Browsing by Author "Wu, Kuowei"

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A Combined Field and Numerical Modeling Study to Assess the Longitudinal Channel Slope Evolution in a Mixed Alluvial and Soft Bedrock Stream
(2019-04-01)
Wu, Kuowei
;
Yeh, Keh-Chia
;
Lai, Yong G.
;
土木工程學系
;
Department of Civil Engineering
A study approach is developed to assess the longitudinal channel slope under the equilibrium condition as well as the transient evolution of a mixed alluvial-soft-bedrock stream. Both the historical field data and 2D mobile-bed numerical modeling are adopted. The proposed approach is applied to a 14 km reach downstream of the Ji-Ji Weir, Chuo-Shui River, Taiwan, where continuous maintenance works have been carried out to stabilize this reach. In this study, the temporal evolution of the longitudinal channel profile is assessed numerically with three spatial scales: The large (the entire study reach), the medium (four sub-reaches), and the local (cross-sections) scale. The large scale analysis is the approach for purely alluvial streams and is shown to be difficult to use to characterize mixed alluvial-bedrock streams. The local scale analysis shows that the soft-bedrock incision has a widely fluctuating slope, reflecting the compound environmental forcing and complex riverbed setting. With the medium scale analysis, the longitudinal channel profile is found to follow a predictive trend if the reach is partitioned into four distinctive sub-reaches. Characteristics of the dynamic channel slope evolution in different spatial scales are computed and presented. The study results can be used to select the proper locations and types of the engineering stabilizing structures in a mixed alluvial and soft bedrock stream.
Coupled geo-fluvial channel evolution and bedrock erosion modeling on the river in Taiwan
(2015)
Wu, Kuowei
;
Lai, Yong G.
;
Yeh, Keh-Chia
;
Liao, Chung-Ta
;
環境工程研究所
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.
Reservoir Turbidity Current Modeling with a Two-Dimensional Layer-Averaged Model
(2015-12-01)
Lai, Yong G.
;
Huang, Jianchun
;
Wu, Kuowei
;
土木工程學系
;
Department of Civil Engineering
A two-dimensional layer-averaged model is developed and verified to simulate turbidity current characteristics and its sluicing in reservoirs. The governing equations consist of mass and momentum conservation laws for the turbidity current mixture, equations for the sediment transport and bed dynamics, and auxiliary relations for the interactions among clear water, turbidity current, and bed. A finite-volume, unstructured, polygonal mesh method is adopted so that reservoirs with complex terrains may be simulated. Special algorithms are developed to capture the turbidity current front movement through a clear water bed and to simulate turbidity current sluicing through reservoir outlets. The developed model has been tested and verified with both conservative and nonconservative turbidity currents ranging from simple to complex reservoir terrains. Case studies presented include a lock-exchange turbidity current with large eddy simulation and direct numerical simulation results, a laboratory test of turbidity currents, and a physical model of turbidity currents at Shihmen Reservoir in Taiwan. Comparisons of model results with available data show that the developed model, with appropriate calibration, reasonably predicts the turbidity current movement through reservoirs, the resultant sediment deposition along the reservoir bottom, and sediment sluicing through bottom outlets. The study also points to the need for future model improvements. (C) 2015 American Society of Civil Engineers.