Ministry of Construction, Japan, started “Model Project for Comprehensive Countermeasures against Snow Avalanches” in 1990 to protect human lives in towns and villages from snow avalanches not only by constructing facilities but also by rationalizing the warning and evacuation system. In this paper, we report the result of examination about the zoning method of threatened areas, which is one of the main items of this model project. Perla's calculation model was examined to apply the numerical calculation. Since this model has two parameters, combination of which can be selected widely, the relationship between μ and velocity which Schaerer assumed for simplification was applied and unknown parameter became only M/D. M/D value were counted backward to fit runout distance of each past avalanche records. Relationship between various factors and M/D was examined and volume of avalanche was found to have a clear relationship with M/D. Basically it was assumed that M l D could be estimated by volume of avalanche. Slope length was also found to be another influential factor to M/D and was adoped to estimate M/D. A concrete method to determine dangerous area of snow avalanche was proposed with a scale of probabil-ity and case studies were carried out. Although a few problems still remain, the whole method has been almost completed.
Generally they make the master plan for debris flow countermeasure in the area around active volcano. That is called the Volcanic Sabo Master Plan. The object of the Master Plan is the runoff sediment volume by a probability of 1/100 rainfalls and the total runoff sediment volume during 5 or 10 years after the stop of eruption. Especially considering the case that the counter-measure does not execute in the upper part of river basin because of the danger by the volcanic activity, urgent program is also made in the Master Plan. But different phenomena occur in each active volcano, for example, debris flows occur continuously due to the sediment of pyroclastic flow in some volcanos. And the condition of execution is just different in each active volcano, as execution area for countermeasure is limited by the volcanic activity. Considering the various condition in active volcanos, we need the actual countermeasure plan used the idea that we consider not only the total runoll sediment volume but also runoff sediment volume by time sharing. In this paper I proposed the new urgent plan and the provisional plan for debris flow countermeasure in active volcano adding the Master Plan.
Warning and evacuation timing for debris flow has great importance to protect life from debris flow disaster. In this sense, a warning system has been operated since a decade ago in some district. However, it's based on an assumption that debris flow always occures at the time of maximum rain intensity, although considerable amount occures at less rain intensity. Adding to that, the critical rain standard of the warning system tend to be fixed using the method which includes rather empirical “fuzzy” decision of critical line (CL) i. e. the critical rain standard. These factors sometimes cause inaccuracy in warning and evacuation timing. Hence, some improvement to this inaccuracy is proposed in this report by means of “whole snake line judgement” with which the whole precipitation dynamic pattern can be overlooked. And this method is seemed to be successful for more accurate warning and evacuation timing.
Mayon Volcano, 330 km southeast of Manila on the island of Luzon, is a classic composite cone of andesite and basaltic andesite lavas and pyroclastics. The historical eruptions of Mayon occurred 45 times since 1616 till September 1993. In 2 February 1993, Mayon erupted with minor pyroclastic flow which descended along the Bonga gully at the southern sector of the edifice. The area within the previously declared 6 km permanent danger zone was badly hit with 77 human casualties by this event. Smaller pyroclastic flows followed several days after the first eruption also utilizing the Bonga gully. Secondary lahars triggered by tropical rainfalls were also generated after the eruption. About 60, 000 people living within 10 km on the sooutheast side of the volcano has been evacuated. After the disaster, a relocation/resettlement plan was drawn up as a part of Integrated Rehabilitation Project. Many evacuated people, however, have already returned to “barangays” even in the danger zone. Topographic conditions on the southeastern slopes of the volcano, especially along the Bonga gully, were considerably changed and some sabo structures reduced their effects in controlling sediment movement so that forthcoming rainfall might generate disasters by movement of remaining volume of debris. After conducting a field reconnaissance and rapid appraisal, we have made some considerations on structural measures for sediment control, non structural measures as data acquisition and early warning system, and integrated regional development as a measure for revealing disaster resilient society.
The June 15, 1991 eruption of Mount Pinatubo, one of the largest eruption of this centry in the world, produced remarkable volumes of pyroclastic flow and fall deposits. The volumes of pyroclastic flow deposits on the mountain slope was estimated at 5 to 7 km3. The major piracies of river basins occurred a few times in the eastern slope of Mount Pinatubo, and new drainage systems were rapidly re-established within the pyroclastic flow deposits-filled river valleys. While, Lahars which are generated by rain falling on erodible pyroclastic flow deposits pose continuing and grave danger to human lives and property in low-lying area. Among the river basins surrounding Mount Pinatubo, Sacobia/Bamban and Abacan rivers possess a great danger to the outlying areas currently used for residential, commercial and industrial areas. The area of lahar deposition in Sacobia/Bamban and Abacan rivers was estimated at about 15, 000 ha with an average thickness of 2 to 3 m as of the year of 1993. Upon the official request of the Government of the Philippines, the Japan International Cooperation Agency (JICA) organized a study team in November, 1993 to conduct a study for flood and lahar control works in the rivers. The Study Team proposed urgent rehabilitation works which should be undertaken before the onset of the rainy season in June, 1994.