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.
Population of town of Armero in Colombia? How far from the Nevado del Ruiz?
28 000 people. 45 miles from crater.
Describe how the town of Armero was damaged by the volcanic eruption.
two and a half hours after the start of the eruption, a lahar buried Armero. The explosive eruption had rapidly melted much of the volcano’s snow-covered glacier, sending water surging down canyons, picking up soil, volcanic ash, and red-hot rock as it went. More than three-quarters of the town’s citizens perished.
Describe the lahar that traveled from the Nevado del Ruiz volcano and its destruction. How many people killed, how many injured, how many homes destroyed?
Lahar traveled 100 km and left behind a wake of destruction: more than 23,000 people killed, about 5,000 injured, and more than 5,000 homes destroyed along the Chinchina, Guali, and Lagunillas Rivers.
What triggered the 1980 Mount St. Helens eruption?
a massive 2.8km^3 rockslide that was triggered by ongoing volcanic activity.
How far did rockslide travel from 1980 Mount St.Helens eruption?
The rockslide descended and traveled 22 km down the valley.
What did the rapid melting of snow and ice from the eruption induce?
The rapid melting of snow and ice from the eruption induced mud flows which surged down several valleys destroying almost everything in its path.
Describe the Mount St. Helens May 18 1980 eruption sequence.
At 8:32 am a 5.1 earthquake shook loose the north flank of Mount St. Helens, resulting in the largest known landslide in historic time. Removal of more than half a cubic mile of material released pressure and triggered a devastating lateral blast and ash-laden (vertical) eruptive column.
Mount St. Helens 1980 eruption: Within __ minutes the eruption column reached an altitude of __ miles.
10; 12
When did Mount St. Helens erupt often? Describe the eruption on March 19, 1982.
Erupted often between 1980 and 1986. An explosive eruption sent pumice and ash 14 km into air, and resulted in a lahar flowing from crater into the North Fork Toutle River valley. Part of the lahar entered Spirit Lake (lower left corner) but most of the flow went west down the Toutle River, eventually reaching the Cowlitz River, 80 kms downstream.
Describe the correlation between earthquake magnitude and areal extent of landslides.
Small magnitude earthquakes (M<4.0) generate falls over less than 10km^2 in area, while large magnitude earthquakes (M>6.0) can generate avalanches affecting areas 3 magnitudes larger. Liquefaction can easily be generated by smaller earthquakes.
How many landslides can larger earthquakes generate?
thousands of landslides
How do earthquakes trigger rockfalls?
by dislodging loose resting on steep slopes. Larger scale: seismic loads destabilize slopes by inducing shear stresses thereby weakening the internal structure of the slope material. (Alaska, 1964, when 56% of total cost of damage of M9.2 earthquake was caused by quake-triggered landslides.
Describe the largest landslide in Anchorage.
The largest landslide in Anchorage was triggered by the M 9.2 Good Friday Earthquake in 1964, caused substantial damage to numerous homes in the Turnagain-By-The-Sea subdivision.
Describe the quake-triggered landslide in El Salvador on January 13 2001.
The earthquake-triggered landslide in El Salvador on 13 January 2001 killed 585 people. The earthquake magnitude was M 7.7.
Describe the slope stability analysis of the Alaska earthquake-triggered landslide.
A slope stability analysis of the Alaska earthquake-triggered landslide shows that the Factor of Safety decreased sharply due to both an increase in the shear stress and a decrease in the shear strength due to the effects of the earthquake.
How can humans increase the likelihood of a landslide?
- Excavation of a slope at its toe (point at which lowest part of a slope intersects with natural ground line)
- Loading of a slope at its crest.
- Deforestation for construction and development (including clear-cut logging on slopes)
- Irrigation
- Mining
- Water leakage from utilities
- Artificial vibration (Blasting and the movement of heavy equipment during construction and mining can act as triggers for landslides.)
What two activities commonly done during construction, particularly in areas with topographical reflied as it is an effective way to create flat areas to build upon?
excavation of a slope at its toe,
loading of a slope at its crest
What is the purpose of the loading of a slope at its crest in residential construction?
to increase the size of a lot by extending its backyard.
What is commonly done in road construction through mountainous areas, particularly if the material is bedrock? Why bedrock?
Blasting through the base of a slope and creating near-vertical cliffs.
Why do we blast through the base of a slope in road construction through mountainous areas especially if material is bedrock? (i.e. why bedrock?)
Bedrock has a higher shear strength than unconsolidated sediment; however, bedrock still must be evaluated for failure potential as it can be fractured and/or faulted, yielding a lower shear strength.
The next time you are driving north along the Sea-to-Sky Highway, examine the fracture orientation of the rocks near Porteau Cove. What will you see?
