Hazardous landslides from 1996 to 2004 in the Chenyoulan River basin of central Taiwan are studied in order to weigh the impact of their controlling factors and to identify their main trigger. The study shows that small landslides contributed a significant proportion of the total area disturbed in the Chenyoulan River basin and that, after earthquakes, > 80% of the landslides remained confined to hillslopes. Rainstorms afterwards can deliver huge hillslope masses into the river, which can lead to a 13-fold increase in average annual sediment discharge. We believe that the landslide effect after earthquakes continues to expand in the understanding that landslide rates continue to rise when rates of newborn landslides display a saturation condition.
Debris flow hazards are not always easily recognized, particularly on fans that are subject to high magnitude, low-frequency events. Debris flow professionals hear the phrase “I have been living here for so many years and the creek has no debris flow come over the banks”. This statement has often been proven incorrect after detailed investigations. Debris flow hazard recognition is therefore the first, and possibly most important, step in the debris flow hazard analysis.
The remote sensing method has the advantage to investigate vast areas of debris flow events indoor in short time. So it is more cheaper and efficient to evaluate debris flow hazard than other method. In general, stereo aerial photographs is the most common tool for inventories of debris flow due to its higer spatial resolution. Spaceborne multispectral imagers with spatial resolutions of only a few meters have recently been launched, satellite images have become increasingly useful for debris flow delineation.
This paper intend to present the utilization of remote sensing images for recognization, interpretation and analysis of debris-flow. A case study of debris flow triggered by Toraji typhoon in 2001 in Bei-Shih river basin was introduced.
Initiation of a debris flow depends on basic factors and triggering factors. Basic factors are those of geological, geometrical, and hydrological conditions that are close related to a site condition. In order to correlate the relation, research has been conducted for many years in various zones of Taiwan including the Hualian area in eastern Taiwan, the Nantou area in central Taiwan, and the Taipei area in northern Taiwan. It is hoped to find out significant influential factors for debris flow initiation so that the results can be used for future mitigation.
In addition to that, a simplified model to examine debris flow types is proposed. The parameters used in the model are obtained from many soil tests conducted in the past years. The model has the merits of simplicity, being reasonable accurate, and having physical meaning. It will also provide references for designing preventive structures as well as zoning the area influenced by debris flows.
The geology, topography and meteorological conditions in Taiwan provide the essential condition inducing debris flows. The Soil and Water Conservation Bureau identified 485 potential debris flow torrents in Taiwan in 1996, but the number increased to 1420 after typhoon Toraji in 2001. It appeared that follow-up investigation of the debris flow torrents was required due to variations of topography and other conditions. For such a large number of debris flow torrents, it was also necessary to analyze the occurrence potential as well as to assess the priority of mitigation of the potential torrents in order to provide the decision support for hazard mitigation. In this paper, the field investigation of debris flow torrents and the potential analysis were performed, where the potential analysis was based on the potential factors extracted from ground features documented in the database. Model with risk and mitigation factors was established and then combined with the potential evaluation to provide assessment of mitigation priority. The potential model was tested and adjusted, and the final results were verified using historical events. The potential analysis and final assessment of mitigation priority could be provided as decision supports for drafting of hazard mitigation policy.
The purpose of this paper is to establish a method for assessing the risk of debris flow hazard-prone streams. Debris flow risk assessment comprises hazard assessment, vulnerability analysis and capacity assessment. Firstly, hazard assessment can be estimated from gully density, basin areas with slope larger than 15 degree, basin relief, landslide ratio and mainstream length. For the vulnerability analysis, this research estimates the possible loss based on the type of land use. In order to assess the capacity of a community, the questionnaires and interviews of the residents were processed randomly in the hazard-prone areas to learn the way they cope with debris flow hazards. Finally, for the references of decision-making for risk management, the risk maps were producing by ArcView GIS software. In addition, a village, Bo-Ai in Taichung Counties was taken as examples to explain the analysis procedure.
