Morphodynamic and Slope Instability
Observations at Wabush Lake, Labrador
D. Turmel, J. Locat, G. Cauchon-Voyer, C. Lavoie, P. Simpkin,
G. Parker, and P. Lauzière
Abstract Since 1964, Iron Ore Company of Canada (IOCC) has deposited iron
tailings resulting from mining operations into Wabush Lake, Western Labrador.
Bathymetric surveys were carried out between 2004 and 2008 as part of the overall
environmental IOCC plans to maintain safe disposal strategies of tailings into the
lake. In this paper, we present the evolution in distribution and morphological characteristics over the last 4 years of the tailings overlying lacustrine sediments and
bedrock. In 2004, a high definition multibeam survey of Wabush Lake was carried
out for the first time and revealed lake floor features, including a fine network of
channels and a delta foreslope with well-developed knickpoints. Other features such
as older subaqueous mass movement scars and an esker were also identified. The
delta foreslope channels, in 2004, led into a deeper long channel connected with the
deepest part of the lake where turbidite deposition took place. A second bathymetric survey was carried out in 2006. Many of the features seen on the 2004 map were
already buried by the advancing delta front. Results indicate that the 2004 channel
system was almost completely obliterated with the generation of many new others.
Development of the knickpoints is also observed where some are still present and
D. Turmel (), J. Locat, and G. Cauchon-Voyer
Laboratoire d’études sur les risques naturels, Département de géologie et de génie géologique,
Université Laval, Québec, G1K 7P4, Canada
e-mail: dominique.turmel.1@ulaval.ca
C. Lavoie
Department of Geosciences, Hamilton College, Clinton, New York, 13323, USA
P. Simpkin
IKB Technologies Limited, 1220 Hammonds Plains Road, Bedford, Nova Scotia,
Canada B4B 1B4
G. Parker
Ven Te Chow Hydrosystems Lab, Department of Civil Engineering, College of Engineering,
University of Illinois at Urbana-Champaign, IC 61802, USA
P. Lauzière
Iron Ore Company of Canada, Environment Department, P.O. Box 10000, Labrador City,
NL, Canada A2V 2L8
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
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new ones are discovered. A third bathymetric survey conducted in 2008 demonstrates a different spatial arrangement of features. Well developed wider channels
and new knickpoints are observed on the foreslope delta. The sequential analysis
of 2004, 2006 and 2008 surveys proved to be a useful tool to evaluate: (1) the rate
of infilling of the lake, where accumulation took place and by which mechanisms
(2) the evolution of the foreslope delta gradient and, (3) the evolution of instability
areas. Our results indicate that these recently developed techniques are useful tools
for monitoring underwater tailings disposal and stability.
Keywords Landslides • knickpoints • tailings • sediments transport
1
Introduction
Wabush Lake, is located near Labrador City, Western Labrador, Eastern Canada
(Fig. 1). Since 1964, Iron Ore Company of Canada (IOCC) discharges annually a
volume of more than 100 m3 of iron tailings at the southern part of the lake resulting
from mine operations. With such a high sedimentation rate and the closed-like
basin morphology of the lake (Fig. 2), Wabush Lake is a unique large-scale natural
laboratory for studying sedimentary processes in an extremely dynamic environment. This study is part of an ongoing environmental plan of IOCC to maintain and
improve the disposal strategies of iron tailings in the context of sustainable development. In general, tailings are transported to the near shore area of the lake by up
to five pipelines discharging more or less at the same place at a given time. As the
delta progrades, the pipelines are extended and their discharging points are often
moved laterally. However, we can consider that the sedimentation progress into the
Wabush Lake could be characterized as an advancing point source delta.
This paper focus on results obtained from three bathymetric surveys conducted
on 2-year intervals from 2004 to 2008. On this sequential analysis of the lakefloor,
we observed a topset, foreset and bottomset deltaic system, including a complex
network of channels and multiple mass-transport movements such as knickpoints
and landslides, characterized by planar and lateral spread failures. Particular attention will be given to the formation process of the channels and knickpoints in the
southern and central part of the lake.
2
Methods
The multibeam sonar system used during the first survey, during July 23–28th
2004, was a Kongsberg Simrad EM-3000. In 2006 and 2008, a Reson Seabat 8101
system has been used. The surveys were held between July 11–20th in 2006 and
between August 12th and September 2nd in 2008. All three surveys were calibrated
on the same datum and the GPS precision (x, y) was about 1 m horizonal (using
CDGPS in 2004 and 2006 and WAAS in 2008).
437
Fig. 1 Localization of Wabush Lake
Morphodynamic and Slope Instability Observations at Wabush Lake, Labrador
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D. Turmel et al.
