Browsing by Author "Yeh, Keh-Chia"

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3D Simulation of Softrock Erosion in Chuoshui Creek
(2015)
Zhang, Yaoxin
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Jia, Yafei
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Yeh, Keh-Chia
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Liao, Chung-Ta
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Wang, Sam S. Y.
This paper presents a study of soft rock erosion in Chuoshui Creek, Taiwan, using CCHE3D hydrodynamic model. The geological condition of Chuoshui Creek is complex; one reach of the river is flowing through a zone of base rock which is highly weathered and erodible, resulting in tremendous channel incision and headcut propagation particularly by high floods during typhoon seasons. A semi-empirical soft-rock erosion mechanism is implemented, in which the softrock properties, flow shear stress, and stream power, etc. are considered. The erosion rate coefficients of the formulation have been calibrated by field data in separate studies. In this study, the event of typhoon Morakot in 2012 was utilized to simulate the softrock erosion process in a 1.6km-long reach downstream of a large cross-river infrastructure, JiJi Weir, where serious soft rock erosion has occurred, threatening the integrity of the weir. The 3D model performed well under the conditions of highly unsteady, supercritical flows and complex channel topography. Comparing to the field data, the simulation results agreed well with the observations. The calibrated CCHE3D softrock model is then applied for evaluating the effectiveness of planned erosion control structures in the river.
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
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Yeh, Keh-Chia
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Lai, Yong G.
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土木工程學系
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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.
A probabilistic model for evaluating the reliability of rainfall thresholds for shallow landslides based on uncertainties in rainfall characteristics and soil properties
(2017-05-01)
Wu, Shiang-Jen
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Hsiao, Yi-Hua
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Yeh, Keh-Chia
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Yang, Sheng-Hsueh
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土木工程學系
This study aims to develop a probabilistic rainfall threshold estimation model for shallow landslides (PRTE_LS) in order to quantify its reliability while being affected by uncertainties in the rainfall characteristics and soil properties. The rainfall characteristics include the rainfall duration, depth and storm patterns in which their uncertainties result from temporal variation. The effective cohesion of soil, the unit weight of soil, the angle of internal friction, hydraulic conductivity and hydraulic diffusivity are the soil properties represented as the soil parameters in the TRIGRS model in which uncertainties are attributed to spatial variation. After analyzing the sensitivity of rainfall characteristics and TRIGRS parameters to the estimation of rainfall thresholds, the maximum rainfall intensity, storm pattern and soil parameters (soil cohesion, soil friction angle and total unit weight of soil) are retreated as uncertainty factors used in the model development. The proposed PRTE_LS model is used for the reliability assessment of the issued rainfall thresholds in the Jhoukou River watershed, southern Taiwan, to demonstrate its applicability. The results indicate that the corresponding exceedance probability (i.e., underestimated risk) approximates 0.1 on average. In other words, its reliability reaches 0.9. However, issued rainfall thresholds with high reliability might hardly achieve the goal of early warning because shallow landslides can possibly happen before the actual rainfall amount exceeds the threshold. Consequently, the proposed PRTE_LS model can modify issued rainfall thresholds with the specific occurrence probability under the critical safety factors and warning durations being considered. As a result, the proposed PRTE_LS can not only quantify the reliability of the issued rainfall thresholds, but its results can also be referred to in modifying the issued thresholds in order to enhance the early-warning performance.
A systematic approach for the assessment of flooding hazard and risk associated with a landslide dam
(2013-01-01)
Yang, Sheng-Hsueh
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Pan, Yii-Wen
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Dong, Jia-Jyun
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Yeh, Keh-Chia
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Liao, Jyh-Jong
Inundation caused by landslide dams may occur in the upstream and downstream of the dams. A proper flooding hazard assessment is required for reaction planning and decision-making to mitigate possible flooding hazards caused by landslide dams. Both quick and detailed procedures can be used to evaluate inundation hazards, depending on the available time and information. This paper presents a systematic approach for the assessment of inundation hazards and risks caused by landslide dam formation and breaches. The approach includes the evaluation of dam-breach probability, assessment of upstream inundation hazard, assessment of downstream inundation hazard, and the classification of flooding risk. The proposed assessment of upstream inundation estimates the potential region of inundation and predicts the overtopping time. The risk level of downstream flooding is evaluated using a joint consideration of the breach probability of a landslide dam and the level of flooding hazard, which is classified using a flooding hazard index that indicates the risk of potential inundation. This paper proposes both quick and detailed procedures for the assessments of inundation in both the upstream and downstream of a landslide dam. An example of a landslide dam case study in southern Taiwan was used to demonstrate the applicability of the systematic approach.
