Low Impact Development Toolbox

Bioswales are similar to bioretention cells in design and function but are linear elements that can also be used for conveyance and storage in addition to their biofiltration function. They can be used anywhere and are best used on small sites, in urbanized and suburban commercial areas, residential areas, and parking lots.

Cisterns are large rainwater systems installed above or below ground with a much larger capacity than rain barrels. They can store water from multiple downspouts and pavement areas.

Constructed stormwater wetlands are manmade shallow-water ecosystems designed to treat and store stormwater runoff. These wetlands allow pollutants to settle out or to be treated by vegetation. Runoff is slowly discharged over one to three days. Wetlands provide plant and wildlife habitat and can be designed as a public amenity. While constructed stormwater wetlands have limited applicability in highly urbanized settings, they are a desired technique on larger sites with relatively flat or gently sloping terrain. They are also well-suited to low-lying areas, such as along river corridors.

A green roof is a vegetative layer grown on a rooftop that filters, absorbs, and/or detains rainfall. The green roof system typically contains a soil layer, a drainage layer, and an impermeable membrane. Water is captured and detained in the soil and dispersed through evaporation or transpiration by the plants. Green roofs reduce volume and peak rates of stormwater and enhance water quality. Other benefits include reduction in heat island effect, extension of roof life, recreational and gardening opportunities, air and noise quality improvement, and reduced building heating and cooling costs. They can be integrated into new construction or added to existing buildings, including buildings with flat and sloped roofs. This practice is effective in urbanized areas where there is little room to accommodate other LID systems.

Permeable pavement is a durable, load-bearing paved surface designed to allow water to pass through and into an underlying rock base. Due to the prevalence of clay soils in this region, runoff flows through the permeable pavement and is directed to an underdrain, subsurface detention, or rainwater harvesting system. Permeable pavement allows for streets, parking lots, and sidewalks to mimic pre-development runoff conditions while sustaining the functional attributes of the site area they replace. Permeable pavements reduce pollutant loads and control runoff volume and peak flow rates.

Permeable pavement includes a wide range of materials, such as permeable stone pavers, porous asphalt, and porous concrete. These materials can be used as a substitute to conventional pavement on parking areas, roadways, playgrounds, and plazas.

Rain barrels are small systems that guide runoff through a downspout into a barrel that usually holds less than 100 gallons. Rain barrels are typically installed and maintained by single-family homes.

Bioretention cells, or rain gardens, are vegetated depressions layered with engineered soil media that filter pollutants, increase the time water stays on the site, and provides stormwater storage. These systems usually have an underdrain to ensure the cell drains in a reasonable time period. Although they are applicable in most settings, rain gardens are best used on small sites, urban areas, suburban areas, and parking lots.

A stormwater planter box is a bioretention system enclosed in a concrete container that contains porous soil media and vegetation to capture, detain, and filter stormwater runoff. Stormwater planter boxes are lined, contain an underdrain, have various small to medium plantings, and are installed below or at grade level to a street, parking lot, or sidewalk.

Runoff is directed to the stormwater planter, where water is filtered by vegetation before percolating into the ground or discharging through an underdrain. The stormwater is also used to irrigate the tree or other vegetation in the planter box.

In addition to stormwater control, stormwater planter boxes offer on-site stormwater runoff treatment and aesthetic value. Stormwater planter boxes are optimal for urban or streetscape environments.

Underground storage systems capture and store runoff below grade in large chambers. The stored runoff is usually used for irrigation. If the soils are suitable, a portion may also infiltrate into underlying soils. Underground storage may be used for stormwater detention instead of surface ponds. If used under parking, this method of detention can increase the land available for development.

A vegetated filter strip is a band of vegetation, usually a mix of grasses and native plants that acts as a buffer between an impervious surface and a waterway. They are designed to slow runoff from adjacent impervious surfaces, filter pollutants, and provide infiltration (depending upon the permeability of underlying soils). They can also provide aesthetic benefits, stormwater storage, and wildlife habitat. In addition to stormwater management, vegetated filter strips can add recreational value with opportunities to incorporate trails into their design.

Filter strips are best suited on sites that naturally support dense vegetation. Filter strips are best used in treating runoff from roads, roofs, small parking lots, and other small surfaces.

A vegetated swale is a wide, shallow channel with vegetation covering the sides and bottom. Swales are designed to convey and treat stormwater, promote infiltration, remove pollutants, and reduce runoff velocity. Vegetated swales mimic natural systems better than traditional drainage ditches.

Vegetated swales can be used on sites that naturally cultivate a dense vegetative cover and have an appropriate area, slope, and infiltration potential. Swales are most effective when used in a treatment train with other LID techniques. They are widely used to convey and treat stormwater runoff from parking lots, roadways, and residential and commercial developments and are compatible with most land uses.