Flash Flood Study on Mt. Merapi Slope for Improvement of the Monitoring Method

Flash floods with or without debris in the rivers on the Mt. Merapi slope have been causing damage and loss of lives until now even though a series of sabo dams have been built for each river, and warning systems have been established and operated. This study aims to understand the hydrology and hydraulic characteristics in the area, especially of the two important rivers namely Bebeng and Boyong Rivers for flood monitoring improvement. The research verified the results of previous studies on hydrology and hydraulics of the area, approximating flood travel times using a simple formula stepwise, comparing with the result of the analysis using recorded pair data of a precursor rainfall and the related local peak of water level hydrographs. The reference study results show that the South-West slope of Mt. Merapi has a higher risk for flash flood occurrence. Flood travel time in the short upstream reach is well estimated using the simple formula stepwise because the result is similar to the measured one. Based on pairs of precursor rainfall and the related local peak water level data, the local peak water level weakly influences the time lag for those two AWLR Sta. locations. It is proposed to include the flood propagation time in considering the issuance of warning messages.