Kun-Ting Chen

In recent years, frequent heavy rainfall and strong earthquakes have caused numerous landslides, debris flows, and barrier lake disasters in Taiwan. This study applies terrain analysis on real landslide barrier lakes to develop an interpretation method for the potential location of barrier lake induced by a massive landslide. This study applies GIS and a 1 m-resolution DEM to analyze landslide cases. First, a microtopographic feature database for a massive landslide is established, from which types of large landslide masses are identified. Next, the length and width boundaries of the landslide body are determined. The SLBL sliding-surface estimation method proposed by Jaboyedoff et al. (2019) is then applied, assuming the landslide scarp and the outcrop point as the start and end boundaries of the sliding surface, and using a quadratic parabolic equation to infer the intermediate failure surface. Finally, the cross-sectional areas of the segmented transverse profiles along the longitudinal sliding surface are calculated to estimate the landslide volume. After obtaining the landslide volume, the equivalent friction method is used to estimate the horizontal transportation distance of the landslide mass, in order to assess whether the landslide debris can reach the opposite river bank and form a landslide dam that blocks the river. Subsequently, the model proposed by Chen et al. (2014) is applied to infer the dam length. Together with the dam length, water depth, and channel width are used to calculate the minimum blocking volume. Last, by comparing the actual landslide volume with the minimum blocking volume, it is then determined whether a landslide lake can be formed. 25 landslide barrier lake events in the Gaoping River Basin, Taiwan, were used to test the interpretation method for potential locations. This interpretation method can serve as a predictive tool for identifying potential landslide barrier lake sites, thereby reducing the impact of disasters.

Landslide dam hazards have become one of the disaster types of increasing global concern. Once a landslide dam is formed, its typically short lifespan, high uncertainty, and sudden failure can pose severe threats to protected targets both upstream and downstream. The ability to identify locations within landslide-prone areas where landslide dams are likely to form would substantially enhance disaster prevention and mitigation capabilities. This study focuses on historical landslide events and landslide-dammed lake cases. Satellite imagery is employed to quantify key geomorphic parameters, including landslide elevation, slope gradient, and channel width. Both the longitudinal accumulation characteristics along the river channel and the lateral mobility of the landslide mass are considered. Through nondimensional analysis, this study seeks to identify quantitative threshold values linking historical landslide characteristics to landslide dam formation. These thresholds are used to preliminarily assess the likelihood of landslide dam formation in potential landslide hazard areas and to further delineate locations with dam-formation potential. The results are expected to provide a scientific basis for improving early warning systems and disaster prevention and mitigation planning related to landslide dams.

Wild Streams in Taiwan are characterized by steep channel slopes, rapid flow velocities, short channel lengths, and small watershed areas. These streams are highly susceptible to severe compound flood and sediment disasters triggered by typhoons and intense rainfall. To safeguard human life and property in surrounding areas, river management has historically emphasized engineering safety and disaster mitigation. Consequently, ground sills were widely installed to stabilize riverbeds and reduce channel erosion, while the potential ecological impacts of such engineering measures were largely overlooked, often resulting in severe degradation of river ecosystems. With the extensive installation of ground sills, river environments have become increasingly homogenized. The height configuration of the structures can alter hydraulic conditions and longitudinal connectivity, as well as modify the natural diversity of flow types (e.g., shallow flow, riffles, runs, and deep pools), leading to a decline in biodiversity. As a result, aquatic ecosystems and fish habitats have been significantly disturbed, causing degradation of river ecological functions. This has prompted efforts to restore and rehabilitate rivers whose ecological functions have been degraded by human interventions.The object of investigation in this study was the Tengqiao Creek and the Two-Dimensional Habitat Diversity Construction Model was applied to simulate and investigate changes in fish habitat types and the spatial distribution under different discharges and varying heights of ground sills. Habitat diversity was quantified using the Shannon index as the Habitat Diversity Index (HDI). The results indicate that, under all discharge conditions and ground sill heights, habitat types 1(shallow pool) and 2 (medium pool) account for the largest proportion of habitat area within the study reach. When discharge is lower than 0.04 cms and the ground sill height is below 0.5 m, habitat type 3 (deep pool) does not occur; the area of habitat type 3 increases with increasing structure height. The area proportions of habitat types 4 (slow riffle), 5 (fast riffle), and 6 (run) are considerably smaller than those of types 1 and 2, with type 4 occupying a relatively larger area than types 5 and 6. When the ground sill height exceeds 0.75 m, the areas of habitat types 4, 5, and 6 are significantly smaller than those observed under structure heights below 0.75 m. Across all flow conditions, the habitat diversity index increases with increasing discharge but exhibits a decreasing trend as the height of ground sills increases. The findings of this study provide valuable references for future river restoration and ground sill design.

This study proposes a novel assessment model to evaluate the occurrence potential of channelized debris flows (CDFs) in sedimentary rock regions of Central and Southern Taiwan, with a particular emphasis on the characteristics of hillslope debris flows (HDFs) within watersheds. CDFs are significantly related to the occurrence of HDFs in the upper reaches of watersheds, suggesting a high correlation between the potential for both phenomena. The study initially developed a hillslope debris flow (HDF) recognition model utilizing Fisher’s discriminant analysis, based on data from 40 HDF events and 40 landslide events. This model was subsequently applied to identify HDF units within channelized debris flow (CDF) watersheds. Subsequently, a CDF potential assessment model was constructed using data from 32 streams, which included 16 CDFs and 16 non-debris flow streams. Two key indicators emerged as the most effective: “Total WA(_>8)” and “number of WA(_>10).” These indicators achieved an accuracy rate exceeding 84%, significantly outperforming the official assessment model, which had an accuracy rate of 60%. The newly developed assessment models offer substantial improvements in predicting CDF occurrences, enhancing disaster preparedness and sustainable environment development.

