Technical Studies


Implement and Application Study of NCCHE’s River Migration Models (2/3)

Located in the University of Mississippi, the National Center for Computational Hydroscience and Engineering (NCCHE) is founded by several US federal agencies and the US Congressional Appropriation. The NCCHE developed the 1D (CCHE1D) and 2D (CCHE2D) sediment transport models which have been applied to river and coastal problems in the USA and abroad with significant by successful achievements after through rigorous tests and verifications of the models. This project will be carried out in a three-year term in cooperation with the NCCHE for the introduction of the 1D and 2D numerical sediment transport models. With Choshui River chosen as the target of study, the work is shared among NCCHE, Water Resources Planning Institute, Water Resources Agency, and the NCTU Hazard Mitigation Research Center. One of the main purposes of this project is the building of a model base of NCCHE’s hydraulics and sediment transport models. In this year, we applied the CCHE2D to investigate the upstream and downstream bed changes near the JiJi Weir. The study reach is from the upstream Jilu Bridge to the downstream Minchu Bridge. The cross-section geometry measured in 2004 was used as the initial condition for the calibration (2007 bed) and validation (2008 bed) of this model, by considering the rock scouring module and the significant typhoon events during 2004-2008. From the calibration and validation results, CCHE2D model is capable of simulating the flow field, sediment transport, morphology changes and rock scouring within the study reach with unsteady boundary conditions for the flow and sediment. In the model comparison studty, the CCHE2D and EFA2D both gave reasonable and well-trended results. However, the model under-predicted a big scour hole close to the weir, which is partially affected by the vertical scouring. In the erosion control plans, using the same boundary conditions of calibration case, it shows that the deposition would normally appear over the erosion control reaches covered with large rock blocks. The simulated results indicate the widened channel reach would be naturally narrowed again resulted from sediment depsition along one side of the channel. The bedrock incision downstream of the structures for all the plans could be increased. For the short-term mitigation of bedrock incision problems near the Fujo embankment, plan2 to plan5 and plan10 have better results than others. The plans could slow down the flow and protect the eroded main channel. However, due to the uncertainty of the data such as sediment load, its composition, rock scouring mechanism, the accuracy of the numerical model would be affected. It is difficult to propose a long-term countermeasure based on the information and simulation results obtained at the present stage. More information associated with physical experiment needs to be tested in order to improve the model’s reliability. In the building up of the model database, we have maintained the website, developed the digital documents and translated the CCHE2D user’s manual. In addition, the CCHE1D quick start guide was used as the sample for the establishment of the Flash animation as a multimedia teaching tool which would be more user-friendly during the model training sessions.