The need
The Cayenne mountains are known for their sensitivity to land movements, generally of small magnitude, but which can occasionally be of large magnitude, as was the case with the landslide of Mont Cabassou in April 2000 (300,000m3). At least thirty other large-scale landslides, with a volume greater than 10,000 m3were identified during several studies recently carried out by the BRGM. Following the destabilization of the southern slope of the Tigre Mountain in May and June 2021, an initial assessment by the BRGM, carried out as part of its mission to support administrations, then an ANTEA study, co-led by the BRGM and the DGTM (General Directorate of Territories and the Sea), showed that, on this same slope, large-scale landslides were not excluded and could threaten two major road axes in the commune of Rémire-Montjoly. It is in this context that the DGTM requested the BRGM to characterize the areas which will be impacted by such events, according to several scenarios of mobility and volume of materials mobilized.
The results
The study was based on data from pre-existing geological, geomorphological, geophysical and geotechnical work (data acquired by BRGM or whose acquisition BRGM co-led in collaboration with the DGTM and ANTEA Group). These data made it possible to define three destabilization scenarios of the southern slope of the Tigris Mountain, with volumes ranging from 200,000 m3 (event possible within 10 years) at 1,400,000 m3 (exceptional event, but not excluded within 100 years, in particular in the event of an extreme rainy episode).
The BRGM then modeled the propagation of these different destabilization scenarios using the numerical code SHALTOP, developed as part of a collaboration between the IPGP and the Department of Applied Mathematics, ENS Ulm. Two past and documented large-scale landslides, including that of Mont Cabassou in April 2000, were first simulated to determine realistic rheological parameters. Two settings were thus retained for the Tigris Mountain, each simulating the propagation of more or less mobile materials depending on their water saturation.
Thanks to each simulation, the BRGM thus estimated, for the two threatened roads, the impacted length, the maximum thickness and the volume of expected deposits.
Use
In the event that signs of instability are detected on Tigre Mountain, the results of this study will allow communities to adapt the means to be implemented to ensure the safety of people and property and anticipate clearance work. road axes.
