Flash Flood Disaster Reconstruction for Estimating the Available Warning Time, the Case in Sempor River on 21stof February 2020, Mt. Merapi Slope, Yogyakarta Special Region

Flash floods are hazardous events characterized by short response times. The occurrences of flash flood disasters have increased significantly in the last few years, producing a remarkable casualty number globally. On February 21st, 2020, a flash flood occurred in the Sempor River of the Mount Merapi slope, Special Region of Yogyakarta, Indonesia, causing a significant death of high school students. This study aims to reconstruct the river's hydrologic and hydraulic conditions and identify the available warning time based on the flash flood event. This study extracted the catchment configuration from a Digital Elevation Model (DEM) using the GIS technique. A simulation of flood hydrograph at a control point used the HEC-HMS model, the SCS Curve Number Loss method, and the SCS Unit Hydrograph. The simulated flood hydrograph was inputted into the one-dimensional unsteady flow model of HEC-RAS to simulate water depth and flow velocity. The calibration process adjusts both models' parameters by comparing the simulated peak discharge with the surveyed data. The modelling results provide warning time components. The results of this study can support the decision-making in flash flood risk mitigation for the local communities. © Published under licence by IOP Publishing Ltd.