Predicting Rock Scour in an Alluvial River with a Two-Dimensional Model

Most rivers in Taiwan have a steep slope, subject to high flows, and have relatively weak bedrock. Bedrock is often exposed and rock scour is an important river process. For example, the river channel downstream of the Chi-Chi Weir on the Choshui River experienced up to 10 meters of rock scour since the construction of the weir in 2001. The occurrence of deep rock scour has been a major threat to river infrastructure at many other locations in Taiwan. In this study, rock scour models are reviewed and a specific rock scour model is proposed. Among rock scour mechanisms, those due to flow hydraulic force and sediment abrasion are considered to be important. The rock scour model is incorporated into an existing two-dimensional (2D) mobile bed model, SRH-2D version 3. The resultant 2D model is capable of simulating the combined effects of rock erosion and alluvial river erosion and deposition. The new model is applied to the river reach at the Chi-Chi Weir in an attempt to test and verify the applicability of the rock scour model. Findings related to the applicability and limitations of the current rock scour models are reported.