A rain garden is a specially designed depression in the landscape, planted with native species, intended to collect and filter stormwater runoff from impervious surfaces like roofs and driveways. This mechanism allows water to soak into the ground, recharging local groundwater and removing pollutants before they enter the storm sewer system. Building this structure on a slope presents a unique challenge because the downward rush of water can easily erode the basin and wash away the soil and plants. The solution requires transforming the single depressed basin into a series of smaller, level collecting pools to manage the water’s speed and volume, preventing erosion and maintaining a consistent water level.
Assessing the Site and Slope Grade
Determining the precise grade of the land is the first step, as conventional rain gardens are not recommended for slopes exceeding 12% without special design modifications. A slope percentage is calculated by dividing the vertical change in elevation (rise) by the horizontal distance (run) and multiplying the result by 100. This measurement can be taken by placing two stakes a known distance apart, attaching a level string between them, and measuring the vertical drop from the level string to the ground at the downhill stake.
You must also identify the exact path of the fastest water flow, which dictates the orientation of the final terraced system. Mapping the existing drainage patterns and the total catchment area is best done following a heavy rain event. If the native soil’s permeability is low, a larger garden or deeper soil amendment will be required for each terrace. For slopes steeper than 20%, the risk of slope failure and the engineering required to stabilize the terraces often necessitate professional consultation.
Designing Terraces and Berms
The fundamental strategy for building a rain garden on a slope involves terracing, which breaks the single steep incline into a series of smaller, level, stepped sections. This design prevents the water from gaining erosive momentum as it flows downhill. Each level section, or terrace, must be perfectly flat to ensure water ponds uniformly across the entire area, maximizing infiltration before the water moves to the next lower level.
Earthen walls, known as berms, are constructed across the slope to define the downhill edge of each level terrace, acting as check dams to hold the water temporarily. The height and width of these berms are calculated to contain the design storm’s runoff volume without overtopping; a minimum height of six to twelve inches is often required. The horizontal distance between terraces is inversely proportional to the slope; steeper slopes require more frequent, shorter terraces to maintain a manageable elevation difference. This terracing pattern slows the water’s velocity, preventing the development of destructive channels and ensuring the soil has sufficient time to absorb the runoff.
Constructing the Basin and Stabilization Features
The physical work begins by staking out the location of each terrace and berm, working from the lowest point of the slope upward. Excavation should start at the highest terrace first, using the excavated soil to construct the downhill berm of that same terrace. This cut-and-fill method helps level the basin and conserves material, but ensuring the bottom of each terrace is perfectly horizontal is critical for even ponding.
After the soil is placed to form a berm, the downhill face must be thoroughly compacted to prevent structural failure, or a “blowout,” during a heavy rain event. Compaction is achieved by walking or tamping the soil in thin layers, creating a dense barrier that can withstand the hydrostatic pressure of the impounded water.
To manage extreme rainfall, a rock weir or vegetated overflow spillway should be incorporated into the top of each berm, allowing excess water to safely cascade to the next terrace without causing structural damage. The excavated soil is then amended with compost and sand to create a highly porous bioretention soil mix, ensuring rapid and effective infiltration.
Selecting Plants for Erosion Control
The plant selection for a terraced rain garden must prioritize deep-rooted, tough species that provide structural integrity to the engineered slopes. These plants act as a living reinforcement, binding the soil of the berms and the sides of the terraces with extensive, fibrous root systems. Their primary function on the slope itself is stabilization, not just water filtration or aesthetics.
Native grasses, such as switchgrass or inland sea oats, are excellent choices because their root systems can penetrate several feet into the soil, forming dense, underground mats that resist soil displacement. Shrubs with a suckering habit, like certain varieties of dogwood or elderberry, offer additional woody structure to hold the soil on the berm faces. A dense planting strategy, particularly on the compacted berms, is necessary to minimize bare soil patches susceptible to erosion from raindrop impact and surface runoff.