Landslide Susceptibility Mapping of Highway 16 ... - UNBC GIS Lab

GEOG 413: Advanced GIS
Adam Simons
R. Wheate and P. Bai April. 2, 2012
Landslide Susceptibility Mapping of Highway 16 Area
East of Terrace, B.C.
Introduction:
On May 29, 2007 an estimated 30,000 cubic meter landslide occured covering a stretch of
Highway 16 roughly 37km east of Terrace, British Columbia; this being the second slide in the
area in three years (CTV News, 2007). Landslides are a serious natural hazard that can cause
enormous amounts of property damage and, when located near human infrastructure, often
fatalities as it did in the case near Terrace. The purpose of this study was to examine and analyze
the terrain surrounding a stretch of Highway 16 just east of Terrace for its likelihood to fail via
landslide. For this analysis, spatial data for slope, which controls gravitational force‟s effect,
aspect, which controls solar energy and water regimes, distance to water such as streams and
rivers, which acts to cut into slopes weakening them and supplying moisture and sediment in
high flows, and bedrock geology, which is the main structural control, were used.
Data Implemented:
Vector topographic data were gathered for the 103I09 BC mapsheet at 1:50,000 scale
from Geogratis.ca; this being the area containing a large section of Highway 16 just east of
Terrace, B.C.. These data used for this analysis include the line features “streams” and “roads”,
and the polygon feature „water bodies” which contains rivers and lakes. Vector polygon
geological data were gathered from the B.C. Ministry of Energy and Mines website for the region
including Queen Charlotte Islands and the North Coast at a 1:250,000 scale, and a 25m DEM
was downloaded from the UNBC download directory for the 103I 1:250,000 mapsheet.
Methods, Data Analysis, and Results:
The idea for this study originated from an article by Cevik and Topal (2003) reviewed
earlier in the year for the Geography 413 class, whereby a landslide susceptibility map was
generated for terrain around a natural gas pipeline in Turkey using weights for each factor to
assign priority, or influence on the event likelihood. Some of the methodology must be credited
to Vikki St-Hilaire and her similar project from a previous year‟s class, who did a great job with
her landslide hazard mapping in the southwestern B.C. region. Information used to determine the
weights applied to the aspect, slope, and distance to stream/water layers was gathered from a
paper by Dai et al. (2001) which analyzed landslide susceptibility in Hong-Kong, and for the
geological weighting, a government publication entitled, “Chapter 4: Intrusive Igneous Rocks”,
was used along with Dai et al. I did not have significant geological knowledge to apply to this
problem, but I used basic knowledge in assigning priority for which type is most likely to fail.
For example, the geological data contained three types of intrusive rock types for the study area,

and I determined the least likely to fail were the types with the most quartz mineral content in
them, which was determined from the government article. Limestone and calcareous sedimentary
rocks were deemed the most likely to fail, followed by volcanic, then sedimentary. The factors
used in the analysis and the priority values assigned to each class within them are shown below,
with the highest value equalling the most likely to contribute to landsliding, and the lowest
equalling the least likely.
Other factors that could have been included are vegetation type cover, elevation, soil
moisture, land use, previous landslide events, and others (Cevik and Topal, 2003).
Table 1: The four factors that contribute to landslide events and the priority values assigned to each class within the factor.
Higher values = more influence on slide.
Factor Class Priority
Bedrock Geology Quartz dioritic intrusive rocks 1
Rhyolite,felsic volcanic rocks 5
Granodioritic intrusive rocks 2
Limestone,marble,calcareous
8
sedimentary rocks
Volcanoclastic rocks 6
Calc-alkaline volcanic rocks 7
Intrusive rocks, undivided 3
Undivided sedimentary rocks 4
Distance to Streams/Rivers
(2 layers)
200m 1
Slope (°) 0-10 1
10-20 2
20-30 3
30-40 6
40-50 5
>50 4
Aspect N 1
NE 2
E 5
SE 7
S 8
SW 6
W 4
NW 3

The following steps were followed to generate the final landslide susceptibility map output:
- Gathered data from the three sources mentioned earlier for geology, elevation, roads,
streams, and water bodies.
- Converted the ASCII DEM to an ESRI Grid and re-projected each of the vectors to the
DEM projection- NAD83 UTM Zone 9- except for the geology layer which was already
in the correct projection. Coordinate system: GCS North American 1983.
- Clipped the geology layer and DEM to the study area extent given by the streams layer.
- Generated the slope (degrees) and aspect rasters from the DEM layer and
classified/symbolized them based on the classes shown above. The DEM and the
resulting two layers are shown below:
Figure 1: Digital Elevation Model.
Figure 2: Slope.
Figure 3: Aspect.