Fig. 2 Bathymetric maps of 2004 (a), 2006 (b) and 2008 (c) and localization of Figs. 2–6
3
Results
Wabush Lake is subdivided in three different parts: the Southern, the Central and
the Northern Basin (Fig. 2). The 2004 bathymetric survey shows for the first time
the subaqueous part of the tailings delta located in the southern part of the Southern
Basin (Fig. 1a). At that time, most of the transport system was carried out via the
eastern side of the lake.
On the eastern part of the delta frontset, channels of less than 2 m deep and about
20 m wide are observed (Figs. 3a and 5a). This system of channels connects itself
to two wider channels at water depth of around 30 m. The west channel is 50 m wide
Morphodynamic and Slope Instability Observations at Wabush Lake, Labrador
b
c
2004
2006
0
250
500
2008
1000
1500
Meters
2000
N
a
439
Fig. 3 Hillshade from the bathymetric map showing different channels and knickpoints
(arrows)
–3
–9
2004
2008a
a
–15
–21
Esker
–27
–33
–39
–45
(m)
–51
–58
–64
delta shoreline
2006
b
2008b
Slides
Slides
–70
–76
–82
–88
–94
Water depth (m)
Fig. 4 Oblique view of a knickpoint migrating in the western part of the southern basin (see Fig.
1 for location). Part (d) of this figure shows new knickpoints developing in the lake, approximately
300 m from the others knickpoints. (modified from Locat and Lee 2009)
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Fig. 5 Hillshade image of the western part of the central basin. This figure is located in Fig. 1
and 1.5 m deep, whereas the east channel is 25 m wide with a greater depth up to
5 m. This latter one is meandering down to the Central Basin, where it disappears
(Fig. 4a). In the western part of the delta, channels are also observed, but they are
not going down the central part of the lake. An esker of more than 3 km long is also
discovered in this part of the lake (Fig. 4 a–d). An esker is a long and winding ridge
of stratified sand and gravel deposited under a glacier.
Morphodynamic and Slope Instability Observations at Wabush Lake, Labrador
441
Knickpoints are found on the frontset of the delta where turbidity currents are
continuously formed due to the constant flow of tailings into the lake (Figs. 3a and
4a). Knickpoints are topographic depressions, characterized by a steep scarp slope
and a flatter profile downslope. In the 2004 chart, they are formed on a mean slope
of 1.8 (±0.9) degrees and can have a width up to 70 m (mean of 21 m). The mean
scarp-slope of knickpoints is 13.8 (±5.8) degrees. Statistics are from all the knickpoints marked by an arrow on Fig. 3a–c. In the western part of the Southern Basin,
two major knickpoints are also identified (Fig. 4a). These two parallel knickpoints
have a scarp height of 6 m, an upslope of 4 degrees, a downslope of 5 degrees, and
a scarp slope of 30 degrees. Their width is 22 m for the western one and 30 m for
the other one.
In the central basin, two large landslides are visible (Fig. 6). The first one,
located on the western flank, has a slide scarp of 1.2 km long, an average height of
2 m and an estimated volume of eroded sediments of about 0.5 Mm3. The planar
failure occurred on an average slope of 14 degrees. On the eastern flank, the other
landslide is about 400 m long and 600 m wide. The estimated volume of displaced
sediments is about 2.4 mm3 and according its ridge-like morphology is interpreted
as a lateral spread failure (Locat and Lee 2002).
A second bathymetric survey was conducted in 2006. The results show a significant evolution of the morphology of the lake floor since 2004 (Figs. 2b–5b). Using
the spatial position of the 4 m isobath, the delta has prograded, between 2004 and
2006, 150 m in the eastern part of the delta to about 250 m in the western part.
About 40 million tons of sediments have been discharged in the lake between 2004
and 2006.
The 2006 bathymetric data reveals that the single and well-defined channel
reaching the central part of the lake is no longer present, as it was visible in 2004.
Lateral spread failure
Planar failures
Fig. 6 Hillshade of the central basin from the 2004 bathymetric map showing planar failures and
lateral spread failure. This figure is located in Fig. 1
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However, the presence of many channels on the delta foreset is observed (Figs. 3b
and 5b). A new wide and shallow channel is formed on the delta frontset, just where
the subaerial channel formed by the tailings discharge is reaching the shoreline. The
upstream section of this underwater channel has a conical shape, with a width of
more than 250 m. This channel meanders for more than 1 km down into the lake.
