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
The structure of large landslides is particularly complex, due to their geological history and rugged morphology. In order to understand the mechanisms that govern these gravitational movements and thereby better understand the potential risks, we need to map their structures with imaging technologies, characterise the underground flows and analyse the results in relation to each landslide's dynamics.
Understanding the structure of a landslide with geophysics - RenovRisk Erosion Project
Geosciences for a Sustainable Earth BRGM ANALYZE, EXPERIMENT, MODEL LANDSLIDES ON RÉUNION ISLAND Cirque de Salazie Réunion Island This is the Cirque de Salazie, which was created by intense erosion coupled with heavy rains. The surface collapsed, and all these rocks began draining out of the circus. There are two other circuses nearby. Unique to Salazie are its massive landslides. There are a dozen high-volume slides. Two are particularly large, measuring some 250 million m3. Salazie is also unique for its heavy rainfall, receiving about 3 m of water per year. This heavy rainfall, coupled with geological conditions here, explain all the landslides. The problem is that homes are at risk of falling. Houses are located in areas that are moving, creating cracks that pose risks for inhabitants as well as to infrastructure. During Tropical Cyclone Hyacinthe, the pond rose up. Cathrine Ramin lived through the cyclone in 1980. It was nothing but heavy rain for some 15 days. It destroyed everything from the pond to that road you see there. We had to rebuild. There was a gazebo up there? Yes, you see it on old post cards. Hyacinthe took the gazebo. It got buried at the cliff base following a mudslide. There was landsliding beforehand, then the water took everything. Do you find new fissures? - Yes. - When it rains? After spells of heavy rainfall, whole new rifts appear. Using GPS tracking that we've been doing for 20 years, we find landsliding speeds up after rainfall. When it rains heavily, water penetrates into the subsurface, or so we think, and that water causes landsliding and speeds up landsliding. What we want to know is how the water moves and where it penetrates. Does it penetrate through cracks? Does it penetrate at the landslide base, or at some intermediary point? That's important for understanding how everything works. If it remains on the surface, the risks are not the same as if it penetrates and goes deep beneath the landslide. We have an idea of the internal structure of the slide from an aerial geophysical survey we did. We basically did a 3-D scan of the land down to a depth of 300 m. Now we want to confirm our hypothesis that there are two landsliding surfaces, one atop the other, and find out if they move at different speeds. First, we see the landslide in its entirety. Now we want to zoom in on different parts to see if there's water, if there are subsections, and identify what slide type it is in a more accurate way, while testing the hypothesis we developed from the aerial survey. October 2020 Fond de Rond-Point, Cirque de Salazie Passive Seismic Recording Project Phase One of Data Gathering The first data-gathering phase of the campaign was setting up about 100 sensors on the landslide, from the head of the slide all the way down. Sensors: 100 Geophone Seismic Recording Device Geophones have a permanent magnet inside them and a very sensitive coil that takes ground movements and converts them to electrical signals, which get recorded. Each one of the devices will record noise. Those recordings will give us an idea of how fast waves are moving, which helps us map the subsurface, almost like an x-ray. Mapping using passive recording devices November 2020 Fond de Rond-Point, Cirque de Salazie Phase Two of Data Gathering Welcome, all, to phase two of our campaign. Phase two of the project is setting up the same 100 sensors along one line that will cut across the landslide from top to bottom. The reason we want a single, 2-km-long line reading is to get a 2-D vertical image almost like a slice of the terrain, to then get an idea of subsurface hundreds of meters deep. We record waves generated by explosions of our own making. Explosives The explosive will go into boreholes two meters deep so that they're in contact with the subsoil when they explode, generating a mechanical wave that causes the subsoil to vibrate, which we'll record along our line. Nota Bene The explosions have no influence on landslide activity Explosion Number 1 Explosion Number 7 Explosion Number 9 Explosion Number 13 Each explosion generates two waves. Primary waves, which we record first, travel at a given speed underground. Then we record another type of wave, secondary waves. They travel much slower. Primary waves help us to scan the subsoil very accurately. Map generated from primary waves To get an idea of where water is, we use secondary waves, which don't travel through water. Comparing the speeds of primary to secondary waves, we can map where water is and is not. Map generated from secondary waves What we learned about this landslide was first about its high volume. At the back end, it's just under 100 m, from 50 to 75 m. The landside is biggest near the river. There it's some 200 m thick. This project helped us figure out there were two slides atop each other downstream, closer to the river. We're still researching the role each slide plays. We have a base of the slide, this first sliding mass moving. Then a second one moving atop it. Water moves between the two slides near the surface. What we want to know is if that's at different speeds. Final Map The point is to get a highly accurate map of the slide. Then we can model the slide. From there, we can simulate climate change over the long term. If cyclones get stronger, will parts of the slide move faster? Will others destabilize suddenly, causing sudden collapse? Mapping the slide, knowing its internal structure and how water moves within it gives us direct knowledge of risks to infrastructure and the population in Salazie. BRGM thanks Salazie locals for their cooperation while readings were being taken.
