The sandy plain at the foot of the Piton de la Fournaise volcano (La Réunion, 2009). © BRGM - Camille André

RenovRisk: a research project on gravitational and erosion phenomena on La Réunion

03.09.2020
Four multidisciplinary research teams from the BRGM worked together with the University of La Réunion on the RenovRisk-Erosion project to study gravitational and erosion hazards in La Réunion's mountain cirques, on the scale of the Rivière des Pluies / Salazie catchment basins.

The RenovRisk research programme is aiming to study natural hazards associated with tropical cyclones in the southern Indian Ocean and to assess their impacts on economic development in the region.

The research programme comprises 4 interdependent projects: RenovRisk-Cyclones, RenovRisk-Erosion, RenovRisk-Transports and RenovRisk-Impacts, on which the multidisciplinary teams are working under University of La Reunion and BRGM coordination.

The Laboratoire Géosciences Réunion (LGSR) and the BRGM have jointly built up the RenovRisk-Erosion research programme, initiated under the MvTerre project (2003-2008) and MvTerre-2 (2011-2014), with a view to pursuing and perfecting monitoring of gravitational and erosion phenomena in the mountain cirques of La Réunion.

Gravitational and erosion phenomena in the mountain cirques of La Réunion

La Réunion, a volcanic island with a tropical climate, is regularly hit by cyclones. These extreme climatic phenomena bring not only violent winds, but also episodes of intense rainfall that trigger gravitational instability in many places.

Rainfall in La Réunion is often among the highest on record worldwide.

The Ilet landslip at Vidot and the Mare à Poule d’eau landslip in the Salazie cirque, La Réunion. Active erosion of ravines during heavy rainfall events tends to trigger ground instability and movements of land on which people may be living. The yellow contours delimit examples of active erosion zones, the blue contour delimits a stable compartment. © R. Carayol

Land movements are especially active in the Salazie cirque, because of its sheer sides and extremely heavy rainfall. Different phenomena can be observed:

  • Large, slow-moving landslips: 6 are active in the Salazie cirque, with the Grand Ilet landslip as a prime example. The latter is currently displacing a volume of more than 350 Mm3, on which houses have been built, at a rate of several decimetres per year.
  • Catastrophic phenomena: instantaneous localised gravitational movements. These can be landslides displacing tens of thousands of cubic metres of earth, rampart collapses or "déboulés", a local term that refers to a rockfall causing the Ilet edge to retreat almost instantaneously by several hundred metres.

Aims of the RenovRisk-Erosion project

By the end of 2021, the RenovRisk-Erosion aims to:

  • characterise and acquire fundamental knowledge on landslips and rockfalls in mountain cirques;
  • understand landslip mechanisms when movements are triggered by water;
  • model the dynamics of landslips as affected by climate change;
  • detect and monitor the transport of materials destabilised by river spates so that the data can be incorporated into prevention plans for tropical cyclone risks.

Monitoring displacements caused by landslips and rockfalls

To better understand the dynamics of these landslips and assess land movement hazards on La Réunion, the BRGM and LGSR use comprehensive observation networks. Since 2011, high-precision geodetic and topographic surveys and seismic and electromagnetic measurement campaigns have been conducted to investigate the main landslips occurring on a large scale and along the ramparts of the island's cirques. The BRGM is also conducting hydrological and geochemical monitoring of ground and surface water where large landslips are occurring.  A broadband seismic network has been installed to continuously monitor the transport of eroded materials in rivers.

Monitoring ground movements in the mountain cirques of La Réunion. Geodetic measurement campaign. © BRGM

Monitoring the transport of erosion products. Granulometric measurement campaign in the Rivière des Pluies. © Alicia Gonzalez - Thesis

Monitoring ground movements in the mountain cirques of La Réunion. Hydrometric monitoring campaign. © BRGM

The data have shown how groundwater accelerates these landslips during cyclonic events.

Cumulative displacement measured at the centre of the Grand Ilet landslip and modelled from rainfall data (Belle et al, 2013 and 2018). The phases of landslip acceleration coincide with episodes of heavy rainfall, as during cyclone Diwa in 2016 and cyclone Gamède in 2007.

Action undertaken

Acquisition campaigns 

  • June 2019: LiDAR survey of landslips in the Salazie cirque
  • September-October 2019: Geomorphological mapping of gravitational movements in the Salazie cirque 

Dissemination of results

"ReNovRisk-Erosion project - erosion processes and impacts", with financial support from the European Union, France and the Réunion Region.

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