A rain garden is a depression, hole, bucket or pond intentionally created and placed in order to collect rainwater runoff from impervious structures and surfaces like roofs, driveways, parking lots, roads and overly compacted lawns. They help reduce runoff and aid in stormwater infiltration or allow for water to be used for irrigation around the landscape. Many cities have been installing rain gardens and other green infrastructure measures, like green roofs, to reduce the amount of stormwater going into hard stormwater infrastructure and avoid overwhelming those systems. Urban stormwater management systems were built to carry precipitation away from the cities and help mitigate flooding, however, recent large storm events have revealed that additional measures may need to be put in place and real estate is already at a premium.
Rain gardens slow water down, increasing lag time which allows turbidity to settle out and naturally filter water, reducing the pollutant load into the watershed. With low installation costs, little maintenance and the ability to modify them as necessary, rain gardens are becoming increasingly popular as methods of stormwater management.
Ideally planted, a rain garden can be a natural depression cultivated or modified to act as such, or an engineered structure or pond. Cities, like Chicago, have identified depressions, filled them with soil and plants which can thrive in variable conditions and created efficient and effective rain gardens in parking areas and along their streets. When designing engineered rain gardens soil composition is critical; it must be able to absorb and infiltrate water while also retaining sufficient moisture to support plant life. The optimal ratio for soil composition in rain gardens is: 25-40% sand, approximately 50% compost and 15-25% native soil, this will vary based on regional climatic differences and vegetative requirements.
Studies have shown that rain gardens in shopping centers and parking lots in cities in North Carolina, Connecticut, Ohio and Maryland have reduced the volume of runoff anywhere between 44 and 99 percent. But the major benefit of utilizing rain gardens for stormwater mitigation is the minimal maintenance which they require. If plants are chosen correctly they require no pruning, mulching or mowing. Sediments may require removal periodically, but on the whole, rain gardens offer low-cost, efficient, effective solutions to stormwater mitigation which also provide aesthetic value.