You will see that the weaker fracture planes in the cliff face that are likely to accommodate motion, slope toward the highway and resemble daylighted bedding
Worst slope-instability related catastrophes of the 21st century in terms of death toll? Trigger? How many deaths?
2013 Uttarakhand, India. Flood driven. 5700 deaths.
Why is the erosion of the headlands of Point Grey and the subsequent recession of the cliffs that bound UBC a concern for the university since 1960?
the material that UBC is built upon is composed of unconsolidated sands and silts. Wave action easily erodes and undermines the base of the cliffs that support the area where UBC sits.
When was the unconsolidated material that makes up UBC deposited?
during the last glaciation (Fraser Glaciation).
At the UBC location, a _____ year old layer of unconsolidated deltaic and glaciofluvial sediments up to __ meters thick is overlain by a thin layer of ______ year old till with thickness of between _ - _ meters.
~23,000; 65; ~12,000; 0 - 5
What does well-indurated mean?
hardened
Why can the glacial sands at UBC support near-vertical cliffs?
They have been overridden and compacted by subsequent glacial advances.
What significant steps did UBC take since the 1970s to decrease coastal erosion along Wreck and Tower Beaches?
Two berms (artificial beaches) have been constructed. Cliffs behind Museum of Anthropology were cut back to decrease shear stress. Vegetation was also planted to increase shear strength in those slopes.
What is the purpose of a berm? (artificial beach)
Curtail coastal erosion and allow material from previous slides, large or small, to accumulate at the base of the cliffs until a stable configuration can be achieved. Berms effectively widen the natural beach, preventing waves from reaching the base of the cliffs.
Describe the two berms that UBC constructed.
In 1974, a berm composed of sand, gravel, and cobbles overlain by gravel found in natural deposits was constructed for 750 000. However, the ber However, the berm was breached by waves by late Fall 1975. The berm material was eventually transported ~2 km eastward to Spanish Banks by longshore drift by the spring of 1976.
In the spring of 1982, a new berm was completed at a cost of $1.2 million (figure below). This berm consisted of well-sorted cobbles that allowed the energy from the crashing waves to dissipate instead of eroding the finer-grained material eastward. A series of groins (a structure made of concrete, rocks, or wood usually built to preserve the sediments in a beach) made of large boulders were constructed perpendicular to the beachfront in order to prevent the berm material and finer sands from being transported to Spanish Banks by longshore drift. These structures are still visible on the beaches.
When did UBC construct its second Bern? How much did it cost?
1982 spring. $1.2 million
Describe how UBC cut the cliffs behind the Museum of Anthropology in 1989. Why did they do it?
The cliffs were cut to a more gradual slope to decrease shear stress. Fences were built to prevent people from treading onto unstable cliffs sands in order to reach the beach. Vegetation was planted to increase the shear strength of the material; willow stems were embedded into sand using pins driven into the slope. (planted along the contours of the cliffs and anchored to pins driven into sand)
What can you see on Tower Beach?
A relatively large sand fan left behind after a section of the cliff behind Cecil Green Mansion collapsed in 1994
What is a short-term solution for preserving the cliffs and mitigating the effects of coastal erosion at UBC?
use the sands dredged from the Fraser River to cover and widen the beaches so that waves would break further from the cliff base. These sands would be reworked and eroded relatively quickly as demonstrated by the undermining of the 1974 berm
What year did SADE begin construction of the Vaiont Dam?
1956
What is SADE?
private hydroelectric energy company in northern Italy
How tall was the Vaiont dam at time of construction?
265.5 m. Tallest concrete arch dam in the world.
The construction of the Vaiont dam provided work for how many people? How many people lived in the valley?
Provided employment for 400+ people. Valley population: 2000+ people.
What did Giorgi Dal Piaz conclude during his geological investigating?
He concluded that some local detachments of material of the reservoir slopes could be expected, but not serious. These investigations (reservoir slopes) only included the lowermost sections of the slopes.
What did Edoardo Semenza conclude during his geological investigating?
He investigated during the the filling of the reservoir. His investigations contradicted Dal Piaz’s favorable report, uncovering a prehistoric slide of unknown dimensions on the left reservoir slope.
Which slope of the reservoir seemed to have serious problems?
the left reservoir slope
Why were the people responsible for the construction of the Vaiont dam under intense pressure to have the dam tested and certified as operational?
Because plans were underway to nationalize the production of hydroelectric energy in Italy, where existing structures would be sold to the state. For Vaiont, a working plant would be worth much more than an untested plant.
Describe what happened 5 months after the initial filling of the reservoir.
A 2-km long tension crack opened near the top of the slope suggesting that a very large landslide had been mobilized. Later, with the depth of the reservoir at 180 m, a large landslide occurred when 700,000 m3 of material slid into the lake in about ten minutes killing a worker. Lowering of the dam was ordered.