Interpretative cross-section of the Hell-Bourg landslide (Cirque de Salazie, La Réunion) established by combining data from multidisciplinary studies, airborne geophysical surveys (a.) and active seismic surveys (b.). (Rault et al., 2021)
The teams were able to create images of the 3D internal structure of three of the largest landslides in the Cirque (Hell-Bourg, Grand-llet and Ilet-à-Vidot). This was achieved by monitoring the landslides' movements using a Global Navigation Satellite System (GNSS) and analysing data from the ReunEM heliborne geophysical survey of 2014 and the active and passive seismic survey of 2020, as well as data from other geomorphological mapping campaigns.
We now know that these three landslides are constantly moving and even accelerating following intense rainfall events. The landslides are different from each other, in terms of their thickness (which can sometimes exceed 200m) and structure, which can be more or less compartmentalised, along both their width and depth. These complex structures have a direct influence on their movement mechanism.
Using the results
The knowledge acquired concerning the structure of these large landslides can be integrated into the existing hazard maps for ground movements, thus making them more accurate.
The data also provide a better understanding of the origin of these large landslides. Notably, we now know that their structure and mechanisms are directly linked to ancient processes of dislocation of the volcanic island, while their activity is closely linked to the extreme rainfall conditions in the Cirque de Salazie. The results also need to be considered in relation to the impact of climate change on large-scale land movements and the risks that such changes are likely to generate.
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.
Monitoring ground movements in the mountain cirques of La Réunion
Monitoring ground movements in the mountain cirques of La Réunion. Hydrometric monitoring campaign.
Monitoring ground movements in the mountain cirques of La Réunion. Geodetic measurement campaign.
BRGM intervened at our request, as an expert on ground movements and risk prevention. What makes this project stand out is the extensive range of instrumentation used by the BRGM teams to understand this unusual phenomenon, based on an innovative methodology that could be used for practical applications. Indeed, we now have a better understanding of this risk and can take appropriate measures to ensure the safety of the population and protect the area.
C. Rault, Y. Thiery, M. Chaput, P. A. Reninger, T.J.B. Dewez, L. Michon, K. Samyn and B. Aunay, Landslide processes involved in volcano dismantling from past to present: the remarkable open-air laboratory of the Cirque de Salazie (Reunion Island), JGR- Earth Surface (accepted)
C. Rault, K. Samyn, B. Aunay, A. Bitri and M. Delatre, 2021 New Insights from a Multi-Method geophysical Investigation on a Very Large, Slow-Moving Landslide (Hell Bourg, Reunion Island), First Break, Volume 39, Issue 8, p. 71-78. https://doi.org/10.3997/1365-2397.fb2021063
Belle, P., Aunay, B., Bernardie, S., Grandjean, G., Ladouche, B., Mazué, R., Join, J.-L., 2014. The application of an innovative inverse model for understanding and predicting landslide movements (Salazie cirque landslides, Reunion Island). Landslides 11, 343–355. https://doi.org/10.1007/s10346-013-0393-5
Tulet, P., Aunay, B., Barruol, G., Barthe, C., Belon, R., Bielli, S., Bonnardot, F., Bousquet, O., Cammas, J.-P., Cattiaux, J., Chauvin, F., Fontaine, I., Fontaine, F.R., Gabarrot, F., Garabedian, S., Gonzalez, A., Join, J.-L., Jouvenot, F., Nortes-Martinez, D., Mékiès, D., Mouquet, P., Payen, G., Pennober, G., Pianezze, J., Rault, C., Revillion, C., Rindraharisaona, E.J., Samyn, K., Thompson, C., Vérèmes, H., 2021. ReNovRisk: a multidisciplinary programme to study the cyclonic risks in the South-West Indian Ocean. Natural Hazards. https://doi.org/10.1007/s11069-021-04624-w
La Réunion GeoSciences Laboratory, University of La Réunion