A: Stormwater is the surface runoff of rain and snowmelt. In undeveloped areas such as grasslands and forests, the surface flow of water is slowed by vegetation and much of the water seeps into the ground. With urban growth and development, open land is replaced by hard surface areas, such as buildings, streets and parking lots, which are unable to absorb stormwater. Because development results in increased amounts of stormwater runoff, a stormwater drainage system is necessary to channel the runoff out of urban areas and reduce the occurrence of flooding.