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<br />I <br />II <br />I <br />I <br /> <br />DRAINAGE CRITERIA MANUAL (V. 3) <br /> <br />STORMWATER QUALITY MANAGEMENT <br /> <br />,. WHY IS STORMWATER RUNOFF QUALITY IMPORTANT <br /> <br />I <br />I <br />I <br />I <br />I <br />II <br />I <br />I <br />I <br />I <br />I <br /> <br />The public's concerns with stormwater are usually related to flooding, not water quality. People complain <br />when their basements flood or roads become impassable, Very few people are aware of the water quality <br />impacts that stormwater has on our rivers or streams. Elevating or enhancing stormwater quality improves <br />the quality of streams and lakes in the urban area, <br /> <br />1.1 Environmental Impacts of Runoff <br /> <br />Many people are familiar with the environmental impacts from municipal and industrial wastewater <br /> <br /> <br />treatment plants. Few are aware of the existence of a stormwater drainage sewer system much less the <br /> <br /> <br />impact that it may create to the environment. Interestingly, studies have shown that runoff from urban and <br /> <br /> <br />industrial areas contain significant quantities of the same general types of pollutants that are found in <br /> <br /> <br />wastewater and industrial discharges and can cause similar environmental impacts. <br /> <br />The impact of stormwater on streams can be fit into one of four categories. These Include stream <br />hydrology, stream morphology, water quality and aquatic ecology. The extent of impact Is related to the <br />climate, land use, and the measures implemented to address the Impacts. <br /> <br />Briefly, the impacts on streams are, <br /> <br />1. Stream Hvdroloav: Urban development affects the environment through changes in the size and <br /> <br /> <br />frequency of storm events, changes in base flows of the stream and changes in stream velocity <br /> <br /> <br />during events which results in decrease in travel time for runoff. Peak discharges may increase by <br /> <br /> <br />a factor of 2 to 5 times original discharge rates. Less noticeable changes which result from <br /> <br /> <br />hydrologic changes due to urbanization include decrease in infiltration into the groundwater and <br /> <br /> <br />loss of buffering vegetation. This results in the loss of significant pollutant removal mechanisms. <br /> <br /> <br />Loss of riparian vegetation can result in an increase in temperature which affects the biological <br /> <br /> <br />composition of the stream and its chemistry. <br /> <br />I <br />I <br />I <br />I <br /> <br />2. Stream Moroholoav: When the hydrology of the stream changes, it results in changes to the <br /> <br /> <br />physical characteristic of the stream, Such changes include widening, streambank erosion and <br /> <br /> <br />deposition, increased stream elevation due to greater discharge rates and an increased amount of <br /> <br /> <br />sedimentary material within the stream, The stream may widen to accommodate higher flows, this <br /> <br /> <br />results in an increase in sediment loads instream. This can result in changes in the stream bed. It <br /> <br /> <br />may change from coarse.grained particles (pebbles) to fine grained particles (sand). This change <br /> <br /> <br />in habitat can have negative effects on the aquatic life's ability to survive and reproduce. <br /> <br />X.X.98 <br />Urban Drainage and Flood Control District <br />DE Nllg1599.d0cJ98272Q004. DOC <br /> <br />5Q.1 <br />