Arkansas River Channel Capacity and Riparian Habitat Planning Study

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3.2.5 Problem Areas — Existing and Improved Conditions Problem Area 1. Hydraulic modeling of Problem Area 1 indicated that the capacity within the active channel banks is approximately between 750 and 1,000 cfs. This is the flow above which water begins to significantly inundate the overbank areas adjacent to the river ' channel. Several cross - sections within the model indicate flows in excess of 1,500 cfs could cause damage to the adjacent agricultural area. Examination of the profile indicated by the topographic maps shows little in way of varying slopes between adjacent cross- sections. The reach - length weighted average slope for the problem area is 0.0005 (ft/ft). There is a high possibility that the inaccurate mapping information causes this slope, which is represented by the water surface and would change with either better mapping or field surveys. In comparing the 1975 photos provided by the CWCB ' with the 1998 photos, it is obvious that the channel has somewhat similar configuration. The channel exhibits a small change in channel top width. There is an increase in dense overbank vegetational growth of about 80 %. Area 1 also has a very high water table. Some specific areas ' of noticeable change; Station 560+00 to Station 570+00 – Some channel encroachment due to Salt Cedar. Station 625+00 to Station 645 +00 – Noticeable channel encroachment. Problem Area 1 is aggrading, as indicated by the Point of Zero Flow (PZF) plot of the ' Arkansas River at U Junta obtained from-the U'SGS. This is shown, along with some other gage locations, in Figure 6. The effective average channel width is approximately 155 feet. The most efficient portion, in terms of sediment transport, averages about 125 feet wide. ' The channel improvement template for this problem area uses a 120 -foot bottom width for the low -flow channel, with a meander that results in a channel, slope of 0.00128. The 2,500 cfs channel has, a 475 -foot bottom width and follows the valley slope, estimated to be approximately 0.00161. Problem Area 2. Hydraulic modeling of Problem Area 2 indicated that the capacity within the active channel banks is between approximately 750 and 1,000 cfs. This is the flow above which flow begins to significantly inundate the overbank areas adjacent to the river ' channel, Some cross- sections within the model indicate flows between 1,000 and 1,500 cfs could potentially cause damage to the adjacent agricultural area. r 35