What is a swale? Definition, purpose, and how it works.

Swales are a common sustainable drainage feature that provides a simple and effective way to manage stormwater runoff. They are commonly used along roads, parking lots, developments, and other areas where rain falling on hard surfaces needs somewhere to go.

A swale is a shallow channel or depression designed to collect, slow, and convey stormwater runoff. Unlike a conventional drainage ditch, a vegetated swale can also filter sediment and pollutants and allow some water to infiltrate into the soil.

Swales are commonly used alongside roads, parking lots, buildings, parks, and residential developments. Depending on their design, they may be grassed, vegetated, dry, wet, or engineered as bioswales.

In simple terms: a ditch primarily moves water; a swale is designed to manage it.

Unlike a conventional drainage ditch, which is primarily intended to move water from one place to another, a swale can perform several jobs at once: conveying runoff, slowing its flow, encouraging infiltration, filtering pollutants, and reducing erosion.

That’s why swales have become a common feature of Sustainable Drainage Systems (SuDS), green infrastructure, and stormwater management designs around the world.

How does a swale work?

When rain falls on an impermeable surface such as a road, parking lot, sidewalk, or roof, much of that water becomes surface runoff rather than soaking into the ground. A swale intercepts this runoff and gives it a controlled path through the site.

Its shallow shape and vegetation slow the movement of water compared with conventional hard-surface drainage. As water moves through a vegetated swale, sediment can settle out, vegetation and soil can help filter pollutants, and some of the water may infiltrate into the underlying soil.

The result depends on the design of the swale and the conditions at the site, but swales can perform four important stormwater functions:

• Conveyance: Moving stormwater safely from one part of a site to another.
• Infiltration: Allowing some runoff to soak into the soil where site conditions permit.
• Filtration: Using vegetation and soil to capture sediment and some pollutants.
• Flow control: Slowing runoff and reducing erosive flow velocities.

The US Environmental Protection Agency similarly describes vegetated swales as broad, shallow channels designed to reduce stormwater velocity, promote infiltration, and trap particulate pollutants.

What does a swale look like?

Most swales are long, relatively shallow channels with gently sloping sides. Depending on their purpose, they may be covered with turf grass, planted with denser vegetation, designed with an engineered soil layer, or fitted with features such as check dams and underdrains.

A swale’s dimensions aren’t arbitrary. Engineers may need to consider:

• Longitudinal slope: Water needs to move through the swale without creating velocities that cause erosion.
• Side slopes: These affect hydraulic capacity, stability, maintenance, and how easily the swale integrates with the surrounding landscape.
• Bottom width: A wider bottom can spread flow across a larger area, influencing velocity, infiltration, and treatment.
• Soil conditions: Soil permeability affects how readily water can infiltrate.
• Vegetation: Plants can slow flow, stabilize soil, capture sediment, and contribute to water-quality treatment.
• Inlets and outlets: Runoff needs to enter and leave the swale without causing erosion or unwanted ponding.
• Check dams and underdrains: Some designs incorporate check dams to slow water or underdrains where infiltration into the native soil is limited.

Local requirements vary considerably, so swales should be designed according to the applicable stormwater and drainage standards rather than a single universal set of dimensions.

What are the different types of swales?

“Swale” is a broad term, and different regions and design standards classify swales somewhat differently. Three common types are vegetated, dry, and wet swales.

Vegetated or grassed swales

A vegetated swale is a shallow channel covered with grasses or other plants. Vegetation slows runoff, protects the soil from erosion, and can help capture sediment and pollutants.

These swales are often found alongside roads and parking lots or within landscaped areas.

Dry swales

Dry swales are designed so that they do not normally contain standing water between storm events.

They may incorporate engineered soil, check dams, or an underdrain to improve treatment and drainage. The aim is generally to slow and treat runoff while allowing the swale to drain after a storm.

Wet swales

Wet swales are designed for conditions where the soil remains saturated or standing water is present for extended periods.

They typically support wetland vegetation and operate differently from dry swales, where drainage between storms is generally desirable.

