Technical Studies

In recent years, the stability of the western Taiwan’s rivers is an important issue, especially in the downstream reach of weir or dam, where the severe erosion often endangers the safety of hydraulic flood-prevention structures. Most of the mobile-bed models currently used in Taiwan are one-dimensional or two-dimensional. For complex three-dimensional flow field and local scour problems, such as the vicinity of piers, groins, aprons, etc., the use of 1D or 2D model is inadequate, because of the use of section-averaged or depth-averaged velocity. With the adoption of sophisticated 3D model, we can obtain a more objective assessment of local flow phenomenon. Since the development of 3D mobile-bed model is difficult, and the existing 3D models are still at the developing stage in Taiwan, the implement and application study of foreign advanced 3D mobile-bed model is needed.

The National Center for Computational Hydroscience and Engineering (NCCHE), located in the University of Mississippi, is founded by several US federal agencies and US Congressional Appropriation. NCCHE has developed the 3D sediment transport and local scour model (CCHE3D), which has been applied to river and coastal problems in the USA and other countries with significantly successful achievements, after rigorous tests and verifications of the model.

This project is carried out in a three-year term in cooperation with the NCCHE for the implement of CCHE3D in Taiwan. The work is executed among NCCHE, Water Resources Planning Institute, WRA, and Disaster Prevention & Water Environment Research Center, NCTU. The main purpose of this project is to study the local scour mechanism between the hydraulic structures and riverbed. Different field cases in Taiwan are selected to test the capability of the CCHE3D model.

In the case of downstream reach of JiJi Weir, the CCHE3D model has the capability to simulate the soft bedrock erosion and knickpoint migration. The bedrock erosion rate formula proposed by Liao et al. (2013) can be directly applied in the 3D model simulation. The erosion control design plan of downstream JiJi Weir reach has been simulated using the CCHE3D model. It is found that the excavated channel over the softrock bed will still have erosion, but the erosion depth will be much less than the current channel.

In the case of Chungcheng Bridge reach, typhoon Morakot is used to test the unsteady flow module of CCHE3D. The simulated water stages agree well with the measured ones, so the model can simulate the flow field with complex hydraulic structures over the riverbed. In the simulation of flow passing Kaoping Weir’s diversion dike, the 3D vortex near the rubber weir No.1 can be seen. The velocity distribution and flow field in different vertical layers could be the reference for the design of diversion dike in future.

In the model evaluation, the tests of local scour, bedrock erosion, channel migration, and unstedy flow under typhoon events, and complex geomorphorlogy show that the CCHE3D model has satisfactory capability for both experimental and field cases. In follow-up years, the model function would be extended and applied to Taiwan’s rivers.