Browsing by Author "Tsai, Tung-Lin"

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A layer-integrated approach for shallow water free-surface flow computation
(2008-12-01)
Hung, Meng-Chi
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Hsieh, Te-Yung
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Tsai, Tung-Lin
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Yang, Jinn-Chuang
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土木工程學系
A novel semi-three-dimensional (semi-3D) layer-integrated approach was proposed in this study for the shallow water free-surface flow computation. A quadratic shape function proposed to approximate the velocity distribution in the layer ensures the continuity of velocities and shear stresses at interfaces. To discretize the governing equations, the finite volume formulation with staggered grid is used. As the two-dimensional (2D) sub-model for locating the free surface is not needed in this approach, the computational time consumption has been dramatically reduced. The model was verified through the wind-induced circulation in a closed basin by comparing the velocity profiles to the analytical solution. The formation of the velocity profiles due to change of viscosity distribution and the evolution process of water surface slope were also investigated. Further, the eddy viscosity is estimated by a function related to discharge, water depth, and Manning's n under the assumption of parabolic distribution of longitudinal velocity along the vertical direction for open channel flows. Two designed hypothetical cases were conducted to examine the proposed eddy viscosity relation and to demonstrate the capabilities of the proposed model. Copyright (C) 2007 John Wiley & Sons, Ltd.
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.
Conjunction effect of stream water level and groundwater flow for riverbank stability analysis
(2011-02-01)
Chiang, Shih-Wei
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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
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Disaster Prevention and Water Environment Research Center
Variations in pore-water pressure determined by the groundwater table within a riverbank have been investigated and recognized as an essential factor in determining riverbank stability with respect to mass failure. However, the effect of pore-water pressure is taken into account for most of the existing riverbank stability models under some simplified assumptions, and the limitations of predicting ability may arise. To avoid the unrealistic estimation of pore-water pressure distribution, the new approach proposed here is to couple riverbank stability with groundwater flow modeling, and apply this to tackle the conjunction effect between river stage and groundwater table. Moreover, riverbank material characteristics and the influence of infiltration can be taken into consideration via groundwater flow modeling. Two hypothetical examples, stage rising and stage falling, are used to investigate the capabilities of the present study and two representative methods. The simulated results show that riverbank failure is triggered particularly during the falling stage, which has been pointed out by other researchers as well. Furthermore, the riverbank material characteristics predominantly control the occurrence of failure and should be considered regarding assessment of riverbank stability. Additionally, the effects of parameters indicate that riverbanks with soil properties of low permeability or high specific yield with great infiltration intensity during the falling stage have a tendency to riverbank failure.
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.
Effects of soil parameter variation and grid deformation on the numerical simulation of aquitard consolidation
(2018-08-01)
Tsai, Tung-Lin
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Peng, Chen-Chang
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Wang, Xi-Jun
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土木工程學系
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Department of Civil Engineering
Changes in soil parameters such as saturated unit weight (body force), hydraulic conductivity, and elastic modulus must be considered for simulating aquitard consolidation. In addition to changes in soil parameters, grid intervals change the subsequent soil deformation in such simulations. This study incorporated and examined the effects of both the grid deformation and soil parameter variation on the numerical simulation of aquitard consolidation caused by hydraulic head decline in a multilayered aquifer system. According to the nondimensional analysis, the deformation number can be regarded as a dimensionless parameter for assessing whether the grid deformation and soil parameter variation affect this simulation. The results revealed that when the deformation number exceeds 0.01 (i.e., when the aquitard is thick or soft), both the grid deformation and soil parameter variation must be simultaneously considered to increase accuracy. Aquitard consolidation was always overestimated when only the soil parameter variation was considered. However, when only the grid deformation was considered, the aquitard consolidation was either overestimated or underestimated depending on whether the influence of the change in the body force was more substantial than that of the elastic modulus.
Examination of characteristics method with cubic interpolation for advection-diffusion equation
(2006-12-01)
Tsai, Tung-Lin
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Chiang, Shih-Wei
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Yang, Jinn-Chuang
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土木工程學系
;
防災與水環境研究中心
In this study, the use of the characteristics method integrated with the Hermite cubic interpolation or the cubic-spline interpolation on the space line or the time line, i.e., the HCSL scheme, the CSSL scheme, the HCTL scheme, and the CSTL scheme, respectively, for solving the advection-diffusion equation is examined. The advection and diffusion of a Gaussian concentration distribution in a uniform flow with constant diffusion coefficient is used to conduct this investigation. The effects of parameters, such as Peclet number, Courant number, and the reachback number, on these four schemes used herein for solving the advection-diffusion equation are investigated. The simulated results show that the CSSL scheme is comparable to the HCSL scheme, and the two schemes seem insensitive to Courant number as compared with the HCTL scheme and the CSTL scheme. With large Peclet number, for small Courant number the HCTL scheme is more accurate than the HCSL scheme and the CSSL scheme. However, for large Courant number the HCTL scheme has worse computed results in comparison with the HCSL scheme and the CSSL scheme. With small Peclet number, the HCTL scheme, the HCSL scheme, and the CSSL scheme have close simulated results. Despite Peclet number, for small Courant number the CSTL scheme is comparable to the HCTL scheme, but for large Courant number the former scheme provides unacceptable simulated results in which very large numerical diffusion is induced due to the effect of the natural endpoint constraint. For large Peclet number the HCSL scheme and the CSSL scheme integrated with the reachback technique can improve simulated results, but for small Peclet number the HCSL scheme and the CSSL scheme seem not to be influenced by increasing the reachback number. (c) 2005 Elsevier Ltd. All rights reserved.
Flood hazard mitigation in land subsidence prone coastal areas by optimal groundwater pumping
(2019-11-01)
Chang, Yin-Lung
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Tsai, Tung-Lin
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Yang, Jinn-Chuang
;
土木工程學系
;
防災與水環境研究中心
Continuing land subsidence can aggravate flood hazard and diminish the effectiveness of existing flood mitigation systems. This paper presents a study of groundwater- pumping management for mitigating flood hazard in a subsidence infested area in the southwest coast of Taiwan. An optimal groundwater-pumping model is developed for preventing flood hazard from worsening and for satisfying the groundwater demand. In an area where the land is susceptible to pumping-induced subsidence, the study shows that proper long-term pumping strategy to manage spatial-temporal variation of future land subsidence should be an integral part of flood risk management as it can sustain the effectiveness of the flood mitigation measures.
Numerical modeling of rainstorm-induced shallow landslides in saturated and unsaturated soils
(2008-09-01)
Tsai, Tung-Lin
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Chen, Hung-En
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Yang, Jinn-Chuang
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土木工程學系
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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.
Stochastically optimal considering groundwater management land subsidence
(2007-11-01)
Chang, Yin-Lung
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Tsai, Tung-Lin
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Yang, Jinn-Chuang
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Tung, Yeou-Koung
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土木工程學系
This paper presents a stochastic groundwater management model explicitly considering land subsidence. Through the use of response matrix technique and one-dimensional consolidation equation, a deterministic management model is first developed. By Latin hypercube sampling technique, along with numerical subsurface flow simulation, statistical features of unit response coefficients due to random hydrogeologic parameters, including hydraulic conductivity (K) and Lame constants (mu and lambda), are quantified. The first-ordervariance-estimation method is adopted to analyze the uncertainties of drawdown and land subsidence based on which the concept of chance-con strained programming is applied to transfer the original deterministic management model into its stochastic form. The stochastic management model enables the determination of optimal total pumpage subject to the constraints that drawdown and land subsidence do not exceed the allowable values with a specified reliability. A hypothetical example is utilized to demonstrate the applicability of the stochastic model to five cases in which various levels of parameter uncertainty are considered. The results indicate that joint consideration of drawdown and land subsidence is essential, and the proposed stochastic management model can be generally applied for regional groundwater resources management in conjunction with controlling land subsidence.
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
;
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.