Cross-stream structures such as weirs are widely used for water intake in many alluvial rivers in Taiwan. Use of these structures, however, leads to severe scour downstream as rivers in Taiwan typically have steep slopes and are subject to flood flows caused by typhoon. The 7.3-magnitude earthquake in 1999 caused a differential uplift of river channel near several weir sites, which led to accelerated river width and slope adjustments. This study concerns with a reach downstream of Ji-Ji weir that consists of mostly soft bedrock and has experienced significant vertical and lateral erosion. Extensive bank failure occurred during Typhoon Saola in 2012. Vertical erosion of upto 14 meters was recorded since the weir operation in 2000. A number of engineering schemes have been proposed to stabilize the reach or to reduce the erosion in the bedrock exposed reach. In this study, a new two-dimensional geo-fluvial numerical model, SRH-2D, was used to simulate combined vertical and lateral erosion in alluvial stream. The geo-fluvial model is coupled with a bedrock erosion module to predict both vertical and lateral alluvial erosion and bedrock incision. The model is used to simulate a 15.5-kilometer reach of Cho-Shui River over a three-year period. The results are then compared with the field data for model assessment. The simulated results show that the coupled geo-fluvial and bedrock model could correctly capture major erosion and deposition patterns. However, this study also points to the need for future improvement.
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