Trevor English
Swales are essential components in modern drainage design and play a significant role in managing stormwater runoff and promoting sustainable drainage practices. These shallow, vegetated channels are designed to slow down, capture, and filter stormwater, reducing the risk of flooding and improving water quality. Their goal is to work with nature, rather than against it. In this article, we will delve into the concept of swales, their benefits, and how Autodesk InfoDrainage integrates SuDS to help designers optimize swale implementation.
What are swales?
Swales are, in essence, engineered ditches. They are typically shallow, gently sloping channels lined with vegetation, such as grasses or shrubs. Ditches are v-shaped and may not be carefully designed, but swales are different in that they have specific slopes, bottom widths and soil requirements. In fact, these restrictions may be tightly controlled by local, regional, or national regulators, which means they must be carefully and thoughtfully designed. They are often designed to manage water runoff from impervious surfaces like roads, parking lots, and rooftops, but they can be placed anywhere.
The primary functions of swales are:
- Conveyance: Safely transporting stormwater from one location to another
- Infiltration: Allowing water to percolate into the soil, replenishing groundwater
- Filtration: Removing pollutants and sediments from stormwater through vegetation and soil
- Evapotranspiration: Releasing water back into the atmosphere through plant processes
Swales are often filled with vegetation, which helps slow down the velocity of the stormwater, and helps with infiltration of that water into the surrounding ground.
But why use a swale over a more traditional ditch or culvert? Well, by working with nature, there are many benefits.
Benefits of swales
Swales offer numerous advantages over traditional drainage systems. The four main benefits of swales are:
- Flood reduction: By slowing down and capturing stormwater, swales help reduce the risk of flooding in urban areas.
- Water quality improvement: Vegetation and soil in swales filter out suspended solids, sediment and pollutants, enhancing the quality of water that eventually reaches natural water bodies.
- Groundwater recharge: Swales promote infiltration, which helps replenish groundwater supplies.
- Aesthetic and ecological value: Swales can be designed to enhance the visual appeal of urban landscapes and provide habitats for wildlife.
Historically, many drainage designers have handled stormwater by building large culverts and making the surfaces that funnel water towards them impermeable by lining them with concrete. The idea is to forcibly move stormwater to where engineers want it to go, usually past an obstacle or into a subterranean waterway. But moving water away as fast as possible using impenetrable surfaces is not always the best solution.
By utilizing swales and other sustainable urban drainage solutions, sites can not only handle changing stormwater events more effectively and efficiently than with culverts, but they can provide natural areas for vegetation and are naturally a habitat for surrounding birds, insects, etc. It’s an approach that not only seeks to use less concrete but to embrace natural ecosystems.
Different types of swales
Swales can be categorized into three main types: grassed, dry, and wet swales. Each type is tailored to manage stormwater differently based on the environment and desired outcomes:
- Vegetated swales are shallow, vegetated channels designed to convey and treat stormwater runoff. They typically feature native grasses and vegetation, take into account local wildlife, and can help filter pollutants. Sometimes, they’re simply called “grass swales”.
- Dry swales do not maintain a permanent pool of water, making them suitable for areas where prolonged saturation is not ideal. The key to their effectiveness lies in their design, which often includes underdrains to manage water levels during dry conditions.
- Wet swales are constructed to handle larger volumes of stormwater and maintain a permanent pool of water. These swales are maintained below the water table, facilitating the growth of wetland plant species that aid in stormwater treatment. They have saturated soils and aquatic vegetation, unlike dry swales which prioritize infiltration.
What they call these systems around the world
Swales are simple in design, but guidelines differ slightly around the world:
- US: The Environmental Protection Agency (EPA) is generally considered the authority on how to implement swales across America. These systems are often referred to as LiD (Low-impact Development) strategies, but most construction and drainage professionals in the US use the term BMPs (Best Management Practices) – the preferred EPA term – to talk about the actual structures themselves.
- Australia: The acronym WSUDs (Water Sensitive Urban Design systems) is how they talk about sustainability down under. The Government of Western Australia’s Department of Water has a good PDF with construction details.
- UK: They call features like this SuDS (Sustainable Design Systems) in the UK, which makes them sound water-centric and natural. The UK government offers both regulations and grants to build swales, although details may differ in Scotland, Ireland, and Wales.
Modeling swales within InfoDrainage
How might an engineer or hydraulic modeler know that the swale they designed can handle a flooding event? Using apps like InfoDrainage, modelers can simulate stormwater events of any size and know the exact inflows and outflows of a swale.
InfoDrainage provides a suite of tools for modeling and simulating swales and other SuDS components, ensuring that they are optimally sized and positioned within the drainage network. It also allows for detailed hydrological analysis, helping engineers understand how swales will perform under various storm events and conditions.
One of the benefits of using InfoDrainage is that you’re able to quickly generate compliant drainage reports for SuDS and LIDs by utilizing the built-in flexible reporting tool. This means that designing with more sustainable drainage systems and getting them through often antiquated approvals processes at your local authority can be easier than ever.
Designing with swales in InfoDrainage
Inside of InfoDrainage, you can model a range of stormwater controls including wet swales, vegetated swales, and dry swales. What makes the app powerful is that you can link all your SuDs together and see how the entire system works, giving you the opportunity to fine-tune the landscape of your project site.
For example, you may want to model swales in conjunction with French drains, filter trenches, infiltration trenches, trench dry wells, wadis – all with or without under drain pipes. With InfoDrainage, you can very accurately model all these details and determine infiltration rates for not just each SuDs feature, but for the entire site.
Why sustainability is important
What do you do when stormwater runoff overwhelms a lake, causing algae blooms, rotting vegetation, and suffocating fish? You carefully design a new nature-friendly drainage environment like Sasaki did for the University Lakes of Baton Rouge.
Go deeper into sustainable design
In addition to this article on swales, we have articles on infiltration trenches, cellular storage, porous pavement, and bioretention systems – plus plenty of other resources specifically about swales:
Whether you are an engineer, urban planner, or environmental advocate, understanding the role of swales in drainage design is essential:
- We have a Guide to Representing SuDS in InfoDrainage in accordance with the SuDS Manual Ciria 753.
- Our documentation shows you how to input specific constraints to accurately model and size your swale.
- Prefer a video walkthrough? Check out our free course Designing a swale.
- Don’t have a copy of InfoDrainage? We offer a 30-day free trial with no credit card required.
- Are you a student or educator? If so, we have some very good news for you.