Abstract
Abstract. A risk reduction program was developed after debris-flow disaster analysis is conducted using mitigation structures, evacuation measures and community restrained expansion strategy. The risk assessment method delineates hazard zones and analyzes vulnerability and the resilient capacity of an affected area, allowing the prediction of losses of properties and lives, and the corresponding risk. It can also be used to evaluate performance of a risk reduction program. The proposed method was applied to the Songhe community as a case study to assess debris-flow risk and performance of reduction programs consisting of mitigation structures, evacuation measures and a restrained expansion strategy. Total annual risk decreased to $0.01 million from $0.72 million for the No. 1 Torrent and to $0.36 million from $1.22 million for the No. 2 Torrent after mitigation structures were installed, and evacuation measures were implemented based on restrained expansion. Although mitigation structures are costly, they can reduce the size of hazard zones. Delimitating the Designated Soil and Water Conservation Area restrains community expansion and decreases possible losses. Although evacuation measures cannot reduce the size of hazard zones, they effectively increase the resilient capacity of residents. The benefit-cost ratio for mitigation structures exceeds 1.0 for both torrents with an average of 3.87; the benefit-cost ratio for evacuation measures is markedly greater than 1.0. Combining mitigation structures and evacuation measures increases the total benefit with a benefit-cost ratio of 4.38. Analytical results showed that the risk reduction program is cost-effective.
Highlights
Due to its steep topography, fragile geology, seismic activity, and rapid development in mountainous regions, Taiwan is often affected by debris flows
Rickenmann (2001) simulated debris flows for return periods of 10, 100 and 1000 years and classified the affected area into high, medium and low hazard zones based on deposition depth and maximum velocity; the delineation criterion was related to the legal situation in Switzerland
Chen et al (2005) considered community resilient capacity to be the combined effect of legal right to ask for the disaster prevention budget, disaster prevention and response organization, communication capability, warning capability, the ability to pay for mitigation and disaster prevention education
Summary
Due to its steep topography, fragile geology, seismic activity, and rapid development in mountainous regions, Taiwan is often affected by debris flows. Liu and Lei‘(2003) assessed the regional hazard for debris flows based on gully density, mean annual rainfall, and the percentage of cultivated land on steep slopes They estimated vulnerability based on fixed assets, gross domestic product, land resources, and population density as well as inhabitant age, education level and wealth. Zezere et al (2007) analyzed landslide risk and evaluated direct and indirect costs resulting from a road disruption caused by slope movement In their analysis, landslide hazard maps were developed based on a scenario for landslides in the 27 years. This work assesses risk by considering the probability of the occurrence of debris flows, the expected loss, and the resilient capacity, with the vulnerability and the values of elements at risk included in the expected loss calculation. We chose this community for our case study to demonstrate the application of the proposed method
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