Developing a warning system in Ambon city, Indonesia: Rainfall threshold for sediment related disasters

Ambon city of Indonesia is extremely vulnerable to climatic hazards and the frequency of sediment related disasters appears to increase. During 2012 to 2013, more than one hundred of sediment related disasters including landslides occurred especially in the settlement area. The damage was particularly severe in the city and at several sites along the transportation network. The sediment related disasters resulted of hundreds houses destroyed, including 43 deaths, numerous injured people and hundreds people evacuated. Rainfall threshold method is an approach for develop a warning system for sediment related disasters occurence. Two types of rainfall thresholds can be established (Aleotti, 2004): (1) empirical thresholds, based on historic analysis of relationship rainfall/landslide (sediment related disaster) occurrence, and (2) physical thresholds, based on numeric models that take into account the relationship between rainfall, pore pressure and slope stability by coupling hydrologic and stability models. Empirical thresholds were used in this study. Empirical threshold has been considered as collecting rainfall data for sediment related disaster events from 2007 to 2013. The results show that the sediment related disasters occurred in short periods (2 hours) with a high average intensity and longer periods (48 hours) with a lower average intensity. We determined new rainfall thresholds for possible sediment related disaster occurrence with the regression value of I = 83.88D-0.80 (I is rainfall intensity, mm/hr and D is duration, hr). It is expected that the new rainfall thresholds could be used for the development of a warning system in Ambon city.