# Airborne Electromagnetics to Improve Landslide Knowledge in Tropical Volcanic Environments
## 4 Results
### Slope Instability Analyses
#### 4.2.3 Influence of the GWL  
The initial hypothesis following antecedent monitoring and observations is that the two case studies are controlled by the rise of the GWL corresponding to the material saturation. Therefore, several scenarios were tested by gradually increasing the GWL. The scenarios take into account the best calculation cell size (i.e., 10 m), and the best set of geotechnical data. For the La Medaille landslide, the goal is to find the most favorable materials prone to failures, confirming the landslide-triggered hypotheses. Thus, several scenarios were tested with computations of failures (i) for R2 in dry and completely saturated situations (i.e., GWL = 0 and GWL = 1), (ii) for R3 in dry and completely saturated situations (i.e., GWL = 0 and GWL = 1), and finally, (iii) for both types of materials in dry and completely saturated situations (i.e., GWL = 0 and GWL = 1). Once the materials to be considered have been defined, the influence of the GWL is tested by gradually increasing its level from 0 to 1 (with a maximum at 0.5 m below the topography). For the Morne-Figure area, considering the stakes, the main goal is to improve the knowledge about the material of the compartment face and the rise of the GWL to best understand future landslide-prone  
Figure 12: Comparison of results with cell sizes of 5 m and 10 m for the Morne-Figure area. (**a**) Map computed for shallow translational landslides with a cell size of 10 m. (**b**) Map computed for shallow translational landslides with a cell size of 5 m. (**c**) Map computed for moderately deep rotational landslides with a cell size of 10 m. (**d**) Map computed for moderately deep rotational landslides with a cell size of 5 m. (**e**) Statistical tests for the four computed maps. For each map the GWL = 1. Hillshade maps were produced with the Litto3D DTM (IGN, 2010). areas. The maximum depth of the retained GWL takes into account the observations (i.e., the GWL reaches a maximum at 0.5 m below the topography for moderately deep rotational landslides). For the La Medaille area, the results (Figure 13) show that in dry conditions (i.e., with a very low GWL), the landslide is computed as stable with a failure probability under the low class threshold. Only the dacitic screes (R2) on very steep slopes are computed with a high probability of failure. For the same R2 materials located on gentle slopes, by taking into account these materials alone, then the computed failure probabilities are low, even if the GWL = 1 (Figure 13a). Only a very thin area of the upper part and the northern boundary of compartment A are computed with high failure probabilities. The relative error and ROC-AUC are not satisfactory, with values of 0.73 and 0.55, respectively. By taking into account R3 formations, it is possible to better delineate some failure areas (Figure 13b). The tests are better than previous tests (Figure 13a) but not optimum, with a relative error of 0.32 and an ROC-AUC equal to 0.73 (Figure 13b). Nevertheless, the results are not exactly consistent with field observations, particularly the failure area observed upstream of compartment A. Finally, by taking into account both types of materials (R2 + R3), the failure areas observed in the field are well identified (Figure 13c). The relative error and ROC-AUC are acceptable, with values of 0.25 and 0.89, respectively. Figure 14 illustrates the sensitivity of materials when the GWL varies inside of those materials. Therefore, water appears as the engine of recurrent landslide destabilization; the more the GWL increases, the more the number of computed cells with high failure  
Figure 13: Influence of the type of materials introduced to compute failure probabilities with ALICE® for the La Medaille landslide. (**a**) Computations with R2; (**b**) computations with R3; (**c**) computations with R2+R3. Hillshade maps were produced with Helium5 DTM (2013). probabilities increase.