Large-scale landslides in French Guiana: developing a methodology for risk mapping

The need

Over the past ten years, BRGM and the Departmental Authority for Inland and Marine Areas (DGTM) of French Guiana have been carrying out studies to map and characterise landslides in this region, which is subject to strong weathering effects that are rarely found in mainland France. BRGM recently identified a number of landslide events involving several tens to hundreds of thousands of cubic metres. However, until now, there has been no effective methodology in place for mapping the risks in this area.

To remedy this shortcoming, the DGTM asked BRGM to develop a methodology for mapping landslides involving several tens to hundreds of thousands of cubic metres of earth, covering both the areas of origin and propagation, in a similar vein to most recent methodological guides published for mainland France.   

The results

Initially, the study involved identifying and establishing a morphometric database of around thirty landslides using LiDAR data available for the area being studied (Cayenne peninsula). After selecting four landslides with different dynamics or magnitudes, geological maps at 1:25,000 scale were produced of the zones around these landslides. The cartographic data was supplemented by hydrogeological and geotechnical data from various studies available in existing scientific literature.

After that, electrical geophysical and seismic profiles were acquired around the periphery of the landslide events that had been selected. All this data was used to create conceptual hydrogeological and then hydrogeotechnical models of the environment around the selected landslides. These conceptual models were used as a basis for the digital modelling of different types of rupture phenomenon, taking into account different groundwater-level scenarios. The parameters were calibrated against specific profiles for each type of landslide selected, using a limit-equilibrium calculation tool (TALREN®). Once the parameters had been defined, they were entered into a spatially-based rupture model (ALICE®) to define the sectors that could become unstable under certain conditions (influence of the water table on the materials).

The sectors identified were used as a basis for calculating landslide propagation and damage probabilities. The methodology (digital processing chain) therefore combines the calculation of rupture points and of propagation, making it possible to define the probabilities of ground ruptures and the resulting landslide propagations and then determine the spatial scope of damage for types of landslides that are known for their destructive potential in French Guiana.