The La Clapière landslide ( Alpes-Maritimes ) involves 50 million m³. Identified around 1980, the phenomenon showed a gradual acceleration with score seasonal modulations. After a acme in speed in 1987-88 that raised fears of a catastrophic tear, and led to the mining of a diversion tunnel for the river, a deceleration occurred, attributed to better drain in the moving aggregate. The La Clapière landslide is located in the Southern Alps ( Figure 1 ), 80 kilometer north of Nice. It developed on the leave bank of the Tinée, a feeder of the Var, less than one kilometer downriver of the village of Saint-Étienne-de-Tinée. The landslide extends from the valley buttocks, at an elevation of 1100 thousand, to 1760 m. The sum area of the landslide is approximately 1.2 km² and its volume is estimated at around 50 million m3 ( 50 hm3 ). Forty years ago, boulder falls and road deformations attracted care and a phenomenon of instability on the gradient was detected. A monitor of the movements was in position at the end of 1982, by outdistance measurements from the antonym side. In 1984 the road was temporarily diverted and then, in 1986, definitively on the right bank slope of the Tinée river .
A catastrophic break was expected in the late 1980s. To avoid the risk induced by a dam in the valley, a burrow project was studied, following the chew the fat of Minister H. Tazieff in 1988, and built in 1989-1991 under the right bank of the Tinée river. Zones prohibiting access to the foot of the slide and construction in the assume spreading area have been defined. A contingency plan has been put in place. however, deoxyadenosine monophosphate early as 1988, a deceleration of the movement appeared, interrupted by a few seasonal worker peaks, and in 2019 it seems that the main mass stabilized, after a accumulative shift of more than a hundred meters ( see the animation Clapiere.avi ) .
1. An honest-to-god phenomenon reactivated
The slope is marked by the footprint of the Tinée glacier that occupied the valley : steep gradient at the free-base ( about 40° ), slope failure around hill 1600 molarity and softer slope above. The connecting rut of the Rabuons feeder borders the slide to the southeast ( Figure 2 ). The slide has a crown divided into two lobes ( Figure 2 ) ; its foot, at the valley buttocks, gradually covered the road and the alluvial sediment by pushing back the Tinée.
Reading: La Clapière: is the largest French landslide still a major risk? – Encyclopedia of the Environment
annual rain averages 990 millimeter in Saint-Étienne-de-Tinée. The upper part of the gradient, which rises to 3000 thousand, is covered with snow in winter and spring .
1.1. geology
The La Clapière landslide is located on the southwestern edge of the Argentera-Mercantour massif, in contact with the aqueous Triassic. It develops on a gradient composed of metamorphic rocks of the Hercynian age, chiefly gneiss, amphibolites and migmatites : the Annelle series, in which the 80 m thick Iglière formation can be individualized and composed of massive migmatites ( Figure 3 ). The proportion of mica ( biotite ) is high in some levels of the Anelle series, promoting change .
The main structural features are derived from the Hercynian and Alpine tectonic phases [ 1 ]. The Hercynian foliation is by and large parallel to the Tinée valley, i. e. north-west/southeast, and its drop, at astuteness, is 60-80° towards the inner of the massif ( Figure 3 ). In addition to metamorphic foliation, two discontinuity directions have a major role : N10-N20° and N60-N90° .
The presence of gypsum Triassic rocks at the foot of the slide, which would present be masked by alluvial fill, remains a hypothesis, in the absence of boreholes .
After the glacial retreat, dated 13,000 years ago B.P. ( Before Present : origin = + 1950 ) in the Tinée Valley, glacio-alluvial and alluvial formations filled the valley floor to a thickness of several decameters .
From a hydrogeological point of view, a two-layer model can be accepted : one, basal, connecting to the alluvial groundwater of the Tinée and impregnating the foot of the slide, and the other, perched in the decompress zone of the gneisses ( a few surface decameters ), feeding the top of the slide and in particular the amphetamine slide .
