Investigation and numerical simulation of the 22 February 2018 landslide-triggered long-traveling debris flow at Pasir Panjang Village, Brebes Regency of Central Java, Indonesia

On 22 February 2018 at 08:00 am, a landslide occurred in the dense conserved forest area of the Cigunung Sub-River Basin at Pasir Panjang Village, Brebes Regency of Central Java Province, Indonesia. The landslide materials transformed into debris and flowed down along the river channel rapidly with a distance of more than 2 km, causing 18 people dead and 14 people injured. About 8.5 ha of farm fields, two bridges, 507 m of the provincial road, and 25 houses were destroyed due to the impact of debris flow. This paper reports the site investigation of Pasir Panjang landslide conducted in early June 2018 which covers geological on-site measurement, soil sampling for laboratory tests, and mapping using the unmanned aerial vehicle. Results imply that the source of Pasir Panjang landslide came from a highly weathered layer of volcanic breccia which fails from a steep slope. The landslide materials then entrained weathered tuffaceous sandstone and volcanic breccia layers along the river bank. Hydro-meteorological data shows that the two series of antecedent rainfalls in 10 consecutive days may generate the excess pore water pressure and high saturation condition within the landslide and debris flow area. The balancing depth of saturated soil layer then calculated to estimate the pore water pressure by using the slope infiltration distributed equilibrium model at each time interval of rainfall. We used the LS-RAPID numerical model to simulate the integration of failure mechanism of Pasir Panjang landslide and its debris motion behavior. The simulation results show that the slope fails from the unstable area of approximately 28,000 m² with the velocity around 13-14 m/s and the maximum depth of 25-26 m. The landslide materials transformed into debris when hit the provincial road with the velocity of more than 30 m/s and traveling down rapidly to the farm field and civilization area. From this study and investigation, we concluded the enlarging volume, flow velocity, and long traveling of the debris materials of Pasir Panjang landslide controlled by the steep slope topography, highly weathered volcanic breccia, and saturated layers in the vicinity of the river and farm field area. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.