Geomorphological evolution of the Gendol valley following the October 2010 eruption of Mt Merapi (Java, Indonesia)

The Merapi is an active stratovolcano in Central Java with a dense network of rivers located at the bottom of deep radial valleys, which rapidly evolves during and in the aftermath of the regular eruptions. In the wake of the 2010 eruption, the authors have monitored the evolution of a section of the Gendol Valley between 6 and 11 km to the summit of the volcano. In this reach, the pyroclastic density currents of October to November 2010 filled the valley with several tens of meters thick deposits, before being eroded by lahars and anthropic extraction in the quarries located at the bottom of the valley. The combination of these two processes has deepened and widened the valleys, cutting the 2010 pyroclastic-flow deposits and the pre-eruption material. The present contribution provides a quantitative analysis of the erosion in the Gendol valley. The results were acquired through a diachronic analysis of topographic data between 2010 prior to the eruption and April 2014. For this purpose, three topographical datasets were created: 2 DEMs were produced using Structure-from-Motion and LiDAR for respectively September 2010 and August 2012; and one field-survey was conducted in April 2014 with a handheld laser range-finder. The results have highlighted that lahars have heavily eroded vertically and laterally the valley, while the upstream progression of extraction sites locally increased erosion. Extractions are the main morphogen agent in the Gendolvalley but also in many other valleys of Merapi where deposits are present.