A combined geophysical survey on an active debris flow

The Peillettes landslide in the western Swiss Alps is a complex landslide threatening villages of the main Rhone Valley. The slope is between 25 and 30 degrees. Trees cover a large part of the unstable zone which extends between elevations of 1000 to 2100 m; its width is 1000 m about. This slope is a known unstable zone and it was the subject of many studies, considering the potential impact on the neighbouring communities. In spite of many works to increase drainage of the run-off water, the zone remains unstable and it is reactivated after each particularly rainy period. The lower part of the landslide is the most active of the instability. Here a debris flow called le Grand-Tsâble is located few meters away from the closest country cottages. The goal of this work is to evaluate the contribution of several geophysical methods and in particular we will discuss the results of combined geophysical surveys: high-resolution P and S wave seismic reflection (PHR and SHR) and georadar (GPR). We focus on imaging the subsurface of the active debris flow and comparing the results of each method, which are often complementary.