A Study of the Tsunami Effects of Two
Landslides in the St. Lawrence Estuary
R. Poncet, C. Campbell, F. Dias, J. Locat, and D. Mosher
Abstract The Lower St. Lawrence Estuary (LSLE) is a 230 km long by 50 km
wide trough with a broad, flat floor with maximum water depths of 400 m
and “shelves” that sit in water depths of < 60 m. It is partly filled with thick
glaciomarine and post-glacial sediments and lies within close proximity to the
Charlevoix Seismic Zone, the most seismically active region of eastern Canada.
The purpose of this paper is to present the modelled tsunami effects of two submarine landslides from the LSLE. A regional seafloor mapping project revealed
several submarine landslides on the slopes and channel floor of the LSLE. The
tsunamigenic effects of two instability features in the area were investigated. The
features chosen were: (1) a blocky submarine landslide that covers an area of
~3 km2, a run-out distance of 1.2 km and maximum slab thickness of 20 m; and
(2) a lateral spread feature with a 4 km long headwall escarpment and a maximum
slab thickness of 10.5 m, which may be a candidate for a future landslide. Using
a numerical wave tank, the nonlinear shallow water equations were solved for
motions induced by the submarine instability features. The equations are solved
numerically by the finite volume method, and the code is able to model accurately tsunami runup and drawdown.
Keywords Submarine landslide • tsunami • numerical simulation • St. Lawrence
estuary
R. Poncet and F. Dias ()
Centre de Mathématiques et de Leurs Applications, Ecole Normale Supérieure de Cachan
and CNRS, France
e-mail: Frederic.Dias@cmla.ens-cachan.fr; Raphael.Poncet@cmla.ens-cachan.fr
C. Campbell and D. Mosher
Natural Resources Canada – Atlantic, Dartmouth, Nova Scotia, Canada
e-mail: cacampbe@nrcan.gc.ca; dmosher@nrcan.gc.ca
J. Locat
Département de géologie et de génie géologique, Université Laval, Québec, Canada
e-mail: Jacques.Locat@ggl.ulaval.ca
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
755
756
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Introduction
In the past, submarine landslides have not been particularly well researched because
of their inaccessibility and wrongly inferred lack of direct societal consequence.
However, with increasing awareness of the potential of tsunami generation and
increasing development of offshore regions, there is a need for better understanding
of offshore landslide processes, landslide potential and tsunami-generation
capability (Locat and Lee 2002; Bardet et al. 2003). This need is coupled with great
advances in underwater mapping technologies and tsunami modeling capabilities
over the past decade and a half (Bardet et al. 2003). Increased resolution of seafloor
bathymetry combined with improved computer processing have led to increased
sophistication of tsunami wave modeling, permitting better prediction of tsunami
impact potential.
Although massive landslides on the continental margin can prove devastating,
such as the 1929 landslide and tsunami off the Grand Banks of Newfoundland
(Mosher and Piper 2007), it is coastal landslides that can affect local communities
and represent a greater hazard because of their proximity to societal infrastructure
(Mosher et al. 2004; Mosher 2008). These locally generated landslides can directly
impact marine infrastructure, but also can cause tsunamis that impact subaerial
infrastructure with great consequence.
The Laurentian Valley coastal zone of Quebec, along the St. Lawrence River and
estuary, is an area of thick Quaternary sediment accumulation, elevated seismicity
and human habitation. In some areas, the sediments have been leached by meteoric
processes making them geotechnically “sensitive”. Mazzotti (2007) explains higher
than expected seismicity potential in this region because of elevated strain rates
related to ongoing postglacial rebound along the paleotectonic suture that forms the
valley (Fig. 1). Lamontagne (2009) reviews potential seismic triggers to submarine
landslides in this region. Along the banks and submarine slope of the St. Lawrence
estuary and the Saguenay Fjord are numerous examples of mass failure (e.g.
Cauchon-Voyer et al. 2007; Levesque et al. 2006; Urgeles et al. 2001). Most are prehistoric but a few are recent, e.g. 1663 and circa 1860 (Cauchon-Voyer et al. 2007).
Depending upon conditions of failure and location, a modern instability event in
these areas could readily cause damage to underwater structures and generate waves
that impact coastal infrastructure and potentially propagate up river.