The terminology can vary by location and regulatory framework, so the name alone doesn’t necessarily tell you exactly how a particular swale has been designed.

Swale vs ditch vs bioswale vs rain garden

These terms are sometimes used interchangeably, but they describe different approaches to managing runoff.

The boundaries aren’t always rigid. For example, a bioswale is a type of swale designed with a particular emphasis on biological and soil-based stormwater treatment. A rain garden, by contrast, is generally a basin designed to receive and temporarily hold runoff rather than primarily convey it along a linear route.

What are the benefits of swales?

A well-designed swale can do more than simply move rainwater from one place to another. Depending on its design and site conditions, a swale can provide several stormwater management benefits.

Slowing stormwater runoff

Vegetation and the shallow cross-section of a swale can reduce the velocity of runoff. Slower-moving water can reduce erosion and give sediment more opportunity to settle before water moves farther downstream.

Supporting water-quality treatment

As runoff passes through a vegetated swale, vegetation and soil can capture sediment and some pollutants. This can make swales useful as a form of pretreatment before stormwater reaches another drainage feature or receiving water.

Encouraging infiltration

Where soil and groundwater conditions allow, swales can help some runoff infiltrate into the ground rather than sending the entire volume directly into a piped drainage system.

Managing runoff as part of a wider system

Swales can work alongside rain gardens, detention or retention systems, infiltration features, and conventional drainage infrastructure. Combining multiple stormwater controls in this way can create a treatment train that manages runoff at different points across a site.

Adding green space

Because many swales use vegetation rather than concrete, they can be incorporated into landscaped areas and may provide ecological and amenity benefits alongside their drainage function.

These benefits aren’t automatic. The performance of a swale depends on its design, construction, site conditions, vegetation, and long-term maintenance.

Where are swales used?

Swales can be incorporated into many different types of development.

Common applications include:

• Alongside roads and highways
• Around parking lots
• Within residential developments
• On commercial and industrial sites
• Alongside pedestrian and cycling infrastructure
• In parks and public spaces
• Around buildings and other impermeable areas

Their relatively simple geometry makes them particularly useful where there is enough land available to manage runoff at the surface rather than immediately directing it underground.

What are sustainable drainage systems called around the world?

Swales are used as part of broader approaches to managing stormwater more sustainably, but the terminology used to describe these approaches varies around the world.

In the United Kingdom, the term Sustainable Drainage Systems (SuDS) is widely used for approaches that manage surface water while considering factors such as water quantity, water quality, amenity, and biodiversity. The UK government offers both regulations and grants to build swales, although details may differ in Scotland, Ireland, and Wales.

In the United States, terms such as green infrastructure (GI), low-impact development (LID), and best management practices (BMPs) are commonly encountered in stormwater management. The Environmental Protection Agency (EPA) is generally considered the authority on how to implement swales across America.

In Australia, water-sensitive urban design (WSUD) is commonly used to describe an integrated approach to managing water within the built environment. The Government of Western Australia’s Department of Water has a good PDF with construction details.

Other countries and regions have developed their own terminology, standards, and approaches. These terms aren’t necessarily interchangeable, but they share many underlying principles, including managing rainwater closer to where it falls and using natural or engineered processes to reduce and treat runoff.

Swales can play a role in many of these systems. Rather than treating stormwater solely as something to move quickly into underground pipes, a swale keeps part of the stormwater management process at the surface, where water can be slowed, conveyed, filtered, and potentially infiltrated.

When is a swale the right choice?

Swales can be effective stormwater controls, but they aren’t appropriate everywhere.

A designer first needs to understand where the water is coming from, where it needs to go, how much runoff the site produces, and what happens during larger storm events.

Site conditions such as topography, soil, groundwater, available space, expected flow rates, vegetation, downstream drainage, and local design requirements can all affect whether a swale is suitable.

Swales may be less suitable where space is extremely constrained or slopes make safe conveyance difficult. Poorly draining soils, high groundwater, contaminated runoff, or very large contributing drainage areas can also require a different design approach or additional controls.

