We are all familiar with rain: water falling from the clouds onto the ground. Although rain itself is easy to identify, what happens to rainwater after it reaches the ground is less obvious and dependent on the type of land cover where it rains. Undeveloped areas are made up of natural expanses that allow most of the rainwater to soak into the ground. A small amount slowly runs off into ditches and creeks. This runoff is an important source of fresh water to replenish wetlands, creeks, and lakes.
In the country, nature, not man, handles and protects the water supply. However, as land is developed into urban areas, parts of the ground are replaced by roads, buildings, and other hard surfaces that no longer allow rainwater to soak into the ground. Instead, the rainwater now rapidly pours into our creeks and streams. In this urban setting, man, not nature, must control the surface water as well as protect the water supply.
Stormwater is the runoff that results from rainfall. As this water flows over construction sites, farm fields, lawns, driveways, parking lots, and streets, it picks up sediment, nutrients, bacteria, metals, pesticides, and other pollutants. Unlike sanitary sewers that go to a treatment plant, most stormwater is discharged directly to local water bodies. Increasing amounts of impervious surfaces in urban areas, such as roof tops, driveways, parking lots, and streets, decreases the ability of the water to soak into the ground, thus increasing the potential for flooding from greater volumes of runoff entering the city’s storm sewer system at a faster rate.