What are SuDS?
Surface water drainage is a critical element of flood risk management and mitigation.
The rate and volume of surface water run-off from developed sites and urban areas with extensive impermeable surfacing is typically far greater than an equivalent undeveloped greenfield site.
Historic drainage systems were designed to remove and discharge surface water run-off to the nearest watercourse as quickly as possible. This method of surface water management has worked without many problems for decades. But as urban areas grow ever larger, and climate change is resulting in storm events of increasing frequency and intensity, two problems have arisen:
- An increase in flood risk in watercourses and rivers as large volumes of run-off are flushed rapidly into the receiving system.
- Historic sewers and drains that were not designed to manage significant surface water flows are inevitably proving inadequate. This can result in increased surface water flooding and sewer surcharge in areas away from watercourses.
In order to combat this dual threat Sustainable Drainage Systems (SuDS) are now widely adopted and considered standard practice when designing drainage systems for both new and existing development.
Sustainable Drainage Systems (SuDS) aim to reduce pressures on drainage infrastructure, slow the flow to watercourses (“Source Control”), and mimic the natural hydrologic regime within a catchment. All of this can reduce the likelihood of flooding, whilst simultaneously providing additional visual, biodiversity and amenity benefits.
The drainage hierarchy sets out the preferential discharge destination for surface water run-off. Only where it is not practicable or possible to manage surface water run-off using the methods at the top of the hierarchy, should subsequent methods be considered.
- Collect run-off for re-use – Rainwater can be collected and used domestically for gardening and flushing toilets.
- Infiltration – Surface water is drains directly to ground.
- To a surface water body – Surface water should be conveyed into a nearby watercourse or water body.
- To a surface water sewer, highways drain or another separate surface water system.
- To a combined sewer – a last resort when no alternative options are available.
SuDS systems can be utilised regardless of where surface water run-off is ultimately discharged to.
SuDS can take many forms:
- Bioretention (e.g. rain gardens)
- Tree pits
- Filter drains
- Attenuation / Infiltration Basins
SuDS can be multi-functional, enhance biodiversity, improve water quality and provide educational opportunities.
There is now a significant amount of literature and case studies relating to the implementation and effectiveness of SuDS measures as can be found on the following websites:
How can FPS help?
FPS can help develop and design SuDS schemes for both new and existing development.
As part of any new development in England and Wales, the Lead Local Flood Authority is likely to require that SuDS are considered within the design. SuDS and surface water drainage design can also be incorporated within a Flood Risk Assessment (FRA) where required.