2
Regional Geological Setting
The St. Lawrence Estuary trends parallel to a faulted contact between Appalachian
and St. Lawrence Platform bedrock to the south and Grenvilian basement to the
north (Fig. 2) (Campbell et al. 2008). The contact forms the Laurentian Channel
trough which varies from a half graben to graben structure, bounded by normal
faults to the north and south (Tremblay et al. 2003; Pinet et al. 2008). Thick
deposits of Quaternary sediments partially fill the Laurentian Channel trough and
there is evidence that most of the Quaternary sediments in the Estuary were
Fig. 1 Calculated postglacial rebound strain rates and seismicity (black dots) of eastern North
America overlain with major tectonic elements (from Mazzotti 2007). The Appalachian front
passes up the St. Lawrence valley and through the study area
Fig. 2 Map of the St. Lawrence Estuary showing the Matane and St-Siméon study areas. White dashed
lines delineate contact between major bedrock units (boundaries modified from Pinet et al. 2008)
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deposited during the last deglaciation (Syvitski and Praeg 1989; St-Onge et al.
2008). The modern seafloor morphology can, therefore, be attributed to recent
glacial excavation and erosive processes, as well as much older regional tectonic
elements.
The trough-like morphology of the St. Lawrence Estuary provides an excellent
natural laboratory for studying submarine instability features. The deepest part of
the trough is only 440 m deep and is, therefore, very accessible. Shelf areas flank the
north and south sides of the trough in water depths of 60 m or less. The slopes that
mark the transition from shelf to estuary floor range from > 4° along the northern
slope and 1°–3° along the southern slope. A number of large rivers enter the estuary
from the north and have built deltas and submarine fans.
2.1
Study Sites
Two submarine landslide locations were selected in the Lower St. Lawrence
Estuary in order to model the tsunamigenic effects of the landslides. The study sites
are described below.
2.1.1
Location 1 – Matane
The Matane location is interpreted as a lateral spread feature. The location has been
described previously by Campbell et al. (2008). Regionally, the slope off Matane
has the lowest gradient in the study area. Multibeam bathymetry data reveal a fissure
on the seabed in the area in ~ 200 m water depth approximately 7 km offshore from
the town of Matane, Québec (Figs. 2 and 3). The main fissure is 4 km long, 180 m
wide and 15 m deep with en echelon fractures present at the western portion of the
fissure (Fig. 3). Approximately 800 m downslope, compressional ridges are present
(Figs. 3 and 4). Gas venting features (pockmarks) are common on the seafloor in
this area. High resolution seismic reflection data show an area of acoustic wipeout
below the fissure (Fig. 4). Downslope from the fissure, seismic reflections are
continuous with the exception of the upper 10 m which is incoherent and undulates. The disturbed interval is conformably overlain by ~2 m of acoustically stratified
sediment, giving the feature an age of ~500 years through correlation with dated
piston cores in the area (Campbell et al. 2008).
The Matane location was selected because it provides a unique opportunity to
model the potential tsunamigenic effects of an area that may fail in the future.
Because the area has not failed completely, it is possible to accurately determine the
parameters d and m for the models (see below). If the seabed fails at this location
and a slide develops, the slab thickness would likely be 10–11 m and the seabed
would fail along the transition between homogeneous glaciomarine clay and postglacial silty clay. Campbell et al. (2008) have shown that seabed failure in the area
is common at this stratigraphic level.
A Study of the Tsunami Effects of Two Landslides in the St. Lawrence Estuary
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Fig. 3 Perspective view of high resolution bathymetry of the lateral spread feature at the Matane
site. Note the graben developed at the headwall of the slide and the compressional folds at the toe
(modified from Campbell et al. 2008)
Fig. 4 High resolution seismic reflection profile across the Matane site. The slide decollement is
approximately 11 m below the seabed at the transition between homogeneous glaciomarine clay
and post glacial sediments (modified from Campbell et al. 2008)
2.1.2
Location 2 – St-Siméon
The St-Siméon location is interpreted as a blocky coastal landslide along the
northern flanks of the St. Lawrence Estuary (Figs. 2 and 5). Along this section
of the St. Lawrence Estuary, the northern slope exceeds 15° and the seafloor drops
dramatically to a flat channel floor at a depth of ~90 m (Fig. 5). Multibeam bathymetry
mapping reveals that much of the seafloor at the base of the steep northern slope is
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Fig. 5 High resolution bathymetry map of the St-Siméon site. The inset shows a depth profile across
the landslide. The slope of the headwall exceeds 15°. Individual slide blocks are up to 20 m thick
covered with landslide deposits. The St-Siméon slide comprises large, intact blocks
up to 20 m thick and up to 1 km2 in area. The deposit is likely composed of glacial
till which is the dominant surficial material in the area. The failure may have had a
sub-aerial component as well, given its close proximity to the shore.
The St-Siméon location was selected because it appears to be representative of the
style of failure that has occurred along much of this part of the Estuary. The deposit
is located just a few hundred meters offshore from the village of St-Siméon (Fig. 5).
