Analysis of Flash Floods Behaviors in Sentani, Jayapura Regency, Papua

Flash floods are flows in very large numbers and occur suddenly, flowing from the upstream to the downstream at a very high speed. This study aims to analyze the parameters that influence the occurrence of flash floods in Sentani and to analyze flood behavior Kemiri river and Flafouw river. Hydrological analysis based on maximum daily rainfall data for the last 10 years (2009-2019), and hydraulic analysis on secondary and primary tertiary channels (S. Kemiri) on river geometry then simulated by modeling using HEC-RAS software to calculate one-dimensional water level profiles, in each section of the Kemiri river. The planned flood discharge using the Nakayasu method obtained Q10th = 51.798 m3/s kemiri river with t = 1.615 hours while the Flafouw Q10th river = 94.804 m3/s and t = 1.669 hours. The flood inundation when the disaster occurred was 1.00 - 4.00 meters with inundation time < 24 hours, but the sediment (sand, stone, and logs) rose until it reached the roof of the house. The results of field observations of the classification of flood hazard levels in several points in Sentani are classified as moderate hazard class (0.75 - 1.50 m), as well as according to the Directorate General of Water Resources (2010) including the medium class (0.50 - 2.00 m). However, it was not only the impact of the inundation that occurred, what was even more devastating was the very fast speed of water flow from the upstream of the river, which destroyed several settlements, and several other infrastructure. The worst condition was that it (3.00 - 4.00 m), namely in the BTN Gajah Mada Housing, Dobonsolo Village, classification of high hazard areas.