Browsing by Author "Chen, Hung-En"

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A Layer-Integrated Model of Solute Transport in Heterogeneous Media
(2014-12-01)
Chen, Hung-En
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Lee, Hui-Ping
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Chiang, Shih-Wei
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Tsai, Tung-Lin
;
Yang, Jinn-Chuang
This study presents a numerical solution to the three-dimensional solute transport in heterogeneous media by using a layer-integrated approach. Omitting vertical spatial variation of soil and hydraulic properties within each layer, a three-dimensional solute transport can be simplified as a quasi-three-dimensional solute transport which couples a horizontal two-dimensional simulation and a vertical one-dimensional computation. The finite analytic numerical method was used to discretize the derived two-dimensional governing equation. A quadratic function was used to approximate the vertical one-dimensional concentration distribution in the layer to ensure the continuity of concentration and flux at the interface between the adjacent layers. By integration over each layer, a set of system of equations can be generated for a single column of vertical cells and solved numerically to give the vertical solute concentration profile. The solute concentration field was then obtained by solving all columns of vertical cells to achieve convergence with the iterative solution procedure. The proposed model was verified through examples from the published literatures including four verifications in terms of analytical and experimental cases. Comparison of simulation results indicates that the proposed model satisfies the solute concentration profiles obtained from experiments in time and space.
Effect of Rainfall, Runoff and Infiltration Processes on the Stability of Footslopes
(2020-05-01)
Chen, Hung-En
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Chiu, Yen-Yu
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Tsai, Tung-Lin
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Yang, Jinn-Chuang
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防災與水環境研究中心
To analyze the effect of runoff on shallow landslides, a model coupling one-dimensional rainfall-runoff and two-dimensional infiltration was established to simulate rainfall, infiltration, and runoff processes. Based on Bishop's limit equilibrium method, the slope failure of a hypothetical footslope was studied. First, conditions with and without inflow were compared. The results reveal a remarkable difference in factors of safety (FS) between the two conditions, suggesting that considering the effect of runoff is crucial for landslide modeling. In terms of a series of tests of the various magnitudes, durations, lag-time, and peak position of the hydrograph, analyses show that larger inflow leads to more accumulated infiltration and triggers landslides earlier. A long-term duration inflow decreases the stability more than short intensive inflow does. With subsequent surface inflow, slope failure may occur after rainfalls stop, owing to the inflow, and the shape of inflow hydrographs could slightly affect the variance in FS. Results also indicate the necessity of considering the surface runoff when using a numerical model to analyze landslide, particularly on a footslope.
Investigation of the Influence of Rainfall Runoff on Shallow Landslides in Unsaturated Soil Using a Mathematical Model
(2019-06-01)
Chiu, Yen-Yu
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Chen, Hung-En
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Yeh, Keh-Chia
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土木工程學系
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Department of Civil Engineering
Infiltration and groundwater have been widely considered as the main factors that cause shallow landslides; however, the effect of runoff has received less attention. In this study, an in-house physical-process-based shallow landslide model is developed to demonstrate the influence of runoff. The model is controlled by coupling the shallow water equation (dynamic) and Richards' equation. An infinite slope stability analysis is applied to evaluate the possibility of regional landslides. A real, small catchment topography is adopted as a demonstration example. The simulation illustrates the variations of runoff and the factor of safety (FS) during a storm. The results indicate that, after the surface becomes saturated, the FS may keep varying due to the increasing pressure head, which is caused by increasing surface water depth. This phenomenon most likely occurs downstream where the slopes easily accumulate water. The depth of the surface water may also be a factor of slope failure. Therefore, it is essential to increase the accuracy of calculating the runoff depth when assessing regional shallow landslides.
Numerical modeling of rainstorm-induced shallow landslides in saturated and unsaturated soils
(2008-09-01)
Tsai, Tung-Lin
;
Chen, Hung-En
;
Yang, Jinn-Chuang
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土木工程學系
;
Department of Civil Engineering
For the assessment of shallow landslides triggered by rainfall, the physically based model coupling the infinite slope stability analysis with the hydrological modeling in nearly saturated soil has commonly been used due to its simplicity. However, in that model the rainfall infiltration in unsaturated soil could not be reliably simulated because a linear diffusion-type Richards' equation rather than the complete Richards' equation was used. In addition, the effect of matric suction on the shear strength of soil was not actually considered. Therefore, except the shallow landslide in saturated soil due to groundwater table rise, the shallow landslide induced by the loss in unsaturated shear strength due to the dissipation of matric suction could not be reliably assessed. In this study, a physically based model capable of assessing shallow landslides in variably saturated soils is developed by adopting the complete Richards' equation with the effect of slope angle in the rainfall infiltration modeling and using the extended Mohr-Coulomb failure criterion to describe the unsaturated shear strength in the soil failure modeling. The influence of rainfall intensity and duration on shallow landslide is investigated using the developed model. The result shows that the rainfall intensity and duration seem to have similar influence on shallow landslides respectively triggered by the increase of positive pore water pressure in saturated soil and induced by the dissipation of matric suction in unsaturated soil. The rainfall duration threshold decreases with the increase in rainfall intensity, but remains constant for large rainfall intensity.
Threshold of Slope Instability Induced by Rainfall and Lateral Flow
(2017-09-01)
Chen, Hung-En
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Tsai, Tung-Lin
;
Yang, Jinn-Chuang
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土木工程學系
;
Department of Civil Engineering
In this study, a two-dimensional numerical landslide model was developed to investigate the effects of the amount of rainfall and lateral flow on induced slope failures. The Richard's equation was used to evaluate pore water pressure distribution in response to moisture content variations induced by rainfall and infiltration in soil mass. The slope stability was then assessed using the limit equilibrium method of slices, and the moment equilibrium was considered. Several hypothetical cases involving various rainfall amounts and durations were simulated using the proposed model to investigate the possible tendency toward slope instability caused by rainfall time-series processes. After the rainfall conditions were analyzed, rainfall patterns were categorized as uniform, intermediate, advanced, and delayed types. Furthermore, the lateral flows running through the upstream and downstream boundaries of a slope were analyzed to investigate the lateral effects on the hillslope. The results indicated that the lateral flow may increase or reduce the groundwater table and, thus, accelerate or reduce the occurrence of slope failure. In addition, several rainfall threshold curves that accounted for the rainfall amounts, durations, and patterns were developed and appeared more realistic and to approximate real conditions more accurately than those created using one-dimensional landslide modeling do.