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APPENDIX C • <br />DRAINAGE REPORT <br />EXISTING DRAINAGE SITUATION <br />Tributary Areas <br />The site lies adjacent to the west side of the South <br />Platte River (Figure A ). The South Platte at this point <br />has a drainage area of 4713 square miles, which includes <br />the major tributaries of Clear, Bear, Cherry, Plum, Sand, <br />and First Creeks, and the majority of the Denver Metropo~i- <br />tan arcs. The U.S. Geologic Survey (USGS) has main~ained <br />a stream gage near the site since May, 1926. <br />Records of average daily and instantaneous discharges <br />are available since November, 1933. The minimum flow re- <br />corded was 4.v cfs on April 1, 1950; the maximum, 33,000 <br />cfs, was re~ordcl curing a f~nnd on May G, 1973. Major <br />floods are Known to have oc~._ad in 1921, 1933, 1942, 1965, <br />and 1973. The 100-year flood presently estimated, has a <br />peak discharge of approximately 30,000 cfs. This dischar~ <br />would have a 1 percent chance of being equale3 or exceeded <br />in any given year. <br />The po tion of the mining site north of Henderson Road <br />intercepts Brantner Gulch, which is presently undeveloped. <br />The Gulch concentrates flow at Riverdale Road where the tri- <br />butary area is approximately 9.7 square miles (Figure B~. <br />The gulch is intermittent, draining east from its head- <br />waters near the intersection of East 120th Avenue and <br />North ldashinyton Gtrcet. Much of the basin is with'. the <br />present municipal boundaries of the City of Thornton and <br />development is ].ikel~~ If the basin were to develop fully, <br />the 100-year flood would have a peak discharge estimate' ~t <br />4,300 cfs. <br />An additional 0.5 square miles could contribute over- <br />land storm runoff to the sits (Figureg ). Normal flows <br />from this area are intercepted by Lower Clear Creek and <br />the Brantner Ditches. The Lower Clear Creek I ch terminates <br />at kenderson Road and water in the ditch is diverted into <br />the Brantner Ditch. Flows from severe storms could over- <br />flow both ditches and drain eastward onto the site. During <br />such storms, additional storm water could be imported into <br />t}ie mining site by the Lower Clear Creek Ditch which inter- <br />cepts a considerable area south and west of the site. <br />Flood Plain <br />The site lies mostly within the 100-year flood plai.i of <br />the South Platte River. T~:~o flood plain s*.udies have already <br />been under ,ken. The first was by the U.S. Army Corps of <br />-23- <br />