Some of the knickpoints visible in 2004 are still present in 2006 (Fig. 3b) and
new ones were discovered. In the western part of the Southern Basin, the two knickpoints described earlier are still present, but their dimensions have evolved since the
first multibeam survey (Fig. 4b). The western channel has migrated toward the
coast line by about 20 m. The overall morphology of this knickpoint is similar to
the 2004 description (i.e. same width, depth and slope angles). By 2006, the eastern
channel has migrated upstream by about 175 m. A landslide is also visible in the
scar of the knickpoint. The volume of displaced material, at that time, is estimated
at ∼76,000 m3.
Differential bathymetry maps (2006–2004, Fig. 7a) show that most of the tailings were deposited within the southern basin during this time interval. Tailings
accumulation in the central basin is <0.5 m/year, whereas it reaches 5 m in the
southern basin and >15 m near the delta shoreline. An example of the significant
change in the sediment dynamics is the complete infilling of the main channel of
the southern basin (shown in red in Fig. 7a).
The 2008 bathymetric data (Figs. 2c–5c) shows a very different morphology
compared to the 2006 survey. This is mainly due to a major change in the disposal
pattern on the delta: the mine has concentrated the routing of the pipelines to fill
the western part of the lake. Approximately 50 m of sediments have been discharged into the lake between 2006 and 2008. Discharge rates between 2006 and
2008 were about 25 m/year, where it was of 20 m/year between 2004 and 2006.
There was significant deposition in the western part of the lake, reaching as
much as 10 m in some area, which led to the progradation of the delta over a large
portion of the esker. Some new channels are visible where the two knickpoints
presented earlier were located (Fig. 4c). However, due to the very high sedimentation rate, it is difficult to link these channels to knickpoint migration. On the other
hand, in the same area, two new major knickpoints are presently forming along the
esker (Fig. 4d). These knickpoints are ∼5 m high, present a downslope of 5 degrees,
a scarp slope of about 30 degrees and an upper slope of 4 or 5 degrees. These
knickpoints look very similar to the ones described earlier.
A very different spatial arrangement in the channels can be observed when comparing the 2008 bathymetric chart to the 2004 and 2006 charts. In 2004, there were
two main channels grading down to the central basin, whereas in 2006 several small
channels were present in the Southern Basin, but none of them reached the central
basin. In 2008, there is one main channel (Figs. 3c and 5c) and two smaller channels following the eastern and the western flank of the eastern part of the Southern
Basin. The main channel has a width of >500 m for the first 150 m and narrows
down to a width of 160 m for the following 500 m. Several hydraulic jumps are visible in this channel and some ripples are also visible. The two smaller channels are
intermittent and they are not connected to the central basin. There are also less
Morphodynamic and Slope Instability Observations at Wabush Lake, Labrador
2004-2006
a
443
2004-2006
b
Northern basin
N
Deposition
–20 - –5
Central basin
–5 - –2
–2 - –0,5
–0,5 - 0,5
0,5 - 1,5
1,5 - 3
3-5
5-7
7 - 10
Southern basin
10 - 15
0
750
1500
3000
4500
Meters
6000
Fig. 7 Differential bathymetry maps. Deposition in meter
knickpoints visible in 2008 than in the previous two surveys. This difference may
be due to the presence of a main channel draining most of the sediment supply in
the deeper part of the lake.
In the central basin, debris from the western flank landslide are now mainly
buried under the tailings. No landslides were identified in this sector of the lake
between 2004 and 2008 and no channel has developed in the central sector of the
lake.
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The differential bathymetry map for 2006–2008 (Fig. 7b) shows that most of the
sediment accumulation took place in the western part of the central basin, where
the shoreline prograded for >400 m, while it has moved <75 m in the eastern part of
the southern basin. This is mostly due to changes in the disposal strategies. The
2006–2008 differential map shows less accumulation in the western sector of the
lake, while there has been >2 m of accumulation in the central basin, compare to
<0.5 m of accumulation between 2004 and 2006.
4
Discussion
Wabush Lake is a very dynamic environment with significant change in the already
high sedimentation rates due to the tailing disposal practices of IOCC that have
been studied. The morphology of the lakefloor indicates that the sedimentation in
the lake mostly occurred by avalanche process from the delta topset to the frontset
and by hyperpicnal flows such as turbidity current in the deeper parts of the lake.
The latter one has been also observed in other similar environments such as the
ones created in laboratory (e.g. Kostic and Parker 2003). According to Normark
et al. (1979) and Mitchell (2006), knickpoints are initiated by a change in turbidity
current characteristics over an erodible bed. The 2004–2006 differential bathymetric chart shows that these turbidity currents did not supply sediments in the distal
portion of the deltaic complex, as less than 1 m of sediments were deposited in the
Central Basin. On the other hand, the 2006–2008 differential map shows sediments
accumulation of more than 2 m located more than 6 km away from the delta (Fig. 7b).
