HR: 11:50h
AN: H42B-07    [Abstracts]
TI: Geomorphic Adjustment of a Gravel-bed Stream Meander During a Large Flood
AU: * Clayton, J A
EM: claytonj@colorado.edu
AF: University of Colorado, Geography Dept, 260-UCB, Boulder, CO 80309 United States
AU: Pitlick, J
EM: pitlick@colorado.edu
AF: University of Colorado, Geography Dept, 260-UCB, Boulder, CO 80309 United States
AB: Detailed field measurements of channel topography and bed surface grain size were made in a gravel-bed meander near the headwaters of the Colorado River during the summer of 2002. The spring snowmelt discharge for that year peaked at 5.4 cms (34% of normal peak flow) and probably mobilized only the finest fraction of the bed materials. The following year (2003) snowmelt discharges peaked at 27 cms (170% of normal peak flow) and all available grain sizes within the study reach were mobilized. Additionally, during the 2003 peak flow the river avulsed immediately upstream of the study reach, thereby abandoning approximately one half kilometer of the former channel. The abandonment was rapid (probably within a few hours) leaving the bed texture essentially frozen in place at peak flow conditions. These events provided us with a unique opportunity to test the hypothesis that the bed surface texture of gravel-bed rivers becomes finer as the shear stress and sediment transport rate increase. All 70 previous sampling locations were resampled following the stream abandonment. In response to the high flow the surface median grain size (D50) coarsened slightly in the outer part of the bend while remaining nearly constant along the inner part of the bend, resulting in an overall increase from 18 to 21 mm for the study reach. Thus, the bed surface texture persisted despite shear stresses throughout the bend that were well above the critical entrainment value. These data are inconsistent with the hypothesis that bed surface textures become much finer during large floods. Also, the distribution of local depth (h') increases closely matched that of the surface D50 such that the reach-averaged ratio of h' to D50 remained approximately constant despite the differences in runoff from 2002 to 2003. Therefore, our results suggest that in meander bends the stream is able to adjust the surface grain sizes and pool depths to provide roughly constant local shear stress values over time despite being subjected to a range of sediment and water discharges.
DE: 1821 Floods
DE: 1824 Geomorphology (1625)
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting