To prevent disasters by debris flow, it is necessary to predict occurrence of debris flow in advance for warning and evacuation. This study desribes a reformed method of setting standard rainfall by the reform of antecedent rain for the warning and evacuation based on such a view that it is the most proper to use rainfall for debris flow prediction.
In this paper the authors developed a new method of analysis for slope failure. The distribution of matric potential (suction) in soil layer, which is an indicator of soil moisture content, was calculated by finite element method and internalized into Bishop's simplified method of slope stability. imulations with this method were performed on four rectangular rainfalls conditions (10-40mm/hr) which accumulate 480 mm respectively. The transitions of afety factor (Fs) resulted in; (1) minimum Fs values were all equal regardless of rainfall intensity. (2) some delays from the ends of the rainfalls to the time of minimum Fs values occured. (3) the larger rainfall intensity caused the longer delays. Initial conditions of the slope were presented by means of discharge quantities, so rainfall accumulation caused failures were calculated on each condition. The method is available to estimate a catastrophic failure.
Mayon Volcano is an active volcano located in the southern part of Luzon Island, Philippines. It has erupted 44 times since 1616 based on existing records, generating huge amount of unstable sediment. Therefore, lahars and debris-flows often occur on its slope and attack the residences around it. Pawa-burabod River, running down the south-eastern slope of Mayon Volcano, is pointed out as the most dangerous river. Debris-flows running through the Pawa-burabod River has caused severe disasters many times and the sediment from the river is raising the bed of the Yawa River which in turn is flowing through the urban area of Legaspi City. In addition to this, Bonga pyroclastic flow avalanche fan, which was formed during the 1984 eruption, is closely located to the upper reach of the Pawa-burabod River. So, it is feared that the fan would be a huge sediment producing source when it joins the river by means of erosion. The countermeasures to control debris-flow and to decrease sediment coming from the Pawa-burabod River should be revised to include the use of some areas on the slope as sediment retarding basins with artificial facilities. Moreover, comprehensive volcanic disaster prevention and preparedness plans are expected to be carried out with dispatch.