Mechanisms of the landslide movements: displacement and velocity study between 1997.5 and 2006.5
Monday 26 October 2009 by Thomas Lebourg

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The calculation of the motions of all targets of the La Clapière site between 1997.5 and 2006.5 allows to get a more global point of view concerning the whole movement of the landslide. We can notice:

- a general slowdown of all mobile targets, but much less strong in some parts of the landslide as the wrenching part of the North-West lobe (towards the C6 target),
- a major crisis (very high acceleration: till + 50 m/year in the North-East lobe...) visible on every target in 2001. Nevertheless it does not affect all lanslide zones with the same intensity: it is extremely high in the North-East lobe, very high in the main and lower landslides (see the figure below), but less high in the wrenching part of the North-West lobe,
- an important crisis at the beginning of 2003, with less intensity than the 2001 one. It seems to affect the whole targets in a more homogeneous way than the 2001 crisis did,
- many big accelerations then slowdowns between 1998 and 2000, only observed in the North-East lobe.

 

The velocities in meters/year of several targets with regard to the tacheometer during the 1997.5-2003.7 period, with the 2001 crisis in the middle of the graph (the curves have been translated vertically for a better readability. The starting velocities in 2002.7 and the maximum velocities during the crisis are indicated in the graph)

It is interesting to note that the two crises affect first the North-East lobe (and the North-West lobe concerning the 2003 crisis), then from 1 to 2 months later the rest of the slide. This observation is crying for the 2003 crisis, as shown by the following figure. The crisis initiates in the upper parts of the slide, then spreads out according to the direction of the slope till it affects the lowest areas of the site.

 

The velocities in meters/year of several targets with regard to the tacheometer during the 2003 crisis (the curves have been translated vertically for a better readability. The starting velocities in 2002.7 and the maximum velocities during the crisis are indicated in the graph)

The analysis of the phenomenon leads to the following conclusion. The North-East lobe and to a less extent the North-West lobe have velocities highly superior to the rest of the landslide ones and moreover are heterogeneous and unstable. Under conditions favorable to landslide (heavy rainfall for example), these zones may suddenly accelerate. Then they come and hit the lower zones (main landslide) creating an important overweight that generates their acceleration. Finally the crisis spreads out little by little till the Tinée River valley.