French Guiana: study of landslide propagation on the Cayenne peninsula

The need

The Cayenne Mountains are known for their susceptibility to landslides, which are generally small-scale. However, these landslides can occasionally be of a large scale, as in the case of the Mont Cabassou landslide in April 2000 (300,000m³). At least thirty other large-scale landslides, with volumes in excess of 10,000m³, were identified during several recent BRGM studies. Following the landslides on the southern slope of Montagne du Tigre in May and June 2021, an initial assessment was carried out by BRGM, as part of its mission to support local authorities. This was followed by a further study conducted by the ANTEA group, co-managed by BRGM and the Departmental Authority for Inland and Marine Areas (DGTM), which showed that large-scale landslides on the same slope could not be ruled out and could even threaten two major roads in the Rémire-Montjoly municipality. Within this context, the DGTM therefore asked BRGM to characterise the areas that would be affected by such events, according to several scenarios in terms of the mobility and volume of materials involved in the landslides. 

The results

The study was based on data from pre-existing geological, geomorphological, geophysical and geotechnical studies (data which was collected by BRGM or gathered by BRGM in collaboration with DGTM and ANTEA Group). This data was used to define three landslide scenarios on the southern slope of Montagne du Tigre, with volumes ranging from 200,000m³ (possible event within 10 years) to 1,400,000m³ (an exceptional event, but which cannot be ruled out within the next 100 years, particularly in the event of an extreme rainfall episode). 

BRGM then modelled the propagation of these different landslide scenarios using the SHALTOP digital model, developed as part of a partnership between IPGP and the Applied Mathematics Department, at the Ecole Supérieure Normale (ENS) in Paris. Two large-scale, documented landslides, including the Mont Cabassou landslide of April 2000, were first simulated to determine realistic rheological parameters. Two configurations were selected for the Montagne du Tigre, each simulating the propagation of more or less mobile materials depending on their water saturation level.

Thanks to each simulation, BRGM was able to estimate the linear distance affected, the maximum thickness and expected volume of deposits for the two roads at risk.

Using the results

Should any signs of instability be detected on the Montagne du Tigre, the results of this study will enable the local authorities to adapt the measures that need to be implemented to ensure the safety of people and property, and to anticipate road clearance work.

The partners

• DGTM
• Antea Group