An improved genetic programming to SSM/I estimation typhoon precipitation over ocean
(2011-07-30)
Chen, Li
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Yeh, Keh-Chia
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Wei, Hsiao-Ping
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Liu, Gin-Rong
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土木工程學系
This article proposes an improved multi-run genetic programming (GP) and applies it to estimate the typhoon rainfall over ocean using multi-variable meteorological satellite data. GP is a well-known evolutionary programming and data mining method used to automatically discover the complex relationships among nonlinear systems. The main advantage of GP is to optimize appropriate types of function and their associated coefficients simultaneously. However, the searching efficiency of traditional GP can be decreased by the complex structure of parse tree to represent the multiple input variables. This study processed an improvement to enhance escape ability from local optimums during the optimization procedure. We continuously run GP several times by replacing the terminal nodes at the next run with the best solution at the current run. The current method improves GP, obtaining a highly nonlinear meaningful equation to estimate the rainfall. In the case study, this improved GP (IGP) described above combined with special sensor microwave imager (SSM/I) seven channels was employed. These results are then verified with the data from four offshore rainfall stations located on islands around Taiwan. The results show that the IGP generates sophisticated and accurate multi-variable equation through two runs. The performance of IGP outperforms the traditional multiple linear regression, back-propagated network (BPN) and three empirical equations. Because the extremely high values of precipitation rate are quite few and the number of zero values (no rain) is very large, the underestimations of heavy rainfall are obvious. A simple genetic algorithm was therefore used to search for the optimal threshold value of SSM/I channels, detecting the data of no rain. The IGP with two runs, used to construct an appropriate mathematical function to estimate the precipitation, can obtain more favourable results from estimating extremely high values. Copyright. (C) 2011 John Wiley & Sons, Ltd.
Combining satellite data for estimation of rainfall for analysis of a watershed scale
(2011)
Wei, Hsiao-Ping
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Yeh, Keh-Chia
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Liu, Gin-Rong
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Chao, Chun-Chieh
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土木工程學系
The microwave/infrared rainfall algorithm (MIRA) method combines microwave and infrared channels for their respective contributions to the achievement of higher rainfall-correlated and spatial resolution data, in order to acquire appropriate rainfall data for hydrological models. In this study, estimates of rainfall rate using the MIRA method are affected by the time lag between the data acquisition of the microwave and infrared channels. The root-mean-square error (RMSE) for comparing the rainfall rate retrieved by the MIRA method (5-minute time lag between microwave and infrared data) to the ground rainfall rate in Taiwan is 12.23 mm hour(-1) with a correlation coefficient of 0.58. In addition, by comparing rainfall estimated by the TMI-2A12 and the MIRA method for three typhoons, the rainfall estimated by the latter method is more significantly correlated with the ground-observed rainfall.
Coupled geo-fluvial channel evolution and bedrock erosion modeling on the river in Taiwan
(2015)
Wu, Kuowei
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Lai, Yong G.
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Yeh, Keh-Chia
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Liao, Chung-Ta
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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.
Dam overtopping risk assessment considering inspection program
(2008-04-01)
Kuo, Jan-Tai
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Hsu, Yung-Chia
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Tung, Yeou-Koung
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Yeh, Keh-Chia
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Wu, Jian-De
Safety inspection of large dams in Taiwan is conducted every 5 years. The practice does not take into consideration uncertainty of dam conditions. The goal of this study is to determine the optimal dam inspection interval under the consideration of overtopping risk incorporating uncertainty gate availability. In earlier studies, assessment of overtopping risk only considered the uncertainties in reservoir properties and natural randomness of hydrologic events without giving much thought to the availability of spillway gates. As a result, the overtopping risk could be underestimated. In this study, an innovative concept is proposed to evaluate dam overtopping by taking into account spillway gate availability. The framework consists of three parts: (1) evaluation of conditional overtopping risk for different numbers of malfunctioning spillway gates; (2) evaluation of spillway gate availability; and (3) dam inspection scheduling. Furthermore, considerations are given to overtopping risk, inspection cost, and dam break cost for determining the optimal inspection schedule. The methodology is applied to the Shihmen Reservoir in Taiwan and to evaluate its time-dependent overtopping risk. Results show that overtopping risk considering the availability of the spillway gates is higher than the one without considering the availability of the spillway gates.
Deriving landslide dam geometry from remote sensing images for the rapid assessment of critical parameters related to dam-breach hazards
(2014-02-01)
Dong, Jia-Jyun
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Lai, Po-Jung
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Chang, Chung-Pai
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Yang, Sheng-Hsueh
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Yeh, Keh-Chia
Dam-breaches that cause outburst floods may induce downstream hazards. Because landslide dams can breach soon after they are formed, it is critical to assess the stability quickly to enable prompt action. However, dam geometry, an essential component of hazard evaluation, is not available in most cases. Our research proposes a procedure that utilizes post-landslide orthorectified remote sensing images and the pre-landslide Digital Terrain Model in the Geographic Information System to estimate the geometry of a particular dam. The procedure includes the following three modules: (1) the selection of the reference points on the dam and lake boundaries, (2) the interpolation of the dam-crest elevation, and (3) the estimation of dam-geometry parameters (i.e., the height, length, and width), the catchment area, the volumes of barrier lake and landslides dam. This procedure is demonstrated through a case study of the Namasha Landslide Dam in Taiwan. It was shown the dam-surface elevation estimated from the proposed procedure can approximate the elevation derived from profile leveling after the formation of the landslide dam. Thus, it is feasible to assess the critical parameters required for the landslide dam hazard assessment rapidly once the ortho-photo data are available. The proposed procedure is useful for quick and efficient decision making regarding hazard mitigation.
Development and application of 2-D mobile-bed model with bedrock river evolution mechanism
(2014-08-01)
Liao, Chung-Ta
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Yeh, Keh-Chia
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Huang, Ming-Wan
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土木工程學系
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Department of Civil Engineering
Steep slope and severe bed change are the characteristics of mountain rivers. These characteristics often cause the bed armour layer flushed away, make the bedrock exposed, and then increase the channel incision. Most mobile-bed models of past few decades aimed at the sediment transport of alluvial channel. In this paper, a bedrock river evolution mechanism is proposed, and included in a 2-D mobile-bed model, called the explicit finite analytic model (EFA). The EFA model can consider both incision and deposition over the bedrock, by combining a new stream power type of bedrock erosion rate formula with the flow and sediment transport modules. An uplifted reach of Taan River, Taiwan, caused by the Chi-Chi earthquake occurred in 1999 is chosen as the study site. Rapid bedrock incision since the uplift provides a rare chance to explore its mechanism and evolution process. The field and numerical results show that the proposed model has the capability of simulating morphological changes for bedrock rivers. (C) 2013 International Association for Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rights reserved.
Estimating the Risk of River Flow under Climate Change in the Tsengwen River Basin
(2016-03-01)
Wei, Hsiao-Ping
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Yeh, Keh-Chia
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Liou, Jun-Jih
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Chen, Yung-Ming
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Cheng, Chao-Tzuen
This study evaluated the overflow risk of the Tsengwen River under a climate change scenario by using bias-corrected dynamic downscaled data as inputs for a SOBEK model (Deltares, the Netherlands). The results showed that the simulated river flow rate at Yufeng Bridge (upstream), Erxi Bridge (midstream), and XinZong (1) (downstream) stations are at risk of exceeding the management plan's flow rate for three projection periods (1979-2003, 2015-2039, 2075-2099). After validation with the geomorphic and hydrological data collected in this study, the frequency at which the flow rate exceeded the design flood was 2 in 88 events in the base period (1979-2003), 6 in 82 events in the near future (2015-2039), and 10 in 81 events at the end of the century (2075-2099).
Estimating Typhoon Rainfall over Sea from SSM/I Satellite Data Using a Back-propagated Network
(2010-01-01)
Chine, Wen-Huan
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Wei, Hsiao-Ping
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Yeh, Keh-Chia
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Chen, Li
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Liu, Gin-Rong
This paper proposes a back-propagated network (BPN) and applies it to predict the rainfall using meteorological satellite data. In the case study, the back-propagated network (BPN) combining with SGji-Dao SM/I microwave frequency channels was employed to establish a suitable method for estimating rainfall at sea surface during typhoon periods. These results are then verified with the data from four rainfall stations located at Peng-Jia-Yu, Don-, Lan-Yu, and Green Island. From the results, the back-propagated network (BPN) outperforms the traditional multiple linear regression and empirical equations with lower estimating errors for predicting the sea-surface rainfall during typhoon periods.
Impact assessment of sea-level rise and hazardous storms on coasts and estuaries using integrated processes model
(2013-10-01)
Ding, Yan
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Kuiry, Soumendra Nath
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Elgohry, Moustafa
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Jia, Yafei
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Altinakar, Mustafa S.
The study of this paper focuses on local and regional sea-level rise (SLR) and emphasizes complexities in impact assessment of SLR under combined hydrodynamic and morphodynamic conditions induced by extreme events such as hurricanes and typhoons. In terms of integrated coastal/ocean processes modeling, two case studies are presented: The first one is to predict flooding/inundation and erosion in a small-scale estuary in Taiwan induced by a set of SLR scenarios and local extreme hydrological forcing, which includes waves, tides, river floods, and sediment transport; The second case aims at simulating large-scale hydrodynamic responses (i.e. waves and storm surges) to SLR and a hurricane in a region-scale domain covering the northern Gulf Coast. The model is validated by simulating waves, storm surges and morphological changes by using local hydrological conditions. Prediction results show that the variations of water surface elevations, waves, and morphological changes are generally not linearly proportional to the static SLR; the rates of their variable changes are varying with location and topography/bathymetry. To deal with nonlinear features in unsteady and multiscale hydrodynamic and morphodynamic processes in coasts and oceans, it is essential to adopt this integrated-process modeling approach for coastal hazard management in response to local SLR. (C) 2013 Elsevier Ltd. All rights reserved.
Investigation of Morphological Changes in the Tamsui River Estuary Using an Integrated Coastal and Estuarine Processes Model
(2020-04-01)
Hsieh, Tung-Chou
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Ding, Yan
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Yeh, Keh-Chia
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Jhong, Ren-Kai
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土木工程學系
This study is to investigate morphological changes in the Tamsui River Estuary in Taiwan driven by multiple physical processes, such as river flows, tides, waves, and storm surges, and then to study the impacts of sediment flushing operated at the Shihmen reservoir upstream on the river estuary. An integrated coastal and estuarine processes model (CCHE2D-Coast) (Center for Computational Hydroscience and Engineering Two-Dimensional-Coast) was validated by simulating these physical processes in the estuary driven by three historical typhoons in 2008. The site-specifically validated model was then applied to simulate morphological changes in the estuary in response to reservoir sediment flush scenarios from the upstream. For the impact assessment of sediment flushing, a synthetic hydrological event was designed by including a historical typhoon and a typical monsoon. It was found that during the typhoon, the sediments will be mostly deposited in the estuarine river reach of Tamsui and the Wazihwei sandy beach. During the monsoon period, most of the sediments tend to be deposited in the second fishing port of Tamsui, the northern breakwater, and the estuary, while the Wazihwei sandy beach in the river mouth would be scoured by backflow. Simulations of the complex flow fields and morphological changes will facilitate the best practice of sediment management in the coastal and estuarine regions.
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.
Levee Overtopping Risk Assessment under Climate Change Scenario in Kao-Ping River, Taiwan
(2020-06-01)
Wei, Hsiao-Ping
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Su, Yuan-Fong
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Cheng, Chao-Tzuen
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Yeh, Keh-Chia
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土木工程學系
With the growing concern about the failure risk of river embankments in a rapidly changing climate, this study aims to quantify the overtopping probability of river embankment in Kao-Ping River basin in southern Taiwan. A water level simulation model is calibrated and validated with historical typhoon events and the calibrated model is further used to assess overtopping risk in the future under a climate change scenario. A dynamic downscaled projection dataset, provided by Meteorological Research Institute (MRI) has been further downscaled to 5-km grids and bias-corrected with a quantile mapping method, is used to simulate the water level of Kao-Ping River in the future. Our results highlighted that the overtopping risk of Kao-Ping River increased by a factor of 5.7 similar to 8.0 by the end of the 21st century.
Modeling Morphological Changes due to Multiple Typhoons in the Danshui River Estuary
(2015)
Ding, Yan
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Hsieh, Tung-Chou
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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
This paper presents a preliminary study on morphological changes in the Danshui River estuary driven by tides, waves, and river floods during the period of three typhoons in 2008 (Fung Wong, Sinlaku, and Jangmi) using an integrated coastal process model. The submodels for wave, current, and sediment transport were validated by comparing the simulation results with available observation/survey data. Model validation results indicate that the simulated water levels, flow velocities, and wave heights are in good agreement with the measurements at gages, and the simulated bed changes have captured the major features of deposition and erosion in the river, the estuary, and adjacent coasts.
Modeling probabilistic lag time equation in a watershed based on uncertainties in rainfall, hydraulic and geographical factors
(2016-12)
Wu, Shiang-Jen
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Yeh, Keh-Chia
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Ho, Chih-Hung
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Yang, Sheng-Hsueh
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土木工程學系
This study proposes a framework for developing a probabilistic lag time (PLT) equation by taking into account uncertainty factors, including the rainfall factor (i.e., the maximum rainfall intensity), the hydraulic factor (i.e., the roughness coefficient in the river), and the geometrical factors (the catchment area, the length, and the basin slope). The proposed PLT equation is established based on the lag time equation by means of the uncertainty and risk analysis, i.e., the advanced first-order and second-moment approach. Hourly rainfall data in the Bazhang River watershed are used in the model development and application. The results indicate that compared with observed lag times extracted from historical events, the observed ones are mostly located within the 95% confidence interval of the simulated lag times. In addition, the resulting underestimated risk from the PLT equation can reasonably represent the degree of the difference between the estimated lag time and observed lag time. Consequently, the proposed PLT equation not only estimates the lag time at specific locations along the river, but also provides a corresponding reliability.
PIV MEASUREMENTS OF CAVITATION BUBBLE COLLAPSE FLOW INDUCED BY PRESSURE WAVE
(2012)
Yang Sheng-Hsueh
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Jaw, Shenq-Yuh
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Yeh, Keh-Chia
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交大名義發表
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National Chiao Tung University
In this study, a single cavitation bubble is generated by rotating a U-tube filled with water. A series of bubble collapse flows induced by pressure waves of different strengths are investigated by positioning the cavitation bubble at different stand-off distances to a solid boundary. Particle images of bubble collapse flow recorded by high speed CCD camera are analyzed by multi-grid, iterative particle image distortion method. Detail velocity variations of the transient bubble collapse flow are obtained. It is found that a Kelvin-Helmholtz vortex is formed when a liquid jet penetrates the bubble surface. If the bubble center to the solid boundary is within one to three times of the bubble radius, the liquid jet is able to impinge the solid boundary to form a stagnation ring. The fluid inside the stagnation ring will be squeezed toward the center of the ring to form a counter jet. At certain critical position, the bubble collapse flow will produce a Kelvin-Helmholtz vortex, the Richtmyer-Meshkov instability, or the generation of a counter jet flow, depending on the strengths of the pressure waves. If the bubble surface is in contact with the solid boundary, the liquid jet can only splash inside-out without producing the stagnation ring and the counter jet. The complex phenomenon of cavitation bubble collapse flows is clearly manifested in this study.
Preface to the Special Issue on "Climate Changes and Their Impacts in Taiwan"
(2016-10)
Cheng, Ke-Sheng
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Chen, Cheng-Ta
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Hsu, Huang-Hsiung
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Yeh, Keh-Chia
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Lin, Lee-Yaw