Quantifying sediment supply to the river from a tropical slow-moving landslide using photogrammetry: the case study of Grand Eboulis, Réunion Island

Landslides, in general, can release large volumes of sediment into rivers, sometimes causing river blockages, outburst floods, or mudflows. Quantifying sediment contributions from landslides is important not only for the resilient development of population in mountainous settings but also to understand river responses to catastrophic sediment inputs.

This study focuses on the watershed of Rivière des Pluies, in La Réunion, a high-standing volcanic island characterised by a particularly dissected topography. In the upper part of this mountainous watershed, a slow-moving landslide (Grand Eboulis) has been recently shown to be remobilised by shallow landsliding, supplying sediment to the river (Hopquin et al., in prep). Such contributions raise societal concerns for downstream populations exposed to flooding, as well as economic challenges due to the proximity of the island’s main airport to the river’s fan.

In this study, using photogrammetry on historical aerial images, we investigated the volume of sediment transferred by shallow landslides between 1978 and 2011. Landslide scars and deposits were mapped from the computed orthoimages, and associated sediment volumes were estimated using difference of Digital Surface Models (DSMs).

Preliminary results show that over the 33 years, 4.21 ± 0.36 Mm³ of sediment were eroded from Grand Eboulis through shallow landslides. In the river, only 0.64 ± 0.14 Mm³ remained, suggesting that the river exported 3.57 ± 0.5 Mm³ of sediment at an export rate of 108 kM³/yr. As the discharge responsible for such transfer depends on precipitation regimes, we will investigate the relationship between precipitation and sediment contributions, as well as between hydrology and the river transport capacity, at a decadal timescale, for intermediate years: 1984, 1989, 1997, and 2003.