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I I ~ J I 1 II J I B 1 a ] ] ] J I ~ a 11 11 'There are seven brldgeslnthfs reach fncluding the bridge for go If carts in the Englewood Municipal Golf Course. At the Bowles Avenue bridge. bedrock 1s 37 feet below the riverbed. At Bel1eview, it is only 9 feet below and immediately downstream from the wefr at West Union Drive, the Denver .Formation outcrops on the bed of the South Platte River. Local sub. reaches which are very steep are from the Cherry Creek confluence to Twenty-third Street (20 feet per mile), and from Thfrty- eighth Street to the Burlington Northern Railroad Bridge (23 feet per mile). Thesereachesareofspecia11nterest. If the rlverbed 1s not bedrock, then of what material 1s the bed composed in these very steep reaChes? Overall the average slope between drops 1s only 8 feet per mile. There are 39 vehicular or railroad bridges in this reach (Table 2). This is an average of one every 2.8 miles. The foundations of the bridges are on or in the Denver Formation which is for the most part well below the riverbed. However, the rock outcrops periodically on the bed of the river. One subreach where the rock is prevalent is from Dartmouth Avenue to Evans Avenue. Here, the soft weathered upper layer has been grooved by the river's currents. The floodplain contains gravel pits, golf courses, trailer courts, a race track. junk yards, other assorted businesses. homes. and old land- fills. Bear Creek enters the South Platte River approximately 1/4-mile upstream from West Hampden Avenue.' A few hundred feet upstream from the mouth, the creek flows over a rapids where the Denver Formation outcrops. The Corps of Engineers (1983) are in the process of channelizing the South Platte River from the south side of Columbine Valley to West Oxford Avenue to accommodate larger flows without flood damage. The proposed project consists of enlarging the channel by lowering the bed and widening and straightening the existing channel. Sharp bends will be elimi- nated. The materials dredged from the channel will be used to raise the banklines. Both banks will be blanketed with riprap. Three bed grade control structures wlll be built to replace those weirs existing now. Other works are needed to protect old and relocated water, sewer and other utility lines crossing the river. Sh inches of cobblestones will be added to the channel bottom in selected areas to prevent excessive erosion. The local sponsor for the project is the Colorado Water Conservation Board. The U.S. Geological Survey's streamgaging station is located at River Station 1859 + 30 opposite the Centennial Racetrack. West Hampden Ave. to Sand Creek. This is the Urban reach of the South Platte River. Over its length of 13.8 miles. the river falls 159 feet. One or both banks are stabilized with a variety of materials including rubble, riprap, concrete, concrete piles, and timber piles. Although there are local areas where the current is endeavoring to erode its banks, the urban reach of the South Platte River is essentially locked in place. When erosion occurs it needs to be repaired quickly before damage to expensive property happens. The river has been straightened so that its sinuosity is only 1.09. A sharp bend still remains immediately upstream from Sand Creek. In the Urban reach, the South Platte River bed averages 120 feet in width varying from a minimum of 60 feet to a maximum of 180 feet. In general, both banks are built up to the same level. 8ankfull depth ranges from 10 to 22 feet; the average is approximately 15 feet. The Urban reach has been improved for the people's enjoyment (Shoemaker with Stevens, 1981). A bicycle and foot path follows the channel from opposite the Sewage Disposal Plant near Dartmouth Avenue in Englewood to East Seventy-Eighth Avenue in Adams County near Thornton. Provision has been made for rafters and kyakers. The river is free of trash. Ameni- ties are appearing. Sand Creek to East One Hundred-Fourth Avenue. The change of the South Platte River from Urban to Suburban form is subtle and somewhat arbitrary. However, the level of development on the river and its flood- plain does change in the area of Sand Creek. From Sand Creek to East One-Hundred-Fourth Avenue, the river Channel falls 64 feet in6.8mi1es. There is a drop of6 feet across the diver- sion weir for Fulton Oitch, and another 4-foot drop at the rock structure immediately downstream from the "East Eighty-eighth Avenue bridge. The sinuosity of the channel 1s1.18. In this reach there are low levees on one or both banks of the river over most of the dist~nce. The floodplain is approximately 0.75 miles wide. Its principal uses are for agriculture and gravel mining. Former gravel pits are now employed to store water. The density of buildings is low. The levees do not reduce appreciably the inundation of the floodplain caused by the 100-year flood. Only the ponds are well protected. The sharp bends 1n the river have been stabilized with rubble. How- ever, th~ river has been moved lalerdlly 5o~what dlong ib relatively straight stretches. The movement is shown on the Commerce City and East- lake 7.5 minute topographic maps. Pdrt of this movement is the result of gravel mining operations. There are numerous drops in the riverbed level along the Urban reach. Many are small one or two foot drops; others are substantial. Public Service Company of Colorado has a collapsible fabric dam at their Zuni Plant at Thirteenth Avenue in downtown Denver. The vertical drop across thedamisBfeet. The others are l1sted 1n Table 3. Of the total drop of 1S9 feet, approximately one-third (57 feet) occurs at the dr?p.structures. In this reach, the height of the drops were taken from the prellm1nary profile of the South Platte River prepared by the Urban Drainage and Flood Control District in 1976. ~ I I I I I I I I I I I I I I I I I I I