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• APPENDIX C • DRAINAGE REPORT EXISTING DRAINAGE SITUATION Tributary Areas The site lies adjacent to the west side of the South Platte River (Figure A ). The South Platte at this point has a drainage area of 4713 square miles, which includes the major tributaries of Clear, Bear, Cherry, Plum, Sand, and First Creeks, and the majority of the P .ver Metropo~i- tan area. The U.S. Geologic Survey 'USGS) has mai.rained a stream gage near the site since May, 1926. Records of average daily and instantaneous discharges are available since November, 1933. The minimum flow re- corded was 4.4 cfs on April 1, 1950; the maximum, 33,000 cfs, was re::ordcl during a flood on May G, 1973. Major floods are ;mown to have occured in 1921, 1933, 1942, 19..x, and 1973. The 100-year flood presently estimated, has a peal discharge of approximately 30,000 cfs. This discharge would have a 1 percent thane: of being equaled or exceeded in any given year. The portion of the mining site north of Henderson Road intercepts Brantner 'ulch, which i~ ~resently undeveloped. The Gulch concentrates flow at Riverdale ;;oad where the tri- butary area is approximately 9.7 square miles (Figure B~. The gulch is intermittent, draining east from its head- waters near the intersection of East 120th Avenue and North Washington Street. Much of the basin is within the present municipal boundaries of the CiL-y of Thornton and development is likely. If the basin were to develop fully, the 100-year flood would have a peax discharge estimated at 4,300 cfs. An additional 0.5 square miles could contribute over- land storm runoff to the sits (Figurep `. Normal flows from this area are intercepted by Lower Clear Creek and the Brantner Ditches. The Lower Clear Creek Ditch terminates at Henderson Road and water in the ditch is diverted into the Brantner Ditch. Fl s from severe storms could over- flow both ditches and drain eastward onto the site. During such storms, additional storm water could be imported into the mining site by the Lower Clear Creek Ditch which inter- cepts a considerably area south and west of the site. Flood Plain 'Phe site lies mostly within the 100-year flood plai.~ of the South Platte Ri~•cr. Tejo flood plain s*.udies have already been undertaken. The first was by the U.S. Army Corps of -23-