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 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 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. (2014). Typhoon Matmo and Morakot were selected for the model calibration and validation. The simulations agreed well with the measured data.

In this year’s study (2015), an optimal plan is proposed based on the idea that the headcut approaching the JiJi Weir needs to be controlled by a high weir structure to reduce the erosion power of the flow; and channel incision needs to be controlled by a series of lower weirs to reduce the flow erosion power. A straight drop structure near CS-116, four low-headed weirs at CS-115, CS-113, CS-111, and CS-109, and the excavation from CS116 to CS-109 are considered. The bed slope, S=0.0028 is used as a reference slope with which the channel may be stabilized because the lower segment of this river reach is controlled by this slope. The current incision part of the channel has a slope of S=0.0057; with these proposed weirs installed, the sediments may fill up the pools and further deposit between the weirs to form a new stable channel covered with sediments.