Introduction

Taiwan is in an active mountain belt created by the oblique collision between the northern Luzon arc and the Asian continental margin. The inherent complexities of geological nature has created relatively steep hillside slopes and discontinuities prevailing in rock masses on the island. In recent years, the increasing frequency and intensity of major natural events is believed to be closely related to global warming and climate changes. After the 1999 Chi-Chi earthquake, especially, detrimental landslides, debris flows and floods triggered by heavy rainfalls from the Toraji, Mindulle , Aere and Morakot  typhoon events were remarkably severe in both scale and magnitude.

Efforts were initiated therefore to expedite the progress of flood prevention and to improve the life quality of local residents by studying and strengthening the geo-hazard mitigation starting from the upstream watersheds of flood-prone areas. The aims of the study were to utilize remote sensing data, historical events of landslide hazards, geological drilling and field testing in an integral investigation so that the hydrogeologic characteristics of the watersheds can be better understood. It is expected that through the study of the hydrogeological investigation and numerical model simulation, the complex relationship between the landslide mechanisms and hydrogeological conditions can be greatly clarified. Finally, upon completion of the project in the end, a designated database will be established with all project investigation data and integrated results.

Project Contents

For the study of regionalinvestigation and analysis of hydrogeology on slope stability, we have finished (1) rainfall frequency analysis, (2) surface water and groundwater analysis, (3) investigation of hydrogeological characteristics by geological drilling, (4) investigation of soil thickness, (5) isotope composition analysis, and (6) analysis of the influence of hydrogeology on rainfall induced landslides. All of the investigation data were adopted to study the regional rainfall-induced shallow landslides by using the Transient Rainfall Infiltration and Grid-based Slope-stability (TRIGRS) model that couples an infinite-slope stability analysis with a one-dimensional analytical solution for rainfall infiltration. It is expected that with the use of GIS and RS (remote sensing) techniques the rainfall thresholds for landslides during can be improved and enhanced in the study areas.

For the study of active landslide investigation, monitoring, and modeling, we have finished (1) landslide field investigation, testing, monitoring, and analysis, (2) establishment of conceptual model of landslides, (3) analysis of the influence of hydrogeology on rainfall induced landslides, and (4) completion of the hydrogeological and environment geological maps of the active landslides. Field studies, analysis, and monitoring directly at reducing landslide hazards and risk with emphasis on rainfall induced landslide were conducted. For the site characterization, borehole drilling and downhole testings including borehole acoustic televiewer and double packer system to the study of hydraulic properties of fractured rocks were conducted for the hydrogeological investigation of active landslides. The double-ring infiltration test and laboratory tests were also made to obtain the mechanical parameters for improving understanding of landslide behaviors. For the study of active landslide analysis, two-dimensional and three-dimensional analyses were carried out. To improve understanding of active landslide triggering mechanisms especially focused on the rainfall induced landslide, modeling and analysis using two dimensional unsaturated numerical program, Geostudio, and three dimensional numerical program, FLAC3D, were made. It is expected that the landslide initiation, enlargement, or reactivation can be predicted through the physically based models. Finally, the investigation data, case study data, and analysis data will be integrated and then established into the database.