Flash Flood Disaster Reconstruction for Estimating the Available Warning Time, the Case in Sempor River on 21st of 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.

Impact of Extreme Rainfall on Flood Hydrographs

Extreme rainfall is the main factor triggering flooding in various regions of the world including Indonesia. The increase in intensity and duration of current extreme rainfall is predicted as a result of global climate change. This paper aims to analyze the impact of extreme rainfall to the peak discharge of flood hydrographs at a watershed outlet in Palu, Sulawesi, Indonesia. Maximum daily rainfall data for the period 1990-1999 recorded at the Palu Meteorological Station, Central Sulawesi were selected using the Annual Maximum Series Method, and grouped into two types. Type I is the maximum daily rainfall data with extreme events and Type II is the maximum daily rainfall data without extreme events. Frequency analysis was applied to the two data groups using the best distribution method of: Normal, Normal Log, Pearson III Log, and Gumbel to obtain the design rainfall of each data group. In the next stage, the design rainfall transformation into a flood hydrograph is performed using the Nakayasu Synthetic Unit Hydrograph based on a number of return periods in one of the rivers flowing into Palu Bay, namely the Poboya River. The analysis results show that the design rainfall graphs with both extreme rainfall and without extreme rainfall are identical at the low return period and divergent at the high return period with a difference of up to 21.6% at the 1000-year return period. Correspondingly, extreme rainfall has a greater impact at the peak of the flood hydrograph with increasing return periods ranging from -1.28% to 26.81% over the entire return period.