A swale is a shallow, vegetated channel or depression designed to convey and filter stormwater runoff. These engineered landscape features are a common component of sustainable site design, managing water that flows off impervious surfaces like roads, parking lots, and rooftops. By slowing the movement of water and encouraging natural processes, swales reduce the environmental impact of development. This integration of land management with hydrology makes them an effective part of modern stormwater management systems.
The Physical Anatomy of a Swale
The structure of a swale is defined by its geometry, engineered to manage water flow velocity. Most swales utilize a broad, shallow cross-section, often shaped like a trapezoid or a parabola. This shape prevents water from flowing too quickly, which minimizes erosion and maximizes contact time between the runoff, vegetation, and soil media.
The side slopes of a standard swale are kept gentle, typically at a 3:1 (horizontal to vertical) ratio or flatter, which aids stability and allows for easier maintenance, such as mowing. The channel follows a gentle longitudinal slope, ideally less than four percent, ensuring water moves without becoming overly erosive. A dense cover of turf grass or native, water-tolerant plants stabilizes the soil and creates roughness that slows the flow.
The swale depth is usually shallow, often designed to contain a flow depth of only about four inches during smaller storm events. This shallow depth, combined with the low longitudinal slope, maximizes the surface area where water interacts with the filtration media. The bottom width of the channel typically ranges between two and eight feet.
Stormwater Management Function
The primary function of a swale is to manage stormwater runoff through conveyance, infiltration, and filtration. Swales are linear practices that safely transport water away from developed areas, functioning beyond simple drainage ditches. Their design reduces the volume and speed of runoff, which helps mitigate the risk of localized flooding.
The vegetation and soil within the swale work together to clean the water before it enters groundwater or downstream systems. When runoff flows across the vegetated surface, its velocity is reduced, allowing suspended solids and sediment to settle out. Plant root systems stabilize the soil and create macropores, promoting infiltration by allowing water to soak into the underlying soil.
Pollutant removal occurs as water filters through the vegetation and soil media. The dense plant cover traps particulate pollutants, while the soil media and plant uptake remove dissolved contaminants. This biofiltration process sequesters pollutants such as nutrients and heavy metals carried by stormwater from impervious surfaces. This treatment is most effective for the “first flush” of a storm, which contains the highest concentration of pollutants.
Common Swale Types and Design
Swales are engineered with variations to suit different site conditions and water quality goals. A basic dry swale, or grass swale, is lined with turf and primarily designed for conveyance and minor infiltration into the natural soil. These are used where the native soil has a good infiltration rate and the goal is to slow and move water.
Wet swales are designed for locations with a high water table or poor soil drainage, encouraging the channel to remain saturated for longer periods. They utilize wetland-tolerant vegetation, and their pollutant removal relies on settling and nutrient uptake by plants in the marshy conditions. Holding water maximizes the time for treatment before discharge.
Bioswales, sometimes called enhanced dry swales, represent a more engineered and highly effective design. These systems include a specialized, amended soil media mix, often sandy loam with organic compost, placed over a gravel layer and sometimes an underdrain. The engineered soil media significantly enhances the removal of fine sediments and dissolved pollutants through adsorption and filtration. Bioswales frequently incorporate small earthen or rock check dams along their length to further slow the flow, promote settling, and maximize infiltration.
Required Upkeep and Maintenance
Regular maintenance is necessary to ensure a swale retains its capacity for filtration and conveyance over time. A common task involves the routine mowing of grass swales, maintaining vegetation at a height of three to six inches. This height is important because it is tall enough to slow water flow and trap sediment effectively, but not so tall that it impedes flow or smothers the underlying grass.
Accumulated sediment and debris must be removed periodically, especially around inlets and outlets, to prevent blockages. Sediment accumulation in the swale bottom exceeding about one inch significantly reduces the infiltration rate and must be cleared. If sediment is not removed, the swale may lose its function and become a pathway for runoff, concentrating pollutants.
Erosion control is another ongoing task, requiring prompt attention to any rills or gullies that form along the side slopes or channel bottom. Eroded areas should be repaired by restoring the original grade and immediately reseeding or sodding to re-establish a dense vegetative cover. Control of invasive species is also important, as aggressive weeds can displace the intended vegetation and compromise the swale’s designed function.