A three-dimensional numerical model for simulating flow and sediment transport processes in fluvial and deltaic environments.

摘要

Process-based numerical sedimentation models offer a unique approach to quantitatively describe the evolution of sedimentary environments by combining time and stratigraphic records in which sedimentation history is interpreted through simulations based on physics of erosion, transport and deposition of clastic sediment. They provide insights into the dynamics of depositional processes, and therefore help sedimentologists understand the mechanisms that create depositional sedimentary features and predict future behavior of depositional environments. Based on a previously existing model SEDSIM, a new three-dimensional process-based model, SEDSIM2, has been developed to simulate sedimentation processes in fluvial and deltaic environments.; Flow and sedimentation in fluvial and deltaic systems can be simulated using simplified Navier-Stokes equations for open channel flow and empirical relationships between flow and transported sediment. A mixed Lagrangian-Eulerian representation of flow and sediment transport is employed where the momentum equation is solved by a Lagrangian discretization of flow using momentum tracers, and the continuity equations of flow and sediment transport are solved based on a fixed-grid system. An entrainment criterion and a suspension criterion are used to determine the initiation of sediment movement and the manner in which sediment is transported, as characterized by suspended load and bedload. Suspended load and bedload are handled separately, resulting in more accurate calculations for sediment concentrations and transport rates. Computing time is optimized by using a self-adjusting time-extrapolation scheme. Simulated sedimentary sequences provide 3-D distributions of sediment composition contained in detailed numerical grids that can be transformed into time-stratigraphic displays to represent sedimentary deposits formed during successive time intervals of uniform length.; SEDSIM2 accommodates any number of arbitrary grain sizes and densities, and their mixtures in any proportion. The model is capable of handling irregular topographic features, unsteady flow, and variable sediment discharges. It yields equilibrium topographic profiles and steady state flow velocities if discharges do not change significantly. SEDSIM2 demonstrates that surface-flow equations can realistically predict steady stream flows and sediment-transport processes associated with them, and can also simulate floods (highly unsteady flows) and their effects on channel topography.