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

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.
30 November 2020

RenovRisk Erosion: 1st seismic campaign to study the Salazie landslides

Our teams will present the RenovRisk Erosion project at La Reunion. Admire photographs of one of the biggest landslides in the world to take place in an inhabited area, the Salazie volcanic cirque.

In November 2020, a scientific study was undertaken to develop an image of the subsurface and to understand the landslide phenomenon.

© BRGM

Renovrisk : a research programme to study natural hazards associated with tropical cyclones in the southern Indian Ocean

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.

The Ilet landslip at Vidot and the Mare à Poule d’eau landslip in the Salazie cirque

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.

© BRGM - René Carayol

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.

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.
Cumulative displacement measured at the centre of the Grand Ilet landslip

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.

© Belle et al

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.

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

Action undertaken

Acquisition campaigns

  • June 2019: LiDAR survey of landslips in the Salazie cirque
  • September-October 2019: first geomorphological mapping of gravitational movements in the Salazie cirque
  • September-October 2019: second geomorphological mapping of gravitational movements in the Salazie cirque
  • November 2020: passive and active seismic campaign

Dissemination of results

  • October 2019 - Lyon: CFGI Technical seminar on "Devenir des grands glissements" (How large-scale landslips evolve)
  • October 2019 - Nice: Gravitational Hazards Seminar
  • November 2019 - Mauritius: IWS-Mauritius - Computational Geo-Environmental Mechanics: Multiphysics and Multiscale Couplings
  • May 2020 – ISPRS Online Conference - Structure-from-motion processing of aerial archive photographs: sensitivity analyses pave the way for quantifying geomorphological changes since 1978 in La Réunion island
  • February 2021 – Publication Natural Hazards - ReNovRisk: a multidisciplinary programme to study the cyclonic risks in the South-West Indian Ocean
  • March 2021 - colloque GEOFCAN online - Morphology and structuring of a large landslide in a tropical volcanic environment: contribution of geophysics to the characterization of the Hell-Bourg landslide (La Réunion)
  • April 2021 - EGU General Assembly 2021online - Structure-from-Motion applied to historical aerial photographs: parameter variability and application on landslide cyclonic evolution on France's La Réunion Island, Indian Ocean
  • April 2021 - EGU General Assembly 2021online - How to reconstruct geometrical characteristics and failure conditions of an historic atypical complex flow-like landslide: example of the Déboulé of La Ravine de l’Eglise (La Réunion Island, France)
Logos of the RenovRisk Erosion project and its financial partners

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

© RenovRisk Erosion