Mechanism And Simulation Of River Basin Sediment Dynamics

Soil erosion and sediment yield start when rain drops reach ground surface and then runoff yields. Splash detached particles are then carried by flow discharge as sediment, which is replenished or reduced by scouring or deposition along hillslopes, different orders of channels and rivers, and finally enter the sea. The life cycle of sediment from detachment in highlands to deposition in rivers or the sea is a continuous physical process. The subject of river basin sediment dynamics aims at the mechanism and integrated simulation of the whole process of sediment yield, transport and deposition, including soil erosion in highlands, non-equilibrium sediment transport in channels and rivers, etc. Taking the coarse sediment source region in the middle Yellow River as the example, this dissertation achieved the simulation of continous sediment processes based on dynamic principles and modeling approaches.Firstly, the mechanism of sediment processes in the coarse sediment source region was analysed on the basis of existing researches. The fundamental of dynamic modeling of sediment processes was pointed out that different geomorphological positions and different sub-processes must be identified and be modeled separately. In the studied region, the sub-models consist of rainfall- runoff-soil erosion model for hillslopes, flow routing and non-equilibrium sediment transport model for channels, and gravitational erosion model for gully regions.The proposed rainfall-runoff model mainly simulates infiltration-excess runoff in hillslopes, and a process-oriented soil erosion model was improved to simulate hydraulic erosion process by taking surface flow intensity, soil erodibility, and micro relief into account. Flow discharge in channels was routed by using a 1D diffusive wave method, and sediment transport was simulated according to non-equilibrium transport of suspended load. An existing gravitational erosion model was integrated in simulation for river basins by proposing several adptive treatments.To integrate and run the sub-models following the sequence of flow routing and sediment transport among simulation units, a binary tree-based methodology of drainage network codification was proposed. Based on the methodology, a PC cluster-based parrellel computing scheme was achieved following the MPI standard to provide sufficient computing capacity for simulation of large-scale basins.Further more, the integrated model system was calibrated and applied in a watershed and a river basin. The results of hydrographs and sediment graphs in hydrological stations proved effectiveness of the model, and more results were provided including the distributions of hillslope erosion, gravitational erosion, and channel erosion, and their propotions.Finally, simulated results were used to explain the cause of scale effects in sediment processes, and such was revealed that gravitational erosion occurs in specific scales and contributes most to scale effects. Different scenarios were set to analyze the responses of soil erosion and sediment dynamics on the change of rainfall pattern, and it was found out that if rainfall amount decreases and intensity increases, runoff will decrease with amplitude larger than that of the decrease of soil erosion.