We herein report an example of pyroclastic density current avulsion on 14 June 2006 at Merapi, Indonesia. Four discrete series of multiple dome collapses led to the generation of four individual block-and-ash flows into Kali Gendol valley. All four pyroclastic density currents locally overflowed the channel margins to devastate cultivated terraces along each side of the box-shaped canyon while propagating as much as 2.2 km within adjacent tributaries. The largest destruction was caused by the second and third pyroclastic density currents. Both of these flows partially spilled out of the Gendol valley at a travel distance of 4.9 km, bypassing a sabo dam just upstream of the village of Kaliadem and leaving the village almost completely destroyed and buried by several meters of overbank deposits.
The main mechanism of flow avulsion on June 14 was the overflow of up to 20 vol% of the dense, basal part of the pyroclastic density currents onto interfluves. The relative proportions of valley-escaped material increased dramatically in the succession of each of the four pyroclastic density currents. The main geometric parameters controlling flow avulsion and their critical values were quantified through high-resolution real-time kinetic–global positioning system (RTK-GPS) data of the pre-event topography for each of the 14 June flows. This case highlights the way in which a sabo dam can significantly increase the potential of flow avulsion. Key lessons are derived for the future hazard mitigation in valleys subject to volcanic mass flows. Flow-observational and geometric data are combined into a model to derive the kinematic characteristics of the basal, valley-ponding avalanche and the valley-escaping veneer flows, which are otherwise hidden by overriding clouds of elutriated ash.