Browsing by Author "Pan, Yii-Wen"

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A case of rapid rock riverbed incision in a coseismic uplift reach and its implications
(2013-02-15)
Huang, Ming-Wan
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Pan, Yii-Wen
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Liao, Jyh-Jong
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土木工程學系
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Department of Civil Engineering
During the 1999 Chi-Chi earthquake (M-w = 7.6) in Taiwan, the coseismic displacement induced fault scarps and a pop-up structure in the Taan River. The fault scarps across the river experienced maximum vertical slip of 10 m, which disturbed the dynamic equilibrium of the fluvial system. As a result, rapid incision in the weak bedrock, with a maximum depth of 20 m, was activated within a decade after its armor layer was removed. This case provides an excellent opportunity for closely tracking and recording the progressive evolution of river morphology that is subjected to coseismic uplift. Based on multistaged orthophotographs and digital elevation model (DEM) data, the process of morphology evolution in the uplift reach was divided into four consecutive stages. Plucking is the dominant mechanism of bedrock erosion associated with channel incision and knickpoint migration. The astonishingly high rate of knickpoint retreat (KPR), as rapid as a few hundred meters per year, may be responsible for the rapid incision in the main channel. The reasons for the high rate of KPR are discussed in depth. The total length of the river affected by the coseismic uplift is 5 km: 1 km in the uplift reach and 4 km in the downstream reach. The downstream reach was affected by a reduction in sediment supply and increase in stream power. The KPR cut through the uplift reach within roughly a decade; further significant flooding in the future will mainly cause widening instead of deepening of the channel. (C) 2012 Elsevier B.V. All rights reserved.
A comprehensive mechanics-based model to describe bedrock river erosion by plucking in a jointed rock mass
(2016-03)
Li, Kuo-Wei
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Pan, Yii-Wen
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Liao, Jyh-Jong
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土木工程學系
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Department of Civil Engineering
Plucking is an erosion mechanism usually occurring to sub-meter rock blocks in a bedrock channel subjected to intense water current. The removal of a rock block because of plucking can be either a result of impulsive plucking or accumulative plucking. A plate-like block is more likely to escape owing to impulsive plucking, while a pillar-like block is more likely escape because of accumulative plucking. The potential of impulsive plucking can be evaluated by examining the maximum uplift displacement within the period of the maximum upward impulse load acting on a typical rock block in a rock mass. The size of the rock block, various sources of uplift forces and the frictional resistance of lateral joints play major roles in determining whether accumulative plucking will occur. The present work proposes a mechanics-based model that simulates the kinematics of a rock block subjected to an inclined jet flow. A non-dimensional index APP is derived to assess the potential of accumulative plucking. In addition, a comprehensive approach is proposed to evaluate both the potential depths of the scour hole in a plunge pool and the incision in a downstream channel subjected to a prescribed jet flow passing a spillway or an overflowing weir. Finally, a case study adopting the proposed method is presented to demonstrate and to verify the applicability of the comprehensive approach.
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.
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.
Discriminant analysis of the geomorphic characteristics and stability of landslide dams
(2009-09-15)
Dong, Jia-Jyun
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Tung, Yu-Hsiang
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Chen, Chien-Chih
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Liao, Jyh-Jong
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Pan, Yii-Wen
Landslides can cause the formation of dams, but these dams often fail soon after lake formation. Thus, rapidly evaluating the stability of a landslide dam is crucial for effective hazard mitigation. This study utilizes discriminant analysis based on a Japanese dataset consisting of 43 well documented landslide dams to determine the significant variables, including log-transformed peak flow (or catchment area), and log-transformed dam height, width and length in hierarchical order, which affect the stability of a landslide dam. The high overall prediction power (88.4% of the 43 training cases are correctly classified) and the high cross-validation accuracy (86%) demonstrate the robustness of the proposed discriminant models PHWL (with variables including log-transformed peak flow, and log-transformed dam height, width and length) and AHWL (with variables including log-transformed catchment area, and log-transformed dam height, width and length). Compared to a previously proposed "DBI" index-based graphic approach, the discriminant model AHV - which uses the log-transformed catchment area, dam height, and dam volume as relevant variables - shows better ability to evaluate the stability of landslide dams. Although these discriminant models are established using a Japanese dataset only, the present multivariate statistical approach can be applied for an expanded dataset without any difficulty when more completely documented worldwide landslide-dam data are available. (C) 2009 Elsevier B.V. All rights reserved.
Landslide monitoring with interferometric SAR in Liugui, a vegetated area
(2019-08-01)
Shih, Peter Tian-Yuan
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Wang, Huijung
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Li, Kuo-Wei
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Liao, Jyh-Jong
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Pan, Yii-Wen
Interferometric SAR analysis with a time series of images obtained from satellite has been proven to be a useful monitoring scheme for geomorphological changes, particularly related to the vertical dimension. The success of this technique relies on the coherence between the images. This imposes severe challenges for applications in vegetated areas. In this study, the landslide process of a site located in Liugui, southern Taiwan, is investigated by a time series of SAR images collected from ALOS PALSAR and Sentinel 1A. This landslide monitoring site is heavily vegetated. From the Persistent Scattering (PS) analysis, ALOS PALSAR presented more extracted points. The wavelength of the band may be a decisive factor. The time series of ALOS images spans across the deadliest typhoon impacted Taiwan in recorded history, Typhoon Morakot on 8 August 2009. It is clear that there are more PS points extracted from the time series after the event as compared to before Typhoon Morakot. This is largely explained by the increase in the amount of bare earth after the disaster. From the interferometric SAR analysis, the development of headward erosion could be observed. Although the number of PS points is limited, the understanding of erosion and landslide progress in the area could still be improved while integrating with the digital elevation model collected with airborne lidar, historic aerial photos, and images collected with UAS (Unmanned Aviation Systems). Together with the in situ measurements, the colluvium depth ranges from 2 - 40 m in the area. The surface instability is expected to persist.
Large paleo-rockslide induced by buckling failure at Jiasian in Southern Taiwan
(1970-01-01)
Yang, Che-Ming
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Kang, Keng-Hao
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Yang, Shun-Hsing
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Li, Kuo-Wei
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Wang, Hui-Jung
The major regions of Taiwan consist of high-relief terrain exposed to frequent typhoons and earthquakes. Recently, thousands of potential large-scale landslides have been delineated via remote sensing analysis and field surveys. However, little attention has been paid to the stability of colluvium, which is a potential hazard. This study investigated a paleo-rockslide site in detail and proposes a geological model to explain the failure mechanism. The studied area is in Jiasian, Southern Taiwan, and the site is a dip slope consisting of Miocene sedimentary rocks. Detailed topographic features, including residual slip surfaces, platforms, scarps, and colluviums, are identified using the slope map of a 1- m airborne remotely sensed digital elevation model. Platforms beneath the residual slip surfaces represent the topographic features of the paleo-rockslide, an interpretation validated by field works and core analysis. The platforms consist of a displaced rock mass (DRM) with a gentle bedding dip angle and interbedded structure that has been preserved. A geological model is established according to the data compiled; outcrops of DRM and colluviums on bedrock were delineated in the geological map and profiles. Outcrops of shear off, buckling, and compressive ridges within the alternating sandstone and shale are distributed at the toe of the residual slip surfaces. A geological model with identified topographic features is adopted to establish the geomorphological evolution of the site. A buckling-induced rockslide model in the dip slope of the interbedded layers is proposed, and potential future slope failure scenarios can be evaluated using the geomorphological evolution model.
Lessons from a failure case of an excavated floodway supported by precast cantilever pile walls
(2016-07-15)
Cheng, Meng-Hsiung
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Huang, Ming-Wan
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Pan, Yii-Wen
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Liao, Jyh-Jong
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土木工程學系
The Gangweigou floodway, across the Quiren and Rende districts in Tainan, Taiwan, was designed to bypass partial discharge in the Gangweigou River during heavy rainfall events for the purpose of flood control. This flood way was an excavated open channel supported by single or double rows of precast RC cantilever pile walls on each side of its banks; these piles were installed by the jetting-assisted method. Twenty days after the completion of the floodway, it was put to work for the first time during a heavy rainfall. Unexpectedly, the cantilever pile walls in several sections failed and caused the collapse of the slope behind the walls. This paper explores the failure mechanism and the real causes of failure in this case. Based on the conditions of hydrology and hydraulics, failure processes and the results of stability analyses, it was concluded that ignoring a possible bed incision or bank erosion was likely the primary problem with the improper design. The most critical condition was when the water level in the floodway quickly dropped, which resulted in rapid drawdown. The failure of the case could have been avoided if both the rapid drawdown condition and the potential of the channel bed incision were considered. (C) 2016 Elsevier B.V. All rights reserved.
Logistic regression model for predicting the failure probability of a landslide dam
(2011-01-10)
Dong, Jia-Jyun
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Tung, Yu-Hsiang
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Chen, Chien-Chih
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Liao, Jyh-Jong
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Pan, Yii-Wen
Landslides may obstruct river flow and result in landslide dams; they occur in many regions of the world. The formation and disappearance of natural lakes involve a complex earth-surface process. According to the lessons learned from many historical cases, landslide dams usually break down rapidly soon after the formation of the lake. Regarding hazard mitigation, prompt evaluation of the stability of the landslide dam is crucial. Based on a Japanese dataset, this study utilized the logistic regression method and the jack-knife technique to identify the important geomorphic variables, including peak flow (or catchment area), dam height, width and length in sequence, affecting the stability of landslide dams. The resulting high overall prediction power demonstrates the robustness of the proposed logistic regression models. Accordingly, the failure probability of a landslide dam can also be evaluated based on this approach. Ten landslide dams (formed after the 1999 Chi-Chi Earthquake, the 2008 Wenchuan Earthquake and 2009 Typhoon Morakot) with complete dam geometry records were adopted as examples of evaluating the failure probability. The stable Tsao-Ling landslide dam, which was induced by the Chi-Chi earthquake, has a failure probability of 27.68% using a model incorporating the catchment area and dam geometry. On the contrary, the Tangjiashan landslide dam, which was artificially breached soon after its formation during the Wenchuan earthquake, has a failure probability as high as 99.54%. Typhoon Morakot induced the Siaolin landslide dam, which was breached within one hour after its formation and has a failure probability of 71.09%. Notably, the failure probability of the earthquake induced cases is reduced if the catchment area in the prediction model is replaced by the peak flow of the dammed stream for these cases. In contrast, the predicted failure probability of the heavy rainfall-induced case increases if the high flow rate of the dammed stream is incorporated into the prediction model. Consequently, it is suggested that the prediction model using the peak flow as causative factor should be used to evaluate the stability of a landslide dam if the peak flow is available. Together with an estimation of the impact of an outburst flood from a landslide-dammed lake, the failure probability of the landslide dam predicted by the proposed logistic regression model could be useful for evaluating the related risk. (C) 2010 Elsevier B.V. All rights reserved.
Mechanics and response of a surface rock block subjected to pressure fluctuations: A plucking model and its application
(2014-03-13)
Pan, Yii-Wen
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Li, Kuo-Wei
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Liao, Jyh-Jong
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土木工程學系
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Department of Civil Engineering
Plucking (removal of rock blocks) is often the dominant mechanism for producing a scour hole on riverbeds comprised of heavily jointed rocic masses being subjected to pressure fluctuations from a jet flow. This paper explores the mechanics and response of a surface block subjected to pressure fluctuations. First a particle-flow simulation was conducted to demonstrate how repeated pressure fluctuations are able to gradually remove rock bridges in discontinuities surrounding a rock block, if a pressure fluctuation's intensity is substantial. As a consequence, these weak planes may become fully persistent. The block's uplift speed then depends on the pressure differences on the opposite (horizontal) faces, and the frictional resistance of the lateral discontinuities. This paper proposes a theoretical framework to model the mechanics and response of a rock block subjected to a sinusoidal pressure fluctuation. This model can be applied to estimate the development of a scour hole, through plucking, during a specific flood event. An example demonstrates the applicability of the proposed approach in predicting the potential depth of the scour hole. (C) 2013 Elsevier B.V. All rights reserved.
Rapid channelization and incision into soft bedrock induced by human activity - Implications from the Bachang River in Taiwan
(2014-07-22)
Huang, Ming-Wan
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Liao, Jyh-Jong
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Pan, Yii-Wen
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Cheng, Meng-Hsiung
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土木工程學系
Many Taiwanese river channels have been affected by human activity, such as engineering works and gravel mining. Environmental conditions such as high seasonal precipitation and young sedimentary bedrock exaggerate the narrowing and incising of river channels. The Bachang River is located in southwestem Taiwan, and the 11-km-long channel at its midstream reach has been incised to a maximum depth of approximately 30 m within several decades. Therefore, the morphologic evolution of the Bachang River in southwestern Taiwan provides an illustration of channel changes in response to human activity. The historical data used for this analysis of morphology change include one set of topographic maps and six sets of aerial photographs from the past 100 years. The channel changes are analyzed qualitatively and quantitatively and include the channel planform, longitudinal profiles, channel incision, channel width, and channel cross sections. The results indicate that remarkable channel changes have occurred since the 1980s as a result of human influence. The 11-km-long study reach transformed from an alluvial-type channel to a gorge-type channel within several decades. The maximum accumulated depth of the channel incision is approximately 30 m, with a meter-scale annual average incision rate; the maximum width has been decreased to approximately one-sixth of the original width (448 m). The process of channel evolution is divided into four stages, and we have concluded that the causes of channel change include six factors from two categories: natural factors that include geological conditions, hydrological conditions, and the process of bedrock erosion; and human factors that include gravel mining, lateral structures, and levees. The initiation of channel evolution is triggered by human factors. Finally, we discuss the potential future channel evolution and lessons learned from the case study. (C) 2014 Elsevier B.V. All rights reserved.
設置攔河堰引致岩盤沖刷之機制與評估研究(2/2)
(2010)
潘以文
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Pan, Yii-Wen
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國立交通大學工學院防災工程研究中心
攔河堰之設置因改變河川坡度，造成上游泥砂淤積，下游水流能量增加而引致局部沖刷或形成沖刷坑。傳統之沖刷議題係著眼於河床顆粒材料而未及於岩盤，國內、外對於攔河堰下游的岩盤沖刷機制及評估方法尚未有具體有效之研究成果。而岩盤之沖刷具有快速及不可逆之特點，近年如集集堰及石岡壩等皆有岩盤沖刷之情勢，增加管理單位之困擾。本計畫分兩個年度針對岩盤之沖刷力學機制進行研究，進而提出可行之沖刷深度評估方法以及研擬抗沖蝕穩定工法，以供規劃設計單位參考，防範相關問題於未然。延續第一年度攔河堰下游岩盤抗沖蝕能力調查、沖刷機制及抗沖蝕能力探討、沖刷深度評估方式探討等初步成果，本年度將探討影響攔河堰下游岩盤沖刷機制之必要條件、提出岩盤沖刷深度評估方法，最後對現有及未來的攔河堰下游岩盤沖刷穩定策略提出建議。