A rain garden is a specialized, landscaped depression designed to intercept and manage stormwater runoff from hard surfaces like roofs and driveways. This system functions as a biological filter, slowing the rush of water that typically flows into storm drains and local waterways. The overall goal is to reduce the volume and velocity of surface runoff while significantly improving water quality by filtering out common urban pollutants.
Physical Components That Make It Work
The rain garden relies on a physical structure, often called a bioretention system. The central feature is the shallow basin, engineered to temporarily hold incoming stormwater in a ponding area, usually six to twelve inches deep. This depression slows the water’s momentum, allowing time for the filtering process to begin.
The soil mixture within the basin is highly specific, replacing dense native soil with an engineered media to enhance porosity and drainage. This media is typically a blend of sand, compost, and topsoil, designed to promote rapid infiltration. A high sand content, sometimes reaching 30 to 60 percent, ensures that water can quickly soak into the ground.
A perimeter berm, a small raised edge, is constructed on the downhill sides of the garden to contain the water within the basin. This edge ensures that the full volume of runoff is held in the ponding area, maximizing contact time with the filtering media.
Native plants are chosen for their tolerance to brief flooding and dry periods. These deep-rooted plants are instrumental, as their root channels create pathways deep into the soil, maintaining a high infiltration rate over time. Plant selection is often zoned, placing species that tolerate constantly “wet feet” at the basin’s center and more drought-tolerant varieties along the drier edges.
The Hydrological Filtering Process
Runoff is typically channeled from impervious areas through a swale or downspout into the shallow basin. This initial capture dramatically reduces the volume of water entering conventional stormwater systems, mitigating potential downstream flooding.
Once captured, the water is held temporarily in the ponding area, where infiltration begins. The engineered soil media allows the water to soak into the underlying ground at an accelerated rate. The system is designed to fully drain, or achieve drawdown, within 24 to 48 hours following a storm event to prevent the area from becoming a stagnant wetland.
Pollutant removal occurs through three simultaneous mechanisms: physical, chemical, and biological. Physical straining is the first line of defense, where the soil media and mulch layer trap suspended solids, dirt, and debris as the water passes through. This simple filtration removes pollutants bound to these particles, such as phosphorus.
The second mechanism involves chemical processes, primarily adsorption, where dissolved pollutants like heavy metals adhere to the surfaces of soil particles and organic matter. The third mechanism is biological degradation, facilitated by plant roots and dense microbial communities. These microbes actively break down organic contaminants, such as petroleum hydrocarbons, and facilitate nutrient cycling. Plant roots also take up nutrients like nitrogen and store them in their biomass, effectively removing them before the water recharges the groundwater supply.
Choosing the Best Location
Proper placement ensures the rain garden effectively manages the intended water volume and drains correctly. The first step involves assessing the drainage area—the total size of the impervious surface feeding the garden, such as a roof or driveway. The rain garden size is then calculated as a small percentage of this catchment area, typically three to ten percent, to handle the expected runoff.
The land’s slope is also a determining factor, with a gentle grade between one and fifteen percent being most favorable for installation. A moderate slope allows water to flow naturally into the garden without causing significant erosion, although slopes exceeding twelve percent may require additional modifications like terracing.
Proximity to structures must be managed to prevent water damage to foundations. A rain garden should be located at least ten feet away from building foundations and basements so infiltrating water does not compromise the structure. A setback of 25 to 50 feet from any septic system is also required to avoid overloading the drain field.
Before digging, a percolation test must confirm the soil’s drainage capacity. This test ensures the soil is not overly compacted clay, which would prevent the necessary drawdown within the 48-hour window. If the soil drains too slowly, amendments will be required to maintain the system’s function.