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I o I I I I I I I I I I I I I I I I I 29 30 V. flUVIAL MORPHOLOGY acrossltsbed. The overall slopes for the five subreaches are listed in Table 9 along with the average slopes between major drop struc. tures. The complete profile is shown 1n Plan and Profile in the Appendix. - Except for the section lr:r.,cdiately dcwnstrtam from Chatfield Dam, the overall slope decreases in the downstream direction. The average slope between diversion dams and drops for four reaches is approximately 7 feet per mile. The upstream Suburban reach 1s much steeper being 12 feet per mile. General A general description of the reach of the South Platte River from Chatfield ~m to Baseline Road irrrr.ediately north of Brfgnton is provided in Chapter II. There. the river was divided 1nto three types depending on the general characteristics of the riverine environment. The division was made mainly on the influence of man--his works, his trash, and his modi- fication of the channel, its bars and vegetation. The upstream and down- stream reaches were called Rural because they still have many features of the pristine river. Possibly, SubruraT would have been a better choice because nowhere in the Metropolitan Denver area is the river in the truly rural setting. The reach through the heavily populated area including downtown Denver was designated the Urban section. This portion is flanked on both sides by Suburban subreaches. In its present-day fonn, the South Platte River is a slightly sinuous single thalweg chaMel. In this chapter, the fluvial morphology of the river is described in more esoteric terms. These include length, sinuosity, slope, width, depth and geometry of bends. Inaddftion,thereisinformationonthe lateral movement of the river in one of its Rural reaches, and on the occurrences of bars and vegetation. Width. As shown in Figure 10, the bankfull width changes from an average of 372 feet between Sand Creek and One.Hundred-Fourth Avenue to only 192 feet in the Urban reach including downtown Denver. Moreover, the top width varies greatly in all but the Urban reach. The top width is greatest at sharp, short radius bends where much of the point bars are vegetated. The bottom width is that portion of the riverbed width which is sand or gravel and is not vegetated. The bottom width is substantially less than the bankfull width, being only 120 feet on the average in the Urban reach and 250 feet downstream from Sand Creek. Most of the water and sediment is transported on or over the bottom width. Sedimentation often occurs in the other portions of the river cross-section. ~ep~h. From the 1983 maps, the vertical distances from the high <Indlo.. <IMS to the water surface were detennined every 50C feet along the river. The average of these depths for the five reaches are listed in Table 10. The depth to the low-flow water surface is greatest in the Urban reach (13.4feet) and least in the Rural reaches (6.8 and 6.5 feet). In determining high and low bank elevations, levees and othe~ arti- ficial fills or excavations were ignored. For example, where there 1S a levee along the bank, the elevation of the bank was assumed to be the same as the floodplain landward from the levee. In crossings where the main current moves from one side of the channel to the other, the level of the thalweg is only a foot or so below the low-water surface. In bends, the depth of flo~ 1s greater,.maybe as much as 3 to 6 feet. Comparison of the water prof1le on 16 Aprll 1~83 with thalweg elevations obtained from cross-sectional surveys done 1n pre- vious years yielos no definitive answer as to how far the riverbed is below the low-water surface. An estimate of 2 feet on the average would seem appropriate. !n places where there are diversion dam~ and drop structures, the bankfull depth is much greater downstream from the ~tructures than upstream. For example, at the 8rantner Ditch Diversion Dam (R1ver Station 508 + 00), the bankfull depth is approximately 2.5 feet upstream and 9.5 feet down- stream for the concrete weir. Throu9hout the entirestudyreach,thehighbank 1s on the average 1.5 feet above the low bank. The variation from reach to reach is no more Hydraulic Geometry An alluvial river can be described by the dimensions and shapes of its boundaries. "nown as its hydraulic geometry, these are delineated below. unless otherwise stated, all dimensions were obtained from Delta Aerial Surveys' 2-foot contour maps prepared from aerial photographs taken on 16 April 1983. Thereafter, the spring flood in 1983 has modified the river ali9nment and rearranged bars in some local reaches. Len~th. From Chatfield Dam to Basel1ne Road inrnediately north of Brighton, t e length of the South Platte River is 41.2 miles along the thalwe9(deepestpartoftheriver). Thelengthsofthefivesubreaches are given in Table g. The river stationing was taken from older maps and profiles made prior to 1983 and listed 1n Table 1. The thalweg shown on . the 1983 map is different mainly in the Rural subreaches. The Urban sect10n is 13.8 miles long; the Suburban 12.4 miles and the Rural 15.0 miles. Conse- quently all three types are almost equally represented. Sinuositv. The river is somewhat sinuous, more so in the subreaches less affected by development. Overall, the valley length is 34.5 miles making the sinuousity 1.19 (ratio of thalweg length to valley length). The sinuosity of the Urban reach is only 1.09 Slope. In the study reach, the South Platte Rwer falls ~32 f-;et in 41.2 miles or 10,5 feet per mile. Basically the riverbed slope 1S un1form between tributaries except for the diversion'dams and drop structures built