- Generated a straight-line distance raster from both the stream layer (containing all small
mountain streams) and the water body layer (containing the main Skeena River which the
highway follows and small mountain lakes) and classified/symbolized as shown in table
1 (i.e.,

- After the five raster layers were generated and reclassified to show weights (priority),
they were input to the raster calculator in ArcMap with the expression:
landslide_susceptibility = 0.20*[geology] + 0.20*[slope] + 0.20*[aspect] +
0.20*[distance_water] + 0.20*[distance_streams],
the initial output of which is shown in figure 7. For the final map output, the resulting
classes created by the summation of each layer were reclassified/symbolized to a fiveclass
system from “Very High” susceptibility to “Very Low”, as shown in figure 9.
- A second map was generated using information from Cevik and Topal (2003) whereby
each factor themselves, not the classes within each factor, were ranked on likelihood to
trigger an event. The authors rank geology as the most important factor, followed by
slope, distance to streams, with aspect being the least important. The raster calulator
equation is shown here:
landslide_susceptibility = 0.35*[geology] + 0.25*[slope] + 0.10*[aspect] +
0.15*[distance_water] + 0.15*[distance_streams].
This result was not used because of the odd patterns caused by the over-weighting of the
geology polygons, but the rough output is shown in figure 8.
Figure 7: Rough output of landslide susceptibility using
constant weights. Lighter areas represent more susceptible
terrain; dark, less susceptible.
Figure 8: Rough output of landslide susceptibility using
varied weights from Cevik and Topal (2003).

Figure 9: Final map result with overlying vector layers roads, streams, and water bodies. Further information is provided in
the formal PDF map.
Conclusion:
As shown in the final PDF map attached with this report, as well as in the rough picture
provided above, the majority of the highly landslide susceptible terrain (shown in red and
orange) is located along the south-facing stream gullies running down the valley walls which
lead towards the Skeena River and Highway 16. This also happens to be how the landslide in
2007 occured; forming in an upper valley and running a few kilometers down-slope to the
highway. While the area immediately adjacent to the main road does not appear at risk of sliding,
this map clearly shows there is risk in the nearby mountains that could affect transportation, and
even river flow if large enough.

References:
Literature:
Cevik, E. and Topal, T. (2003). GIS-based landslide susceptibility mapping for a problematic
segment of the natural gas pipeline, Hendek (Turkey). Environmental Geology, 44: 949-
962. DOI 10.1007/s00254-003-0838-6.
Chapter 4: Intrusive igneous rocks. Retrieved April 1, 2012.
http://www.cedd.gov.hk/eng/publications/sheet/doc/g4_chapter_4.pdf
CTV.ca News Staff (2007, May 29). Northern B.C. town still cut off after landslide. Retrieved
April 2, 2012, from CTV News website.
http://www.ctv.ca/CTVNews/Canada/20070529/bc_mudslide_070529/
Dai, F.et al. (2001). Assessment of landslide susceptibility on the natural terrain of Lantau
Island, Hong Kong. Environmental Geology, 40, 381-391. doi: 10.1007/s002540000163.
Digital Data Sources:
British Columbia Ministry of Energy and Mines. File directory: GeoFile 2005-5: Digital
Geology Map of B.C.- Tile NN8-9 North Coast and Queen Charlotte Islands/ Haida Gwaii.
http://www.empr.gov.bc.ca/Mining/Geoscience/PublicationsCatalogue/DigitalGeologyMaps/Pag
es/DigitalGeologyMapCoverage.aspx.
Geogratis.ca. National Topographic Data Base. File directory: bndt_103i09_shp_en.zip.
http://ftp2.cits.rncan.gc.ca/pub/bndt/50k_shp_en/103/i/.
UNBC GIS Lab Data Download. http://www.gis.unbc.ca/resources/data_download/get_data.php.

Landslide Susceptibility Map for Highway 16
East of Terrace, B.C.
By Adam Simons
Skeena River
Highway 16
0 2.5 5 10 Kilometers
Created April 2, 2012
UNBC, Prince George, CA.
Data from 103I09 1:50,000 mapsheet
NAD83 UTM Zone 9 projection
Legend
Streams
Roads
Rivers and Lakes
Landslide Susceptibility
Very High
High
Moderate
Low
Very Low