According to our results, it seems that large channels may serve as a bypass for
turbidity currents that will carry sediments quite farther. As described above, the
2004 and 2006 charts do not show well-defined channels in comparison with a large
channel described on the delta frontset of the last bathymetric map. The smaller
channels as mapped in the earlier charts may restrain distal deposition.
Our results show many knickpoints mostly located in shallow areas of the lake,
less than 30 m deep, but also at a depth up to 70 m. According to the hypothesis that
large sedimentation in the deepest part of the lake is due to hyperpicnal flows sedimentation and these currents can travel a distance of more than 5 km to reach the
deeper parts of the lake, we proposed that the knickpoints are initiated by turbidity
currents. The example of knickpoint migration presented in Fig. 3 shows that this
migration process may mobilize significant volume of material in a mode similar
to an infinite slope (slab) failure process. It may be possible to identify them in
instability areas. More geotechnical investigations on these knickpoints are in
process.
In the deepest part of the lake, two landslide scars are visible (Fig. 6). The first one
is located on the western flank of the trench and is characterized by planar failures.
The resulting debris is now largely buried by the intensive tailing discharges. This
landslide occurred on a relatively steep slope, and may have been triggered by
sediment accumulation at the top of this slope or by the nearby mining activities.
Morphodynamic and Slope Instability Observations at Wabush Lake, Labrador
445
Geotechnical investigations will be necessary to better understand these planar
failures, and coring in the debris may be helpful to approximate the age of this
landslide. The other landslide present in the Central Basin seems older. A lateral
spread failure implies a weak layer of sediments where failure plane occurred. The
development of such a failure plane implies toe erosion or a large earthquake.
However, there have not been any major historical earthquakes reported in this area.
The option to account for a triggering mechanism for this landslide is erosion of the
material at the toe of the slope. As seen on Fig. 4, large channels are present near
this landslide in 2004 and 2006.
These channels may have created significant toe erosion of the slope if the turbidity currents were strong enough. This would imply that this landslide occurred
when the mine was operating and this could be validated using detailed coring and
dating estimation of sedimentation rates. Coring of lake floor sediments at this site
is required and may help to better understand this landslide, as some seismites
layers may be encountered in the lake. In such a case, this may provide some relative
estimates of the seismic hazard for this area.
5
Conclusion
Wabush Lake is an active sedimentary environment resulting of the dynamic disposal of mine tailings. Three bathymetric surveys have been conducted since 2004.
With more than 100 m3 of tailings discharged into the lake each year, the delta progrades at a rate of > 100 m/year. Turbidity currents are generated at the mouth of
the so-formed river. These hyperpycnal flows may reach a distance of more than
5 km from the river mouth. According to the 2008 bathymetric chart, these flows
seem to be channelized by one major channel, which allow them to develop further
and transport sediments in the central basin of the lake. However, this process did
not take place in 2006 and 2004, where little sedimentation occurred in the distal
part of the lake and where only small channels were observed.
Geophysical surveys also reveal a complex lakebed morphology, involving
multiple mass-movements as knickpoints and landslides characterized by planar
and lateral spread failures. Knickpoints are mostly found in the shallower part of
the lake, even if some were developed as deep as 70 m. Our results also show that
some of these knickpoints have migrated upstream. Planar and lateral spread failures are found in the central basin. The location and morphology of the area where
planar failures are observed needs to be investigated in more detail, particularly to
investigate the potential role of nearby mine blasting on the stability of lacustrine
sediments. Lateral spread failure may be due to an old earthquakes or by the erosion at the toe of the slope due to the passage of turbidity currents. Sampling and
geotechnical investigations are necessary to be able to understand these failures.
Sequential multibeam surveys is an efficient tool for evaluating the rate of infilling
of the lake, to detect deposition centers, as well as to describe the evolution of the
instability areas.
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Acknowledgements This project was founded by the Iron Ore Company of Canada. We thank
the Fonds de recherche sur la nature et la technologie du Québec (FQRNT) for a student grant to
Dominique Turmel. We also thank S. Flynn and J. Clark (IOCC) for their support during fieldwork
and M. Garcia, C. Guilmette, F. Kinden, P.-E. Lord, R. O’Keefe, M. Sansoucy and M. Wong for
their assistance during this project. Finally, authors would like to acknowledge both reviewers,
Gwyn Lintern and Patrick Lajeunesse, for their constructive remarks that have helps improving
this paper.
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