Sediment discharge by sheet erosion has been measured at Sakurajima volcano on slopes made up of bare land and revegetation. Characteristics of sediment discharge and the effects of revegetation on sediment discharge have been examined. The results are as follows. The yearly specific sediment discharge on revegetated slopes was about 2% for those on bare land slope. In cases where rainfall intensities were of the same degree, the values for the median diameter of the sediment discharge on revegetated slopes were about 50% of those on bare land slope. The infiltration capacity, Manning's coefficient of roughness, and the dry density of soil in the surface layer have been increased by incremental plant cover. Moreover, results of analyses using a runoff model and a sediment discharge formula shows that revegetation caused a decrease of the area contributing to the occurrence of Hortonian overland flow.
Evacuation is one of the most important aspects among the disaster response activities by local municipalities in time of severe weather. Japanese national laws put responsibility of evacuation order on local mayors. Decision making by local mayors and their subordinates has been supported both by warning provided by various sectors of national government and by hazard and risk information uncovered by researchers. A series of field interviews to 24 local municipalities across the nation was conducted to clarify the decision structure in terms of perceptive costs and losses. Number of personnel actively put into operation in pre-disaster stages and expected losses in local officials' cognition were numerated. Major obstacles to issue an evacuation order were listed out. These factors, analyzed by a decision tree, reveal what hinders evacuation judgment. Uncertainties entailing to expected casualties and inaccuracy of warnings keep local officials from the issuance of evacuation order through higher costs of explanation. The costs of explanation can be reduced by combined efforts to clarify expected losses and to raise accuracy of warnings. Importance of further research on risk communication based on quantitative hazard identification and risk analyses was brought to our attention.
Debris flow disaster in the area around Tsurumi Town, Ohita Prefecture, Japan, occurred during a heavy rainfall over 600mm from November 27, 2003 to November 29, 2003. Antecedent precipitation of more than 300mm, and additional severe hourly rainfall of about 90mm were brought onto the area. We have investigated the circumstances of the disaster in Tsurumi Town and confirmed the occurrence of the fluidized phenomena there. While some houses were damaged by debris flows, nobody was killed. Although there is not usually a heavy rainfall in this season, it showed that the debris flow could occur at any time depending on weather and environmental conditions. However, it is very characteristic that the rich forest there has resisted the enlargement of the extent of sediment produce and sediment yield even as a result of this heavy rainfall.
Serious damage concerning to the human life and artificial structure in central Taiwan according to the 921 Chi-Chi earthquake (M 7.3) occurred near Chi-Chi on September 21, 1999. This earthquake caused uplift of active surface fault, and occurrence of many slope failures in the steep mountainous area. The number and area of slope failures were surveyed just after the earthquake by Taiwan Agricultural Committee. The purpose of this study is estimating the total area of newer slope failures caused by 921 Chi-Chi earthquake origin, except for the former existing slope failures. To distinguish between former existing bareland and newly occurred slope failures, the SPOT/HRV satellite images just before and after the earthquake were analyzed using the method of simple orthophotography. The result obtained from this method yields lesser value compared with the actuality because of being limited by the minimum resolution (20m square) of SPOT/HRV satellite images. As the result, the area of the slope failures triggered by 921 Chi-Chi earthquake are 2.1% (in north area) and 4.3% (in south area) .
PIV (Particle Image Velocimetry) is a method for detailed motion analysis of fluid velocity. The aim of this study is to examine the applicability of PIV to slope failures. The example was the slope failure occurred in Koyae region, Miyazaki Pref., Japan, captured on VHS videotape. In order to raise analysis accuracy, parameter setup, preprocessing and the post processing methods of the images were examined. The results analyzed by the present PIV method correspond with previous researches. However, some problems still remain in terms of full practical use, and we also suggest a solution method.