Landslide!
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Vancouver Island Region:
Uniquely vulnerable
Vancouver Island is largely covered by mountains, rising to about 2,200 metres above sea level. The terrain is rugged and over-steepened, so combined with high rainfall (falling primarily in winter months), seismic activity from the Juan de Fuca and Explorer plates (see http://www.pgc.nrcan.gc.ca/pgchome.htm for information about Vancouver Island’s seismic environment) and glacier related sediments left on steep slopes, makes Vancouver Island uniquely vulnerable to landslides. In general, steep slopes are those greater than about 30o, however, that amount may be lower under certain conditions.
Landslides! When
slopes fail
Sediment, rock and debris moving down-slope under the influence of gravity are called landslides and are common on Vancouver Island. Landslides vary widely in size, type, and frequency and may impact lives, property, streams, aquatic and animal habitat. Landslides may be natural or man-made, but generally require steep slopes and a trigger mechanism such as rain, earthquakes or just time.
Several types of landslides occur including: rock falls, debris slides and debris flows and slumps.
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Marine sands and plastic clays
Sediments in the Vancouver Island region are generally coarse (silts, sands and gravels). However, a small percentage of them are fine and contain what are known as plastic soils (some silts and clay). These finest sediments are related to old sea levels, and consequently occur primarily along the coastline. Plastic soils are unique in that they are strong when dry, but lose strength when saturated (sufficiently wet) and flow as a liquid along very gentle slopes. Plastic soils typically have a sticky greasy feel, and portend of stability problems to come.
In addition, along both sides of the Georgia Basin, deep surficial sediments of unconsolidated sands, sands and gravels, and lenses of finer materials were deposited as a pro glacial outwash feature in the Pleistocene. These deep deposits, typically described as the Quadra Sands, are notoriously unstable and vulnerable to wave erosion. There are examples of landslides from these deposits in many of the urban areas along both sides of the Georgia Basin, and notable examples include the cliffs above Wreck beach at the University of British Columbia, Island View beach in Victoria, the Wilmar Bluffs in Courtenay and below Cilaire at Departure Bay in Nanaimo.
Slumping
Failures: Landslides where we live
Along the east coast lowlands,
landslides are less common, but perhaps more costly.
In the inevitable attempts for a better view, many of us place ourselves
at risk by building a house or structure too close to a slumping riverbank or an
eroding ocean side cliff. These
threats can seem stable for years, and then suddenly fail with renewed activity.
Slumping is a rotational failure in deep sediments, and while the process is less common across the region, it is very common along the interface of the land and water. Consequently slumping occurs at shorelines on lakes and oceans, and at river edges. This is particularly common on the east coast of Vancouver Island where urban development is the greatest in the region, but where steep slopes don’t normally register as a conscious problem. These are also often the areas with marine sediments and clays. Water and vegetation management are often key issues on these propertiies.
Debris slides and debris flows: Bringing the mountains down
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Debris slides and debris flows are
extremely rapid (>5 m/second; the worlds fastest sprinters run about 10
m/second) shallow landslides that break up or flow down-slope as water content or
velocity increases. They are
typically quite large (~7,000 m2) but range from the very small to
many kilometres in size. In
general, debris slides and debris flows occur in our steep uninhabited terrain
as a result of saturated soils and are often related to storms or earthquakes.
They are a threat to people and infrastructure when they block roads, dam
streams or hit remote structures. However,
some residents live on debris flow fans (at the base of the Beaufort mountain
range for example, near Port Alberni) and have to contend periodically with the
risk of debris flows. Debris flows
and debris slides are more commonly a concern to environmental values (including
fish and fish streams) and forestry resources. Some communities have taken special measures to protect themselves from debris flow hazards. Port Alice, for example, has constructed a large debris flow training berm and has a warning system for debris flows that might reach the town. |
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Can
forestry cause landslides?
The wet steep rugged slopes of the West Coast and central Vancouver Island Ranges are continually subjected to landslides both naturally and related to human manipulation. Logging and road building in these areas often result in landslides. An intact forest acts as both an additional stress on the slope and as a strengthening agent. Roots may knit together loose soil and debris, and evapotranspiration removes moisture from the soil. Roads may undercut or overload the slope, while logging removes the supportive network of roots.
Landuse, including forestry, road building, urban development and land clearing, has substantial impact to both the frequency of these events, or the potential for damage. The Forest and Range Practices Act is designed to minimize forestry related disturbances (http://www.for.gov.bc.ca/tasb/legsregs/frpa/frpa/frpatoc.htm).
Rock Falls: The sky is falling!
Rock falls are the falling, bouncing and rolling of rocks down a steep surface, typically a cliff. They are a less common hazard to most communities in the Vancouver Island region, but remain a hazard none the less. Some communities, Zeballos or Horne Lake, for example, are substantially more vulnerable to rock falls because of their close proximity to steep failing rock cliffs. Rock falls are also a hazard to transportation corridors throughout the Canadian Cordillera, including the Vancouver Island region.
Rock falls range from small individual stones, to very large events many thousands of square meters in size. Failures are often associated with freeze thaw action (warm winter days and melting snow coupled with a couple of cold winter nights), and occasionally earthquakes.
Fortunately, the majority of rock falls occur in the remote alpine zones of the region and have little impact to people.
The following link is a report that describes the mass
movement potential for Vancouver Island:
http://wlapwww.gov.bc.ca/wld/documents/techpub/rr01/rr01_geom_vi.html
