Temporal Changes in Runoff Characteristics of Lahars After the 1984 Eruption of Mt. Merapi, Indonesia

After a volcanic eruption, in situations where pyroclastic material generates thick ground cover, even small amounts of rainfall can trigger lahars; this effect sometimes continues for many years. For hazard mitigation against lahar disasters after an eruption, it is essential to evaluate the current risk of occurrence and estimate any possible temporal changes for the future. Putih River is one of the rivers where lahars occurred frequently after the 1984 eruption of Mt. Merapi. In this study, the characteristics of lahars and floods in the Putih River after the 1984 eruption and their change over the years were analyzed, focusing on the runoff characteristics of lahars. Irrelevant rainfall and discharge data for analyzing runoff characteristics of lahars were excluded in preprocessing. The magnitude and occurrences of lahars decreased annually during the four years following the eruption. The maximum runoff rate of lahars was approximately 12 during the 1984–1985 rainy season and decreased yearly after this. A judgement graph was employed to track the temporal changes of lahar- triggering rainfall characteristics. For the 1984–1985 and 1985–1986 rainy seasons discriminant lines, which discriminate between rainfall events triggering lahar flow with peak discharge > 900 m 3 /s and other rainfall events, were drawn on the judgement graph.