Precursory Characteristics of Flash Flood Occurrence in Small Catchment of Upper Brantas River

Indonesia’s flash floods have increased significantly since the flash flood in Bohorok, North Sumatra, in 2003 and Jember, East Java, in 2006. Such flash floods have caused negative impacts, loss of human life and damage to vital infrastructures such as houses, roads, bridges, or other public facilities. Understanding the factors driving flash floods and their mechanism at the outset is essential in mitigating the negative impacts mentioned above. This paper presents the results of the analysis of the flash flood characteristics of several hydraulic parameters under conditions just before the flash flood incident, including the intensity of the rainfall, the time of concentration, and the time lag of the occurrence. As the object of the study, a flash flood event was selected in Batu City, Upper Brantas River, which occurred on November 4, 2021. The results show that the lag time of the flash flood occurrence at 40 minutes magnitude at Bulukerto Village of Batu City, the application of Kirpich for the time of concentration equation performed better than the Ventura and Watt & Chow equations. Further assessment of precursory characteristics for other similar small catchment and hydro-meteorological conditions is still the subject of beneficial study or research.