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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 />7 <br /> <br />Hazard Area Delineation <br /> <br />The Dry Creek flood hazard area is narrow and well defined <br />for almost the entire 6.2 mile length of the channel. It <br />averages approximately 300 feet in width through the uppermost <br />five miles and widens to an average of 700 feet slightly up- <br />stream of Valmont Drive. Downstream of the Union Pacific Rail- <br />road bridge it broadens out into shallow sheet flow due to the rela- <br />tively flat ground slopes near the Dry Creek confluence with <br />Boulder Creek. The flood hazard area has localized wide spots <br />at many locations which are generally associated with either <br />backwaters caused by crossing structures or heavy meandering <br />of the low flow creek channel. <br /> <br />The bridge at Baseline Road can completely convey flood <br />flow without overtopping. However 1 the consecutive culverts <br />at North 75th Avenue and the C & S Railroad are of inadequate <br />capacity and result in lo~alized flooding of North 75th Avenue. <br />Flood flow which cannot be handled by the structures backs up <br />and flows north on 75th Avenue underneath the C & S Railroad <br />viaduct. Ground slopes immediately north of the viaduct <br />enable this excess flow to return to the Dry Creek channel <br />within a short distance. About 2/000 feet downstream of the <br />C & S Railroad culvert a small farm bridge is also overtopped as is <br />the Arapahoe Road bridge another 1,000 feet further downstream. <br /> <br />Downstream of Arapahoe Road the well defined flood hazard <br />area remains narrow for approximately three miles, even though <br />two small structures in this reach are overtopped. From this <br />location to the Union Pacific Railroad bridge it widens due to <br />the backwaters of the structures at Valmont Drive and the <br />railroad tracks. As flood flow overtops the railroad bridge, <br />the major portion of it remains out of bank and covers the <br />gently sloping ground with shallow sheet flow until it reaches <br />Boulder Creek. <br /> <br />Drawings Dl through DID depict flood water elevations in <br />and profile and provide a continuous delineation of the Dry <br />Creek flood hazard areas. <br /> <br />plan <br /> <br />~ Leonard Rice Consulting Water Engineers.lnc <br />