Riverbed variation as influenced by natural and human-interfered phenomena of Gendol River, Mt. Merapi Area, Indonesia

Rivers that originated from Mt. Merapi have a significant role as conveyance channels to migrate the sediment produced by volcanic activities. Depending upon the number of sediment volume accumulated at the upstream of the rivers, as well as the rainfall trigger, the rivers may experience debris flow occurrence which in some extent produces significant destructive power. The accumulation of sediment in the upper part of the rivers depends on the volcanic activity of Mt. Merapi, and unfortunately, its direction is unpredictable. The migration of sediment from the upstream towards downstream rivers is dominated by two mechanisms, i.e. through natural flow employing bed load transports and debris flows, as well as flow and through human-related activities by sand mining activities. This paper evaluates the riverbed variation due to the migration of sediment due to both natural phenomena such as debris flow occurrences triggered by the heavy rainfall and the human-interfered phenomena such as sand mining. The sediment control structure of GE-D5 at Gendol River and rainfall station of ARR Kaliadem were selected objects of observation. Study on the topographical change during the period of 23 October 2019 through 22 January 2020 as affected by the rainfall and the sand mining activity was then carried out. The riverbed variations were analyzed through the photogrammetry method at two different conditions, i.e. at the beginning of the rainy season and the middle of the rainy season. The discrepancy of results of the formation of new storage at near upstream of GE-D5 is presented and discussed. Moreover, the results of the evaluation contribute the highlight on how to carry out the proper sediment mining management at upstream of GE-D5 of Gendol River. © 2020 22nd Congress of the International Association for Hydro-Environment Engineering and Research-Asia Pacific Division, IAHR-APD 2020: "Creating Resilience to Water-Related Challenges". All rights reserved.