HR: 0800h
AN: H41F-0841    [Abstracts]
TI: Sediment size of surface floodplain sediments along a large lowland river
AU: * Swanson, K M
EM: kswanson@berkeley.edu
AF: Department of Civil Engineering, U.C. Berkeley, Berkeley, CA 94720, United States
AU: Day, G
EM: Geoff.Day@riotinto.com
AF: Department of Earth and Planetary Science, U.C. Berkeley, Berkeley, CA 94720, United States
AU: Dietrich, W E
EM: bill@eps.berkeley.edu
AF: Department of Earth and Planetary Science, U.C. Berkeley, Berkeley, CA 94720, United States
AB: Data on size distribution of surface sediment across a floodplain should place important constraints of modeling of floodplain deposition. Diffusive or advective models would predict that, generally, grain size should decrease away from channel banks. Variations in grain size downstream along floodplains may depend on downstream fining of river bed material, exchange rate with river banks and net deposition onto the floodplain. Here we report detailed grain size analyses taken from 17 floodplain transects along 450 km (along channel distance) reach of the middle Fly River, Papua New Guinea. Field studies have documented a systematic change in floodplain characteristics downstream from forested, more topographically elevated and topography bounded by an actively shifting mainstem channel to a downstream swamp grass, low elevation topography along which the river meanders are currently stagnant. Frequency and duration of flooding increase downstream. Flooding occurs both by overbank flows and by injections of floodwaters up tributary and tie channels connected to the mainstem. Previous studies show that about 40% of the total discharge of water passes across the floodplain, and, correspondingly, about 40% of the total load is deposited on the plain - decreasing exponentially from channel bank. We find that floodplain sediment is most sandy at the channel bank. Grain size rapidly declines away from the bank, but surprisingly two trends were also observed. A relatively short distance from the bank the surface material is finest, but with further distance from the bank (out to greater than 1 km from the 250 m wide channel) clay content decreases and silt content increases. The changes are small but repeated at most of the transects. The second trend is that bank material fines downstream, corresponding to a downstream finding bed material, but once away from the bank, there is a weak tendency for a given distance away from the bank the floodplain surface deposits to slightly coarsen downstream. We also find that sand is present (about 4%) in these surface sediments out to 1 km from the channel bank. These trends are not consistent with simple lateral transport models, and other factors, including effects of flocculation, local flow patterns, and possibly dry season wind effects may matter.
DE: 1820 Floodplain dynamics
DE: 1825 Geomorphology: fluvial (1625)
DE: 1861 Sedimentation (4863)
DE: 1862 Sediment transport (4558)
DE: 5419 Hydrology and fluvial processes
SC: Hydrology [H]
MN: 2007 Fall Meeting