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Geotechnical News • June 2017
GEOHAZARDS
comparing the 2006 and the post-event
DEMs. Glacial retreat in the years
preceding the collapse, in addition
to faulting, deformation, and smaller
failures, were all evident in the lower
flank from SfM analysis.
Details of the volume and mechan-
ics of the 2010 Mt Meager landslide
are expected to be released shortly
in an upcoming paper in Landslides
(Roberti et al., in press). At this point
we can say that we improved on the
original volume estimate of Guthrie, et
al. (2012), that the landslide is some-
what larger than previously supposed,
and that, using SfM, we were able
to make a more precise description
of both causal mechanisms and the
mechanics of the collapse.
2010 debris avalanche deposit
For the geomorphological study of
the 2010 Mt Meager landslide deposit
we took pictures from a low-level
helicopter traverse with a digital
Canon SLR camera. We processed 712
images, where poor quality images
were excluded (blurred, overexposed)
and, when necessary, the helicopter
skids masked out. The resulting ortho-
photo and DEM have a resolution of
8 cm/pixel and 0.34 m/pixel (point
density at 0.64 pts/sq m), respec-
tively. GCP planimetric coordinates
were retrieved from a 0.5 m GeoEye
orthophoto while vertical information
was from a 25 m resolution British
Columbia Terrain Resource Inventory
Mapping (TRIM) DEM of 18.3 m
average accuracy.
The precise distribution of different
debris avalanche facies was mapped
at 1:1000 scale (Figure 2) and depos-
its related to different water contents
phases were recognized (Roberti et al.,
2017). The deposits were correlated
water-rich and water-poor phases. The
water-rich phase superelevated and left
relatively thin scattered distal deposits.
The water-poor phase was confined to
the valley bottom and stopped rapidly
leaving thick hummocky deposits.
1948-2006: The deformation before
the collapse
We documented more than a half-cen-
tury of geomorphological evolution
of the south flank of Mt Meager from
historical aerial photos, orthophotos
and DEMs. A rich archive of histori-
cal vertical aerial photos is available
at the Geography Department Library
at UBC (University of British Colum-
bia). The study area was captured in
1948, 1964, 1973, 1981, 1990, 2006.
We digitized the aerial photos and
processed them with PhotoScan to
reconstruct the diachronic evolution of
the three-dimensional geometry of the
flank. The frames were scanned at 800
dpi with a standard A3 scanner, orient-
ing the strip flight direction parallel
to the CCD array of the scanner. To
georeference the models, planimet-
ric coordinates were retrieved from
SPOT-10 m resolution imagery while
the vertical information was obtained
from the 25 m BC TRIM DEM.
To test the precision of the carto-
graphic products, we took control
points from a Lidar DEM acquired
during summer of 2015. The 2006
and the post-collapse DEMs were
compared using a pixel-wise differ-
ence, and calculating the volume of
the failed mass. It was also possible to
calculate the volume and the geom-
etry of the sub-failures, interpolating
the superficial faults between the two
DEMs. The error in the volume calcu-
lation was assessed by comparing the
two surfaces outside of the collapse
scar, where no change was expected
and calculating the same relative error
for the volume estimate.
In addition, we mapped the extent
of the glacier tongue at the base of
the failed slope on each orthophoto.
Uncertainties in this exercise related to
inherent errors in the base map, to the
manual tracing of the glacier perim-
eter, and difficulty of distinguishing
between ice, snow and snow-covered
ice. We used the inherent pixel preci-
sion of the orthophotos to buffer the
perimeter of the glaciers. The preci-
sion of the glacier area was calculated
as the root of the squared sum of the
buffered areas.
An interpretation of the 2006 topogra-
phy is shown in
.
Figure 2. Detail of the 2010 Mt Meager debris avalanche deposit. High reso-
lution mapping allowed the identification of different stress regimes: exten-
sion (light blue) at the west corner; shear (purple) in the central part and at
the sides; and compression (red) at the front and between the two lobes.
Modified from (Roberti et al., 2017).
