach
There are three main concepts in SuDS design that require the use of flow controls: ‘source control’, the ‘sub-catchment’ concept and the ’management train’. All three characteristics require flow controls to enable the containment and treatment of pollution and efficient storage of runoff.
The use of flow controls allows the management of runoff from development to mimic natural drainage and the delivery of a ‘controlled flow of clean water to the environment’.
Most pollution is washed from hard surfaces by the first 10-15mm of runoff and often called the ‘first flush’ event. This volume is within the 1 in 1 or 1 in 2-year return period runoff rainfall event and can be considered everyday rainfall in most parts of Britain. Source control features must capture this ‘first flush’ volume to trap silts and provide primary treatment of pollution before onward flow to the next part of the system.
Blue/green roofs and permeable pavements can be designed to collect, clean and store all the rain that falls on these surfaces up to the 1 in 100 rainfall event plus any allowance for climate change. They can therefore provide ‘source control’ cleaning and full storage requirements within the construction profile.
Where runoff is managed in the landscape a ‘source control’ structure, like a forebay basin or grass filter strip, will be required to provide trapping of silt and primary treatment of pollution. Whereas a feature like the filter strip collects runoff in a diffuse way, the forebay basin may need a flow control with overflow to achieve the same result.
In all cases the first stage in the design sequence needs to hold the ‘first flush’ or treatment volume long enough to trap silts and begin the treatment process. Where this ‘source control’ volume can be defined by a flow control it can be included in the required storage volume for any site.
In many locations the collection and treatment of the ‘first flush’ volume ‘at source’ can be isolated from most of the larger rainfall events, providing clean water for multifunctional amenity space or wildlife habitat with a low flow conveyance route carrying relatively clean water to a final outfall.
The Sub-catchment Principle
In many locations the sequence of runoff collection, treatment, conveyance and storage is not linear. Housing development and larger transport infrastructure projects are good examples where runoff can be collected, cleaned and stored in separate locations before delivery to a final storage feature and outfall.
Sub-catchment design ensures that ‘first flush’ volumes of polluted runoff from discreet adjacent areas, with different arrival times, do not mix before onward travel in the management train.
The ‘sub-catchment’ concept has multiple benefits. Each discreet area of collection, cleaning and storage can be considered a ‘sub-catchment’ and the storage volume, managed by a flow control, part of the overall storage volume for the development. Each sub-catchment ensures that only relatively clean water is released to downstream SuDS features, providing areas of safe public access, and that the size of storage requirements towards the end of the ‘management train’ is reduced.
Sub-catchment design is particularly important where space is limited and enables the integration of SuDS within new development, re-development and retrofit SuDS design. The design of ‘sub-catchments’ requires shallow, easily managed flow controls at the outflow of each sub-catchment and each SuDS storage feature to provide the benefits described above and to demonstrate that they are achieved both initially and in the future.
The Management Train
The ‘management train’ concept enables a sequence of treatment and storage structures, located in series, to provide sufficient stages of cleaning to meet the requirements set out in the SuDS Manual.
The ‘management train’ is usually represented in a linear form with ‘source control’ at the beginning and each treatment stage providing an increasingly clean volume of water. However, the sub-catchment concept can provide multiple sources of clean water at the head of the management train sequence. Where necessary, any feature may be lined to prevent pollution of groundwater.
Two or more sub-catchments, comprising source control features discharging through flow controls to local amenity and/or wildlife SuDS. with a final multifunctional open-space feature for larger rainfall events.
This non-linear model may suit housing or larger, complex development. Flow controls to each SuDS feature uses open space efficiently and reduces the size of the final storage structure.
Common features along the ‘management train’ include various ‘source control’ structures like permeable pavements, blue/green roofs, filter strips and forebay basins, followed by some form of storage structure like a basin, wetland or pond. Wherever possible they are linked to the next part of the system by conveyance in vegetated swales and channels that have large volume capacity that continue the cleaning process. In urban areas or SuDS retrofit, conveyance may be by short pipe connections or open paved channels but always with exceedance pathways.
Each storage feature along the SuDS ‘management train’ sequence requires a flow control to hold runoff long enough for silt settlement to enable pollution treatment and also to demonstrate the storage of rainfall to meet ‘greenfield’ or other agreed rates of runoff. Flow controls also enable SuDS (notably permeable paving) on sloping sites and offer protection to downstream drainage assets.