Dynamics of lahar-affected river tributaries of the Progo river after the 2010 Mt. Merapi eruption

Riverbeds in the Mt. Merapi area are dominated by gravel originating from Mt. Merapi. The 2010 Mt. Merapi eruption has formed a riverbed consisting of sediments of various size. Since then, the morphology of the rivers has changed dynamically, depending upon the natural and the anthropogenic factors. Most of the rivers drain into the Progo River, whereas the main river system of Progo River drains into the Indian Ocean. The dynamics of the tributaries of volcanic rivers along the Progo River are the major concern to identify the capacity of the tributaries against potential lahar flow disasters that may occur. This paper presents the results of a series of observations of several tributaries of the volcanic rivers, taking into account the natural flow phenomena as well as the sediment mining activity along the river under investigation. The investigation included the hydraulic simulation, which employed a numerical model. The results of the simulation showed that the downstream part of the Progo River is influenced by sediment from Mt. Merapi through its tributaries. These also suggest cost-effective river tributary maintenance to mitigate the risks of lahar flow disasters.