The 1965 Mahavel Landslide (Reunion Island, Indian Ocean): Morphology, Volumes, Flow Dynamics, and Causes of a Rock Avalanche in Tropical Setting

In May 1965, a main landslide occurred in a deeply incised valley of Piton de la Fournaise volcano, in Reunion Island. This event occurred one day after heavy rainfalls and was consequently interpreted as a mud/debris flow. We take advantage of several sets of historical photographs to reappraise this event. They show that the collapse of a large part of the valley headwall produced a rock flow that dropped 1,740 m and traveled 5,050 m. The surface morphology suggests that the landslide produced a main flow that evolved in a secondary flow in the distal part. The main flow deposit is composed of a matrix-rich fragmented facies and a jigsaw-fracturated facies. The matrix presents a gravely-to-silty granulometry and fractal dimensions ranging between 2.472 and 2.865. Volumes of the collapsed material (59.7 +/- 3.1 Mm(3)) and the deposit (46 +/- 13 Mm(3)) were determined from a photogrammetric approach. We estimated velocities ranging between 86 and 44 m s(-1) along the flow path. Our simulations with SHALTOP reveal that the landslide geometry and velocities are well reproduced with frictional Coulomb rheology. We thus interpret the 1965 Mahavel landslide as a main rock avalanche rather than a mud/debris flow. We propose that the trigger of the 1965 avalanche, and of the smaller events in 1995 and 2001, is the water infiltration related to the intense rainfalls, which systematically preceded by one day each collapse. Finally, the succession of dry and wet years before each collapse event could promote favorable conditions to failure. Plain Language Summary In 1965, a main landslide occurred in a deeply incised valley of Reunion Island (Indian Ocean). It followed heavy rainfalls and was consequently interpreted as a main mudflow. We used historical photographs, deposit descriptions, and numerical simulations to reappraise this event that fortunately did not have any casualties. We show that about 60 Mm(3) of volcanic rocks suddenly collapsed and produced a rock avalanche that traveled over a total distance of 5 km. The flow traveled at velocities ranging between 86 and 44 m s(-1). We interpret this landslide as a rock avalanche instead of a mudflow. Our work finally suggests that the annual precipitation variability could be a main conditioning factor that promotes scarp collapses and consequently enhances erosion. The increase of precipitation variability with climate warming could therefore increase the occurrence of large collapses and rock avalanches in mountainous areas.