8110 - Laserfiche WebLink

Home Browse Search

channel morphology. Deposition of sediment on vegetated bars at the following Ouray cross e sections: OCW-2B (1 ft), OCW-4 (0.5 ft), OCW-6 (1 ft), and LEB-27 (1 ft) may be permanent. Some continued channel narrowing at these cross sections should be expected. Within the active channel, the following cross sections were subject to large sand bar deposition: OCW-2A, SHB-lO, WYB-ll, WYB-l1.5, LEB-2D, LEB-25, LEB-27, JOB-3D, JOB-31. Vegetation could become established on these newly-formed or enlarged sand bars. However, during the low flow winter season some erosion of the bars should occur and by spring, much of this deposit may be reworked back into the channel. Cross sections LEB-27 and JOB-3D were very dynamic. These were wide cross sections which experienced 6 to 10 ft of scour on one side and 6 to 8 ft of deposition on the other side. The thalweg shifted from one side of the river to the other. The scour and fill experienced in these two cross sections is required to maintain the active channel. These cross sections should be monitored in the future to evaluate the interaction between channel scour and deposition processes and the potential encroachment in the channel by vegetation. Cross sections WYB-12, -13 and -14 had been previously surveyed in 1992, 1993 and 1994. These were plotted together with the 1995 surveys and presented in the Ouray data report volume. Since 1992 there have been bed variations of 3 ft to 5 ft across all three cross sections. At the present time the elevation of the bed in each cross section appears to be within the range of variation determined through the previous three years of surveys. No significant overall changes are apparent in these three cross sections. It should be noted that these cross sections e are relatively stable, uniform and narrow. They are located in a particularly stable reach of river near the bluffs. Channel Response in Canyonlands Reach In the Canyonlands study reach, a total of 10 cross sections experienced some fill, 5 cross sections displayed net scour and 7 cross sections were relatively unchanged from spring to fall. The cross section plots are presented in Appendix A. Most of the lower cross sections were subject to some sediment deposition. The upstream cross sections displayed either some scour or no net change. The relationship between scour and fill in the Canyonlands reach is controlled by the location of significant sand storage in the channel (slugs). It is observed that most of the cross sections scoured during the peak and then filled. In Canyonlands, sand bar deposition was observed at the following cross sections: V AB- 5 (2 ft), VAB-9 (0.5 ft), and QAB-16 (1 ft). In addition, the vegetated bars between cross sections MCB-l and MCB-4 and cross sections ANB-12 and ANB-13 also exhibited deposition during runoff. Because the bars are densely vegetated, sediment deposition on these bars may be permanent unless high flows sufficient to scour the vegetation occur. If deposition from 0.5 to 2 feet can occur on bars during a year where the peak discharge in this reach was only slightly greater than the average peak discharge, there is potential for further channel narrowing at these e 27