Influence of bedrock topography on the kinematics of
two clayey landslides in the Trièves (French Alps)
U. Kniess1, G. Bièvre1,2, D. Jongmans1, E. Pathier1, T. Villemin3, S. Schwartz1
1
LGIT, Université Joseph Fourier, Grenoble Cedex 9, France
2 CETE de Lyon, Laboratoire Régional dʼAutun, Autun cedex, France
3 LGCA, Université de Savoie, Chambéry, France
1 Introduction
!
The two large adjacent landslides of Avignonet and Harmalière, affecting thick clayey quaternary deposits, are located in the Trièves
area (French Alps). This 300 km2 depression area is located within the
French alpine foreland at about 40 km south of the town Grenoble
(Fig. 1). This area is covered by a thick Quaternary clay layer (up to
200 m), which was deposited in a glacially dammed lake during the
Würm period (Monjuvent 1973). The clay layer has covered the
paleo-topography of the ground made of Jurassic limestone and overlying cemented old alluvial layers located along paleo-valleys of the
river Drac. This results in an irregular surface of the base of the clay
deposits, as shown on the cross-section of Figure 2. In March 1981 the
Harmalière landslide was triggered after a heavy rainfall provoking
the melting of the snow cover (Moulin & Robert 2004). No such dramatic effects were observed for the slow moving Avignonet landslide.
!
Figure 1: Simplified geological map (modified from Jongmans et al.,
2008) showing the locations of the Avignonet and Harmalière landslides. The dashed line shows the cross-section of Figure 2 and the
black box the extent of Figure 3, which is the area investigated by Lidar.
Figure 2: Top: Geological cross-section located in Figure 1. Bottom:
Detailed cross-section showing the lower part of the slope with the location of the boreholes and of the slip surfaces (inclinometer data).
Dashed lines hint the rupture paths (Modified from Jongmans et al.
2008).
910.0
2 Landslide Kinematics
911.5
0
70
500
600
6431.5
!"#$%&%'(
A‘
Lake Monteynard
6431.0
6430.5
A
6430.0
B
'
#'+
6429.5
*+,
)*
6429.0
100 mm/y
B‘
'()*+,-./0120+3(4
Three techniques have been used to analyze the kinematics of the
two landslides:
GPS measurements have been performed biannually since 1995
at 25 points on the Avignonet landslide. No GPS measurements are
available for the Harmalière landslide.
Aerial photographs covering the two landslides are available
since 1948 with a periodicity of 8 years and at a scale of about
1:30,000. These aerial photos (not shown here) has been used to
track geomorphological features evolving within the Harmalière
landslide.
An airborne Lidar laser scan was performed in November 2006
deriving a point cloud with an average density of 6 pts per m2. A
second campaign is planned for spring 2009.
The two landslides show different kinematics. Harmalière shows
activity since 1948, a fast sliding event in 1981 and a continues
headscarp regression until today up to the limit of Avignonet.
On the other side, Avignonet didn‘t show major events since
1948, but a continues slow (<13cm/y) movement up to now. Also
the surface roughness indicates much less activity in Avignonet
than in Harmalière. How to explain this different kinematics?
911.0
500 m
N
&!!
10 mm/y
3 mm/y
$!!
A‘
"!!
!
"!!
#!!! 5-3,+/6)0124
#"!!
B
%!!
$!!
"!!
GPS
A
%!!
'()*+,-./0120+3(4
Figure 3: Top: Shaded Lidar DEM (light direction is from NW,
horizontal resolution of 2m) covering the two landslides (the dotted lines indicate their limits). The white circles show the position of the GPS stations, and the thin straight lines represent the
11-years average horizontal velocity measured by GPS. The thin
curved black lines are height contour lines (in meter above sea
level, the map coordinates units is kilometers and the geodetic
reference system is Lambert93 (the official French system). Bottom: Elevation profiles AA’ and BB’, their positions are shown in
the top figure.
910.5
B‘
!
"!!
#!!! 5-3,+/6)0124
#"!!
!
3 Bedrock Topography
Seismic noise measurements were conducted for determining the
thickness of the clay layer overlying the seismic bedrock. The single station method (also called the H/V technique) was used at 78 locations. It
consists in calculating the horizontal to vertical spectral ratios (H/V) of
seismic noise records and the resonance frequency of the soft layer generally appears as a peak on the H/V curve (Bard 1998). A low resonance
frequency corresponds to a thick soil layer and vice-versa. In the investigated area, the measured H/V curves exhibit peaks in a frequency range
between 0.58 Hz and 3.63 Hz (see Fig. 4 for the H/V curves and Fig. 5
for measurement location). Sensitivity tests have shown that the main
parameter controlling f0 is the clay thickness. On the contrary, no change
in the f0 value has been found when passing the landslide headscarp.
These results support the use of resonance frequency measurements for
determining the clay thickness.
