How can engineers use nature to protect people from flooding?

When you talk about sustainable drainage systems (SuDS), it can mean different things to different people.

Generally, SuDS follow the principles of natural drainage.

They manage surface water as near to its source as possible, and use a variety of methods to slow the flow, attenuate (reduce the source) and infiltrate (absorb).

But for some this means using human-made, or engineered structures, and for others, it's about natural landscape features.

These are known as engineered SuDS and landscaped SuDS, respectively.

These drainage solutions have their pros and cons, and can be used to provide resilience to climate change and the extreme weather that's associated with it.

And what's more, they can be combined.

Landscaped SuDS

Landscaped SuDS include natural features such as attenuation ponds, rills (small streams) and swales (shallow channels).

These are integrated into the landscape and collect water during periods of high rainfall before letting it slowly soak into the ground.

Planting with hardy and drought-resistant vegetation can be used to slow flow and naturally cleanse water.

The natural features of landscaped SuDS are also ideal for boosting biodiversity and creating better places to live with attractive open areas and increased amenities.

The wellbeing value of SuDS is well-recognised.

It creates more opportunities for physical activity and spaces for relaxation in order to reduce stress and improve mental health. It also improves air quality.

All of these benefits are especially true when the features form an integral part of the overall landscape design.

Cost and space can be a challenge

While landscaped SuDS have many benefits, the downside is that they can take up significant areas of land, making them unsuitable for some sites.

Cost is a big implication for developers when allocating large areas of land to such features.

Maintenance and take-up of landscaped SuDS have also prevented them from being implemented in some situations. 

These natural drainage solutions that are integrated into the overall landscape design appear to be the ultimate answer to our climate change issues.

But in many situations, a more pragmatic approach is needed.

Engineered SuDS

Engineered SuDS include permeable paving, geocellular attenuation and infiltration/soakaway tanks.

These options are ideal for high density, commercial and industrial developments.

Geocellular tanks are commonly used to manage surface water runoff either as a soakaway or storage tank. They 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.

They're often also perceived as lower risk than landscaped SuDS, as capacity and flow rates can be guaranteed, allowing for accurate modelling of different situations by engineers.

Geocellular tanks are often used as a solution with proven qualities and performance to reinforce and enhance planning applications, therefore enabling planning developments to proceed.

Engineered SuDS can help manage water quality, but do little to address the requirement for SuDS to provide of biodiversity and amenity.

However, they're suitable for use underneath public spaces such as play areas and thus allow optimal use of the space available.

Engineered SuDS tend to offer a more predictable and lower risk maintenance plan that, in the past, water companies have been more willing to adopt.

But the adoption of SuDS is set to change with the Schedule 3 coming into place - and a more holistic approach is gaining traction.

A combined approach to climate resilience

A greater understanding of the SuDS solutions available will allow engineers and landscape architects to use a combined approach to surface water management on any given site.

Blending engineered and landscaped SuDS allows each to play to its strengths and provide a range of benefits for all stakeholders while ensuring optimal resilience to climate change.

There isn’t necessarily a single correct answer to SuDS design to help increase climate resilience.

But there may be several options available, depending on the design criteria and the opportunities and constraints of the site.

Landscaped SuDS can often be used for source control, which manages rainfall where it falls, while engineered SuDS will provide a more robust and high capacity option for storage of rainfall.

For example, a green roof uses a vegetation layer grown in a substrate with a drainage layer below to promote storage, attenuation and evapotranspiration.

Meanwhile, it provides benefits like thermal comfort and biodiversity.

For extreme weather events in urban areas, where there may not be enough space available to store the water needed through landscaped SuDS alone, adding engineered SuDS is essential.

Civil engineers face the challenge of adapting to the changing SuDS landscape by incorporating a thoughtful and balanced approach.