Describe what happened a year after the second filling began in 1961.
One year later, the 700 m level was reached completing the second water test, and the filling of the reservoir stopped. This was an important milestone as the federal law to nationalize hydroelectric production in Italy was finally issued in December 1962. Meanwhile, the water level was lowered stopping at 648 m on early 1963.
Describe the third filling of the reservoir.
Slope movements resumed. Water level was quickly dropped to bring the slope under control. However, unlike previous experiences where such action helped halt the slope movements, this time velocities continued to accelerate.
Describe the bleak situation in early October 1963 written by Alberico Biadene, the new director of the facility. Why did he write the letter?
Letter addressed to call back from vacation his chief of construction.
Translational speeds within the landslide have increased. Ruptures in the terrain and road, evident inclination of trees in the area over La Pozza, enlargement of the large scarp around the mountain, motion of points near Pineda (which had been stable). Water level going down, intended to reach 695m in level to create a safety margin in the eventuality of waves.
When did the landslide at Vaiont dam occur? How fast was the landslide?
at night, on Oct 9 1963..
How much rock collapsed in the Vaiont dam landslide?
260 million m^3 of rock
Velocity of the Vaiont dam landslide?
30 m/s northwards, completely blocking the gorge
How far did the landslide travel up the opposite bank?
140 m
How big was the wave of water from the landslide in Vaiont dam? How much of it was pushed over the dam?
50 million m^3 wave of water. Half of it was pushed over the dam.
Describe the wave that fell out of the Vaiont dam.
The water then fell vertically, 261 meters into the narrow valley forming a 70 m high wave traveling at about 100 km/h. At about 22:40, about 1 minute after the detachment of the landslide, a wind began to blow across Longarone and mud began to rain down. Three minutes later the wave reached the centre of the valley and with a height of still 30 m, hit Longarone and a series of neighbouring towns. The numbers vary, but approximately 1,900 people were killed in the towns below the dam.
Approx. how many people died from the Vaiont dam tsunami in the towns below the dam?
1900 people
Aftermath of the Vaiont dam disaster?
2 days after the disaster, ENEL/ SADE released a statement saying that “the landslide was not predictable”
When did the slopes east of Oso, Washington collapse? Why did it collapse?
March 22, 2014 at 10:37am. 45 days of heavy rain.
What was the slope east of Oso, Washington composed of?
layers of glacial sediments, which sent mud and debris south towards the Stillaguamish River.
Consequences of the Oso Mudslide?
An entire rural neighborhood drowned in the mud. 43 deaths, 49 homes destroyed, and other buildings too. Mudslide dammed Stillaguamish River, flooded highways and settlements upstream. US $10 million loss. Deadliest single landslide event in the history of the US.
Deadliest single landslide event in US history?
Oso Mudslide, on March 22 2014, 10:37am.
What was controversial about the Oso Mudslide disaster?
The controversial part of this disaster was that government officials insisted that “This was a completely unforeseen slide. This came out of nowhere”, when in fact geologists, engineers, and even local residents were very much aware of the history of slides in the area. A local newspaper article questioned “Unforeseen’ risk of slide? Warnings go back decades” while a government official stated “A slide of this magnitude is very difficult to predict. There was no indication, no indication at all.”
Describe the landslide history in the Oso, Washington region.
Indeed, the history of slides in the area is long. 1937: aerial photographs show active landslides; 1949: A large landslide (300 m long and 800 m wide) affected the river bank; 1951: Another large failure of the slope; the river was partially blocked; 1988: Erosion of the toe leads to another slide. River pushed South again; 1999: US Army Corps of Engineers report warns of “the potential for a large catastrophic failure”; 2006: large slide blocks the river, a new channel is cut to alleviate flooding; 2010: Study commissioned by the county, warns hillside above a local road is “…one of the most dangerous in the county”.
Describe the report by the Geotechnical Extreme Events Reconnaissance (GEER) after the Oso Mudslide. (important: the 2 interconnected events of the slide?)
In a report by the Geotechnical Extreme Events Reconnaissance (GEER), a team of scientists and engineers tasked to evaluate natural disasters, the 2006 landslide likely was a recursor for the much bigger slide. This slide created an uncompacted layer of rocks and soil that readily soaked up water and was thus more prone to failure. The 45 days of heavy rains prior to the disaster soaked up the region. According to the report, the slide was in fact two interconnected events: (1) the debris field from the 2006 landslide liquefied. (2) That process sent a mass of mud and debris shooting across the valley, taking with it support for a large chunk of the mountain. The mountain side fell away and became part of the slide.
What is the GEER (Geotechnical Extreme Events Reconnaissance)?
a team of scientists and engineers tasked to evaluate natural disasters
