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Yang, J.C., 1986

Numerical simulation of bed evolution in multi-channel river systems

Bibliographic Reference

Yang, J.C., 1986, Numerical simulation of bed evolution in multi-channel river systems: Iowa City, Iowa, University of Iowa, Ph.D. dissertation, 205 p.


The purpose of this investigation is to develop a methodology for computing quasi-steady/unsteady, one dimensional, water flow and nonuniform sediment transport in arbitrarily complex systems of alluvial-river channels. The computer-based numerical model includes river-bed degradation and aggradation, the dependence of hydraulic resistance on flow and sediment properties, and hydraulic sorting and armoring of the bed material. The numerical algorithm is based on a split-operator approach, in which the water-flow simulation and sediment transport processes are computed separately but iteratively within a computational time step. Two computer codes baptized BRALLUVIAL (quasi-steady) and CHARIMA (unsteady) have been developed. For BRALLUVIAL, the modelling approach adopted rests on an assumption of quasi-steady flow, under which wave propagation effects are taken to be of secondary importance at the time scale of bed evolution processes. The effect of looped flow paths intervenes primarily in the solution of the flow energy equation. This code is best adapted for simulating long-term bed-evolution problems. For CHARIMA, the full de St. Venant equations are used for the water flow in the looped-channel system. This code is best suited for short-term simulation in which the unsteady effect of water flow has to be taken into account for bed evolution, such as for rapidly rising floods and flow in tidal deltas. The limitations of these codes are discussed through the quantative/qualitative evaluation of the nonuniform sediment transport processes and through assessment of modelling techniques as applied to a complex prototype example (Susitna River, Alaska). With the use of flow stabilization procedures, the CHARIMA code can also be used for the same quasi-steady flow simulation as BRALLUVIAL. Although a relaxation scheme is needed for BRALLUVIAL when looped-channel systems are simulated, the CPU (central processor) time required is only half that needed for CHARIMA. A computational technique has been satisfactorily used to minimize the dry-bed problem which often occurs in the simulation of looped-channel networks. Two case studies are presented to show the applicability and validity of these codes (Missouri River, USA; Cho-Shui River, Taiwan).

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