The thickness data obtained over the whole area were then subtracted
from the Lidar DEM and were spatially interpolated (kriging with an exponential variogram model correction) to produce a topographical map
depicting the bottom of the varved clays (Fig. 5).
The map of Figure 5 reveals that the paleo-topography upon which
clays were deposited was very irregular, with elevation variations of
more than 170 m. The major feature is the presence of a complex shape
depression, approximately striking N-S and bordered to the East by a
ridge culminating at an elevation of about 620 m. To the South, this
ridge disappears (Fig.5). Below the Avignonet slide, the ridge of compact layers continuously extends perpendicularly to the global slide motion and could act as a buttress, preventing deep rupture surfaces to develop (Fig. 2). On the contrary, the Harmalière landslide developed over
the lower elevation zone (Fig. 5) and its southeastward motion is parallel
to the contour lines of the paleo-topography, highlighting the control of
the depression zone, interpreted as a Drac paleo-valley, on the landslide
geometry.
Figure 4: Spectral ratios (H/V) of ambient vibration measurements at the two points (HV1T1,
HV21) shown in Figure 5.
!
Figure 5: Paleo-topography of the bottom of the
clay layer be-low the Avignonet and Harmalière
landslides. The location of the seismic noise measurements is shown by black dots. Dashed lines indicate the landslide limits. Lake Monteynard's elevation is 480 m asl.
4 Conclusions
The two adjacent earthslides of Avignonet and Harmalière, which developed in the same clay deposits under similar weather conditions, were found to exhibit distinct kinematic behavior in terms of dis-placement
magnitude and motion direction. The presence of a ridge of hard formations (compact al-luvial layers and
bedrock) along the western shore of the lake was pointed out both by seismic data and geological observations. This buttress probably prevents the development of a deep active slip surface at Avignonet and explains the higher displacement rates observed at the landslide toe, the soft clayey material being expelled
over the rigid formations. The bedrock buttress disappears to the South and this is interpreted as resulting
from the presence of a paleo-valley of the river Drac, as suggested by seismic data and alluvial layer outcrops. This paleo-valley, which allowed the deposit of thick clay layers, could have controlled the initiation
and the unexpected Southeastward motion of the Harmalière landslide. The material eroded at the top is
moving downward and feeds a funnel shaped track zone through which the landslide material flows to the
lake with a regular slope. In the Avignonet and the Harmalière cases the seismic bedrock topography influences the landslide kinematics and seems to control the difference in evolution of the two adjacent slides.
References
Bard, P.-Y. 1998. Microtremor measurements: a tool for site effect estimation?, Proceeding of the Second International Symposium on the
Effects of Surface Geology on Seismic Motion 3: 1251-1279. Yokohama, Japan.
Bonnefoy-Claudet, S.; Cornou, C.; Bard, P.-Y.; Cotton F.; Moczo P.; Kristek J. & Fäh, D. 2006. H/V ratio: a tool for site effects evaluation.
Results from 1-D noise simulation, Geophys. J. Inter. 167: 827-837.
Comegna, L.; Picarelli, L.& Urcioli, G. 2007. The mechanics of mudslides as a cyclic undrained-drained process. Landslides, 4, 217-232.
Eilertsen R., Hansen L., Bargel T., Solberg I-L. 2008. Clay slides in the Målselv valley, northern Norway: Characteristics, occurrence, and
triggering mechanisms, Geomorphology, 93, 548-562.
Glenn, N. F.; Streutker, D. R.; Chadwick, D. J.; Thackray, G. D. & Dorsch, S. J. 2006. Analysis of LiDAR-derived topographic information
for characterizing and differentiating landslide morphology and activity, Geomorphology, 73, 131-148
Jongmans, D.; Bièvre, G.; Schwartz, S.; Renalier, F.; Beaurez, N. & Orengo Y. 2008. Geophysical investigation of a large landslide in glaciolacustrine clays in the Trièves area (French Alps), Engineering geology, doi:10.1016/j.enggeo.2008.10.005.
Malet, J.-P., van Asch, Th.W.J., van Beek, R., Maquaire, O. 2005. Forecasting the behaviour of complex landslides with a spatially distributed hydrological model, Natural Hazards and Earth System Science, 5 (1), pp. 71-85.
Moulin, C. & Robert, Y. 2004. Le glissement de Harmalière sur la commune de Sinard, Proceedings of the workshop Ryskhydrogeo, Program Interreg III, La Mure (France).
Picarelli, L. ; Urciuoli, G., & Russo, C. 2004. Effect of groundwater regime on the behaviour of clayey slopes, Can. Geotech. J. 41, 467484.
van Asch, T.W.J., Van Beek, L.P.H., Bogaard, T.A 2007. Problems in predicting the mobility of slow-moving landslides. Engineering Geology, 91, 46-55