HR: 16:30h
AN: H44D-03    [Abstracts]
TI: Self-Formed Meanders (With Cutoffs) in a Laboratory Flume
AU: * Braudrick, C A
EM: xian@berkeley.edu
AF: UC Berkeley Dept. of Earth and Planetary Science, 307 McCone Hall, Berkeley, CA 94611 United States
AU: Leverich, G T
EM: glenjam@yahoo.com
AF: San Francisco State University Department of Geosciences, 1600 Holloway Ave, San Francisco, CA 94132 United States
AU: Leverich, G T
EM: glenjam@yahoo.com
AF: Stillwater Sciences, 2855 Telegraph Ave, Berkeley, CA 94705 United States
AU: Sklar, L S
EM: leonard@sfsu.edu
AF: San Francisco State University Department of Geosciences, 1600 Holloway Ave, San Francisco, CA 94132 United States
AU: Dietrich, W E
EM: bill@eps.berkeley.edu
AF: UC Berkeley Dept. of Earth and Planetary Science, 307 McCone Hall, Berkeley, CA 94611 United States
AB: The development of a mechanistic understanding of channel geometry and morphodynamics has been inhibited by the inability to create self-formed, freely meandering, single thread channels in a laboratory flume. By being able to reliably generate such channels, studies of the influence of sediment supply and flow dynamics as well as bank strength on channel morphology can be experimentally explored. We have found that the key experimental controls are: 1) ratio of bank strength to boundary shear stress exerted on the bank; 2) bedload and suspended load rates; and 3) variable flow discharge. We have been able to create meandering channels in a sand bedded laboratory flume using alfalfa sprouts. The alfalfa sprouts decrease the bank erosion rate so that bank erosion would occur at approximately the same pace as bar growth. The addition of coarse suspended load was necessary to cause deposition on bars to grow to the floodplain height. The sprouts contributed to deposition by creating a rough floodplain surface. Steady discharge failed to produced meandering, apparently due to the lack of suspended load deposition on the bar surface. The channels were created in a 3.6-m wide and 6.1-m long flume with an adjustable slope set at 0.01. We introduced both bedload (sand) and suspended load (crushed silica) into the top of the flume, which has an initial channel with either one or two bends carved into the floodplain. Runs lasted between 1 and 4 hours and occurred once per week. Alfalfa seeds were spread evenly outside the low flow channel following each run and are allowed to grow between runs. With the same material and flow conditions, the channel rapidly braided without the alfalfa sprouts. Braided was also favored under steady flow conditions. Under dynamic flows with banks strengthened by sprouts, the resulting experimental channels had many of the features observed in meandering streams such as oxbow lakes and meander cutoffs. The cutoffs occurred during overbank flows when high flow channels were reoccupied. As the portion of the flow passing through the reoccupied channel increased, an upstream-propagating headcut was initiated. Once the headcut propagated past the upstream junction with the main channel, sediment deposition blocked the upstream end of a secondary channel. The cutoffs became oxbow lakes when rapid bar growth promoted lateral channel migration away from the downstream junction with the cutoff channel. With these results in hand we are completing the construction of a larger flume in which we will set forth experiments on the influence of sediment supply, discharge magnitude and duration, grain size and bank strength on channel geometry.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: Fall Meeting 2005