Technical Studies

Continuous river channel migration at the area of the Jhong-jheng Bridge of the Tou-chien River by rapid bedrock erosion could be a great threat to the function and the safety of cross-river infrastructures. According to historical hydrological data, the typhoon Soulik in 2013 was the most significant hydrologic event in the period from 1991 to 2015. This event resulted in a peak discharge close to 100-year return-period discharge Q100 (8290 cms). This event destroyed the 9th pier of the Jhong-jheng Bridge and widened the river channel at right side of the abutment. Therefore, proper countermeasures at this area to prevent rapid bedrock erosion are needed. Taking the area of the Jhong-jheng Bridge of the Tou-chien River as a full scale model, this two-year project aims to accomplish the following four objects: (1) to understand the processes and phenomena of the channel migration; (2) to determine the river channel variation under long term and short term conditions by the mobile-bed numerical simulation; (3) to demonstrate the “partially grouted riprap” method for stabilizing rock riverbed eroded channel; (4) to recommend the countermeasures at the area of the Jhong-jheng Bridge of the Tou-chien River. The following conclusions can be drawn based on the tasks accomplished in this year. 1. According to the data of cross-section survey and digital surface model in this study reach, the river migration can be divided into the following stages. (1) The initial stage of the Jhong-jheng Bridge completion (before 1986): sediment deposition occurred in this area. (2) Protection Works are built on the downstream of the Jhong-jheng Bridge in sequence (1986 to 2012): the maximum erosion depth downstream of the drop structures is up to 15 meters, and there is no significant change of riverbed elevation in the upstream of the Jhong-jheng Bridge. (3) A new channel was developed at the right-hand side of the protection works (after 2012): The Typhoon Soulik widened the new river channel, and sediments were transported from upstream to downstream. The event caused the headward erosion of rock river-bed and channel knickpoint migration. 2. Based on the results of site investigation, the erosion processes of soft rock river-bed included (1) trenching incision and plucking , which were observed from the Jhong-jheng Bridge to 200 meters upstream of the Bridge; (2) uniform incision, plucking, bedload abrasion occurred from.200 meters upstream of the Jhong-jheng Bridge to the Sin-jhong-jheng Bridge: 3. The CCHE2D computer program was applied to simulate the channel variation in this area. The preliminary calculation results show that CCHE2D can be adopted for study the bedrock erosion behavior in the area. 4. Partially grouted riprap consists of specifically sized rocks that are placed above the riverbed and grouted together with grout filling 50% or less of the total void space. In contrast to fully grouted riprap, partial grouting increases the overall stability of the riprap installation unit without sacrificing flexibility or permeability. This project plans to demonstrate the partially grouted riprap on the upstream channel of the Jhong-jheng Bridge next year, and compile a guideline for constructing partially grouted riprap on Taiwan river channel in future.