1.2. history of slope destabilization
The date by cosmonucleic acid of rocky escarpments on the slope, which would mark the inaugural manifestations of destabilization, provides an age of 10,000 years B.P. or about 3000 years after deglaciation. The first base ruptures corresponding to the current cutting would date back to 3600 B.P. ( -1650 years ) .
On the 1938 aeriform photograph, a begin of a high escarpment is visible, corresponding to the northwest lobe of the current landslide. In the 1950s, the two lobes formed a clean, high escarpment while the talus cone of La Clapière sensu stricto was enriched with many blocks .
It was around 1976 that the activity of the talus cone and the deformations of the road located at the animal foot of the slope alerted the authorities ; following a visit by Jean Goguel to the site, a note from the Laboratoire des Ponts et Chaussées de Nice in 1977 questioned the being of a movement involving a large separate of the slope. distance measurements confirmed the overall bowel movement ( Figure 4 ), with score seasonal fluctuations in relation back to rainfall or snowfall, and a clear tendency towards acceleration ( Figure 5 ) .
A new rupture under the northwest lobe developed in 1987, regularizing the upstream escarpment in an bow ( Figure 2 ). During the winter of 1987-88, speeds of 10 cm/d were reached at some points ( Figure 5 ). however, the drift then slowed quickly .
A significant regression upriver of the Rabuons lobe occurred around 1989, accentuated in 1997, delimiting the upper slide ( about 5 hm3 ), which gradually advanced over the chief slide ( Figure 2 ) .
The slowdown, even marked by peaks during identical wet periods, peculiarly in the upper landslide, has continued until nowadays when movements are first gear ( less than 20 cm per year in the main landslide since 2010 for most of the targets ). Since the early 2010, significant displacements of the NW lobe have been observed, but without regression upstream .
After a accumulative displacement of 100 to 130 m, the slide is now subject to surface corrosion, which degrades in particular the upstream escarpment ( Figure 6 ) .
2. A slip observed and measured from every angle
2.1. distance measurements
A system of distance measurements by infra-red from the antonym side has been operational since 1983 [ 2 ]. The measurements were acquired by two distance meters, located 1 kilometer apart, for several years, then by a unmarried device. Between 30 and 50 targets are targeted in and around the swoop .
Variations in humidity and atmosphere temperature in the valley interrupt electromagnetic waves. To correct the leave errors, measurements are taken each time on 3 or 4 fixed targets located on the periphery of the skid. For distances ranging from 600 to 1800 meter, the accuracy is estimated at between 3 and 6 millimeter after correction .
In summation, geodetic measurements were carried out once a year to determine the 3D displacement vectors .
2.2. Digital Differential Terrain Models
The exploitation of two digital terrain models ( DTMs ) taken at different dates gives an judgment of the altitude variations Dz of the surface at a given detail ( x, y ). Serratice [ 4 ] thus compares 1970 and 1997 DTMs from aeriform photograph : the ( apparent ) swelling in the lower region of the slide and the ( apparent ) settling in the upper separate are well highlighted ( several decameters ), but the book balance shows a very modest variation, less than 1 % .
2.3. mundane and satellite images
In addition to the classical multi-date photo-interpretation, which makes it potential to identify significant evolutions in the err [ 5 ], a quantitative exploitation of forward pass photograph has been carried out by different authors .
Delacourt et alabama [ 6 ], after rectifying the forward pass photograph of 1983, 1991 and 1999, estimate the horizontal displacements of the respective landslide sectors ; in particular, between the two extreme dates, the Iglière ’ south bar increased by 160 m in the NNW incision and by 120 m in the SSE ; the foot advanced by 115 m .
The picture correlation technique was used by Delacourt et alabama [ 6 ] with aerial photograph from 1995 and 1999. The authors obtain a map of the horizontal displacements between these two dates .
3. Kinematic and mechanical models
3.1. The ancient deformations
The slope of La Clapière is made up of gneiss whose steering is analogue to that of the valley and whose foliation has a hard dip towards the inside of the slope. however, we observe that this dip gradually decreases as we approach the coat until it becomes horizontal, or even invert, around the err area ( Figure 3 ). This change in dip was explained either by a gravitational phenomenon – mowing: the banks would tip under the effect of gravity and the gradient – or by a fold of tectonic origin, the two phenomena being able to combine .
