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