3
The VOLNA Code
The VOLNA code is an efficient numerical wave tank, which solves the nonlinear
shallow water equations on unstructured triangular meshes, with an emphasis on
tsunami and storm surge numerical modeling. It uses a finite volume method,
blending a second order approximate HLLC spatial Riemann solver together with
a third order Runge-Kutta time integration algorithm. The code was described and
validated following the standards described in Synolakis et al. (2008). By using
unstructured grids, flow computations can be carried out in almost any area.
Moreover, the resolution of the computational domain can be adjusted to the simulation features. Here, the meshes are thoroughly refined near the landslide areas
A Study of the Tsunami Effects of Two Landslides in the St. Lawrence Estuary
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Fig. 6 Computational mesh used for the Matane landslide scenario. The respective typical characteristic lengths of coarse and fine triangles are 1 km and 30 m
(Fig. 6), achieving orders of magnitude savings in terms of grid size, compared to
Cartesian grids with similar fine resolution.
VOLNA has been validated against several analytical solutions and wave tank
experiments. In particular, it is able to accurately reproduce wave runup and drawdown, as well as the generation of a tsunamigenic wave from seafloor motion.
Numerical simulations of landslide generated tsunamis have been carried out
with VOLNA, for the Matane and St-Siméon scenarios. The bathymetry was interpolated on the computational mesh using natural neighbors’ interpolation (Sibson
1981). The landslide induced seafloor motion is included in the code by using an
analytical Gaussian-shaped mass moving with constant acceleration (Liu et al.
2003). In both cases, the mesh size was approximately 150,000 triangles.
4
Results
In the Matane case, the tsunamigenic wave stays mostly confined near its generation zone, and only small disturbances (between 4 or 5 cm) propagate downstream
or upstream. The tsunami takes approximately 15 min to reach Godbout on the
opposite side, where the maximal surface elevation can vary between 10 and 40 cm,
depending on local bathymetry features. Figure 7 represents the evolution of the
water height at two different locations. In Matane, one would expect a leading
depression wave. The fact that the landslide occurs so close to the shore probably
explains why the leading depression wave is not seen. Figure 9 represents a snapshot of the free surface at two different moments: one just after the beginning of the
landslide, and one just before the tsunami hits the opposite side.
In the St-Siméon case, the landslide mass is higher, and the estuary shallower.
Hence, the wave hitting the shore on the opposite side of the St. Lawrence River is
more important, i.e. up to 5 m high. The first wave takes 5 min to reach the opposite
side. In Fig. 8, the free surface elevation at St-André is plotted, which is across the
St. Lawrence with respect to St-Siméon. One can see two snapshots of the water
elevation in Fig. 10.
Fig. 7 Evolution of the water height near Matane (left), and near Godbout (right), on the opposite
side of the St. Lawrence river with respect to Matane. Note the difference in vertical scales
between the left and right plots
Fig. 8 Free surface elevation at St-André (on the opposite side of the St. Lawrence with respect
to St-Siméon)
Fig. 9 Free surface elevation (in meters) at times 1 min (left) and 10 min (right) for the Matane case
A Study of the Tsunami Effects of Two Landslides in the St. Lawrence Estuary
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Fig. 10 Free surface elevation (in meters) at times 100 s (left) and 300 s (right) for the St-Siméon
case
High precision inundation simulations, using both refined bathymetric and
topographic data, might be useful in assessing risks for coastal communities, and might
be carried out in the future with VOLNA. Moreover, it would also be interesting to
simulate, in the St-Siméon case, the propagation of the tsunami all the way to
Québec City, and obviously to model the actual seafloor motion, instead of using
empirical formulas to infer the initial wave profile (Synolakis 2003).
At this stage, we emphasize that no conclusions can be drawn, in particular to
support or invalidate the past occurrence of tsunamis in the St. Lawrence Estuary.
It is also too early to draw any conclusions on the emergency preparedness of
communities on both shores. With additional research, we may find that some of
the assumptions we have used are too crude. Moreover, no sensitivity analysis has
been performed. We do not know how the various parameters of the landslide (size,
aspect ratio, slow creep versus sudden movement, etc.) affect the resulting wave.
There is a strong need for additional research.
Acknowledgments This is Geological Survey of Canada contribution 20090102. The third
author acknowledges the support from the EU project TRANSFER (Tsunami Risk And Strategies
For the European Region) of the sixth Framework Programme under contract no. 037058 and the
support from the 2008 Framework Program for Research, Technological development and
Innovation of the Cyprus Research Promotion Foundation under the Project A⌺TI/0308(BE)/05.
5
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