HR: 1340h
AN: H53B-0476    [Abstracts]
TI: Hydraulic Response of Colorado River Rapids to a Reworking Flood
AU: * Magirl, C S
EM: magirl@usgs.gov
AF: U.S. Geological Survey, 1675 W Anklam Rd, Tucson, AZ 85745 United States
AU: Webb, R H
EM: rhwebb@usgs.gov
AF: U.S. Geological Survey, 1675 W Anklam Rd, Tucson, AZ 85745 United States
AB: Rapids on the Colorado River in Grand Canyon are formed primarily by the accumulation of coarse sediment from tributaries. Frequent debris-flow events in these tributaries contribute alluvium that constricts the river, accelerates local fluid velocities, increases the water-surface fall through the rapids, and raises the water-surface elevation in the upper pool. In turn, large main-stem floods rework fresh debris-fan deposits by removing the smaller coarse sediment and repositioning the largest particles into a stable matrix on the bed of the rapid. While morphologic changes to debris-fan surfaces in response to main-stem floods are widely studied in Grand Canyon, relatively little is known of the specific changes to the hydraulics of a given rapid. The turbulent and dangerous nature of rapids makes in-situ measurements challenging. The current study, however, successfully measured hydraulics within the core of several rapids. These rapids had been steepened by debris-flow event within the previous seven years. Using a boat-mounted fathometer and acoustic doppler velocimeter (ADV), point measurements of water-surface elevation, bathymetry, three-dimensional velocity, and turbulence were made at a collection of moderately-sized rapids in Grand Canyon. The boat was a 19-foot J-snout with a 50-hp Mercury outboard motor capable of maneuvering to nearly any location within each rapid. A three-person crew onboard operated the instrumentation. Two crewmembers on shore operated a survey instrument to continually record the boat position on the river. To characterize hydraulic changes in response to a main-stem flood, repeat measurements were made at each site both before and after the 1,200 m3/s controlled flood of November 2004. While this exercise is only a first step in quantifying the fluid dynamics within a rapid, the hydraulic data collected offers a unique insight into the response of coarse-grained alluvial deposits to floods in fast-moving bedrock-controlled rivers. The study also begins to characterize the interaction between coarse-grained sediment transport, hydraulics, and contributing channel morphology within the Colorado River.
DE: 1821 Floods
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: Fall Meeting 2005