Dynamics and controls of a tropical slow moving landslide measured by remote sensing: the study case of Grand boulis, Runion Island.

Landslides are efficient erosion processes that release large volumes of sediments in rivers, posing threats to nearby population when remobilised during large flood events. Thus, understanding the dynamics and controls of landslides and quantifying the volumes involved in subsequent sediment transfer are crucial for resilient development in mountainous settings. In this study, we use the very high standing island of Runion (Indian Ocean) as a natural laboratory to investigate the interaction between landslides, sediment transfer and climatic forcing. This volcanic island, characterised by mountainous landscapes resulting from intense river incision and frequent landslides, shows physical erosion and chemical weathering rates exceeding most of active mountain belts (Gayer et al, 2019). Here, we focus on studying the Grand boulis landslide short-term (i.e bi-monthly) and mid-term (i.e multi-decennial) dynamics using SAR imagery and photogrammetry. First, we computed cumulative displacement maps and time series using the AMSTer software and Sentinel-1 StripMap images acquired every 12 days between 2017 and 2021. These time series reveal velocity fluctuations, including acceleration and/or deceleration episodes, correlated with extreme climatic events such as intense precipitation and drought episodes. Furthermore, from 2017 to 2021, Grand boulis exhibited a characteristic slow-moving landslide behaviour with a consistent eastward movement of up to 14 cm/year, coupled with continuous subsidence of up to 9 cm/year. The mid-term dynamics of Grand boulis, spanning from 1950 to 2011, was investigated using a series of nine Digital Surface Models (DSM). Each DSM was derived from historical aerial photographs obtained by the French National Institute of Geographic and Forest Information (IGN) and processed through the photogrammetric workflow of Lucas and Gayer (2022). Preliminary findings indicate numerous catastrophic failures in or near the steepest slopes of the landslide that experience the highest velocity gradients. The sediment volumes involved in such events range from a few tens of thousands to millions of cubic meters, predominantly released into the nearby river and transported away from the landslide. Our results suggest that the Grand boulis landslide is in an active mobilisation phase with a continuous slow displacement primarily influenced by extreme climatic events and with catastrophic failures that release large volumes of sediments in the nearby river.