Green Infrastructure Practices
There are many ways to use green infrastructure to meet stormwater requirements in Washtenaw County. Each Best Management Practice below is included in our Rules and can offer a variety of benefits to a site depending on current natural features, land uses, and site needs.
All stormwater management systems, green and gray, require maintenance to function, prevent flooding, and improve water quality. It is helpful to understand the specific needs of different BMPs in the planning and design process to make sure these beneficial systems will be cared for in the future.
Bioretention Systems use plants and soil to capture and infiltrate runoff. These systems are flexible in design and can vary in complexity according to site conditions. Locally adapted vegetation, amended soils, and an aggregate base will allow large volumes of stormwater to infiltrate at sites without highly permeable soils while also treating pollutants from runoff.
Rain Gardens are shallow basins that use plants and soil to collect and infiltrate runoff. These gardens are typically constructed with a controlled overflow outlet without underdrains. Locally adapted vegetation that is specially selected and soils amended with compost provide infiltration and treat pollutants from runoff.
Bioswales are shallow channels with vegetation that are designed to slow, filter, infiltrate, and convey runoff. They are densely planted with a variety of locally adapted grasses, shrubs, and/or trees and do not have underdrains. Check dams can be installed to improve sediment capture and increase the time of concentration.
Infiltration Basins are designed to temporarily capture, temporarily store, and infiltrate runoff into naturally permeable soil over several days. A site that is 5-50 acres can be designed with a single, large basin or several small basins that are constructed or natural features.
Subsurface Infiltration Beds are large reservoirs located underground that are designed to capture, temporarily store, and infiltrate stormwater over several days in the surrounding soil from an area up to 10 acres. While they are typically made up of a rock storage bed underground, other common systems include pre-manufactured pipes, vaults, and modular structures. These systems were developed as alternatives to basins and trenches for sites that have limited space and can be located under paved areas or grassy open spaces.
Infiltration Trenches are shallow, excavated trenches that are designed to capture, temporarily store, infiltrate and convey runoff from an area up to 5 acres. They are typically linear and filled with a rock storage bed below the surface. Water infiltrates into naturally permeable soils
Pervious Pavements are designed to allow stormwater and runoff to filter through the paved surface to a stone reservoir below where it is stored until it can infiltrate into the soil. Variations include pervious concrete, porous asphalt, permeable paver blocks, and reinforced turf. It is well suited for parking lots, sidewalks, playgrounds, plazas, and similar uses.
Vegetated Filter Strips are permanent, maintained strips of vegetation that are meant to slow and infiltrate runoff and filter pollution. These systems require sheet flow and are commonly used with level spreaders. Filter strips are often designed where runoff is directed from parking lots or roadways.
Green Roofs are conventional rooftops that have a thin layer of vegetation designed to capture, temporarily store, and reduce stormwater. While traditional roofing materials shed nearly all rainfall as runoff, a green roof is able to absorb and evapotranspire a portion of that rainfall.
Detention & Retention Basins are designed to capture, temporarily store, and treat runoff for pollutants while controlling the rate of stormwater discharge to prevent downstream flooding. These stormwater storage systems use locally adapted plants and can range from being mostly dry between storm events to having a permanent pool of water. They are used at sites with soils that do not allow for infiltration.
Water Harvesting & Water Reuse are constructed systems that capture and store stormwater and/or runoff for use at a later time. These systems help reduce runoff volume and water quality impairment downstream while providing beneficial uses on-site. Common examples include rain barrels, cisterns that collect water which can be reused for irrigation, or greywater.