Channel Dynamics, Sediment Transport, and the Slope of Alluvial Fans: Experimental Study1

We present the results of an experimental study of alluvial fan sedimentation under conditions of constant inflow water discharge Qw, sediment supply Qso, and median grain size D. The study was designed to complement and test a recently formulated model of alluvial fan sedimentation and to emphasize the interactions between, and controls on, flow channelization and equilibrium fan slope. Flow channelization and fan sedimentation were studied under conditions of nearly steady, uniform aggradation. Steady conditions were achieved by imposing a steadily rising base level, just in balance with the average sediment aggradation rate. Experimental inflows covered a wide range of conditions, allowing examination of the effects of variations in Qw, Qso, and D in both bedloadand suspension-dominated environments. Experimental results were most consistent with an expanding-flow channel model. Key experimental findings successfully and quantitatively predicted by the expanding-flow theory include: (1) straight to slightly concave radial profiles of bedload-dominated fans; (2) distinctly convexo-concave profiles of suspension-dominated fans; (3) a strong, inverse relationship between Qw and fan slope; (4) a strong, but secondary, relationship between Qso and fan slope; and (5) near-independence of D and fan slope so long as transport stage is high and bedload transport dominant. However, potential scale effects in the experiments arose from reduced flow Reynolds numbers and incorrect geometric scaling of channel widths; no confident conclusions regarding the debate over the relative importance of ‘‘sheetfloods’’ and braided channel flows can be drawn from the experimental data. Extrapolation to field scale is best accomplished through appropriate application of the theoretical model herein confirmed against experimental data.