Technical Studies


Implement and Application Study of CCHE3D Mobile-bed Model (2/3)

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 fields 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). After rigorous tests and verifications, the CCHE3D model has been applied to river and coastal problems in the USA and other countries with significantly successful achievements.

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 hydrodynamics and morphodynamics between the hydraulic structures and riverbed.

The sub-domain method were applied to simulate the flow and sediment transport near the dike installed near banks of a complicated braided channel reach from Mingchu Bridge to Zhangyun Bridge. With the sub-domain method, more detailed CFD analysis can be performed for local areas of interests; and, local mesh refinements are easily fulfilled.

The soft-rock erosion rate formula by Liao et at. (2013) is implemented and modified to have side slope erosion capabilities. In Taan River case, the bedrock bank erosion model is applied to simulate the soft rock erosion in 2005-2009 and 2009-2012 periods. The characteristics of the two period of erosions are significantly different. The model can reasonably simulate the erosion processes through the comparison between the simulation and DEM data are adequate.

The bedrock module was used to evaluate the erosion reduction plans in the reach from Mingchu Bridge to JiJi weir, where serious erosion and channel incision problem exists. The bedrock module was based on the shear stress method proposed by Liao et al. (2013). For the one-weir installation plan, the single weir located at downstream CS112 only has limited control effects and the whole channel still suffered from serious erosion. For both two-weir and three-weir installation plans, the bed erosions were not stopped completely, which implies that additional complementary engineering measures are still required.