Modeling framework for sediment deposition, storage, and evacuation in the floodplain of a meandering river: Theory

A theory for the movement of suspendible sediment through the floodplain of a simple actively migrating river is developed. The floodplain is represented as a series of sediment storage reservoirs, each incorporating multiple bends of a river. Independent models for floodplain sediment deposition and for subsequent removal of sediment by lateral channel migration allow the volume of sediment stored in the floodplain (and the associated average floodplain elevation) to change over time and down the valley. At a given quasi-steady discharge, deposition depends on a simple one-dimensional partition between channel and floodplain discharge and on suspended sediment concentration (one size class) which varies along the reach depending on lateral supply and floodplain history. Sediment supply is specified at the upstream end of the reach using a suspended sediment rating curve. Long-term average overbank deposition rates are approximated by weighting multiple steady state solutions using a flow duration curve. The channel bed elevation of the system is assumed to be fixed, and tributaries are handled using simple lateral source terms. The interaction between the submodels is such that under constant boundary conditions, the channel evolves toward a state of geomorphic equilibrium where the volume of sediment added to the floodplain by overbank deposition just balances the volume removed by channel migration. The model is easily adapted to track conservative suspended sediment tracer material concentration as such material moves through the channel/floodplain system.