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I B m ] ] ~ a ~ ~ q l ~ I : I I I I ~ J I 13 14 I I I I I I I I I I I I I I I I I I I The riverbed width varies widely in this reach. The average bottom width 1s 250 feet. Where the channel is straight and wide, there are num- erous tightly spaced (overlapping) alternate bars. Also. there are three middle bars. On wide bends, there are point bars. Where the channel ;s narrow there appears to be no point or alternate bars. There 1s no real distinction between the high bank and low bank. The average bank height is 9 feet with little variation except where there are levees. owners and are not integrated into any kind of general flood control sehe According to the flood hazard maps prepared by Gingery and Associates (19~) only one levee protects a small part of the floodplain from submergence by , the 100-year flood. Three-qua~ters of a mile wide on the average, the floodplain is used pr1marlly for agr1culture. There are several gravel pits, the Adams County Fairgrounds, a landing strip and few buildings. . The floodplain land ~emote from the river is lower than along the r1verbanks, which causes dralnage problems. Between Henderson and Brighton, the land at the riverbank 1s, on the average, 2.5 feet higher than in the back area. On the right or east side of the river between the East One-Hundred. Fourth Street bridge and Henderson some of the flOQdplain slopes away from th~ river, some has no lateral slope, and the remainder has been disturbed by m1ning operations. On the other side, the floodplain slopes towards the river in general. All tributaries flow downstream on the floodplain before emptying into the South Platte River. The test holes taken in the river at the bridge sites indicate that bedrock is approximately 20 feet below riverbed level. However, one must anticipate that the surface of the Denver Formation is very erratic. The ~.S. Geol~gical Survey's streamgaging station at Henderson is located at Rl~er Statlon 376 + 00 (Sheet 6, Appendix 1) approximately one-quarter m11e upstream from Henderson Road. In addition to Sand Creek, with a catchment area of 189 square miles, Clear Creek enters the South Platte River in this reach. The Clear Creek confluence is approximately one ml1e downstream from Sand Creek. The catchment area for Clear Creek is approximately 600 square miles. There are six bridges between Sand Creek and East One-Hundred-Fourth Avenue. Two are twin span Interstates and one is owned by the Union Pacific Railroad. The available records show that bedrock (the Denver Fonmation) is lSto20feetbelowriverbedlevel. Surprfsingly, there are less vehiCular bridges now than in the past. The bridge at East Seventy. Fourth Avenue was destroyed and not replaced. ." East One-Hundred-Fourth Ave. to Baseline Road. The downstream reach 'of the South Platte River is Rural inform. Itfallsg2feetinariver distance of 11.4 miles. The sinuosity is 1.26. There are two weirs which divert water from the river into the 6ranther Ditch. At the intake therc is a 10-foot drop in the riverbed level, whereas at the 6righton Ditch intake the drop is only 3 feet. There are only two bridges in the central portion of the reach and one at each end. The river here is actively eroding its banks and its bends are moving lateral1y. There are large gravel point bars where the bends are sharp. In these locations the river is very wide. Where the channel is straight, there is a pattern of lateral bars with varying spacing. The alignment of the river in the vicinity of River Station 240 + 00 on Sheet 7 of Appendix 1 may be affected by the gr<lvel mining which has left a very large pond on the right side of the river. The average bankfull width of the South Platte River in the Rur<ll reach is 350 feet. The maximum width is approximately 800 feet at <l loc<l- tion where there is a lateral bar and a middle bar. The minimum width is approximately 130 feet at one of the crossings. The mean bed width is 250 feet. In the floodp~ain hazard study for Adams County, the stationing of the South Platte R1ver was determined by measuring the distances along the low-flow channel. This is the common method developed by those who workw!th navigable rivers. Intheserivers,thelow-flowchannelisof great 1nterest because it governs the size of vessel which can run the river For the South Platte River, 1t is suggested that the stationing should be . made along the cente~ line of the bankfull river or midway between the ve~etated ba~ks. Th1S ~ype of stationing is more appropriate for rivers in Wh1Ch the maln concern 1S flooding. High flows follow the alignment of the bankfull channel more so than the low-flow channel. Flood levels calculated using the low-flow channel stationing are higher than levels calculated using the straighter alignment. Because the river is free to move in most placc~, the ban~ height varys widely. Generally, the high bank is on the side where the current is eroding the barlk. Thelowbankisoppositewheredepositsllreoccurring. The average bankfull depth at the high bank is g feet. The average low bank depth is 1 foot lower. The standard deviation for bankfull depth is 2 feet. Numerous levees follow along short reaches of the river bank and criss-cross the floodplain. These levees are efforts of individual land