HR: 08:30h
AN: H41G-03    [Abstracts]
TI: Observations on Alluvial Fans with Relevance to Recent Sediment Transport
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 M
EM: kschmidt@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd MS 973, Menlo Park, CA 94025 United States
AU: Miller, D M
EM: dmiller@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd MS 973, Menlo Park, CA 94025 United States
AB: Steep (slope $>$ 0.01) alluvial fans are widely written about, but there are almost no quantitative field studies of the fluvial channels moving water and sediment across them. For this reason, it is difficult to apply existing mechanistic models of fan evolution (e.g., Parker et al., 1998). It follows that we have little ability to quantitatively predict the effects of regional changes in water flux, sediment supply, or vegetation from ongoing anthropogenic and climatic changes on fans, particularly in arid lands. We have begun a field program to quantify fluvial sediment transport across alluvial fans by measuring the hydraulic geometry and bed texture of channels. The goal of the measurements is to parameterize sediment transport in a way that predicts the observed pattern of slope reduction, often from ~0.07 at fan heads to ~0.02 at downfan margins. In the Mojave Desert of California, we find that alluvial fan channel bankfull depths are largely 0.4-1.0 m at fan heads, decreasing to 0.1-0.2 m at distal fan 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, and thus 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%. Plots of reach gradient versus sand content from these channels agree with experimental flume data from Ikeda and Iseya (1988) with similar hydraulic geometries, suggesting a general relation where similar loads can be transported at lower slopes because of the role that increased fines play in reducing the threshold for sediment transport. However, our attempts to model this role using reduced nondimensional critical shear stress values at higher sand concentrations (Wilcock & Crowe, 2003) with conventional excess shear stress sediment transport formulas result in underpredictions of the observed downfan slope decline. Although the relation between reach slope and sand cover suggests a strong role for sand cover in reducing fan slope, calculations indicate a strong component of bedload deposition is also necessary to match observed slope patterns. These uncertainties about the relative roles of sediment load and grainsize illustrate that much remains to be understood about alluvial fans before we can predict even first-order responses to climatic or anthropogenic change.
DE: 4558 Sediment transport
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting