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The Minbaklu Bridge, an essential segment of the southern cross-island expressway in Taiwan, was destroyed owing to debris flow in the Yusui Stream. To elucidate the development of debris flow in the Yusui Stream and the bridge collapse event, this study proposed a hybrid model of the discrete element method and depth-averaged model to simulate the event through three stages: formation of the natural dam, debris flow triggered by dam braking, and bridge collapse owing to the debris flow. The simulation results obtained indicate that the proposed model can provide reasonable simulations of the formation of natural dam in both time line and spatial distribution. Further, it indicated that the erosion developed at the river bed of the flow region, and illustrated the failure process of the Minbaklu Bridge, which revealed that it was uplifted before being pushed away. Furthermore, the final deposition of the alluvial fan deposition was demonstrated as well. Overall, the proposed hybrid model provides a useful tool for investigating debris flow and interaction with engineering structures. [ABSTRACT FROM AUTHOR]

On August 7, 2021, a disastrous rainfall-induced landslide occurred at Mt. Silabaku in the Yusui Stream source area, Taiwan. The huge deposits from the Silabaku landslide severely damaged the Minbaklu Bridge, and minor collapses in the Yusui Stream disrupted the only major road, Expressway No. 20. More than 400 residents from the three adjacent villages were stranded for 19 days in the mountainous area. The analysis of rainfall data, seismic records, topographic and geologic data, and historical orthoimages of the slope conditions, landform evolution, and current status of the Silabaku landslide revealed that the landslide resulted from slate avalanche induced by heavy rains, with local failures during Typhoon Morakot in 2009. Based on the high-resolution topographic data of light detection and ranging and an unmanned aerial vehicle, the debris deposits accumulated along the Yusui Stream were estimated to be above 8 million m3. In addition, a fan volume of 2 million m3 changed the flow line of the Laonong River, and the thick deposits pose a considerable threat to the safety of the nearby villages and infrastructures. [ABSTRACT FROM AUTHOR]

Outburst flooding after a landslide dam breach causes global fatalities and devastation. Information on the timing, magnitude, and location of the landslide dam is crucial to hazard assessment. Despite recent efforts, successful real-time detection of landslide dams in mountain valleys and dam breakages is rare. Here, we present a series of seismic analysis including landslide detection, identification of landslide dam formations, and monitoring of dam breaches. We show the working of our analysis on a recent landslide dam that occurred in eastern Taiwan. The results indicate that our seismic analysis provides important information on the location and magnitude of landslides and the dam forming based on data acquired from a regional broadband seismic network. Furthermore, we see that the failure of the landslide dam is directly caught by the riverside seismic signals. To provide warning times for impending floods to downstream areas, we believe that proximal high-quality seismic signals along the river channel are viable options for an operational real-time monitoring system, for landslide dams occurring in mountain valleys. Our work can be a starting point to raise awareness in the community. [ABSTRACT FROM AUTHOR]

Mega-earthquakes and extreme climate events accompanied by intrinsic fragile geology lead to numerous landslides along mountain highways in Taiwan, causing enormous life and economic losses. In this study, a system for rapid slope disaster information integration and assessment is proposed with the aim of providing information on landslide occurrence, failure mechanisms, and subsequent landslide-affected areas to the highway authority rapidly. The functionality of the proposed system is deployed into three units: (1) geohazard rapid report (GeoPORT I), (2) multidisciplinary geological survey report (GeoPORT II), and (3) site-specific landslide simulation report (GeoPORT III). After landslide occurrence, the seismology-based monitoring network rapidly provides the initial slope disaster information, including preliminary location, event magnitude, earthquake activity, and source dynamics, within an hour. Within 3 days of the landslide, a multidisciplinary geological survey is conducted to collect high-precision topographical, geological, and remote-sensing data to determine the possible failure mechanism. After integrating the aforementioned information, a full-scale three-dimensional landslide simulation based on the discrete element method is performed within 10 days to reveal the failure process and to identify the areas potentially affected by subsequent disasters through scenario modeling. Overall, the proposed system can promptly provide comprehensive and objective information to relevant authorities after the event occurrence for hazard assessment. The proposed system was validated using a landslide event in the Central Cross-Island Highway of Taiwan. [ABSTRACT FROM AUTHOR]

Newmark displacement analysis (NDA) has been widely used to study the initiation and kinematics of earthquake-induced landslides. The classical NDA for earthquake-induced wedge slides assumes that the wedge is a rigid block (assumption A1). The vertical and horizontal accelerations perpendicular to the sliding direction are neglected (assumptions A2 and A3). Moreover, the friction angles (friction coefficients) of the sliding surfaces are assumed to be constant during sliding (assumption A4). This study evaluates the influences of these four assumptions on the kinematics of huge wedges adopted, where the shear stress could not be identical to the sliding direction. Different rock wedge shapes subjected to synthetic seismic loads are modeled, and the sensitivity of removing unnecessary assumptions from the analysis is evaluated. A case study of the earthquake-induced Daguangbao landslide (wedge slide) is provided to assess the influence of the rigid wedge assumption and adopted strengths along wedge planes on the sliding kinematics. Our study shows that the rigid wedge method (RWM) incorporated into NDA (A1) always yields an unconservative prediction of permanent displacement comparing to maximum shear stress method (MSSM), particularly for huge, narrow wedges. The MSSM, which assumes that the shear stress perpendicular to the sliding direction equals the shear strength, should be considered owing to wedge deformation if huge rock masses are involved. Neglecting the vertical acceleration (A2) induced an underestimation of permanent displacement, particularly for narrow wedges with a gentle plunge of the intersection line. Although horizontal acceleration perpendicular to the trend of I-Line does influence the NDA results for asymmetrical wedges, the effect (A3) is insignificant. Assuming a constant strength on the wedge plane (A4) only predicted stick-slip behavior but not a frequently observed high-mobility phenomenon of huge landslides. Finally, the rapid and long run-out sliding of the Daguangbao landslide can be successfully predicted if the velocity-displacement dependent friction law is appropriately incorporated into NDA using MSSM. • Assumptions of the classical Newmark displacement analysis for a wedge slide are evaluated. • Considering inertial force and applying seismic loads on the base of the sliding wedge. • A deformable wedge incorporating velocity-weakening strength yields higher mobility. • Neglecting vertical acceleration yields underestimated permanent displacement. • Remove unrealistic assumptions; the kinematics of the Daguangbao landslide can be predicted. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Landslides have caused extensive infrastructure damage and caused human fatalities for centuries. Intense precipitation and large earthquakes are considered to be two major landslide triggers, particularly in the case of catastrophic landslides. The most widely accepted mechanistic explanation for landslides is the effective-stress dependent shear strength reduction due to increases in pore water pressure. The Chashan landslide site, selected for the present study, has been intensively studied from geological, geophysical, geodetic, geotechnical, hydrological, and seismological perspectives. Our seismic monitoring of daily relative velocity changes (dv/v) indicated that landslide material decreases coincided with the first half of the rainy period and increased during the latter half of the rainy period. The geodetic surveys before and after the rainy period identified vertical subsidence without horizontal movement. The results from the multidisciplinary investigation enabled us to draw a conceptual model of the landslide recovery process induced by water loading. Where all sliding materials were stable (safety factor > 1.0), unconsolidated landslide colluvium and impermeable sliding surfaces trapped the seepage water to form a water tank, provided that compact forces were acting on the materials below the sliding boundary. The vertical force of compaction facilitates an increase in the cohesion and strength of landslide materials, thereby increasing the landslide materials' stability. We demonstrated that the recovery process periodically occurs only under the combined conditions of prolonged and intense precipitation and the related stability conditions. [ABSTRACT FROM AUTHOR]

