Different ground conditions reveal different underlying geology and site history, including past quarrying or made ground. When considering a site for potential development, often very local and variable conditions mean that surface water must be managed in the most effective way possible in accordance with the setting.
Developers need to use the pre-planning application phase with Local Authorities to ascertain drainage requirements, including where Sustainable Drainage (SuDS) may be an important factor. While a full SuDS strategy is not required at this stage, it is important that site conditions are understood and will help in ongoing discussions with the Authority.
SuDS manage peak runoff after a rainfall event to prevent flooding and often try to mimic what would happen if there were no development on site. Hard surfaces from buildings, paving and driveways usually create flashier runoff into sewers and water courses which can become overloaded downstream.
A combination of SuDS solutions local to the development manages this flow. Ideally, surface water should infiltrate into the soil directly but this may not always be possible as a single solution. Authority planning teams will be looking for infiltration SuDS schemes to be quoting adequate rates of water infiltration that do not increase flood risk on site or elsewhere.
Clay based ground conditions, such as those in parts of London, make solely relying on infiltration far harder. It may be possible that some parcels sit also on gravels and could take some infiltration, but it is more likely that water would need to be discharged from the site. Authorities will look to set the water discharge rate as close to the rates experienced if it were a greenfield site in its natural state, although the degree of reduction will be set by the Lead Local Flood Authority (LLFA) for existing urban areas, not always the planning authority.
Slowing the flow
Slowing surface water flow into existing sewer systems and adjacent water courses is another key element in the SuDS mix. Attenuation, the process of stemming the peak flow from flood events, will rely on developer project teams calculating the total water needing to be stored from typically a 1 in 100 year 6 hour storm, as per the Defra non-statutory guidance.
Attenuation can be managed through multiple mechanisms, which can work together to slow water flow. While large storage tanks could be an attractive consideration for developers, planning authorities will usually view this as a last resort and may consider it as long as other SuDS features are added.
Flow controls from a tank or pond limit what comes out into a surface water course or sewer at a rate agreed with the LLFA / Local Authority. The smaller the estimated discharge rate, the more would have to be stored.
Good SuDS design makes good communities
Cost effective features can be built into the estate design that are not only aesthetically pleasing but can enhance property value and play an important role in slowing water flow. Rain gardens and living roofs increase the surface area for water absorption. Imaginative permeable paving and infiltration basins enhance public spaces and increase amenity value.
This reflects well on the quality of the development from a marketing perspective for both developer and retained project architect. Download our SuDS information Guide to see how SuDS management trains work.
Many urban SuDS schemes have to connect to sewers. The key is understanding what the impact will be on already constrained, Victorian-designed sewer capacities, and to ensure that there is no unrestricted sewer connection.
Understanding the benefit of SuDS
Developers and supporting property professionals must therefore consider how an individual LLFA / planning authority will view the drainage strategy on site. This varies considerably by Authority. Understanding the ground conditions are the first port of call to inform the mix of drainage features needed.
There is a clear benefit to spending time implementing the right SuDS: it reduces flood risk for residents, adds amenity and quality and can raise the value of property with attractive environments, especially in urban areas.
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