A swale is a shallow, level-bottomed trench dug along the contour of a slope to manage water runoff. It is paired with a berm, a mound of earth constructed immediately downhill using the excavated soil. The purpose of this earthwork is to intercept surface runoff and slow its flow across the landscape, rather than channeling water away quickly. By holding the water, swales allow sufficient time for it to infiltrate the ground, recharging groundwater and increasing soil moisture. This process reduces the risk of erosion and prevents nutrient-rich topsoil from washing away during heavy rain events.
Site Assessment and Contour Marking
Before excavation, assess the site’s topography and soil composition to ensure the swale functions correctly. Swales operate most effectively on gentle slopes, typically less than 4%, with slopes under 2% being ideal for maximizing infiltration. Steeper gradients cause water to flow too quickly, potentially leading to erosion within the channel. Soil type is also a consideration; highly permeable soils absorb water faster, while dense clay soils may require a shallower design to promote absorption.
Contact the local utility notification center (811 in the United States) before digging to ensure no underground pipes or lines are present. Choose the swale location where water naturally flows or collects, positioning it at least 10 feet from building foundations. Once the location is clear, the path of the swale must be marked along a contour line, meaning every point shares the same elevation.
Find the contour using a surveying tool, such as an A-frame, water, or laser level, to establish points of equal height across the slope. The trench bottom must be completely level to ensure water spreads evenly for maximum infiltration, preventing flow to a single low point. For residential projects, trenches are often 1 to 2 feet deep and 3 to 5 feet wide. These dimensions should be tailored to the expected runoff volume, with the marked contour line serving as the center of the trench and berm structure.
Excavation and Berm Construction
Digging the swale trench begins by excavating soil along the marked contour line, ensuring the bottom remains level across its entire length. Maintaining a level base facilitates the even spread and ponding of water when runoff occurs. The trench is typically dug with a vertical back wall on the uphill side to maximize the surface area interfacing with the undisturbed soil. Hand tools like shovels are sufficient for smaller projects, but larger properties may require heavy machinery like excavators.
As the trench is excavated, the removed soil is immediately placed along the downhill edge to construct the berm. The berm acts as a retaining wall for the captured water on the uphill side. It should be built high enough to contain the maximum expected water volume. The downhill side should be shaped with a gentle slope, ideally no steeper than a 3:1 ratio (three horizontal units for every one vertical unit) for stability.
Once the soil is mounded into the berm, it must be compacted to prevent erosion and washing back into the trench. Compaction can be achieved by walking over the berm repeatedly or using a mechanical plate compactor for a dense, stable structure. This ensures the berm has the structural integrity to hold the water and resists slumping or breaching under pressure.
Stabilization and Overflow Management
Immediately after the swale and berm are constructed, the exposed earth must be stabilized to prevent erosion, which is a high risk until vegetation establishes itself. The surface of the berm, in particular, should be covered with a layer of organic material such as wood chips, straw mulch, or a fast-growing cover crop. This protective layer mitigates the impact of raindrops and slows surface flow, allowing the soil underneath to remain in place during initial storm events.
Planting provides long-term structural reinforcement for the earthwork, with deep-rooted species being particularly effective at anchoring the soil of the berm. Nitrogen-fixing plants, grasses, and shrubs are often recommended for the berm itself, as their roots penetrate deeply, improving soil health and stability. The trench floor, which is the infiltration zone, should also be planted, typically with water-tolerant grasses or groundcovers that can handle occasional inundation without dying.
An important feature for safety and function is the installation of an overflow mechanism to handle storm events that exceed the swale’s capacity. This is achieved by building a level spillway or overflow weir at the lowest end of the swale, where the contour line naturally terminates. This spillway must be level and armored with materials like rock or dense vegetation to safely direct excess water away from the swale without causing erosive channels in the surrounding landscape. The controlled overflow prevents the water from breaching the berm at an unmanaged point, which could lead to significant downstream damage.