Évolution géomorphologique de la vallée Boyong à la suite de l'éruption du 22 novembre 1994 du volcan Merapi (Java, Indonésie) / Geomorphological evolution of the Boyong Valley following the 22 November 1994 eruption of Mt Merapi, Java, Indonesia

Abstract Lahars accelerate the geomorphological evolution of the channels during or after a volcanic event. This phenomenon was observed and monitored in the Boyong Valley (Java, Indonesia), which channelled block-and-ash pyroclastic flows from Merapi volcano on 22/11/1994. Retention dams allowed us to carry out a morphometric study of the channel, which was updated after each lahar event. The capacity of the dams was sufficient to allow the totality of the volcaniclastic sediments to be trapped after the first rainy season following the eruption. The lahars progressively reduced the riverbed gradient and the sinuosities of the Boyong channel. Sixty percent of the pyroclastic material was removed within the six months following the Merapi dome collapse, from two combined processes: downcutting of the pyroclastic-flow deposits in excess of 12 m and lateral spreading of channel banks, up to 15 m. Downstream from the 1994 pyroclastic flow deposit, two hydro- geomorphological channel segments were distinguishable: a distal segment where depositional process is continuous, and an unsteady transitional zone, where erosional processes alternate with depositional processes.