The rivers in Taiwan are fast flowing with high discharge rates, especially following heavy rain. Thus, riverbeds suffer from severe erosion during the typhoon season and sometimes flash floods caused by the rapid rising of the river stage and massive sediment yield. Such events can not only damage hydraulic structures and people's property but can even lead to the loss of human life. To further understand the scouring problem in Taiwan, the study utilizes multiple downscaling simulations with continuous in-situ monitoring to assess riverbed scouring. First, the HEC-HMS hydrologic modeling system is used to simulate the runoff on the watershed scale. Then the WASH123D model is utilized to understand hydraulic routing on the river scale. Finally, the SRH-2D and CCHE3D mobile bed models are applied to simulate the local scouring of the riverbed on a local scale. In-situ monitoring at Fengshan Creek in Taiwan started in June 2018, including the installation of the Acoustic Doppler Current Profiler (ADCP) and float-out devices in the riverbed. Here, numerical simulations are integrated with field monitoring to study local scouring. The results reveal consistency between the ADCP and float-out device monitoring results for water level, flow velocity, and scouring depth with the multiple downscaling simulations. Furthermore, this study found that most erosion was generated at peak times and that more obvious down-scouring occurred during flood recession when the lessening of the flow velocity along the river generated a more substantial down-erosive kinetic energy. That usually happens in the turbid rivers in Taiwan when the riverbed has been severely disturbed, inevitably resulting in severe scouring. The study also confirms that multiple downscaling simulations can dramatically reduce the computational resources needed while effectively maintaining simulation accuracy. In addition, integrating numerical simulations with field monitoring is an effective way to evaluate riverbed erosion.
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