Debris flow velocity due to the collapse of natural dam

Natural dam on Way Ela river was a landslide formation of Ulakhatu mount located at Negeri lima village, Distric Leihutu, Ambon island on 13^th July 2012. The natural dam blockage the valley for 300 m wide and 200 m height with total 1,000 m dam area and providing 87 million m³ volume of water storage. Slope failure trigger by slide area on the deeper surface structure leading to landslide. High risk landside increased by the high rainfall intensity of 432 mm/day and causing the failure of natural dam Way Ela. This study aimed to analyse debris flow velocity of the landslide of natural dam. Analysis performed based on debris during using geometric slope with empirical model system. PMF Syndrer-Alexeyev and GAMA-1 method and PMP frequency analysis equation method demonstrated that rainfall intensity data along with recurrent period of 100 years of Q_peak resulting debit volume of 448.85 m³/d. Furthermore, velocity estimation also showed that on 215.66 elevation high, allowable slope degree of (i) 0,005 %, average width of Way Ela river of 79.97 m and Manning’s grain roughness of 0.004 μm would resulting debris flow velocity of 10.98 m/s, and distance from the landslide centre point to the river mouth was 2.5 km. Those data leading to achieve arrival flow time to the residential area of 3.8 minutes. This study results potential as a base data for disaster mitigation due to high risk of debris flow velocity presented by Way Ela river.

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