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1 ] ] ] J ~ ~ :I J ~ ~ ] ~ ~ ~ ~ m I ~ 23 24 I I I I I I I I I I I I I I I I I I I T~e flood frequency distribution for floods at Henderson is shown in Figure B. For the period before. the closure of the Chatfield Dam, the lOO-year flood used to be approximately 32,000 cubic feet per second. The estimates made by the Corps of Engineers (1974) of 'OO~year flood subsequent to Chatfield closure remain virtually the same as before closure. IV. SEDIMENT Bed~teria1s Flow Duration Between Chatfield Dam and Brighton, the South Platte River was an alluvial river before man-made development caused changes. That is, for the most part. the bed and banks of the river were composed of materials carried by the river. Now revetment covers many portions of the bankl1ne; the bed has degraded to bed rock in several locations, some of which are shown on the riverbed profile in Appendix 1. In the upper reaches near Chatfield Dam where the river is in its rural form and parts of the bed are visible because they are dry during low flow, material on the bed of the river varies. In locally steep subreaches, the bed is gravel, with particles up to 3 inches in diameter. In the locally flat subreaches, the bed fs coarse sand. In the suburban reaches through littleton, Sheridan and Englewood, some of the small variations in riverbed slope are the result of the concrete, rubble, or gabion diversion dams and other check structures placed on the bed of the South Platte River. The deposits immediately upstream of the higher dams are medium or coarse sand. Gravel appears downstream and on bars or locally steep reaches. In the urban section of the South Platte River, the riverbed is not readily visible. The few samples of bed material collected by the Corps of Engineers (Schneider, 1983) between Colhx Ave. and fifteenth Street in 1977 were medium and very coarse sand. Often there 1s a layer of muck on top of the granular materials. Most certainly there is gravel also in other parts of the reach. In the vicinity of Sand Creek and Clear Creek, the riverbed is coarse to very coarse gravel. Farther downstream the bed materials become finer again. The size gradation curves for the samples of bed materials obtained from the South Platte River and its major tributaries by the Corps of Engineers are shown in figure 10. The locations where the samples were obtained are listed in Table 7 along with the characteristics of the bed. For the South Platte River, the curve numbers increase in the downstream direction. for the most part, these data were collected in September 1977. It is clear that the bed material ranges widely from medium sand to very coarse gravel. Some local sections have armored with cobbles from 3 to 6 inches in diameter. Test pits 3t04feetdeepindicatethatcobbles ex1st under the bed also, but not everywhere. As described in the Geology Section, the South Platte River is flowing on or incised into sand and gravels deposited in the recent and tertiary Pleistocene. These deposits contain many cObble.sized particles. For Bear Creek, the bed samples range from coarse silt to very fine gravel. The bed slope for Bear Creek is controlled by rock outcrops of the Denver formation. There is such an outcrop irrmediately upstream from the The mean daily flow duration curves for the U.S. Geological Survey's streamgaging stations at Denver and Henderson have been prepared. Shewn in Figure g, the period for Denver is 1.926 to 1982 inclusive (20al9 days). The year 1926 was the first in a series of dry years which heralded the change to lesser streamflow in the South Platte River. For Henderson, the record be9ins in the 192B water year and contains 20,089 days. The flows for selected percent of time e~ceeded are given below. Flow Duration Percent of Time Flow is Exceeded Flow, ft3/s Denver Henderson 75 70 50 25 10 87.0 96.2 ,,, 318 656 91.1 107 198 41' 777