Dynamics of Morphology Changes in Progo River Due to Lahar Transport from Merapi Volcano

This study aims to comprehensively examine the dynamics of river morphology changes influenced by the occurrence of two distinct lahar floods which occurred in the years 1975 and 2010 respectively. In order to achieve the stated objectives, temporal images representing the conditions of the study area several years after the occurrence of the lahar floods were obtained and used to examine river morphology changes. Furthermore, temporal data from remote sensing and UAV were collected and used to analyze morphology changes of river valleys in the study area. The analysis was carried out using three parameters namely river slope, riverbank morphology, and river curvature. River slope was 0.74, showing a relatively flat gradient, and this condition resulted in a meandering river morphology. Morphology of riverbank was observed to have the “U” shape, providing more surface area for river water to drain. Additionally, the sinuosity ratio (SR) during the two lahar flood periods were found to be dynamic. During the 2010 lahar flood period, the SR pre-lahar flood was 1.92, which increased significantly to 2.65 eight months after the event, and decreased drastically to 1.60 one year later. It is important to comprehend that the main factor responsible for these changes during the period was flood. The obtained results showed that the SR value for the next decade, starting from 2010, experienced consistent fluctuations. These fluctuations provided evidence that river morphology was always experiencing changes even without major triggers. For the analysis of the 1975 lahar flood period, the SR value was obtained from the PBB maps three years before the event, five years post-event, and from an analysis of Google Earth images ten years after the disaster. Similarly, the SR in this period was observed to fluctuate significantly, and the meander became simpler in the tenth year after the occurrence of flood. The results of this study provide evidence that spatial analysis with temporal images is more efficient and economical for studying the dynamics of river morphology.