ABSTRACT
Fast geomorphic transients may exhibit the coexistence of bed-load near the bottom, and suspended load in the upper portion of the flow, whenever a wide-range variability of the shear stress is encountered. These processes represent a challenging task for modelers, since the usual representation of the flow as a mixture may result unsatisfactory. A two-phase depth-averaged model able to account for both bed-load and suspended sediment transport is proposed in the present paper. The mathematical model is derived from mass and momentum conservation equations, separately for liquid and sediment phase, with a further distinction between sediment transported as bed-load and suspended load. The bed-load layer thickness is assumed to vary with the actual shear stress, whereas changes of the areal concentration are described by the differential equations, thus achieving a variable sediment concentration in the bed load layer. The entrainment/deposition of sediment from the bed towards the bed-load layer is evaluated by a formula based on a modified van Rijn mobility parameter, while for the suspended sediment a first-order exchange law is considered. A numerical finite-volume method is employed for the simulation of a benchmark experiment with wide-range variability of the Shields parameter. Even without any detailed calibration, simulated and experimental results reasonably agree.
