A swale is a shallow, channeled depression designed to slow and redirect surface runoff water. This engineered feature serves as a gentle, vegetated channel, contrasting with traditional pipes or ditches. The primary function of a swale is to manage stormwater naturally, reducing erosion and increasing water absorption into the ground. Determining the correct depth is essential, as it directly impacts effectiveness and stability. This guidance focuses on the variables that influence optimal depth and the practical techniques for achieving the correct dimensions.
Understanding the Purpose of a Swale
The fundamental function of a swale is to intercept concentrated surface runoff and reduce its velocity. Slowing the water provides an extended opportunity for it to infiltrate the soil rather than becoming destructive runoff. This process helps recharge local groundwater supplies and maintain soil moisture for nearby vegetation.
A swale is distinct from a simple drainage ditch, which is designed primarily for the rapid conveyance of water. Swales, particularly contour swales, are built nearly level along the land’s elevation line to spread the water across a broad area. This allows water to sink slowly into the earth, preventing the erosive force of rapid flow. Swales lined with dense vegetation, often called bioswales, also filter pollutants before the water enters the ground.
Variables Determining Optimal Depth
The necessary depth of a swale is customized based on site-specific factors that dictate how much water needs to be held and for how long. These factors ensure the swale provides adequate storage volume without compromising stability.
Volume Requirements
The volume of water the swale must temporarily contain is a major consideration. This volume is calculated from the expected rainfall intensity and the size of the upstream drainage area. A larger watershed or an area prone to intense storms requires a greater cross-sectional area to hold the calculated runoff, translating into a need for a deeper or wider channel.
Land Slope
The slope of the land also influences the required depth, especially the swale’s longitudinal slope. Steeper longitudinal slopes increase water velocity, necessitating a design that incorporates features like check dams to slow the flow and increase residence time. If the slope is too steep, the swale must be deeper or include more frequent check dams to prevent erosive speed.
Soil Permeability
Soil permeability, or how quickly water soaks into the ground, is the third defining factor. In highly permeable soils, such as sandy loams, the swale may not need to be as deep because water infiltrates rapidly. Conversely, dense clay soils have low permeability, requiring a design that maximizes temporary storage depth and surface area for sufficient absorption time.
Standard Depth and Measurement Techniques
For typical residential or small-scale applications, swale depth is generally shallow, ranging from 6 to 12 inches, measured from the lowest point of the channel to the natural ground level. Engineering designs for commercial or municipal areas may specify depths up to 2 feet to handle larger volumes. The depth is linked to its width, contributing to the necessary width-to-depth ratio, often kept at 4:1 or 6:1 (horizontal to vertical) for stability and to maximize the surface area for infiltration.
The depth must be uniform across the length of the swale to ensure water is distributed evenly and does not concentrate in low spots. The most crucial measurement is ensuring the swale’s base is perfectly level, or “on contour,” from end to end. This precision is achieved using tools like a string line level, a laser level, or a homemade A-frame level.
To use an A-frame level, the tool is calibrated to find the exact level line. This line is then used to walk the contour across the landscape. Stakes mark this level line, guiding the excavation to create a channel bottom with a uniform elevation. A level base ensures that when water enters the swale, it spreads thinly across the entire length rather than flowing toward a single, lower point. A slightly lower overflow spillway is designed into the berm to handle excess water during extreme events, allowing water to exit as a non-erosive sheet flow.
Construction and Upkeep
Once dimensions are established, construction involves excavating the soil and placing it on the downhill side to form a raised berm. The excavated soil should be “keyed” into the existing downhill grade, meaning the bottom layer is integrated with the subsoil for stability. This creates a stable mound that prevents collected water from breaching the downhill side.
The swale must be stabilized immediately after shaping to prevent erosion before the first rainfall. This is done by planting the swale and the berm with dense, deep-rooted vegetation, such as turf grass or native perennial species. The roots anchor the soil structure and maintain the integrity of the swale’s shape.
Routine upkeep is required to maintain the full water-holding capacity. This involves periodically removing accumulated sediment and debris that can reduce the effective depth. Inspections after heavy rainfall are important to check for signs of erosion or breaches in the berm and to repair any damaged vegetation.