Generalized sorting profile of alluvial fans

Abstract:

Alluvial rivers often exhibit self-similar gravel size distributions and abrupt gravel-sand transitions. Laboratory experiments suggest these sorting patterns are established rapidly, but how - and how fast - this convergence occurs in the field is unknown. Here we examine the establishment of downstream sorting patterns and channel geometry in a km-scale alluvial fan. The sharp transition from a canyon to an unconfined, channelized fan provides a well-defined boundary condition and also a strong discontinuity. The channel changes from deep and entrenched at the fan apex to shallow and depositional over a short distance, exhibiting non-equilibrium behavior. The resulting gravel fining profile is not self-similar; the particle size distribution narrows until approximate equal mobility is achieved. Further downfan, the gravel-sand transition appears to exhibit a self-similar form; field data collapse with laboratory experiments when downstream distance is normalized by the location of the transition. Results suggest a generalized sorting profile for alluvial fans as a consequence of the threshold of motion and non-equilibrium channels.