Deadly Landslide and Debris Avalanche in Abikar Village, Farsan City, Chaharmahal and Bakhtiari Province, Iran.

Abstract On the evening of April 1, 1998, near Farsan city, Chaharmahal and Bakhtiari province, Iran, along the Labad river, the southern edge of the valley of Mt. Kino slid down into the Labad river and buried Abikar village under several meters of debris on the opposite flank of the river valley. The overall casualties and damages claimed the life of 54 people (20 men, 30 women, and 4 infants), the death of 1300 livestock, and the destruction of 40 hectares of farmlands and orchards. The observation of several pieces of extraordinary evidence in the landslide area, such as the lack of a landslide dam or barrier across the river after the slope sliding, evidence of high velocity debris flow, the transformation of some debris material by jumping into the far end of debris flow, and the effects of a severe storm in front of debris flow mass, have made this phenomenon a concerning complex landslide event for further research. The causes for long debris-flow run out have been discussed in some works. Most of these studies have pointed out the effect of air cushion formation under the debris collapsed into the river valley that facilitates the long-distance transport of detrital flows. Another rare mechanism for interpreting the long-distance travel of rock boulders is blocks and boulders displacement in the form of jumping, which might be due to the energy from collisions between heavy rocks and the hard bottom of the river, breaking them down into smaller pieces and making them jump into the far distant points. Field evidence observed in Abikar landslide indicates that the occurrence of this landslide and the consequent events were a combination of these two mechanisms. Part of the debris has moved in a rapid flow, and the other part has jumped to the opposite flank of the river over the debris flow.The deadly landslide in Abikar followed an exceptional rainfall that reached 190 mm during a week prior to the landslide event. In this event, the southern flank of the Labad river valley that composed of limestone, shale, and marlstone layers with a slope angle of 75 degrees and the highest point of 1200 m from the bottom of the river, fell down into the valley and mounted the opposite bank of the river with the deposit height exceeding 100 m. Some observed evidence of the field study showed that the toe erosion by the river as well as intensive rainfall could be the main triggering factor for this landslide. After the fall of southern block (1200 m height, 600m width and average thickness of 30 m), most detrital material moved toward the opposite flank by following the ground surface with a high shear force that cut buildings, gardens, and other people properties to the depth of approximately 6 m. Observing a transported walnut tree with the root and trunks at the end part of the debris confirms the high shearing energy of the debris. The uprooting of shrubs and clear-cutting of the entire vegetation cover to the distance of 100-200 m in a vast area in front of the moving debris flow confirmed the generation of a severe typhoon in front of the debris as well as the generation of high-pressure air cushion under the debris. Another interesting observed evidence confirming the movement of some material by jumping is the presence of large masses of source rock on the top of the deposit and at the end of the debris without any sign of mud contamination of the debris matrix.