HR: 1340h
AN: H53D-0507    [Abstracts]
TI: The influence of variable discharge on channel morphology and rates of change in an experimental meandering floodplain river
AU: * Leverich, G T
EM: glenjam@yahoo.com
AF: San Francisco State University, Department of Geosciences, San Francisco, CA 94132 United States
AU: * Leverich, G T
EM: glenjam@yahoo.com
AF: Stillwater Sciences, 2855 Telegraph Avenue, Berkeley, CA 94705 United States
AU: Braudrick, C A
H53D-0507 AF: University of California, Berkeley, Department of Earth and Plantary Science, Berkeley, CA 94720 United States
AU: Sklar, L S
H53D-0507 AF: San Francisco State University, Department of Geosciences, San Francisco, CA 94132 United States
AU: Dietrich, W E
H53D-0507 AF: University of California, Berkeley, Department of Earth and Plantary Science, Berkeley, CA 94720 United States
AB: Current efforts to restore floodplain river channels are limited by our poor understanding of the controls on channel width and other morphologic attributes of meandering floodplain rivers. Restoration projects are commonly implemented downstream of dams where the supply of both water and sediment has been drastically altered. An emerging restoration strategy involves down-sizing the channel to restore a suite of active geomorphic processes including lateral migration of the channel and floodplain construction by bar growth and overbank deposition. Common channel design methods emphasize selection of the appropriate bank-full discharge and rarely consider the role of higher magnitude flows in shaping the channel and influencing rates of morphologic change. Here we report preliminary results of physical modeling experiments in which we have created laterally migrating stable-width channels in a laboratory floodplain basin. In these ongoing experiments we are exploring the influence of variable discharges on channel geometry and rates of planform evolution. In particular, we focus here on the use of hydrographs with peaks that result in over-bank flooding and sediment deposition. We use alfalfa and a mix of sand and silt to develop bank cohesion and assist bar growth while simultaneously varying discharge following predetermined hydrographs. The experiments are carried out in a 6.1 m long by 3.6 m wide flume with an adjustable slope set to one percent. A channel with constant width and sinuosity is initially carved into the floodplain prior to an experimental set of runs. We feed sand and silica silt, which move as bedload and suspended load, respectively, at the channel entrance. We use a point gage to measure flow depth and water surface slope, document planform evolution using overhead digital photos taken every minute, and create topographic maps with a line-laser photogrammetry technique. Preliminary results are consistent with our hypothesis that a variation in flow is required to maintain a stable width channel by growing bars up to floodplain height, stabilizing banks by depositing fines upon the floodplain and growing levees. Steady flow leads to channel widening and eventual braiding due to bar disconnection with the floodplain and thus creates a positive feedback cycle of runaway deposition and downstream sediment starvation. This phenomenon can be suppressed by a variation in flow magnitude, provided that bars have connected fully to the floodplain and well-developed levees are able to form, which help to stabilize channel banks. We will continue these experiments in the existing model floodplain and in a newly-constructed larger basin where the effects of entrance and exit conditions can be minimized.
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: Fall Meeting 2005