Armoring and Sorting Simulation in Alluvial Rivers

Abstract

Formulations for bed armoring and hydraulic sorting of bed sediments in alluvial streams are developed, and an application to Missouri River bed evolution is presented. A relation for the areal coverage factor for armoring on a plane bed is first formulated as a function of degradation depth and bedmaterial characteristics, and is then modified by introducing correction factors for the effect of bed forms and the stochastic nature of sediment motion. For the simulation of hydraulic sorting, bed sediments are assumed to be mixed homogeneously in a mixed‐layer depth approximately equal to average bedform height, and the composition of mixed‐layer materials is adjusted to satisfy the continuity equation for each sediment size fraction. These formulations are applied in a 20‐year simulation of bed degradation in the middle Missouri River reach downstream of Gavins Point Dam. It is found that the simulated bedsediment composition, including the two‐layer structure comprising the top armor coat and the underlying finer sediments, is in good agreement with the observed Missouri River bed characteristics.