Coupled modelling of alluvial flows

Full recognition of the physical coupling of water and sediment phases of alluvial flows leads to the need for a concurrent solution of the water-muting and sediment-routing components in mobile-bed river modeling. In this paper the coupled and uncoupled methods of alluvial stream modeling are compared. A newly developed coupled model of one-dimensional riverine water and sediment flow, called COUPFLEX, is briefly introduced. Several schemes in discretization of the sediment continuity equation, often used in uncoupled approaches, are mentioned. Two elaborate flume tests on river degradation are briefly described. Experimental data is used to compare the performance of COUPFLEX with that of the uncoupled approaches. This comparison highlights the advantages of the coupled method over the traditional uncoupled method. Performing several numerical tests, some aspects of numerical modeling of alluvial flows are briefly dealt with. These include problems resulting from the use of equilibrium relations in nonequilibrium situations; the dominant role of empirical sediment transport and hydraulic resistance formulas in the accuracy of the answers from any alluvial flow model; significance of the computational time step; violation of the sediment mass conservation law; and the use of approximate water-routing models.