8028 - Laserfiche WebLink

Home Browse Search

1 e Between January 25 and 28, 1997, two teams worked alternating 8-hour shifts, to measure the relative water and ice stage at seven cross-section locations (Figure 15) at approximately 1 hour intervals from just prior to the first peak until the changes in the measured stage indicated the first trough of the fluctuating flows had passed through each cross-section location. At the Chew, Jensen, Bonanza, and Ouray Bridges, the distances from the ice or water surface to fixed reference points on the bridge rails were measured. At the remaining three sites, ice elevation was measured using surveying levels and stadia rods. The latter method proved more accurate. Figure 17 plots the observed changes in the river stage with time. An aerial inspection on the afternoon of January 28 revealed that the small iee jam below Chew Bridge had broken up and that the top of the ice cover had moved from RM 310.8 to 309.8; otherwise, ice conditions appeazed similar to the January 23-25 observations (Figure 14). Air temperatures had moderated during the January 25-28, 1997 period, and the snow and ice around the edges of open leads was becoming wet. Shore cracks were also becoming wet in places. From January 29-30 the field teams made ice observations and measurements after several waves resulting from peaking operations had passed through the study reach. The ice thickness measurements were repeated at the same cross-section locations as under steady flows and the results of these measurements are summarized in Table 4 and Figure 16; individual ice thickness measurements are presented in Appendix A. The most significant change was the movement of the location of the leading edge of the ice cover 4.8 miles downstream, from RM 310.8 to RM 306. This movement occurred in two stages: the first movement was from RM 310.8 to 309.8 by 1500 hours on January 28, 1997, and the second was movement of the leading edge from RM 309.8 to 306 early on January 29 (Figure 18). The ice motion detector at RM 308.2 indicated that the ice cover at that location broke up at 0600 hours on the morning of January 29. A team arrived at this location five hours later to find an open channel with small ice pieces and floes floating past. The post-breakup water level was approximately 4 ft lower (based upon examination of the location of the shear walls of ice remaining along the shoreline [White and Zufelt, 1994]) than the pre-breakup ice surface elevation, attributable to the increased conveyance of the river after removal of complete ice cover. One mile downstream, between RM 307.2 and RM 306.0, the river made a gradual transition from completely open to entirely ice covered. There was little ice debris at the leading edge location at RM 306.0, so ice pieces had either melted in transit or had been carried beneath the upstream edge of the ice cover. Within this transition reach, at 1140 hours on January 29, 1997 a 100-ft-long ice jam spanned the open portion of the channel at RM 307.0. Floes colliding with the upstream edge of the jam, passed beneath the accumulation, and emerged at the downstream end. Downstream of the new leading edge location (RM 306.0) the condition of the ice cover appeared unchanged from conditions that existed during the steady flow observation period of January 23-25. During a subsequent aerial inspection of the study reach on February 20, 1997, it was observed that most of the ice cover present in late January had released or melted. The ice motion detector located above the Bonanza Bridge (RM 290.4) recorded the timing of the ice release to be 0335 hours on February 5, 1997. The detector located within the Ouray Wildlife Refuge failed to properly record the time of ice breakup. 13