| dc.description.abstract | The capability to assess the hazard of debris flows is important because it frequently occurs every few years in the Merapi area, especially during the rainy season. In this area, the rainfall stations have been developed since 1973, and the rainfall data have been stored. The objective of this study is to determine the rainfall thresholds for triggering the debris-flow occurrence in the Merapi area. Besides, the available rainfall data are first evaluated with homogeneity tests to ensure the reliability of the data for determining the rainfall thresholds.
Firstly, the homogeneity of the rainfall series data was established by using four absolute homogeneity tests: the SNHT for a single break (Alexandersson, 1986); the Buishand range (BR) test (Buishand, 1982); the Pettitt test (Pettitt, 1979); and the Von Neumann ratio (VNR) test (Neumann, 1941). It was found that among 25 rainfall stations within the area, 24 (96%) were indicated as homogeneous, and only one (Mranggen) was considered as inhomogenous. Hence, the rainfall stations in the Merapi area are significantly homogeneous and do not need missing data analysis to complete the missing values in the series.
Secondly, the rainfall thresholds were developed by using I-D thresholds from Guzzetti et al., (2007) with power-law threshold curve. It was revealed that the debris flows in the Merapi area are initiated at continuous rainfall of maximum 12 hours, with a rainfall intensity of about 11.8 mm/hour. This result indicates a similar trend with the global I-D threshold (Caine, 1980), but a different trend with the regional I-D threshold (Guzzetti et al., 2007). This finding suggests that the debris-flow initiation in the Merapi area is caused primarily by rainfall which highly correlated to the rainfall characteristics (intensity and duration).
A further study of the homogeneity test can be done with other methods to decrease the error or uncertainty that may emerge in the rainfall data. Also, the development of more comprehensive I-D thresholds can be done by combining the rainfall with other factors such as topography, geological conditions, land-use, and human interference. | |
