Merapi's Lahars: Characteristics, Behaviour, Monitoring, Impact, Hazard Modelling and Risk Assessment

Lahar is an Indonesian term, and Merapi volcano is arguably one of the most renowned lahar producers worldwide. Frequent and voluminous lahars at Merapi result from a combination of four factors: (1) large volumes of pyroclastic debris, in particular dome growth and collapse block-and-ash flows (BAF); (2) high frequency of eruptions; (3) abundant rainfall with intensities and durations regularly exceeding critical thresholds, and (4) a dense drainage network incising the steep summit topography. The majority of Merapi's lahars are rain triggered, occurring for two to four years after moderate, magmatic BAFproducing eruptions; however, during large eruptions, liquefaction of BAF deposits in the river channels can also cause lahars. Merapi lahars have moderate sediment concentrations of 20-50 vol.%, mean velocities between 2.5 and 7.5 m/s, moderate peak discharges (< 600 m(3)/s), and runout distances rarely exceeding 20 km. Beyond 20 km, hyperconcentrated flows and muddy floods propagate across the ring plain and through the city of Yogyakarta (637,000 people). Post-2010 lahar damage assessments revealed that most weak masonry buildings on Merapi can be destroyed by dilute laharswith low velocities c. 3 m/s and dynamic pressures < 10 kPa. At least 372,000 of the 1,300,000 people living within a 30 km radius of Merapi's summit are located in the path of potentially harmful flows conveyed by radial rivers. Despite limited resources, lahar monitoring at Merapi has considerably improved since the 2010 eruption. Warning systemsbased on 24 lahar monitoring stations have been implemented, supporting efficient evacuations when lahars did occur. Building on previous, semi-empirical, operational hazard-zone maps following the 2010 eruption, we present the first FLO-2D lahar simulations over LiDARderived topography. A risk mapping assessment encompassing physical and socio-economic factors at the village scale along rivers likely to be affected by lahars and BAFs provides a risk map for future lahars at Merapi.