HR: 0800h
AN: H51E-0806    [Abstracts]
TI: Convergent Hydraulics and Knickpoint Migration in an Incising Gravel-Cobble River
AU: * Wyrick, J R
EM: wyrick@rowan.edu
AF: Rowan University, CEE Department 201 Mullica Hill Rd, Glassboro, NJ 08028, United States
AU: Pasternack, G B
EM: gpast@ucdavis.edu
AF: University of California, Davis, LAWR Department One Shields Avenue, Davis, CA 95616, United States
AB: Regulated gravel-cobble rivers are known to incise, but the mechanism of incision is not well documented by process-based research. Widespread use of simple "stream power law" equations assumes that incision is caused by continuous downcutting that increases with discharge. To provide an alternate explanation that recognizes the inherent non-uniformity of natural channels, we hypothesize that regulated rivers experience waves of migrating knickpoints that retreat through riffles during low flow when riffle crests function as supercritical weirs and are rejuvenated by floods that downcut the intervening width-constricted pools. To test this new hypothesis, monitoring was performed on the rapidly incising 7-km Timbuctoo Bend of the lower Yuba River, CA, where 463,000 cubic meters of sediment have been scoured out in just the last seven years alone. Direct field measurements of velocity and depth fields were obtained at three migrating, horseshoe-shaped knickpoints in this reach. Also, detailed channel DEMs were obtained at different stages of knickpoint migration to track geomorphic change over seven years. At one of the knickpoints, a special torque sensor was deployed to map near-bed lift and drag stress components. Velocity fields reveal convergent hydraulics controlled by the horseshoe morphology that focus scour in the upstream center of the U-shape. At a relatively low discharge, supercritical flow and near-critical standing waves were observed. Furthermore, the peak near-bed drag stress that was directly measured exceeded 1000 Pa during this relatively low discharge regime, which explains why the bedforms are retreating so rapidly. These direct measurements were compared to similar measurements previously reported for horseshoe waterfalls analyzed in a flume, and will aid in determining the real mechanism for knickpoint migration and channel incision in regulated gravel-cobble rivers.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: 2007 Fall Meeting