4.12 STORMWATER SYSTEflS - SUBPROJECT 12 



Site preparation and construction of OCS facilities, especially large 

 ones in natural drainage areas, can drastically affect the extent, content 

 and course of stormwater runoff. 



Excessive alteration of the land surface in coastal area developments 

 is the cause of considerable pollution and other disruption of coastal 

 tvater ecosystems. Engineering and land use management actions can resolve 

 many of these problems, particularly recent innovations in the structural 

 and nonstructural handling of the stormwater runoff. This discussion 

 addresses structural, or engineered, approaches. Nonstructural watershed 

 management approaches are discussed elsewhere. 



4.12.1 Summary 



Changes in the natural pattern of land drainage are presumed to have 

 adverse effects on water quality. Such changes include development in 

 floodplains, filling or devegetation of drainage-ways, alteration of 

 natural land grades, straightening or channlization of watercourses, land 

 clearing, and land surfacing. Loosened soils may erode and be washed 

 into water basins along with silt, heavy metals, fertilizers, pesticides and 

 other deleterious matter. Extensive areas covered with a paved surface 

 causes runoff of freshwater in surges which may affect salinity and currents 

 as well as water quality. Development planning should respect the 

 principle of retaining the system of land drainage in as nearly the natural 

 pattern as possible. OCS-related developments should restore water cycles 

 so that water leaves the altered area in virtually the same quantity and 

 rate of flow as existed before the project. 



In a developed area, the amount of stormwater runoff increases 

 proportionately with the extent of impervious surfaces and rooftops [71]. 

 For an average acre of land in the United States, around one-third of the 

 precipitation received is given off to the atmosphere and the remainder 

 will eventually find its way to a watercourse and in the end to the sea. 

 Coastal waters are strongly governed by this runoff and by its chemical 

 and particulate content and rate of discharge. Changing the natural 

 rate or volume of flow by more than 10 percent should be considered 

 unfavorable and avoided wherever possible. 



Elimination of pollution in runoff water is a national goal to be 

 accomplished by 1985. Runoff water has been analyzed and may have higher 

 biochemical oxygen demand (BOD) and greater concentrations of various 

 pollutants than domestic sewage [109][110] (Table 25). Sources of 

 contamination include litter, automobile oil and grease, spillage from 

 bulk chemical storage, and exposed dirt piles at construction sites. 



Pollution as well as altered runoff flows can be corrected. The 

 basic requirement of a stormwater system is to retain precipitation within 



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