Braided-Stream Sediment Transport Rates from an Alluvial Fan Diffusivity Model

Abstract

The paper estimates bedload sediment transport rates associated with braided-stream channel/bar deposition and erosion by modifying an alluvial fan surface morphology model. Alluvial fan surface morphology is related to sediment deposition rates using the quasi-steady analytical model developed by De Chant and others (1999). Using the physical basis and mathematical form of the fan surface profile model, a closed-form inverse problem is posed to obtain a single bar-pool sediment deposition rate. This quasi-steady rate is based solely on local channel geometry (slope) and sediment diffusivity. Typical fan sediment diffusivity values are estimated from hillslope studies, flume experiments, and simple open-channel flow relationships. The order of estimated fan diffusivities is approximately 1,000 m 2 /yr for laboratory scale (1- to 10-m) experiments, which is a reasonable value for many active bar/channel deposits. Analytical models indicate that sediment diffusivity is a strong function of time and space; thus, it is not possible to scale directly between laboratory and field measurements. The sediment flux model provides good overall comparison with the available data sets, suggesting that local sediment diffusivity models are linked to hydraulic process–based relationships.