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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />SECTION THREE ADected Environments and Environmental Consequences <br /> <br />districts of the city on its way to the South Platte River. Water that flows from Pawnee Creek <br />along Highway 6 also enters the western side of Sterling and follows existing drainage systems <br />through residential and business areas of the city prior to flowing into the South Platte River. <br /> <br />Flooding in the vicinity of Atwood is increased when the overland flows encounter the UPRR <br />embankment. The embankment does not have a conveyance system that allows the water to flow <br />past the embankment. This causes the overland flows to pond on the upstream side of the <br />embankment, resulting in flooding in Atwood. The ponded water remains in Atwood until the <br />flow in Pawnee Creek reduces to a level that allows the ponded water to be discharged via the <br />road drainage system along Highway 6. During the 1997 flood, it was estimated that the Atwood <br />overflow was approximately 20,500 cfs. This caused water levels in Atwood to raise to a level <br />that topped the UPRR embankment by approximately 2 feet, at which time the embankment was <br />washed out and the ponded water flowed through the breach into the South Platte River. <br /> <br />3.9.1 Alternative 1 . No Action <br /> <br />No activities would occur with the No Action Alternative; therefore, this alternative would not <br />affect the hydrology or water quality of Pawnee Creek or the South Platte River. <br /> <br />3.9.2 Alternative 2 . Improve Routing of Pawnee Creek Flood Flows (Proposed <br />Action) <br /> <br />The effects of Alternative 2 on hydrology are different at each of the three locations where <br />project features would be constructed: Highway 6IUPRR bridges over Pawnee Creek, Riverside <br />Cemetery, and Atwood. Effects of the alternative at each of these locations are discussed below. <br /> <br />Flood depths upstream of the Highway 6IUPRR bridges would be reduced by 4 to 6 feet. The <br />larger bridges would be able to convey the 1 OO-year flood event without ponding water upstream <br />of the bridges (URS Greiner Woodward Clyde 1999). <br /> <br />The facilities at Riverside Cemetery would increase flood depths upstream of Highway 6 by <br />approximately 0.1 to 0.4 feet due to the construction oflevees, floodwall, and road closure <br />system (URS Greiner Woodward Clyde 1999). This increase would primarily occur within and <br />immediately adjacent to the constructed conveyance system. This alternative would prevent <br />flood flows up to the 100-year flood event from entering Sterling and, thus, significantly <br />decreasing the potential for flood damage within the city. <br /> <br />This alternative would reduce flood depths by 3 to 6 feet at Atwood. Although limited flooding <br />could still occur in Atwood, the duration of the flooding that could occur would be greatly <br />reduced by this alternative (URS Greiner Woodward Clyde 1999). <br /> <br />Even for flood events which exceed the design event, this alternative would greatly minimize <br />flooding at all three locations. In addition to less flow, the duration of flooding would be <br />reduced. For the 1997 flood event, the alternative would have reduced the duration from <br />45 hours to 20 hours (URS Greiner Woodward Clyde 1999). <br /> <br />3-13 <br />