Sustainable drainage systems (SuDS) mean different things to different people. For some they tend to be manmade or engineered structures and for others they are about natural landscape features. Both can be used to help our built environments to become more resilient to the effects of climate change and the associated extreme weather events including flooding. But which is likely to provide the optimum solution?
The principles of sustainable drainage involve mimicking natural drainage by managing surface water as close to the source as possible with a variety of methods used to slow the flow, infiltrate and attenuate. This can be done using a ‘grey infrastructure’ approach with engineered or manmade solutions or a more natural approach which utilises landscape features. In this post, we look at the pros and cons of each and which approach is likely to provide the best outcomes.
Attenuation ponds, rills and swales are just a few examples of SuDS that can be integrated into the landscape to collect water during periods of high rainfall before letting it slowly soak into the ground. These features can also be planted with hardy and drought resistant vegetation which can be used to both slow the flow and clean the water. These types of features are ideal for creating SuDS which comply with the four pillars of SuDS design as set out in the Ciria SuDS manual. When designed correctly, these natural features can also help to boost biodiversity and create attractive open areas for amenity which are known to reduce stress and improve mental well-being and increase physical activity of the local community.
Despite the many benefits of landscaped SuDS, they often require large areas of land making them unsuitable for some sites. The allocation of large areas of land to such features can have serious cost implications for developers. In the past maintenance and adoption issues of landscaped SuDs have prevented them from being implemented in some situations. However with the renewed focus on biodiversity and climate change, the value of landscaped SuDs as blue green infrastructure is likely to become more widely recognised.
Engineered SuDS include permeable paving, geocellular attenuation and infiltration tanks and are ideal for high density, commercial and industrial developments. They are commonly used to manage surface water runoff and are usually modular and flexible, robust and easy to install. The key benefit with them is the large volume of water that can be stored in a relatively small site. Capacity and flow rates can be guaranteed allowing for accurate modelling of different situations by engineers. This has led to them being perceived as a lower risk solution by engineers. Their proven qualities and performance are useful to enhance planning applications ensuring that they can proceed.
Whilst, these types of engineered SuDS help to manage water quantity, they do little to comply with the other four pillars of SuDS designs as outlined in the Ciria SuDS manual Their suitability for use underneath public spaces such as play areas do therefore allow optimal use to be made of the space available.
A combined approach
A combined approach that utilises both landscaped and engineered SuDS is likely to provide the maximum capacity of the system combined with a range of benefits for all stakeholders giving optimal resilience against climate change and extreme weather events. This is most likely to happen when engineers and landscape architects have a full understanding of the options available allowing them to employ a the optimal combined approach that allows each of the systems to play to its strengths.
There may be several options for SuDS design that will help to improve climate resilience and the best option will depend on the design criteria and opportunities and constraints of the site. Source control is often provided by the landscaped aspect of the system, managing rainfall where it falls and the engineered components will provide a more robust and high capacity option for storage of rainfall. For example a green roof utilises a vegetation layer grown in a substrate with a drainage layer below to promote storage, attenuation and evapotranspiration meanwhile providing benefits like thermal comfort and biodiversity. For extreme weather events in urban areas, where there may not be space available to achieve the required conveyance capacity through landscaped SuDS alone, and the addition of hard SuDS is essential.
The Ofwat codes for adoptable sewers came into force in England from April 2020 allowing a route for adoption of some landscaped and engineered SuDS. This allows for them to be properly considered and integrated into the urban design process, providing schemes that meet surface water management requirements and improve climate resilience whilst also contributing a range of other benefits. It looks possible that Schedule 3 of The Flood and Water Management Act 2010 may finally come into force setting mandatory standards for delivery and adoption of SuDS in new developments and ending an automatic right builders have to connect into existing sewers. A conditional, SuDS-first approach would ensure new sites don’t overwhelm sewers already nearing capacity with rainwater running off new hard surfaces.