The convex geometry of the slope, left by the glacier, is favorable to a destabilization of the lower part of the slope. The glacier was a foot stop and its dissolve caused an asymmetry in the massif, as has often been observed in frigid valley. however, destabilization seems to have started well after the melt of the Tinée glacier. A climatic factor and possibly a all-out gypsum dissolving may have contributed to the knowledgeability of ruptures.
Field observations and the interpretation of forward pass photograph led Follacci [ 3 ] to a model of the development of the rupture, heterogeneous in time and space. The tear beginning develops in the SE lobe, weakened by the nearby presence of the Rabuons valley. It then reaches the NW lobe, above Iglière ’ s cake and finally over the entire stature ( Figure 7 ) .
It was only around 1990-1991 that the upper and lower speeds converged, indicating the transition to an overall rotational slip .
The trope of a fracture surface developing as it gradually “ smooth ”, as proposed by Follacci [ 2 ], can frankincense be retained ( Figure 6 ). In accession, the Iglière ’ randomness bar, made of massive migmatite, was a strengthener for the slope whose progressive tear may have allowed the acceleration of the 1980s. It should be borne in mind that “ harshness ” on a sliding open can typically have a height of 1 to 2 % of its length in the direction of the slope, which represents here more than ten meters .
3.2. The holocene slither phase : a bit of mechanics
Considering the accumulative supplanting, it can be assumed that after 1988 the pseudo-circular fracture surface formed. We do not have any boreholes in the slide but we can estimate the depth of the slide at about 90 m, using the drop of the vectors moving along the slope .
A backward analysis assuming a boundary equilibrium ( without slick ) leads to residual mechanical parameters : coherence deoxycytidine monophosphate ‘ roentgen = 0 and friction angle φ ’ roentgen = 32.5° .
The kinetic energy of the dislocate ( ½ MV² ), during the 1987-8 crisis, was less than 1 Joule, but the day by day electric potential energy loss was about 100 MJ. The enormous electric potential energy reserve is consequently consume entirely in the form of deformation/rupture within the moving mass and heat along the skid surfaces .
If we compare the morphology of the slope between 1970 and 1992 ( 100 thousand translation, compared to a breaking length of 1000 meter ), the decrease in locomotive torsion could explain, at least in depart, the slow after the 1987 bill. But a hydrogeological cause can besides be invoked .
4. The importance of the hydrogeological component
Seasonal variations in rush were obtrusive in the first base years of measurements ( Figure 8 ) and attempts to correlate speeds and precipitation ( or Tinée flow ) have increased, for scientific purposes such as monitor and warn .
Given the complexity of hydrogeology in a rocky massif such as La Clapière, the copulate between the water supply P ( rain or bamboozle thaw, even if the consideration of snow dissolve is credibly fallible ) and the sliding amphetamine V was simulated using unlike types of “ black boxes ” or grey boxes. To better reflect the P → V command, the process is divided into a hydrogeological part “ P → H ” and a mechanical part “ H → V ”. All models thus consider a pseudo-piezometric degree H calculated as the difference between the water provide ( precipitation P, or better percolation I ) and drain. It is estimated H on day n+1 from H on day n and P on the former days. The mechanical part of the models uses different laws that plainly link rush or acceleration to height H .
All these models allow to obtain a thoroughly agreement between calculations and measurements by adjusting their parameters to the measurements, but this calibration must be renewed frequently. In accession, the models are unable to predict major changes in slope behavior, peculiarly the 1987 bill speed. This seems to confirm that a significant change in hydro-mechanical behavior has occurred. similarly, in 2001 or 2013, the “ jolts ” observed ( Figure 5 ) could be explained by temp clog .
This weakness of the models therefore makes it impossible to predict a catastrophic break by this type of method acting ; however, the pastime in making a calculate within a few days remains, as any discrepancy between the forecast and the measure may be an indication of a deepen in behavior .