The key point is that a swale is part of a drainage system, not an isolated landscape feature.

How are swales designed?

Design starts with understanding how water behaves across the site.

Engineers typically consider the contributing catchment, rainfall, runoff volumes and rates, existing topography, downstream constraints, soil conditions, and the performance expected from the swale.

From there, variables such as swale length, bottom width, depth, side slopes, longitudinal slope, vegetation, check dams, infiltration characteristics, and outlet configuration can be evaluated.

The design also needs to account for what happens when conditions exceed the normal design case. Where does water go if the swale reaches capacity? Could that exceedance flow affect buildings, roads, or neighboring property?

These questions are why hydraulic and hydrologic analysis often becomes an important part of swale design.

Best practices for designing and managing swales

There is no single swale design that works for every site. Dimensions, vegetation, materials, and supporting drainage features need to respond to local conditions and applicable design standards. However, several principles can help guide effective swale design.

Start with the site’s natural drainage

Before determining the shape or dimensions of a swale, understand how water already moves across the site. Topography, low points, contributing catchments, existing drainage paths, and downstream constraints should inform where a swale is located and how it connects to the wider drainage system.

Design for everyday rainfall and larger storms

A swale needs to perform during the events it is designed to manage, but designers should also consider what happens when its capacity is exceeded.

Identifying safe exceedance routes can help prevent larger storms from redirecting water toward buildings, roads, neighboring properties, or other vulnerable areas.

Keep flow velocities under control

If longitudinal slopes or flow rates become too high, water can damage vegetation and erode the swale itself. Designers can adjust the geometry or use measures such as check dams, where appropriate, to reduce velocities.

Consider infiltration early

A swale’s ability to infiltrate water depends heavily on the underlying soil and groundwater conditions.

Site investigation can help establish whether infiltration is appropriate and how quickly water is likely to drain. Where infiltration is limited, a swale may still provide useful conveyance and treatment, but the design may require an underdrain or another means of managing water.

Choose vegetation for the conditions

Plants in and around a swale may experience both wet and dry periods, along with varying flow velocities and pollutant loads.

Vegetation should therefore be selected for the local climate, soil conditions, expected inundation, maintenance requirements, and hydraulic role of the swale—not simply for appearance.

Think about maintenance during design

A swale that performs well when newly constructed may behave differently after years of sediment accumulation, vegetation growth, or erosion.

Designers should consider how maintenance teams will access the swale, remove sediment, manage vegetation, inspect structures, and keep inlets and outlets functioning.

Treat the swale as part of a system

A swale may receive runoff from roads, roofs, parking areas, or upstream drainage infrastructure and discharge into another stormwater control or drainage network.

Its performance therefore affects—and is affected by—the rest of the system. Understanding those interactions is particularly important on larger or more complex sites.

How engineers model swales

Historically, many drainage designers have sought to handle stormwater by building large culverts and making the surfaces that funnel water towards them impermeable by lining them with concrete. The idea here 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.

For simple situations, engineers may be able to evaluate swales using standard hydraulic calculations and local design guidance. More complex sites can require hydraulic modeling to understand how a swale interacts with the rest of the drainage system.

A model can help answer questions such as:

• How much runoff enters the swale during a particular storm?
• How quickly does water move through it?
• How much water infiltrates?
• Does the swale exceed its capacity?
• Where does excess water go?
• How does changing its dimensions affect the wider drainage network?

Autodesk InfoDrainage is one tool engineers can use for this type of analysis. It can represent wet, dry, and vegetated swales alongside other stormwater controls and conventional drainage infrastructure, allowing designers to assess the system as a whole.

Swale maintenance

A swale needs ongoing maintenance if it is going to continue performing as designed. Maintenance requirements vary according to the design and vegetation, but common tasks include:

• Removing litter and debris
• Removing accumulated sediment
• Managing grass and other vegetation
• Inspecting for erosion
• Repairing bare or damaged areas
• Keeping inlets and outlets clear
• Inspecting check dams and other structures
• Checking for unwanted standing water or signs that infiltration has deteriorated

Maintenance is particularly important because sediment, debris, excessive vegetation, or erosion can change how water moves through the swale over time.

