ROUTING WASHLOAD THROUGH CHANNEL NETWORKS

As part of a larger study funded by EPA, we have developed a numerical model for routing washload through channel networks. The model treats the network (and its associated alluvial valleys) as a series of connected reaches, with each reach being hundreds of channel widths in length. Processes represented within each reach include advective transport from upstream, exchange with the bed, and advective transport out of the reach. Inputs from upstream are assumed to be known, either as boundary conditions or from a previous computation at the adjacent upstream reach. Exchanges with the bed can either add or remove washload from the reach, depending on the imposed bed stress. If the imposed bed stresses are low, washload is deposited either in the hyporheic zone or as a layer on top of the bed if pores in the bed are already filled with fines. If the imposed bed stresses are high enough, washload may be removed from the bed surface or from the hyporheic zone. A simple mass balance equation connects all these processes, and is used to compute the concentration of washload leaving the reach. Our washload model along with bedload model are intended to be a module of a complete watershed scale model to simulate the evolution of river channels in response to watershed scale changes in land use and climate change.