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There is a general lack of understanding on the detailed processes and mechanism of low-angle translational block slides. The September 20, 2018 Naga landslide, with a volume of 27 M m3 and a runout distance of 1.34 km, provided new insights on the initiation, transport, and deposition mechanisms of this type of landslide. Drone photogrammetry, video footage, satellite images, slope stability analyses, and field evidence revealed that the landslide occurred as multiple block detachments along a series of tension cracks that formed and grew progressively prior to the main failure. Predominance of intact upright blocks traceable to the distal end of the deposit indicates dominant translational motion. Facies within the Naga landslide deposit revealed that at least three processes (slide, dry flow, and fall) occurred during the main movement. Post-slide processes immediately after emplacement included small avalanches and rockfalls related to the instability of the landslide deposit and the main scarp. Preliminary slope stability analyses showed that the slopes were marginally unstable (FoS 0.67–3.96) even in dry conditions. The low-angle profile of the slip surface (6°) favors stability, but low shear strength of the limestone bedrock and unsupported cut slopes may have contributed to the failure. [ABSTRACT FROM AUTHOR]

Abstract: The role of landslides in generating or enhancing a flash flood event in Aklan, Panay Island, Philippines, resulting from Typhoon Fengshen, was investigated. The flash flood occurred between 1100h and 1400h (GMT+8) of 21 June 2008 and inundated Sitios Agbatan and Abaya in the municipality of Libacao. Peak discharges at Kipot, a channel constriction in Aklan River about 10km upstream, were independently derived using a hydrologic model (302.6m3/s) and Manning''s equation (7000m3/s). The modeled flood peak reached Kipot at 0300h on 21 June 2008 and the communities between 0600h and 1000h on June 21, at the earliest. The actual flood peak arrived at Sitios Agbatan and Abaya between 1100h and 1400h on June 21, at least 1h earlier than the modeled arrival. While the flash flood may have been caused by the preceding rainfall alone (234.4mm from 0600h June 20 to 0600h June 21), the discrepancies in peak discharges and the delayed arrival time suggest the possibility of damming and eventual breaching of the dam. Evidence of the damming was found at the channel constriction in Kipot. A landslide at this site was mapped, characterized, and inferred to have completely blocked the channel and caused the damming. Implications to flash flood risk assessment are discussed. [Copyright &y& Elsevier]

A thorough understanding of the failure initiation process is crucial in the development of physicallybased early warning system for landslides and slope failures. Laboratory-scale slope models were constructed and subjected to instability through simulated groundwater infiltration. This is done by progressively increasing the water level in the upslope tank and allowing water to infiltrate laterally towards the toe of the slope. Physical changes in the slope models were recorded by tilt sensors and video cameras. When the model slope was destabilized, the chronology of events occurred in the following sequence: (1) bulging at the toe, (2) seepage at the toe, (3) initial failure of soil mass, (4) piping, (5) retrogressive failure, (6) formation of tension cracks and (7) major failure of soil mass. Tension cracks, piping and eventual failure are manifestations of differential settlements due to variations in void ratio. Finite element analysis indicates that instability and subsequent failures in the model slope were induced primarily by high hydraulic gradients in the toe area. Seepage, initial deformation and subsequent failures were manifested in the toe area prior to failure, providing a maximum of 36 min lead time. Similar lead times are expected in slopes of the same material as shown in many case studies of dam failure. The potential of having a longer lead time is high for natural slopes made of materials with higher shear strength thus evacuation is possible. The tilt sensors were able to detect the initial changes before visual changes manifested, indicating the importance of instrumental monitoring. [ABSTRACT FROM AUTHOR]

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