Designers should therefore consider how a swale will be accessed, inspected, and maintained from the beginning rather than treating maintenance as an afterthought.

Modeling swales within InfoDrainage

How might an engineer or hydraulic modeler know that the swale they designed can handle a flooding event? They use hydraulic modeling software to understand how features like swales perform as part of a wider drainage network. Using apps like InfoDrainage, modelers can simulate stormwater events of any size and know the exact inflows and outflows:

• Model and simulate swales and other SuDS components, ensuring they are optimally sized and positioned within the drainage network
• Perform detailed hydrological analysis to understand how it will perform under various storm events and conditions
• Link multiple SuDs features together and see how the entire system works, which provides the opportunity to fine-tune a project site.
• Quickly generate compliant drainage reports for inspectors using a flexible reporting tool

For example, engineers may want to model their swales in conjunction with French drains, filter trenches, infiltration trenches, trench dry wells, wadis – all with or without under drain pipes. With InfoDrainage, they can very accurately model the details and determine infiltration rates for not just each SuDs feature, but for the entire site.

Swales are simple—but their design matters

The basic idea behind a swale is straightforward: give stormwater a shallow, controlled path where it can slow down, be filtered, and potentially infiltrate rather than immediately sending it into a pipe.

But making that simple idea work requires understanding the site around it.

Slope, soil, vegetation, contributing runoff, hydraulic capacity, maintenance, and downstream conditions all influence how a swale performs. That’s why swales can range from relatively simple grassed channels to carefully engineered components of larger sustainable drainage systems.

When they’re appropriate for the site and designed well, swales offer engineers another way to manage stormwater at the surface while providing benefits that conventional conveyance alone may not provide.

Go deeper into sustainable design

In addition to this article on swales – as part of our sustainable drainage guide, we have articles on infiltration trenches, cellular storage, porous pavement, soakaways, bioretention systems, rain gardens, and wet ponds and infiltration basins – plus plenty of other resources about stormwater controls:

• 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.

Frequently asked questions about swales

What is a swale?

A swale is a shallow channel or depression designed to collect, slow, and convey stormwater runoff. Swales are commonly vegetated, allowing them to filter sediment and pollutants and, where conditions permit, encourage water to infiltrate into the soil.

What is the purpose of a swale?

A swale helps manage stormwater runoff. Depending on its design, it can convey water, reduce flow velocity, encourage infiltration, capture sediment and pollutants, and provide pretreatment before runoff reaches another drainage feature.

What is the difference between a swale and a ditch?

A drainage ditch is primarily intended to convey water. A swale is generally designed to manage runoff more broadly, often using vegetation, soil, shallow slopes, and other features to slow, filter, and sometimes infiltrate stormwater.

Is a swale the same as a bioswale?

Not exactly. “Swale” is the broader term. A bioswale generally uses vegetation and engineered soils or media specifically to increase stormwater treatment and infiltration.

What is the difference between a swale and a rain garden?

A swale is typically a long, shallow channel designed to move water along a route while managing it. A rain garden is generally a planted depression or basin designed to collect and temporarily store runoff so that it can infiltrate or be treated.

Do swales hold water?

Some do. Dry swales are generally designed to drain between storm events, while wet swales can remain saturated or retain water for longer periods. Whether water remains in a swale depends on its design, soil, groundwater conditions, and recent rainfall.

Where are swales commonly used?

Swales are commonly found alongside roads, parking lots, residential developments, commercial sites, parks, and other areas where runoff from impermeable surfaces needs to be managed.

Can a swale replace a storm drain?

In some situations, a swale can reduce the need for conventional pipes, curbs, gutters, or channels. EPA guidance notes that vegetated swales can form part of a stormwater drainage system and can replace conventional conveyance in appropriate applications. Whether that’s possible on a particular site depends on hydraulic capacity, available space, site conditions, and local requirements.