A swale is a shallow, level ditch dug along the contour lines of a landscape, designed to passively capture and slow rainwater runoff. This earthwork structure prevents water from rushing downhill and causing erosion, encouraging it to spread horizontally and soak deeply into the soil profile. The primary function of a water harvesting swale is water conservation, effectively turning rainfall into a subsurface reservoir that rehydrates the surrounding landscape. By maximizing water infiltration, the swale supports vegetation and raises the water table on the downhill side.
Planning and Siting Your Water Harvesting Swale
The effectiveness of a swale depends entirely on its placement precisely along a contour line, meaning every point along the swale’s bottom must be at the same elevation. This ensures water is held evenly across the entire length of the trench, preventing it from rushing to a low point and breaching the structure. Before digging begins, you must accurately determine the site’s slope and mark the intended path of the swale.
Tools like an A-frame level or a builder’s level are used to identify the precise contour line on the ground. An A-frame level, often constructed from three pieces of wood, allows you to find points of equal elevation across the slope by pivoting the tool. You start at a proposed high point and systematically mark level points across the area where you intend to build the swale.
Initial planning should involve observing how water naturally flows across your property during a rain event to identify key runoff pathways. Swales are not recommended for slopes steeper than a 20-degree pitch, as earth moving becomes less efficient for the water-holding capacity. It is also prudent to site the swale at least ten feet away from any building foundations to prevent potential moisture issues in the structure.
Essential Construction Steps
Once the contour line is accurately marked, the physical construction begins with digging the trench itself. The swale trench, or basin, is typically constructed to a depth between 6 and 18 inches, with a width of 18 to 24 inches for residential applications. The goal is to create a flat, level bottom for the trench, ensuring that water cannot move laterally once captured.
All excavated soil is immediately piled on the downhill side of the swale to form the berm. This mound of earth acts as a dam, holding the water in the trench and preventing it from flowing further downslope. The berm should be compacted and shaped with gently sloping sides, ideally with a slope no steeper than 3:1 (horizontal to vertical), to ensure stability and prevent erosion.
The finished structure consists of the water-holding basin on the uphill side and the raised berm on the downhill side. For residential swales, the width of the berm’s base should be at least four times its height to establish a stable structure. The berm must be continuous along the entire length of the swale, with no gaps that would allow water to escape.
Stabilizing and Activating the Swale System
The final steps involve stabilizing the newly disturbed earth to prevent erosion and activating the system for maximum water infiltration. The berm must be planted immediately to anchor the soil with a network of roots, as bare earth is highly susceptible to washouts during heavy rain. Deep-rooted grasses, shrubs, or trees are excellent choices for planting directly onto the berm, where they can access the stored moisture.
The swale trench itself should be covered with a thick layer of organic mulch, such as wood chips or straw, to reduce evaporation from the soil surface. This mulch layer absorbs water and slowly releases it into the ground. It also prevents sunlight from reaching the soil, promoting the growth of beneficial microorganisms. The organic matter will slowly decompose, enriching the soil and enhancing its water-holding capacity.
For safety and to protect the earthwork from an extreme rain event, a level-sill spillway must be incorporated into the design at the highest point of the swale. This feature is a broad, shallow, perfectly level section of the berm designed to allow excess water to flow out slowly in a thin sheet, rather than concentrating the flow in one spot and causing a destructive breach. Stabilizing the spillway with dense vegetation or rock ensures that any overflow is managed safely, protecting the swale and the land below it from erosion.