Yong G. Lai

Soft Bedrock Erosion Modeling with a Two-Dimensional Depth-Averaged Model

Many rivers in Taiwan have steep slopes, are subject to typhoon-induced flood flows, and contain soft bedrock that is exposed at many locations and easily erodible. The occurrence of extensive bedrock erosion has been a major threat to river infrastructure at many locations. Soft bedrock erosion, therefore, is an important process to consider for river projects in Taiwan. In this study, bedrock erosion models are reviewed. A specific model is proposed by combining two existing models incorporating both the hydraulic and abrasive scour mechanisms. The proposed bedrock erosion model is incorporated into a two-dimensional mobile-bed model, and the integrated model is tested by simulating bedrock erosion downstream of the Chi-Chi weir on the Choshui River in Taiwan. A calibration study is performed to determine appropriate values of the model parameters based on two and a half years of measured data. The model is then assessed based on a verification study that compares model predictions of bedrock erosion of...

Predicting contraction scour with a two-dimensional depth-averaged model

Contraction scour is often encountered in natural rivers due to natural geological controls, bridges, or river restoration structures. Such scour may be better predicted with multi-dimensional than one-dimensional models. The aim of this study is to investigate whether a two-dimensional depth-averaged model is adequate for modelling contraction scour. This study shows that improved predictions are obtained with the present model relative to previous model investigations. The study also shows that the current model is adequate for predicting contraction scour and model results are comparable with those of three-dimensional modelling except for the prediction of aggradation downstream of the contracted channel section.

Coupling a Two-Dimensional Model with a Deterministic Bank Stability Model

Stream bank erosion can be an important form of channel adjustment in unstable alluvial environments and hence should be accounted for in geomorphic studies, river restoration, dam removal, and channel maintenance projects. Recently, one-dimensional and two-dimensional simulation models have become useful tools for predicting channel responses; but most either ignore bank failure mechanisms or implement only simple ad hoc methods. In this study, a twodimensional model (SRH-2D) is coupled with a deterministic bank stability and toe erosion model (BSTEM) to predict channel adjustment and planform development. Herein, the proposed coupling approach is described, along with numerical aspects of the procedures. For test and verification purposes, the coupled model is used to predict bank retreat of Goodwin Creek in Mississippi. A comparison of the model results with the measured data is presented and discussed.

Simulation study of flow through a reach of the Chattahoochee River

A three-dimensional numerical hydrodynamic model was developed by the writers to simulate flows through natural river reaches. The model has been successfully applied to and validated for a number of open channel flows. In this study, it is further applied to a flow through a reach of the Chattahoochee River, near Atlanta, GA, for which field and laboratory measurements were performed. The region of interest contains two piers of the Holcomb Bridge, a pier of an abandoned bridge, an old in-stream water intake, and six pump intakes. Good agreement is obtained between the model simulation and the experimental measurements demonstrating the ability of the model to capture important flow features.

Reservoir Turbidity Current Modeling with a Two-Dimensional Layer-Averaged Model

AbstractA 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 ...

Predicting Contraction Scour with a 2D Model

Contraction scour is often encountered in natural rivers due to channel contraction or river restoration structures. Such scour may be predicted better with multi-dimensional models than 1D models. It has been reported that the 2D depth-averaged model is inadequate for modeling the contraction scour. This study aims to investigate whether other factors may play important roles in the poor performance of 2D models. In the process, an improved prediction using the 2D model is obtained and factors influencing the model prediction are identified. The motivation of the study stems from the fact that a 3D model is too complex and many engineering applications have to rely on 2D models. This study shows that the 2D model is still adequate for predicting the contraction scour.