Compared with debris flows in other areas, debris flows in scenic areas not only seriously threaten residents, tourists, roads, walkways, and other infrastructure, but also cause considerable damage to the landscapes and ecosystems of these areas. Extreme rainfall events in the future will increase the complexities and challenges involved in debris flow control in scenic areas. Currently, the systematic planning of the entire scenic area is not considered in the treatment of debris flows. It is not possible to realize the rapid planning of any debris flow gully control project in a scenic area and to quantify the volume of debris flow material retained by each engineering structure. Based on field investigations and data collected from debris flow control projects in gullies in Jiuzhaigou Valley, China, an engineering planning method for debris flow control projects in scenic areas is herein proposed, and the challenges confronting existing control projects in scenic areas are discussed. Moreover, based on the example of Jiuzhaigou Valley, corresponding control engineering schemes for debris flow gullies in Xiajijie Lake Gully, Zhuozhui Gully, Xuan Gully, Pingshitou Gully, and West-Zhuozhui Gully are formulated. Four control modes for debris flow disasters in scenic areas are proposed, namely, “blocking + deposit stopping,” “deposit stopping,” “blocking,” and “drainage + deposit stopping,” which provide a systematic control strategy for post-earthquake debris flow disaster management in Jiuzhaigou Valley and other similar scenic areas.

Landslide dam formation can be influenced by the erosive capacity of river flow and the dynamic characteristics of the landslide. When the deposition rate of a landslide that reaches a river is higher than the erosion rate of river flow, the landslide can form a dam by blocking the channel. Hence, in this paper, a dimensionless discharge threshold for landslide dam formation considering landslide and river dynamics is established and studied numerically. A two-layer depth-averaged model coupled with an erosion term is presented to simulate river and landslide movements and their interactions. Several numerical cases are simulated to study the influence of landslide and river dynamics on the critical threshold for dam formation by considering some key factors, such as landslide velocity and the angle between the river and landslide transport directions. Through the simulations, three types of landslide intrusion into river can be reflected: a dam forms quickly, a dam forms or does not form close to a critical state, and no dam forms. The results show that these factors together affect the process of dam formation if the difference between the landslide and river discharges is relatively small. All results are helpful to further clarify the formation of such dams for natural hazard prevention under future climate change conditions.

On 8 August 2017, an Ms 7.0 earthquake struck Jiuzhaigou Valley, triggering abundant landslides and providing a huge source of material for potential debris flows. After the earthquake debris flows were triggered by heavy rainfall, causing traffic disruption and serious property losses. This study aims to describe the debris flow events in Zechawa Gully, calculate the peak discharges of the debris flows, characterize the debris flow disasters, propose mitigation countermeasures to control these disasters and analyse the effectiveness of countermeasures that were implemented in May 2019. The results showed the following: (1) The frequency of the debris flows in Zechawa Gully with small- and medium-scale will increase due to the influence of the Ms 7.0 Jiuzhaigou earthquake. (2) An accurate debris flow peak discharge can be obtained by comparing the calculated results of four different methods. (3) The failure of a check dam in the channel had an amplification effect on the peak discharge, resulting in a destructive debris flow event on 4 August 2016. Due to the disaster risk posed by dam failure, both blocking and deposit stopping measures should be adopted for debris flow mitigation. (4) Optimized engineering countermeasures with blocking and deposit stopping measures were proposed and implemented in May 2019 based on the debris flow disaster characteristics of Zechawa Gully, and the reconstructed engineering projects were effective in controlling a post-earthquake debris flow disaster on 21 June 2019.

In this study, we develop a dimensionless assessment method to evaluate landslide dam formation by considering the relationship between the run-out distance of a tributary debris flow and the width of the main stream, deposition thickness of the tributary debris flow, and the water depth of the main stream. Based on the theory of debris flow run-out distance and fan formation, landslide dam formation may result from a tributary debris flow as a result of two concurrent formation processes: (1) the run-out distance of the tributary debris flow must be greater than the width of the main stream, and (2) the minimum deposition thickness of the tributary debris flow must be higher than the in situ water depth of the main stream. At the confluence, one of four types of depositional scenarios may result: (1) the tributary debris flow enters into the main stream and forms a landslide dam; (2) the tributary debris flow enters into the main stream but overflow occurs, thus preventing complete blockage of the main stream; (3) the tributary debris flow enters into the main stream, does not reach the far bank, and sediment remains partially above the water elevation of the main stream; or (4) the tributary debris flow enters into the main stream, does not reach the far bank, and sediment is fully submerged in the main stream. This method was applied to the analysis of 11 tributary debris flow events during Typhoon Morakot, and the results indicate that the dimensionless assessment method can be used to estimate potential areas of landslide dam formation caused by tributary debris flows. Based on this method, government authorities can determine potential areas of landslide dam formation caused by debris flows and mitigate possible disasters accordingly through a properly prepared response plan, especially for early identification.