Laserfiche WebLink
<br />are generated using the same procedures. The impacts of channel changes can be qualitatively, <br />if not quantitatively, predicted. <br /> <br />Problem Areas - Existing Conditions <br /> <br />Problem Area 3. Hydraulic modeling of Problem Area 3 indicated that the capacity <br />within the active channel banks is approximately 1,500 cfs. This is the flow above which water <br />begins to significantly inundate the overbank areas adjacent to the river channel. Several cross- <br />sections within the model indicate flows in excess of 2,500 cfs could cause damage to the <br />adjacent agricultural area. The 3,000 cfs operational discharge was modeled to indicate <br />potential problem areas under current conditions. The flow area extents for this discharge were <br />connected to generate pseudo-floodplains, as described above, and are shown on Sheet 1. <br /> <br />Examination of the profile indicated by the survey shows widely varying slopes <br />between adjacent cross-sections. The reach-length weighted average slope for the problem area <br />is 0.00 I 05 (ftIft), but the individual values range from a relatively steep 0.00505 to an adverse <br />(and also steep) -0.004l5. More disturbingly, the values change sign quite often, with one <br />stretch of sixteen consecutive cross-sections and another of eleven being the longest <br />consecutive runs of positive slope within the model, but more typically averaging three to four <br />sections. This indicates inaccurate mapping information, since these values represent the water <br />surface and the numerous adverse slopes could not exist. Their occurrence is, nonetheless, <br />troubling and could be indicative of disequilibrium. The region in the center of this problem <br />area exhibits considerably more agricultural encroachment than the upstream or downstream <br />ends, but no apparent correlation was observed with the slope changes and adjacent land <br />practices or structures. Also, the sinuosity of the active channel decreases significantly <br />upstream of a tributary at cross-section 3030. Just downstream of this point, the active channel <br />has straightened, leaving a small oxbow lake. It is not known ifthis occurred naturally or was <br />accomplished mechanically. A profile plot of this area is shown in Plate l. <br /> <br />Area 3 is suspected to be aggrading, although this can not be stated with certainty at this <br />point. This is consistent with the apparent downstream movement of an aggradational trend <br />observed in the degradation rangelines. <br /> <br />The "JR2" particle size distribution was used for this sub-reach for sediment transport <br />calculations. The sediment yield for this problem area under existing conditions was 4, 1 00 <br />tonslyr with a mean daily load of II tons/day. <br /> <br />Problem Area 4. Hydraulic modeling of Problem Area 4 indicated that the capacity <br />within the active channel banks is approximately 1,500 cfs. This is the flow above which flow <br />begins to significantly inundate the overbank areas adjacent to the river channel. Some cross- <br />sections within the model indicate flows between 1,500 and 3,000 cfs could potentially cause <br />damage to the adjacent agricultural area. The 3,000 cfs operational discharge was modeled to <br />indicate potential problem areas under current conditions. The flow area extents for this <br />discharge were connected to generate pseudo-floodplains, as described above, and are shown <br />on Sheet 2. <br /> <br />19 <br />