More generally, the sensitivity of the main slip to water inputs can be assessed by the proportion of annual supplanting D to annual input P ( Table 1 ). The decrease in D/P by a factor of 20 to 40 between 1986/1987 and after 2000 is noteworthy .
The development over time of the water control can be interpreted as resulting from a deformation/permeability coupling. thus, the deceleration after the 1987-88 point ( Figure 7 ) would reflect a fairly rapid open of discontinuities, facilitating drain. It should be noted that the secondary coil bill in 1996-97 could correspond, conversely, to a irregular decrease in drain capacity ( sealing of fractures, etc. ) .
5. The generation of a landslide
The question of a possible clogging of the valley arise in the mid-1980s, during the acceleration of a apparent motion that involved about 50 hm3. several simulations of material generation and spread have been implemented, with different approaches : continuous syrupy fluid medium, consecutive elementary volumes sliding with clash, without neglecting simple methods such as the cone method acting ( Figure 9 ) .
6. Messages to remember
- In France, La Clapière was the first major slope movement to benefit from dense and precise monitoring instrumentation, with the Séchilienne (Isère) site being equipped a few years later.
- While this instrumentation was mainly motivated by a concern for public safety, the scientific contribution was very rich, mainly in the analysis of hydrogeological control. It should also be mentioned that, in return, the reliability of operational monitoring is made more reliable thanks to advances in understanding phenomena.
- The history of La Clapière is now almost over, with the truly active phase now giving way to a period of slow degradation by erosion – unless there is a significant upstream decline. The striking fact of this multi-millennial history is that there was no rapid catastrophic movement in the end, mobilizing several tens of millions of cubic meters, an event that seemed very likely in 1987.
- The rather abrupt slowing of La Clapière at the end of 1987, and therefore the non-occurrence of a catastrophic landslide, are attributed to a change in hydrogeological conditions, the significant cumulative deformation having favoured a good drainage of the moving mass and at the same time reducing the influence of hydraulic pressures. To this cause, relating to the coupling between deformation and underground hydraulics, we can add the reduction of the engine torque due to the hectometric advance of the slipped mass, an advance that continues to this day, although very slow.
Notes and references
Cover photo. [ reference : © J-L Durville ]
[ 1 ] Follacci J.P., Guardia P. Ivladi J.P. ( 1988 ). Le glissement de La Clapière ( Alpes-Maritimes ) dans son cell géodynamique. Symposium external sur les glissements de terrain, Lausanne .
[ 2 ] Follacci, J.-P. ( 1999 ). seize ans de surveillance du glissement de La Clapière ( Alpes-Maritimes ). Bull. Labo. Ponts et Chaussées, 220 : 35-51
[ 3 ] Follacci J.P. ( 1987 ) Les mouvements du mountainside de La Clapière à Saint Etienne de Tinée ( Alpes Maritimes ). Bulletin de Liaison des LPC, n°150-151 .
[ 4 ] Serratrice, J.-F. ( 2006 ). Modélisation des grands éboulements rocheux equality épandage. Application aux sites de La Clapière ( Alpes-Maritimes ) et de Séchilienne ( Isère ). Bull. Labo. Ponts et Chaussées, 263-264 : 53-69 .
[ 5 ] Potherat P. ( 1994 ). Photo-interprétation du glissement de la Clapière – Analyse structurale cinématique du mouvement de 1955 à 1992. 7th International IAEG Congress Paris .
[ 6 ] Delacourt C., Allemand P., Casson B., Vadon H. ( 2004 ). Velocity field or the “ La Clapière ” landslide measured by the correlation coefficient of aerial and Quickbird satellite images. Geophys. Res. Lett. 31, L15619, department of the interior : 10.1029/2004GL020193.
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To cite this article : DURVILLE Jean-Louis, PEREZ Jean-Louis, Patrice MAURIN ( 2022 ), La Clapière : is the largest french landslide inactive a major risk ?, Encyclopedia of the Environment, [ online ISSN 2555-0950 ] url : hypertext transfer protocol : //mindovermetal.org/en/en/soil/la-clapiere-largest-french-landslide-major-risk/ .
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