How Can the Morphometric Characteristics and Failure Conditions of a Historic Gully Caused by Intense Rainfall Be Reconstructed?

Abstract

In January 1980, during exceptional cyclonic rainfall, an atypical landslide, called déboulé, rapidly generated the permanent 700 m-long gully of the Ravine de l’Eglise on an inhabited plateau in Reunion Island (Indian Ocean). Retrieving the initial conditions that led to this historical process is both challenging and necessary for understanding the mechanism of gully incision and providing pointers for improving risk mitigation in relation to this phenomenon. In this study, we reconstruct the pre- and post-failure topographies using SFM (structure from motion) applied on archive aerial photographs. Based on the comparison of these digital elevation models, we estimate the volume of material eroded to be ca. 0.63 Mm3. Groundwater level increase, part of the triggering mechanism, is hindcast in the catchment of the gully using a lumped hydrological model. This model shows that in only a fortnight the groundwater level probably rose by 36 m, which could have caused a progressive increase in pore pressure and triggered formation of the gully by retrogressive landslides. We test this hypothesis by considering the pre-failure topography and the hindcast groundwater level in a deterministic model based on limit equilibrium equations to explore ground stability. The evolution of ground stability with a rise in the water table shows that the gully may have extended in a headward direction by retrogressive landslides. This is the first quantitative reconstruction of an exceptional historical event affecting the territory of Reunion Island. The methods used to investigate the Ravine de L’Eglise incision thus offer new complementary insights and challenges for understanding the mechanism and the temporality of gully formation.

Landslide Processes Involved in Volcano Dismantling From Past to Present: The Remarkable Open‐Air Laboratory of the Cirque de Salazie (Reunion Island)

“…(1981). A déboulé is a mix of erosion and a flow process that leads to the extremely rapid formation of a gully on a gentle slope <20° (Figure 7; Rault et al., 2022). Indices of internal erosion in depressions support the idea that the formation of a déboulé may be initiated by internal erosion of the epiclastic material, leading to ground collapse and bank failure.…”

“…Using these data, we delineated rain‐triggered landslides and quantified their volume. This method is promising and provides essential information for landslide hazard mitigation mapping (Gomez et al., 2015; Riquelme et al., 2019; Rault et al., 2020, 2022). For example, it has provided quantitative information on a 100‐year rainfall event in this study.…”

“…The saturation of the terrain during Cyclone Hyacinthe reached the maximum described in the last 50 years due to exceptional precipitation amounts (Rault et al., 2022). Ponds were filled or overflowed, and many streams appeared on the îlets (Advenier & Stieltjes, 1990; Humbert et al., 1981).…”

“…Using these data, we delineated rain‐triggered landslides and quantified their volume. This method is promising and provides essential information for landslide hazard mitigation mapping (Gomez et al., 2015; Riquelme et al., 2019; Rault et al., 2020, 2022). For example, it has provided quantitative information on a 100‐year rainfall event in this study. The comparison of high‐resolution lidar DEMs provided integrated information on morphological changes due to fast and slow landslides (Booth et al., 2018; James et al., 2012).…”