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t;_ -i~~~c~aeiat~ai-_ <br />Figure 1: Watershed imperviousness and the storm runoff coefficient <br />Runoff Coefficient- (Rv) <br />0.9 <br />0.8 <br />0.7 <br />0.6 <br />0.5 <br />0.4 <br />0.3 <br />0.2 <br />0.1 <br />0 <br />0 10 20 30 40 50 60 70 <br />Watershed Imperviousness (~o) <br />flow between urban and rural watersheds after analyz- <br />ing 16 North Carolina watersheds. Simmons and <br />Reynolds26 did note that dry weather flows dropped 20 <br />to 85% after development in several urban watersheds <br />in Long Island, New York. <br />It should be noted that transport-related impervi- <br />ousnessoften exerts a greater hydrological impact than <br />the rooftop-related imperviousness. In residential ar- <br />eas, runoff from rooftops can be spread out over <br />pervious areas, such as backyards, and rooftops are not <br />always directly connected to the storm drain system. <br />This may allow for additional infiltration of runoff. <br />Roads and parking lots, on the other hand, are usually <br />directly connected to the storm drain system. <br />Imperviousness and the Shape of Streams <br />Confronted by more severe and more frequent <br />floods, stream channels must respond. They typically <br />do so byincreasing theircross-sectional area to accom- <br />modate the higher flows. This is done either through <br />widening of the stream banks, downcutting of the <br />stream bed, or frequently, both. This phase of channel <br />instability, in turn, triggers a cycle of streambank <br />erosion and habitat degradation. <br />The critical question is at what level of develop- <br />ment does this cycle begin? Recent research models <br />developed in the Pacific Northwest suggest that a <br />threshold for urban stream stability exists at about 10% <br />imperviousness'•` (Figure 2). Watershed development <br />beyond this threshold consistently resulted in unstable <br />ariu ciwti~~ ci]ariricta, t tic talc anu SeVerlty U1 channel <br />instability appears to be a function of sub-bankfuil <br />80 90 100 <br />floods, whose frequency can increase by a factor of 10 <br />even at relatively low levels of imperviousness.'°•'9=' <br />A major expression of channel instability is the loss <br />of instream habitat structures, such as the loss of pool <br />and riffle sequences and overhead cover, a reduction in <br />the wetted perimeter of the stream and the like. A <br />number of methods have been developed to measure <br />the structure and quality of instream habitat in recent <br />Table 1: Comparison of one acre of parking lot versus <br />one acre of meadow in good condition <br /> <br />Runoff or Water Quality Parameter Parking <br />Lot <br />Meadow <br />Curve number (CN) 98 58 <br />Runoff coefficient 0.95 0.06 <br />Time of concentration (minutes) 4.8 14.4 <br />Peak discharge rate (cfs), 2 yr., 24 hr. storm 4.3 0.4 <br />Peak discharge rate (cfs), 100 yr. storm 12.6 3.1 <br />Runoff volume from one-inch storm (cubic feet) 3450 218 <br />Runoff velocity @ 2 yr. storm (feet/second) 8 1.8 <br />Annual phosphorus load (Ibs/ac./yr.). 2 0.50 <br />Annual nitrogen load (Ibs/ac./yr.). ~ 15.4 2.0 <br />Annual zinc toad (Ibs/ac./yr.) 0.30 ND <br />Key Assumptions: <br />Parking lot is 100% impervious with 3°~ slope, 200 feet flow length, <br />Type 2 Storm, 2 yr. 24 hr. storm = 3.1 inches, 100 yr. storm = 8.9 <br />inches, hydraulic radius = 0.3, concrete channel, and suburban <br />Washington 'C' values. <br />Meadow is i% impervious with 3% slope, 200 foot flow length, good <br />vegetative condition, B soils, and earthen channel. <br />~__. <br />--' ~t - - - l i ~d1 ~', ~ ~aA _ . u ~=i ~ a ~ "i; lk x ~ ~ I ~ I ~' ~ °Y? ~ 101 <br />