The 2024 Hualien earthquake on April 3, eastern Taiwan, with a magnitude of 7.2, resulted in 18 fatalities and 1,155 injuries. Road networks suffered extensive damage, with 55 roadside landslides exacerbating the disruption. Preliminary mapping identified over 3,239 landslides with sizes ranging from 62 to 524,113 m2, primarily clustered in areas experiencing peak ground acceleration exceeding 250 cm/s2. Four landslide dams were formed post-event, including one near a hydropower station. The following impacts heighten concerns about hazards such as frequent unstable slope failures, debris flows caused by intense rainfall, and subsequent coseismic rockfalls occur due to strong aftershocks. [ABSTRACT FROM AUTHOR]

Heavy rainfalls or strong earthquakes may induce catastrophic landslides and affect the topographic evolution in a mountainous region. The major parts of Taiwan are high relief terrains exposed to frequent typhoons and earthquakes. Recently, thousands of large-scale potential landslides have been delineated via remote sensing analysis and field survey. However, not much attention has been paid to the slope-failure recurrence of the colluvium resulted from paleo landslides. This study aims to investigate a large-scale paleo landslide site in detail. The studied slope locates near Jiasian in southern Taiwan. The slope is a dip slope consist of Miocene sedimentary rocks. Site investigation includes topographic interpretation, field geology, geophysics tests and borehole exploration. A geologic model was established according to the compiled information; outcrops, paleo displaced rock masses and colluviums covered on bedrock were delineated in the proposed model. Detail geomorphic features including scarps, displaced rock masses, residual slip surfaces and colluviums were identified using the data of 1m- resolution airborne LiDAR. The geologic model together with the identified geomorphic features can be used for the evaluation of slope stability. [ABSTRACT FROM AUTHOR]

Ancient coseismic landslides indicate prehistoric earthquakes and may also be a measure of their intensity. Their identification in the landscape remains, however, challenging. We used field geologic observations of source areas of recent rainfall-induced and earthquake-induced landslides to define a morphometric index that can distinguish between these two types of triggers. Morphometries of 129 landslides from tropical, subtropical and temperate zones developed in a variety of bedrock types were described by using the following parameters: i) depletion index (I d), i.e. the ratio between the length of the depleted part and the length of the source area; ii) relative slope height (H rel) and iii) source area angle (φ S), i.e. the angle between the top of the line connecting the scarp top with the toe of the source area and the horizontal plane. They were obtained using globally available digital elevation models and satellite images. Their combination into an Index of Potential Trigger (I PT) successfully distinguished between the two types of landslide triggers with an overall accuracy of 87%. The rainfall-induced landslides had a significantly lower average I PT value of 0.01 as compared with 0.37 for the coseismic landslides. The index of potential trigger may be a useful and readily implemented morphometric tool for preliminary identification of prehistoric coseismic landslides recorded in the current topography. • A tool for coseismic landslide recognition is presented • The landslide source areas were used to distinguish triggering mechanisms • Global DEMs and satellite images were used to derive Index of Potential Trigger • Effective morphometric tool also for prehistoric landslide events [ABSTRACT FROM AUTHOR]

Earthquakes can generate seismic disturbances that propagate vast distances and trigger landslides that can achieve high-speeds. It remains difficult to identify the co-seismic ground motion of these landslides and their triggering earthquakes. In this paper, we report on the analysis of co-seismic ground motions generated by the initiating fracture and deposition impact of the Tsaoling landslide. The landslide, with a source volume of 125 × 10 6 m 3 , was triggered by the 1999 Chi-Chi earthquake in Taiwan. The ground motion was recorded by a strong ground motion station, CHY080, near the scar area. The polarization of the seismic waves indicates that the peak acceleration was parallel to the dip direction. Modified ensemble empirical mode decomposition (EEMD), with additional clustering analysis, was applied to decompose the seismic signals. Two instances were found in the seismic records of a series of peculiar wave packets, with the first being associated with the landslide initiation and the second the landslide impact on the deposit valley. To confirm the first landslide breakage, the decomposed signals were compared with the predictions of the analytic elastic wave model and Newmark analysis. The landslide impact was verified with a computational fluid dynamic simulation. Comparison between the EEMD decomposed signals, elastic wave theory, Newmark rigid body analysis, and numerical simulation demonstrates the claimed landslide motion. [ABSTRACT FROM AUTHOR]