ABSTRACT
Releases from municipal storm and wastewater sewer systems are called combined sewer overflow, or CSO. Rainwater runs off of roofs and impervious surfaces, and flows into street drains into surface waters. This is how a typical Municipal Separate Storm Sewer System or MS4 functions. Any discharge into a storm drain other than stormwater is an illicit discharge. A variety of techniques may be used to retain rainwater onsite. A French drain permits water infiltration through the use of a gravel-filled pit that collects excess rainwater. A swale is formed by creating a depression in the land which will carry the water away from the property. Rain gardens are planted areas that allow rainwater to permeate into the ground rather than run off into surface waters. Bioretention is the usage of plants for water infiltration. Permeable pavement materials allow water to infiltrate into the soil, recharging groundwater. Green roofs or vegetated roofs absorb as much as an inch of rainfall per hour. Extensive green roofs are typically 2–4 in. deep, accommodating shallow-rooted plants. Intensive green roofs may be 2 ft deep or more, accommodating plants having deeper roots. Green walls are sometimes known as vertical gardens. They do not provide stormwater amenities that green roofs do.
