Modelling stratigraphy-based gravel bed river morphodynamics

In numerical models of gravel bed river morphodynamics the bed material is described as a mixture of sand and gravel. In these models sedimentation processes are generally simplified by dividing the channel bed deposit in two regions: one or more mixed layers whose particles can interact with the bedload, and the substrate whose grain size distribution may change in the vertical and streamwise direction for channel bed aggradation and degradation only. The main limitations of such active layer based models are that 1) sediment entrainment, as well as deposition, is limited to a discrete top layer of the bed, and 2) they can only describe processes in which the vertical sediment fluxes are dependent on the temporal variation of the channel bed elevation. The continuous Parker-Paola-Leclair framework for sediment continuity was derived to overcome these limitations, and here we present its first implementation at field scale. In particular, we implement a simplified version of the continuous framework; we apply it to an idealized gravel bed reach of which the parameters are inspired on the Trinity River, California, USA; and we show that 1) the continuous framework is able to reasonably reproduce the long term evolution of a gravel bed river reach, 2) 10,000-year-long simulations can be performed in a reasonable time on a personal computer, and 3) the results of the simplified continuous model are very similar to the results of an active layer model in which active layer (vs. bedload) material is transferred to the substrate during channel bed aggradation.