Storm water is defined as storm water runoff, snowmelt runoff, and surface runoff and drainage. Storm water management is important in urban water systems, including water supply systems and wastewater systems. With increasing residential, commercial, and industrial development, stormwater has become an important issue.
Growing urbanization has a significant impact on the surrounding environment, creating problems such as nonpoint sources of water pollution. Because of changes in land-use patterns, pollutants in developed areas build up during dry periods and are washed off as runoff passes over land surfaces. Nonpoint sources account for about 45%, 76% and 65% of the degradation of estuaries, lakes, and rivers respectively (EPA 1989). In comparison, municipal and industrial point source discharges under National Pollution Discharge Elimination System (NPDES) control account for about 9-30% of the degradation of these water sources.
In contrast to our complex urban environment, the hy-drological cycle shown in many hydrology textbooks is rather simplistic. Modification of natural drainage paths, damming of waterways, impoundment of water, reuse of stormwater, and implementation of new stormwater management processes result in highly intricate hydrological processes. The development of storm water runoff and its possible superimposition on dry weather flow in combined sewer systems are summarized in Figure 9.19.1. A detailed urban drainage subsystem is shown in Figure 9.19.2.
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