LA L2: Landslide Causes and Triggers Flashcards
Landslides: cause vs. trigger?
A cause is a factor that makes slopes susceptible to movement without actually initiating a landslide.
A trigger is the event that actually initiates movement by driving the F_S < 1.0.
(i.e. a trigger is the single event that starts the slope material in motion; landslide may have several causes, but ONE trigger)
List some common examples of landslide triggers.
earthquakes
volcanic eruptions
intense, short-period rainfall
mining and quarrying
irrigation
vegetation removal
excavation of a slope at its toe
loading of the slope at its crest
and, loud noises or vibrations
Identify some types of landslide causes.
geological, mechanical, hydrological, morphological, biological, physical, and anthropogenic (human)
What are considered causes? (in landslide)
Factors that decrease slope stability when the F_S ≥ 1.0 or when shear strength is greater than shear stress
Where does slope failure occur?
Slope failure occurs at a critical point where shear stress is greater than shear strength and the FS < 1.0.
Some causes can have an accumulating affect on a slope and, therefore, LOWER/HIGHER? F_S through time (as you go to the right in the Figure above). Also notice that in some cases slope stability can (increase/decrease)? through time as denoted by an increase in the F_S. An example of this is the drying out of the material composing the hill slope after a heavy rain storm.
lower; increase
What is trigger frequency?
Trigger frequency is how often an event occurs that sets off a landslide. It can be natural, human induced, or a combination of both.
Define erosion.
Erosion is the transport of material away from its source.
What are two ways erosion can cause slope failure?
- The driving mass becomes greater than the resisting mass.
2. The slope steepness at the base of the slope increases such that shear stress becomes greater than shear strength.
Most common agent of erosion?
water. The base of cliffs along water and cut banks of rivers are areas where there can be an increased potential for landslides as a result of the erosion of material.
Describe glaciers as an erosion agent
Glaciers are an extremely efficient erosive agent and leave steep-sided glacial valleys such as at Howe Sound, which is prone to landslides.
How do large storms trigger large landslides?
Large storms, either through heavy rainfall events or large pounding waves, can work to rapidly erode and undercut coastal slopes
Describe the UBC Grand Campus Washout of 1935.
heavy rains and melting snowfall eroded a deep ravine across the north end of the campus. The campus did not yet have storm drains, and surface runoff went down a ravine to the beach. When the University carved a ditch to drain flooding on University Avenue, the rush of water steepened the ravine and eroded it back as fast as 3 m/hour. The resulting gully eventually consumed 70,000 cubic metres, two bridges, and buildings near Graham House. The University was closed for four and a half days. Afterwards, the gully was filled with debris from a nearby landslide, and only traces are visible today
How does vegetation influence slope stability?
by affecting the shear strength of the materials comprising the slope. Plant roots increase soil cohesion and strength by helping to bind loose materials. In addition, roots decrease surface erosion and help to absorb excess water through evapotranspiration.
How do plant roots increase soil cohesion and strength?
by helping to bind loose materials.
How do roots decrease surface erosion and help to absorb excess water?
through evapotranspiration
How do negative effects of vegetation affect land? What are some negative effects of vegetation on landslides?
These effects may increase shear stress or reduce shear strength and include: wind stress, uprooting events, tree weight acting on the slope, and bedrock fracturing by roots (similar to the effect of water on weathering of rocks, see item D3 below). The magnitude of these effect vary with many factors such as vegetation type (trees versus grasses, bushes, ground cover, etc.) climate, and soil type.
What can removal of vegetation from slopes cause?
Instability of slopes
What kind of slopes are landslides more likely to occur on? Why?
landslides are more likely to occur on slopes that have been clear-cut or completely ravaged by fire, between about 4 to 15 years after logging or burning. This is because it takes time for the remaining root structures to degrade.
Once decay has occurred, soil cohesion decreases and shear failure surfaces can develop at the interface between … ?
the soil horizon and the underlying bedrock material
How can water increase shear stress in parts of the slope?
An excessive amount of water within a slope increases its mass, increasing shear stress in parts of the slope
How can fluid pressure affect slope stability?
