Author(s): Yutaka Gonda; Shusuke Miyata; Masaharu Fujita; Djoko Legono; Daizo Tsutsumi
Linked Author(s): Djoko Legono
Keywords: Mt. Merapi; Lahar; Rainfall-runoff; Runoff ratio; Lahar-triggering rainfall
Abstract: After an eruption of the volcanic mountain, in case the pyroclastic material covers the ground thickly, the situation that even small amount of rainfalls trigger lahars, continues for the long period. For the hazard mitigation against lahar disasters after the eruption, it is essential to determine the danger of occurrence of them and estimate its temporal change. Putih River is one of the rivers where lahars occurred frequently after the 1984 eruption of Mt. Merapi. In this study, the characteristics of lahar in the Putih River after the 1984 eruption and their change over the years were analyzed, focusing on runoff characteristics of lahar. Exclusion of inappropriate rainfall data and discharge data for runoff characteristics analysis was conducted as preprocessing. Magnitude and occurrence number of lahar decrease with years during four years after the eruption. Maximum runoff rate was approximately 12 during 1984—1985 rainy season and decreased with years. A few hypotheses that describe the mechanism that runoff rate became much greater than 1 were introduced. However, high spatial resolution and high accuracy rainfall data are necessary in order to figure out its mechanism. In case of 1984—1985 and 1985-1986 rainy season, rainfall, which triggered lahar flow with peak discharge > 900m³/s were discriminated on the judgement graph wherein maximum 10 minutes’ rainfall within a rainfall event is an abscissa axis and antecedent rainfall with half-life 24 hour at maximum 10 minutes’ rainfall is an ordinate axis.
Year: 2018