The Prediction of Debris Flow Based on Eruption and Rainfall Event for River Infrastructure Mitigation: Study Case Opak River, Sleman Regency

Mount Merapi is a volcano active in Indonesia. The eruption that occurred in 2010 was a major eruption with a return period of 100 years. Dominant debris to the Opak – Gendol Watershed; the biggest debris flood occurred in 2010 – 2011. One of the disaster mitigations on the Opak River is the Sabo Dam infrastructure. Currently, in the upper reaches of the Opak River, there are 5 Sabo Dams. In 2022, 2 additional Sabo Dams were built, namely OP RRC4 and OP RRC3a. This study will be used modelling 2D HEC-RAS Non-Newtonian before the construction of 2 Sabo Dams and after the construction of 2 Sabo Dams with river geometry measured in 2020 and flood discharge as measured from rainfall triggering debris flow on 03 January 2011, from this rainfall a 6-hour rain distribution was carried out using the PSA coefficient to be input in the HEC-HMS, the hydrological parameters used on 03 January 2011, were used to calculate the Q100 flood discharge. The results of modelling at the observation point using the Q100 showed that the elevation of the debris flow after the presence of the new Sabo Dam series could be reduced by up to 47.21 %, Sabo Dam effectively reduces the rate of debris flow to reduce the potential debris flood.