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

Abstract

In May 1965, a main landslide occurred in a deeply incised valley of Piton de la Fournaise volcano, in Réunion 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 Mm3) and the deposit (46 ± 13 Mm3) 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.

Past volcanic activity predisposes an endemic threatened seabird to negative anthropogenic impacts

“…First, the current deep valleys cutting the southern flank of Piton des Neiges started to incise PN4 around 30 kyr 69 . In Réunion Island, the intense and ongoing erosion results from river incision and valley scarp collapses 70 , 71 . Such scarp collapse events could have limited the colony development.…”

“…These anthropogenic impacts are not unique to Réunion Island and human-related population collapses/extinctions are well-documented for Procellariform species, including the Hydrobates leucorhous on Grand Colombier Island 4 , the Pseudobulweria rupinarum on Saint Helena Island 74 , Puffinus olsoni on the Canary Islands 78 , and Puffinus boydi on Bermuda 79 . Current anthropogenic impacts may be compounded by the ongoing loss of nesting habitat via erosion, as illustrated by the recent collapse of Bras de Mahavel 70 .…”