A Fully Coupled Sediment-laden Shallow Water Model And Its Applications To Computational Morphodynamics In Qiantang Estuary And Dongliu Waterway

This paper presents a depth-averaged fully coupled sediment-laden shallow water model and its applications to a tide-dominated symmetrical funnel-shaped estuary and a waterway characterized by three central bars and four sub-waterways,with a special attention to the bed morphological changes.The governing equations of the model are derived by depth-averaging Reynolds-averaged Navior-Stokes equations,which include the mass and momentum conservation equations for the water-sediment mixture and the mass conservation equations respectively for sediment(carried in the flow),the salinity and the bed materials.The depth-averaged governing equations are solved by the finite volume method.The HLLC approximate Riemann solver is used to estimate the numerical fluxes across the face between neighboring triangular cells.The bed deformation and sediment compositions are updated at the cell center.The bed slope is estimated using bed elevations defined at the cell nodes.In order to improve the computational efficiency of the model,Open MP parallel technique and local time step scheme are used.A series of idealized,theoretical and experimental flows are numerically simulated to validate the model and demonstrate the capability of the model(e.g.,the well-balanced property,treatment of the wet/dry fronts,shock-capturing ability),and the quantitative accuracy in reproducing water depth,flow velocity and bed morphological changes.Afterwards,the model is applied to an estuary.The main geometrical parameters of the estuary,and the upstream and boundary conditions are specified with reference to the Qiantang River Estuary(QRE),where the existence of a giant 'sandbar'is well known.It is shown that a maximum turbidity zone can form due to interactions of the riverine discharge and the tidal flow.Strong deposition occurs in the maximum turbidity zone,whereas erosion occurs otherwise.This erosion/deposition pattern leads to the formation of a sandbar,of which the magnitude and position are comparable to the observed 'sandbar' in the QRE.As the sandbar grows,an increasing trend of the tidal range from the estuary mouth towards upstream is established.This variation trend appears also similar with field data observed in the QRE.Sensitivity analyses of key parameters(e.g.,river discharge,seaside sediment input and erosion coefficient)within a reasonable range lead to quantitative differences,but the above understandings hold true.Finally,the model is applied to the Dongliu waterway of the Yangtze River.It is shown that the present model can successfully reproduce the pattern of sediment erosion/deposition occurred in this waterway,facilitating further applications in the Yangtze estuary,which is also characterized by three central bars and four sub-waterways.Sensitivity analyses of key parameters shows that the local roughness and sediment-laden capacity formulas lead to the qualitatively same but quantificationally different morphologic evolution process.