HR: 1340h
AN: H53B-0459    [Abstracts]
TI: The Problem of Alluvial Fan Slopes
AU: * Stock, J D
EM: jstock@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd MS-973, Menlo Park, CA 94025 United States
AU: Schmidt, K
EM: kschmidt@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd MS-973, Menlo Park, CA 94025 United States
AB: Water and debris flows exiting confined valleys have a tendency to deposit sediment on steep fans. On alluvial fans, where water transport predominates, channel slopes tend to decrease downfan from ~0.08 to ~0.01 across wide ranges of climate and tectonism. Some have argued that this pattern reflects downfan grainsize fining so that higher slopes are required just to entrain coarser particles in the waters of the upper fan, while entrainment of finer grains downfan requires lower slopes (threshold hypothesis). An older hypothesis is that slope is adjusted to transport the supplied sediment load, which decreases downfan as deposition occurs (transport hypothesis). We have begun to test these hypotheses using detailed field measurements of hydraulic and sediment variables in sediment transport models. On some fans in the western U.S. we find that alluvial fan channel bankfull depths are largely 0.5-1.5 m at fan heads, decreasing to 0.1-0.2 m at distal margins. Contrary to many previous studies, we find that median gravel diameter does not change systematically along the upper 60- 80% of active fan channels. So downstream gravel fining cannot explain most of the observed channel slope reduction. However, as slope declines, surface sand cover increases systematically downfan from values of <20% above fan heads to distal fan values in excess of 70%. As a result, the threshold for sediment motion might decrease systematically downfan, leading to lower slopes. However, current models of this effect alone tend to underpredict downfan slope changes. This is likely due to off- channel gravel deposition. Calculations that match observed fan long-profiles require an exponential decline in gravel transport rate, so that on some fans approximately half of the load must be deposited off-channel every ~0.25-1.25 km downfan. This leads us to hypothesize that alluvial fan long- profiles are largely statements about the rate of deposition downfan. If so, there may be climatic and tectonic information in the long-profile, but a mechanistic theory for downfan deposition rate will be needed.
DE: 1815 Erosion
DE: 1824 Geomorphology: general (1625)
DE: 4558 Sediment transport (1862)
SC: Hydrology [H]
MN: Fall Meeting 2005