The March 25 and 29, 2016 landslide-induced debris flow at Clapar, Banjarnegara, Central Java
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The Clapar landslide induced debris flow consisted of the Clapar landslide occurred on 24 March 2017 and the Clapar debris flow occurred on 29 March 2017. The first investigation of the Clapar landslide induced debris flow was carried out two months after the disaster. It was followed by UAV mapping, extensive interviews, newspaper compilation, visual observation and field measurements, and video analysis in order to understand chronology and triggering mechanism of the landslide induced debris flow in Clapar. The 24 March 2016 landslide occurred after 5 hours of consecutive rainfall (11,2 mm) and was affected by combination of fishponds leak and infiltration of antecedent rain. After five days of the Clapar landslide, landslide partially mobilized to form debris flow where the head scarp of debris flow was located at the foot of the 24 March 2016 landslide. The Clapar debris flow occurred when there was no rainfall. It was not generated by rainstorm or the surface erosion of the river bed, but rather by water infiltration through the crack formed on the toe of the 24 March 2016 landslide. Supply of water to the marine clay deposit might have increased pore water pressure and mobilized the soil layer above. The amount of water accumulated in the temporary pond at the main body of the 24 March 2016 landslide might have also triggered the Clapar debris flow. The area of Clapar landslide still shows the possibility of further retrogression of the landslide body which may induce another debris flow. Understanding precursory factors triggering landslides and debris flows in Banjarnegara based on data from monitoring systems and laboratory experiments is essential to minimize the risk of future landslide.
KeywordsLandslide Debris flow Infiltration Antecedent Fishponds Clay
We thank all colleagues from the Department of Environmental Geography and Transbulent Research Group, Universitas Gadjah Mada, for the helpful discussions and their support in the field. We also thank BIG (Indonesian Geospatial Agency) who provided the DEM and high-resolution satellite imagery (Pleiades imagery). The work was supported by Hibah Penelitian Kerja Sama Luar Negeri Ministry of Higher Education number 2245/UN1-P.III/DIT-LIT/LT/2017.
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