Lesson 8: Hazards Flashcards
What are avalanches?
The sudden release and movement of vast amounts of snow down a mountainside under the influence of gravity.
Lessons from the past:
Snow avalanches are one of the most destructive forces in nature, 1000s have been killed over centuries. Avalanches in the European Alps became a sizeable problem between the 16th and 18th centuries, when…?
Increasing population and the widespread cutting of mountain forests coincided with the increasing snowfall and glacial advance of the Little Ice Age.
Lessons from the past:
One of the greatest avalanche disasters in Europe cam during the First World War, when…?
When a series of enormous snow slides on the Austrian-Italian front killed 10,000 soldiers in a single day.
Lessons from the past:
In NA, the first major problems with avalanches arose during the 1800s Gold Rush era, when…?
Prospectors flooded into the mountain W, and numerous mining towns were established.
Lessons from the past:
The earliest recorded avalanche fatalities in Canada occurred in the E, rather than the west. What happened?
1782, when 22 people from an Inuit settlement near Nain, Newfoundland and Labrador, perished.
Nain was founded in 1771, most N and largest community in Nunatsiavut and gateway to Torngat Mountains national park.
Lessons from the past:
The deadliest avalanche in Canada occurred..?
In Rogers Pass, at the height of the Colombian Mountains in BC: On March 4, 1910, 58 workers were killed as they were clearing a section of railway.
Snowpack:
Define snowpack.
When snow falls in mountains, it accumulates in layers within the snowpack, which is the total amount of snow on the ground.
Snowpack:
Different weather conditions and snowfall events create different types of layers in the snow over the course of a winter season. The stability of snowpack is influence by…?
How well the different layers of snow adhere to one another and the surface on which they fell.
Snowpack:
This bond and anchorage of snow layers, called ___ ___, resists the downslope force of gravity, which is called ___ ___.
Shear strength
Shear stress
Snowpack:
When shear stress outweighs shear strength…?
Aunstable mass of snow breaks loose, creating a snow avalanche.
Avalanches range in size from small ___ that wouldn’t harm a person to large powerful slides capable of destroying forests or even small villages.
Sluffs.
There are 2 principle types of snow avalanches. What are they?
- Loose-snow avalanches
- Slab avalanches
The distinction between the two types is based on the cohesiveness of the snow.
Loose-snow avalanches:
Loose-snow avalanches have little internal cohesion. They’re also sometimes called ___-___ avalanches, because…?
Point-release
Because they start when a small amount of loose snow slips and begins to slide down a slope, setting additional snow in motion.
Loose-snow avalanches:
Describe loose-snow avalanches.
- Initiate at a point, and tend to grow wider as they slide.
- Occur much more frequently in freshly fallen snow on steep slopes.
- Generally shallow, small, and cause little damage. Scores can occur during a single snowstorm. However, in the spring, when the snow is wet and heavy, loose-snow avalanches can gain enough momentum and mass to cause serious damage.
Slab avalanches:
Slab avalanches occur much less frequently, but are considerable more dangerous. When do they occur? What 4 things do you generally need?
When a plate or slab of cohesive snow begins to slide as a unit before breaking up.
- A slab of snow (typically dense mass)
- weak layer (less cohesive strength) beneath
- Steep slope (>30)
- Trigger
Slab avalanches:
Slab avalanches can lie teetering on the verge of release for days or even months. What can trigger failure in sensitive weak layers below?
Most avalanches are triggered when slopes are loaded by additional or new snow. “Natural trigger” can also be warming temps, rain, rock fall, cornice failure, earthquake…
“Artificial triggers” are caused by people or wildlife.
Majority of avalanches occur on slopes between …?
36-39 degrees, slopes greater than 60 degrees usually are too steep to hold snow, continuous sluffing keeps them clean.
Avalanche Characteristics:
Slab avalanched can originate in all types of snow. The main distinguishing characteristic is that…?
The snow breaks away with enough internal cohesion to act as a single unit before breakup.
Avalanche Characteristics:
What is a crown?
The area of release is marked by a distinctive upper fracture line, or “crown”, which is perpendicular to the slope and extends down to the sliding surface, or “bed surface”.
Avalanche Characteristics:
Slide paths are a common feature on the landscape, recognizable due to…?
Lack of trees.
Avalanche Characteristics:
What are the 3 major sections or zones of avalanche paths?
- Starting zone (uppermost part)
- Track (avalanche travel area)
- Runout zone (where debris accumulates at bottom of slope)
Motion and forces:
The speeds at which avalanches travel range widely. Give examples.
E.g. Dry slides can reach speeds of 50 to 200 km/hr.
Motion and forces:
When dry flowing avalanches exceed 35 m/hr, a dust or power cloud of airborne particles of snow is created and moves above the dense flowing part of the avalanche. The forces are greatest in the dense flowing part of the avalanche; but…?
If the slide is big enough, the air blast from a powder cloud can travel fast enough to explode your lungs. They can cause damage well beyond the normal avalanche zone.
Motion and forces:
The violence inside the flowing debris grinds up snow into even finer particles. Even if the snow started light, it can become dense by the time it finally comes to a stop. Why?
Small grains sinter much more quickly than large, and the tiny grains making up avalanche debris can sinter as much as ten thousand times faster than the larger grains of the initial slab. Finally, all of the kinetic E heats up snow just enough to create water on the surface of ice grains so that it seizes up like concrete the instant it comes to a stop.
Motion and forces:
Wet-snow avalanches, on the other hand, tend to generally slide at much slower speeds with no dust cloud, but their impressive mass can still cause great damage. Give an example.
This is especially the case in the spring with large climax avalanches: when the whole of the season’s snowpack may release right down to the ground.
Case study: Avalanche forecasting and safety along the Trans-Canada Highway at Rogers Pass in Glacier National Park with Jeff Goodrich…
Jeff Goodrich is Senior avalanche forecast with Parks Canada
134 avalanche paths within 40 km stretch of highway, up to 14 m of snow at treeline per season
Avalanche forecasting, static defences (avalanche berms channel debris over snowshed), active artillery control program (houzers?)
Landslides:
What are landslides?
The downslope movement of rock and debris.
Landslides:
When do they occur?
When shear stress with a slope outweighs the shear strength of the slope’s rock or sediment layers, causing the slope to fail.
Landslides:
How does vegetation on a slope increase its shear strength?
- Acts as a barrier to slow downslope movement
- Acts as a natural anchor for soils
Landslides:
What decreases the shear strength of a slope?
- Slopes with smooth surfaces, like rock, have low frictional strength, making them slippery
- Adding a lubricant (i.e. water)
Landslides:
What are other ways to decrease slope stability?
Streams and rivers undercutting slopes, making them steeper
Human activities, e.g. clear-cutting forests, mining, road construction, and home building can undercut or overload slopes
How are different types of landslides classified?
- Their material composition
- Their water content
- How they move down a slope
Rockfalls and Topples:
The most basic types of landslides. How do they occur?
When rocks suddenly detach from slopes (Rockfalls occur when a rock detaches and falls freely, bounces or rolls downslope; topples occur when a large piece of bedrock falls off a slope and rotates end-over-end)
Rockfalls and Topples:
Where do they usually occur?
On steep slopes with exposed bare rock. Failure may be the result of weathering, or by faulting of rock, and is often triggered by rain or “free thaw cycles”.
