8170 - Laserfiche WebLink

Home Browse Search

e erience throu ow at hi dischaz es rovide laz a stable backwaters at low dischaz xP ~ g p g ges. For the study baz, the range of dischazges at which water but not through-flow occupied portions of the secondary channel differed from year to year. In 1993, this range was from less than 45m3/s to a discharge greater than 136 but less than 170 m31s (4800-6000 cfs). In 1994, the range of these flows was more narrow and was between 76 m3/s and 136 m3/s (2680-4800 cfs). It is important to note that the minims flow necessary to inundate the secondary channel in 1994 was greater than normal base flows. Thus, the channel was not a backwater habitat at the mandated base flow. Throughflow was initiated in the secondary channel at discharges less than 170 m3/s in 1993 and less than 136 m3/s in 1994. These dischazges, and hence throughflow occurred only during the floods. The study baz experienced more substantial changes in bar topography over the 1993 flood, than the 1994 flood. In 1994 the large scale topography of the bar changed little from that of 1993, but the small scale topographic complexity substantially increased, due to superimposition oflow-amplitude bars (migrating sandwaves) on the edges of the existing deposits. Small, shallow backwaters formed behind these superimposed bars. This pattern was repeated throughout the l O lan reach, with the 1993 flood simplifying channel morphology, and the 1994 flood increasing small scale channel complexity. The Habitat Availability Curves (HACs) for the study bar were different each year of this study. The discharge-dependent habitat relationships had more than one peak each year, and each peak was the product of a specific bar feature. Nursery habitats occurred over a lazge range of discharges, but in the lee of different baz features at different discharges. In 1993, all of the available habitat at dischazges between 50 and 136 m3/s (1765-4800 cfs) was associated with the secondary channel. A second peak occurred at dischazges between 175 and 250 m31s (6180 - 8830 cfs) when small amounts of habitat were available downstream of an emergent portion of the baz. In 1994, discharges of 50-90 m3/s (1765-3180 cfs) produced small habitats associated with superimposed bars along the bar edge. Slightly higher dischazges (100-136 m3/s [3530-4800 cfs]) provided habitat associated with the secondary channel, and discharges of 150-300 m3/s (5300-10600 cfs) resulted in small amounts of habitat downstream of the emergent portion of the bar. The dischazge that maximized habitat availability for this bank-attached compound bar was greater in 1993 than was the dischazge that maximized habitat availability in 1994. There was little available within-channel habitat at dischazges greater than 290 m3/s (10240 cfs) in 1993 and 240 m3/s (8475 cfs) in 1994. At over-bankfull flows (>447 m3/s [15785 cfs)), lazge amounts of flooded bottomland become available. The cross sections were used as input to model patterns of scour and deposition at different levels and patterns of discharge, which were compared to field measurements. The general patterns of scour and deposition simulated by the model were similar to those found in the 1.5 l~n study reach with some specific exceptions. Both the field measurements and the , model produced bar building on the bank-attached bar in response to flood events, with greater amounts of aggradation for higher floods of longer duration. In the model runs, the secondary channel was insensitive to flood hydrograph characteristics, aresult contrary to that of the field- measured cross sections, which showed substantial aggradation in 1993, and scouring in 1994. The point bar experienced more scour than was predicted by the model. Thalweg scour was adequately predicted. Although cross-section data resolution of this study was insufficient to address the response of individual nursery habitats to flood passage, increased topographic resolution for the channel and a new version of this model (J. Nelson, USGS, pers, comm., 1996) ix