In some cases fluid pressure can build in such a way that water can support the weight of the overlying rock mass. This is the case when the slope is composed of sediment where grains are not cemented together and excess water sets the grains apart. When this occurs, friction is reduced, and thus the shear strength holding the material on the slope is also reduced, resulting in slope failure.
What natural phenomena are excessive amount of water within slope and fluid pressure buildup (such that water can support weight of rock mass) often associated with? What can they lead to?
Often associated with heavy rainfall or rapid snowmelt. Can lead to mass wasting.
When does water increase weathering of rocks?
When it flows into rock fractures (when it flows into even the narrowest of rock fractures)
Describe how water increases weathering of rocks.
Water increases the weathering of rocks when it flows into even the narrowest of rock fractures. When water goes through its freeze/thaw cycles, it can expand in volume by 9%. This volume change is a very powerful force that can wedge apart rocks, often causing them to fall from steep slopes in mountains and canyons.
How can water help or hinder cohesion in sediment?
Water in some pore spaces binds particles. Water between all particles keeps them apart and allows them to flow.
Why do landslides occur more frequently after period of heavy rainfall?
water affecting erosion rates, interacting with geologic materials, its affect on pore pressure
What’s one of the most common types of triggering mechanisms?
Storm systems that produce intense rainfall for periods as short as several hours or have a more moderate intensity lasting several days
How many rainfall-triggered landslides in Japan every year? How many deaths?
more than 10 000 landslides. more than 200 deaths
Describe the debris flow of 1999 in Venezuela.
In December 1999, heavy rainfall triggered thousands of debris flows along the northern coast of Venezuela. The death toll was estimated at 30,000 people. Heavy rainfall triggered thousands of debris flows and caused numerous flash floods in a 300-kilometer long area of the north coast of Venezuela. Rainfall amounts of 300 to 480 mm over the towns of Maiquetia and La Guaira on the coast north of Caracas, Venezuela.
Describe the 2005 North Vancouver rainfall-triggered landslide.
[date, increase in Coquitlam reservoir level, after__ hours did it occur, which tropical storm caused it, who did it kill]
A local, rainfall-triggered landslide occurred on January 19, 2005 in North Vancouver. During this rain event, the Coquitlam reservoir level increased by 557 mm between January 17 to 22. After 48 hours of heavy rain from tropical storm Kulap, a debris avalanche swept down an 80 meter slope in a residential neighborhood, destroying two houses, killing a woman, and prompting the Premier to declare a state of emergency.
What makes the North Shore more susceptible to landslides?
its steeper slopes, but it also receives approximately three times more rainfall than what is reported at Vancouver International Airport because of its proximity to the North Shore mountains
Why does the North Shore receive 3x more rainfall than at YVR?
proximity to the North Shore mountains
Describe the relationship between the intensity and abundance of precipitation and landslide type.
Heavy and prolonged rainfall can increase groundwater levels as a result of deep infiltration of abundant water. An elevated groundwater table tends to trigger more deep-seated and slower moving landslides such as slumps versus shallower, more fluid, and faster moving (and more dangerous) flows.
Give an example where two very different landslide types occurred in the same area as a result of different precipitation conditions.
1995 and 2005 La Conchita landslides in southern California
What is deposition of loose volcanic ash on hillsides commonly followed by?
accelerated erosion and frequent mud or debris flows triggered by intense rainfalls. Volcanic eruptions have directly triggered some of the largest historic landslides.
Summit elevation of Nevado del Ruiz volcano?
5389m
What happened on Nov 13 1985 in Armero, Colombia?
partially buried and destroyed by lahars
Describe Nevado del Ruiz.
Located in the Andes mountains of South America, Nevado del Ruiz is the northernmost and highest Colombian volcano with historical activity.
How could the deaths of the 1985 eruption of Nevado del Ruiz have been avoided?
might have been averted if government authorities had heeded warning. Colombian scientists monitoring the volcano had cautioned town leaders of Armero about the increased volcanic activity and even radioed urgent messages on the night of the eruption. But no systemic efforts were made to evacuate the town of 28 000. Authorities likely thought town was out of harm’s way.
