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Abstract: Typhoon Mindulle was the most severe typhoon across the Tachia River watershed following the 921 Chi-Chi earthquake, and resulted in extremely high precipitation of 1431mm that induced many landslides, debris flows, and debris floods in affected areas. This paper analyzes the debris flows in the Songhe area induced by Typhoon Mindulle by employing a numerical model for debris flow simulation. The FLO-2D numerical program was adopted to simulate the flow conditions of debris flows in the Songhe area, including flow depths, flow velocities, and sediment depositions. Comparing the field data with simulation results, this paper defines an Index of Accuracy to examine simulation accuracy. Ignoring the influence of houses in the alluvial fans, the simulation accuracy was 79.3% and 61.4%, respectively for the First and Second Branches of the Songhe Stream. Considering the house effect in the Songhe area raised the simulation accuracy to 82.3% and 75.5%, respectively for the First and Second Branches. In addition, the simulation considering the hydraulic structures established after Mindulle was executed. This study also adopted 24h accumulated precipitation at various recurrence intervals (10, 100, and 500years) to perform scenario simulation of debris flows. Based on the method of Swiss Disaster Degree Classification, consisting of influential intensity and occurrence probability parameters, this study classified the risk degree of hazardous debris-flow areas into three categories, including high, medium, and low. The proposed approach generates the risk distribution map that may be used for setting up a disaster mitigation strategy for the Songhe area. [Copyright &y& Elsevier]

Abstract: Typhoon Mindulle struck northeast Taiwan on July 2, 2004, and brought an abundant air current in the following days that caused serious flood disasters to the mountainous areas in central Taiwan. One of sever flood events during Typhoon Mindulle was about hundreds of hectares of farmland loss induced by an 1620m break of Shan-An levee at the Chenyulan River. This paper examines the nature and causes of Shan-An flood by integrating catastrophic investigation and scenario simulation. The catastrophic investigation of the disaster scene, including field survey, outcrop investigation, and remote sensing image interpretation, was executed to reveal the inundation for disaster response and recovery. SPOT-IV and FORSAT-II satellite images and airborne images provided temporal and spatial information for inundation investigation. The HEC-RAS hydraulic model was applied to estimate the water surface profiles of Shan-An levee around the lower reach of Chenyulan River. Also, the impacts of debris and water in the hyper-concentrated sediment flow were estimated to illustrate the effects of sediment yield on Shan-An levee. The result shows that the levee was designed to sustain the heavy rainfall bought by Typhoon Mindulle but failed to the impact of high concentrated sediment flow induced by sediment yield from the landslide. The seismic-triggered landslide on the left riverside was reactivated by intensive rainfalls to deposit a great amount of sediments on the riverbed that narrowed down the river width, increased the impact force, and diverted water toward Shan-An levee. The levee failure should draw more attention from engineers while designing a new flood control structure in the waterway of Chenyulan River due to the occurrence of the hyper-concentrated sediment flow and landslide residuals after the Chi-Chi earthquake (ML 7.3). In addition, the perceived investigation and analysis of an earthen levee failure provides essential information to setup a proper recovery plan of Shan-An levee and a basin management strategy of Chenyulan River in the future. [Copyright &y& Elsevier]

Abstract: In this paper, technologies from multiple disciplines are used to reconstruct the shape of the Hsiaolin landslide dam, a short-lived landslide dam (SLD), that was triggered by Typhoon Morakot. Here, the formation, failure mode and breaching process of this SLD are investigated. The results indicate that the overtopping time and the debris budget constrained the dam geometry. The inferred volume of the Hsiaolin landslide dam (15.4millionm3) is much smaller than the actual landslide volume (25.2millionm3) even considering the rock-mass dilation and debris entrainment. Meanwhile, a 46% error could be induced if an over-simplified equation (a function of the dam height, length and width) is used to calculate the dam volume. The saddle of the dam crest, which is where overtopping occurred, could be used as a reference to measure the dam height and length. Accordingly, the determined dam dimensions of height (44m), length (370m) and width (1500m) are suggested to be the representative geometrical indices that influenced the stability of the Hsiaolin landslide dam. The flow rate of the dammed river, another variable relevant to dam stability, was determined from a run-off simulation. It is suggested that, instead of the peak flow before the dam formation, the flow rate during the blockage period (2974m3/s) should be used as the training data for building a statistical model for stability predictions. Finally, the low hydraulic gradient at the toe of the dam''s surface and the high safety factor of the dam slope indicate that the piping and slope instability were irrelevant to the failure of the Hsiaolin short-lived landslide dam. It is postulated that overtopping (about one hour after the blockage) dominated the failure process of this heavy-rainfall-induced landslide dam. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to present data on the occurrence of landslides triggered by typhoon and earthquake events, and to discuss the basic earthquake triggering mechanisms involved. Four events of typhoon and earthquake triggered landslides in the Chenyoulan River catchment of central Taiwan during 1996 to 2004 were studied in order to identify their controlling factors. The landslides areas were measured by comparing aerial photos taken over the past 18 years, and sediment discharge was counted at hydrometric stations. Investigation results demonstrate that hillslope geomaterials that had been disturbed by strong seismic shaking induced expansion of landslide areas during subsequent events. Analysis of Normalized Difference Vegetation Index (NDVI) also registered the changes of uncovered conditions on the hillslope caused by landsliding. When flow discharge is lower than 100 m3 s−1, post-seismic sediment concentration is roughly four times higher than pre-seismic concentration with the same flow discharge. This fate indicates that the amount of sediment discharge at normal flow discharge is affected by supplied sediment volume, which tends to increase with seismic activity. [Copyright &y& Elsevier]