Major landslide in Harmalière

(Click on the images to enlarge it)

A landslide occurred around 27 June 2016 in the combe d’Harmalière, in the commune of Sinard (Nord Trièves). This is the reactivation of a well-known clay landslide in the region, a major event of which, on March 7, 1981, caused the displacement of approximately 250,000 m3of materials in Lake Monteynard-Avignon.

The first observations following a field visit on 4 July 2016, allow us to estimate the affected area at approximately 70000 to 80000 m² (approximately 200m. by 400m. ; Fig. 1). Depending on the depth of the fracture surface (still unknown but located between 10 and 40 m deep), the volume of slipped clays could exceed 1.5 million m3. This would make it the largest landslide in the Trièves since 1981. The stakes remain very low because only woods, pastures and fields are affected by the landslide. Upstream, the nearest house is located in the commune of Sinard, at a distance of about 200 m. from the escarpment.

Initial evidence suggests that loud noises were heard on June 27, 2016, between 14:00 and 15:00. A first analysis of the permanent seismic data of the AVM station (located a few hundred meters away ; permanent station of the OMIV Observatory on slope instabilities ; https://omiv.osug.fr/) confirms these testimonies and indicates a number of hourly seismic events increasing sharply from the same time (Fig. 2).

Figure 1 : Emprise du glissement de la combe d’Harmalière.
© Grégory Bièvre
Figure 2 : Nombre d’événements sismiques détectés à la station AVM.
© Agnès Helmstetter

 

 

 

 

 

 

 

Similarly, a first analysis of available satellite images confirms that the major activity of the landslide occurred after June 27, 2016 at 12:40 (Fig. 3).

The top escarpment reaches a height of about 20 m. and many blocks have not yet detached (Fig. 4). It seems almost certain that the activity of the landslide will continue during the summer storms. The ISTerre laboratory will monitor this exceptional event, initially by implementing seismic and geodetic monitoring techniques on the ground. The temporal and spatial characterization of the deformation at the slope scale will be carried out in part by the use of aerial and/or satellite images. Environmental data (meteorology, subsoil hydrology) will be provided by the OMIV Observatory, whose Avignonet site, which is adjacent to the Harmalière landslide, has been equipped since 2006.

Figure 3 : Comparaison d’images satellites entre le 27 juin 2016 à 12:40 et le 7 juillet 2016.
© Pascal Lacroix
Figure 4 : Vue de l’escarpement sommital. L’escarpement à l’arrière-plan correspond à la trace de régression du glissement depuis les années 1980.
© Grégory Bièvre

 

 

 

 

 

 

 

 

 

 

 

 

Following this summer crisis, the site was instrumented with low-cost seismic sensors and GNSS systems and their measurements show that the situation has continued to evolve significantly. Since the summer, two important events have occurred. The first event occurred at the end of November 2016. It was used to record the vibrations when a clay scale (Figure 5) equipped with a velocity meter fell. The comparative analysis of seismic signals between this sensor and a reference sensor located ten metres behind the escarpment is underway.

Then on 29 January 2017 at the end of the day, a second event lasted a few hours. The reference sensor, accompanied by a GNSS system, slid with a scale about 5 m deep (Figure 6), over a height of 10 to 15 m. Figure 7 shows the change in altitude as a function of time of the internal GNSS sensor associated with the seismic station.

Figure 5.
Le capteur sismique quelques jours avant la rupture de l’écaille en novembre 2016. © Grégory Bièvre
Figure 6.
Glissement du 29/01/2017. Le matériel géophysique est dans le cercle rouge. Le nouvel escarpement fait une hauteur de 10 à 15 m. © Mickael Langlais

 

 

 

 

 

 

 

 

 

 

 

 

 

 

All the equipment, which was still working, could be recovered, as well as the data. This is an exceptional data set that will allow the evolution of seismic and mechanical parameters and the kinematics of deformation and rupture to be analysed jointly. The permanent instrumentation of the nearby OMIV observatory (200 m.) will also make it possible to evaluate the influence of environmental control factors (temperature, rain) on the reactivation of this unstable mass. These permanent stations make it possible to monitor the evolution of the slip in real time. The figure showing the micro-seismic activity over the last 30 days is available here : ftp://ist-ftp.ujf-grenoble.fr/users/helmstea/AVIGNONET/fig_lastmonth.pdf.

Figure 7.
Variation d’altitude du capteur GNSS associé à la station sismique. © Agnès Helmstetter

Mapping since the beginning of the summer 2016 reactivation indicates a regression of the main escarpment by at least 30 m. to the north. Since the major crisis of 1981 and depending on the area under consideration, the total decline has been between 50 and 100 m, corresponding to an average rate of decline of about 1.5 to 3 m/year. The first dwellings are located about 200 m away. Although the sliding activity is not continuous over time, they could thus be impacted within a few decades.

Contacts :
Grégory Bièvre - +33 (0)4 76 63 51 73
Denis Jongmans