The data of rainfall events that induced debris flows were analyzed to calculate the corresponding rainfall intensities, effective accumulated rainfall amounts, as well as the values of rainfall triggering index (RTI ) defined as the product of the rainfall intensity multiplied by the effective accumulated rainfall amount, and then to evaluate the spatial distribution of the critical RTI -values for debris-flow warning. The logistic regression method was used to evaluate the potential distribution of debris-flow occurrence, based on the corresponding RTI -values of debris flow events. The proposed model was tested during two rainfall events at Sinyi Town and Ren-ai Town in Nantou County of Central Taiwan, and then applied to evaluate the temporal variations of debris-flow occurrence probabilities for seven towns in Nantou County. The results show that the proposed model could effectively evaluate the spatial and temporal variations of debris-flow occurrence probability during a rainfall event.
This paper introduces the up to date technology of debris flow monitoring stations established by the Soil and Water Conservation Bureau, Council of Agriculture (SWCB). Besides constructing the fixed debris flow monitoring stations, the SWCB also performs the research on the mobilization and module of debris flow monitoring system in order to enhance the monitoring possibility of debris flow events, to reduce the damage of the on-the-spot instruments and to broaden the monitoring limitation. The mobile debris flow monitoring station can be immediately sent to the high potential spot to detect the possible debris flow events according to the prediction route of incoming typhoon from Central Weather Bureau. The module of the debris flow monitoring system may play an important role in decreasing the maintenance cost and increasing the monitoring efficiency due to the portable devices. So far, the SWCB establishes 13 fixed and 2 mobile debris flow monitoring stations around Taiwan. Each station consists of the monitoring sensors, the instrumental cabin (mobile instrument plate for mobile monitoring station), the transmission system and the display system (Formosa Emergency Management Action system, http://fema.swcb.gov.tw). The purpose of setting up these monitoring stations are to collect real-time debris flow information in the field. This information may help us to understand the physical mechanism of debris flows and to improve the accuracy of the current debris flow warning system based on rainfall thresholds. Besides, it also can be utilized as references for sabo engineering design, countermeasures of slopeland disaster mitigation and academic research.
The Uncertainty of reoccurrence frequency and the unpredictability of the event scale make debris-flow mitigation and countermeasure techniques less standardized as we anticipate. Engineers are required to adapt the concepts of active and passive defense system to cope with specific debris-flow cases so that safety concern is fulfilled and ecologic habitats are maintained. The main objective of this paper is to convey the design concept and rational of debris-flow countermeasures. Setting design standards or recommendations is not the intention of the paper.
This paper presents the study on the occurrence of debris flows from the Da-Chia river watershed after Chi-Chi earthquake through the use of remote sensing data, DEM, and rainfall data. The remote sensing data include aerial photographs and satellite images of five temporal stages which are the stage before Chi-Chi earthquake, the stage after Chi-Chi earthquake, the stage after typhoon Toraji, the stage after typhoon Minduli, the stage after typhoon Airi and Hytarng. Results obtained show that the regional debris flow problems in Da-Chia watershed were mainly caused by the huge amount of sparsely deposited materials from landslides triggered by Chi-Chi earthquake. Several typhoon events with heavy rainfalls moved the sparsely deposited materials into gullies and then induced the debris flows. It is also demonstrated that the regional debris flows that range from Techi Dam to Ma-an Dam are strong related to the spatial distribution of rainfalls.
The geological structures of Taiwan are complex, weak and young. The rock slopes of many river valleys were damaged by earthquakes and landslides, which caused a great deal of cracks and unstable rock blocks. The torrential rain brought by typhoons usually induced serious debris flow. In the recent years, with the increasing of population density and the development of economy, the scope of human activity has been expanded to the mountain district. For the reason, debris flow disaster has been the nightmare of us. This paper presents the case of remediation construction in Sitou. And with the presentation, it is advantage to the engineers to take the lessons as a reference in their plan, design, and construction works to against the disasters.