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Lecture 14 Mass Wasting/Mass Movements

Veronica McCann・44 minutes read

Mass wasting, or landslides, occurs when large amounts of earth move down slopes due to factors like gravity, weathering, and saturation from rainfall, posing risks evidenced by historical events such as the 1959 Hebgen Lake landslide. The study of these phenomena includes assessing slope stability and implementing preventive measures, underscoring the importance of monitoring and understanding the conditions that lead to such destructive occurrences.

Insights

  • Mass wasting, or landslides, is primarily driven by gravity, and its occurrence can be significantly influenced by weather conditions, such as heavy rainfall, which increases soil saturation and destabilizes slopes. The 1959 Hebgen Lake landslide serves as a historical example of how natural disasters can trigger deadly mass wasting events, emphasizing the importance of understanding the risks associated with certain geographic areas.
  • Research conducted at the U.S. Geological Survey's landslide flume in Oregon has demonstrated how quickly conditions can change, as adding just five inches of simulated rain can lead to soil liquefaction and subsequent landslides within a short period. This highlights the critical need for predictive models that can anticipate landslide occurrences based on varying soil types and moisture levels.
  • The impact of human activities, such as construction on unstable slopes and vegetation removal, can exacerbate the risk of landslides, as seen in cases like the condemned home in Athens County, Ohio. Recommendations from the U.S. Geological Survey stress the importance of proper drainage, ground assessments, and planting vegetation to help stabilize slopes and mitigate potential hazards.

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Recent questions

  • What is mass wasting?

    Mass wasting, also known as mass movement, refers to the process by which large amounts of earth material move down a slope due to gravitational forces. This phenomenon can occur in various forms, including landslides, mud flows, and avalanches, and is often triggered by factors such as heavy rainfall, earthquakes, or human activities that destabilize slopes. The movement can vary in speed and type, with some events occurring slowly over time, while others can happen suddenly and with great force, leading to significant geological and environmental impacts.

  • How do landslides occur?

    Landslides occur when the forces acting on a slope, primarily gravity, exceed the shear resistance that holds the materials in place. This imbalance can be caused by several factors, including increased water saturation from heavy rainfall, earthquakes, or the removal of vegetation that stabilizes the soil. When the soil or rock becomes too heavy or loose, it can begin to slide down the slope, resulting in a landslide. The speed and type of movement can vary, with some landslides being rapid and destructive, while others may be slow and gradual.

  • What causes slope instability?

    Slope instability is primarily caused by a combination of gravitational forces and various environmental factors that weaken the structural integrity of the slope. Key contributors include excessive rainfall that saturates the soil, earthquakes that shake the ground, and human activities such as construction or deforestation that remove vegetation and alter the landscape. Additionally, the composition and thickness of the soil play a crucial role; for instance, clay soils can become slippery when wet, increasing the likelihood of mass wasting events. Understanding these factors is essential for assessing and mitigating the risks associated with slope instability.

  • What are the types of mass wasting?

    Mass wasting is categorized into three main types based on the speed and nature of the movement: flow, slide, and fall. Flow involves the slow movement of materials, such as soil creep, where soil gradually shifts down a slope. Slides, like rock slides, occur when a mass of rock or soil moves down a slope more rapidly, often triggered by saturation or destabilization. Falls, such as rock falls, happen when rocks break off and fall freely due to gravity. Each type has distinct characteristics and can vary significantly in speed, from slow movements to rapid, destructive events.

  • How can landslides be prevented?

    Preventing landslides involves a combination of engineering techniques and environmental management practices aimed at stabilizing slopes. Recommendations include avoiding construction near steep slopes, conducting thorough ground assessments to understand soil composition and stability, and ensuring proper drainage systems to manage water runoff effectively. Additionally, planting vegetation can help anchor soil and reduce erosion, while creating gradual cuts instead of steep ones can enhance slope stability. Implementing these strategies is crucial for minimizing the risk of landslides and protecting both human life and infrastructure in vulnerable areas.

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Summary

00:00

Understanding Mass Wasting and Its Impacts

  • Mass wasting, also referred to as mass movement or landslides, involves the movement of large amounts of land down a slope, which can occur due to factors like earthquakes or heavy rainfall. Examples include mud flows, landslides, and avalanches.
  • A notable historical event occurred in 1959 at Hebgen Lake in Montana, where an earthquake triggered a landslide that buried a campsite, resulting in fatalities. This illustrates the ongoing risk of mass wasting in certain areas.
  • The primary driving force behind mass wasting is gravity, which pulls materials down slopes. The stability of these slopes can be affected by weathering processes that break down rocks into smaller pieces, making them more susceptible to movement.
  • There are three main types of mass wasting movements: flow (slowest, e.g., soil creep), slide (e.g., rock slides), and fall (e.g., rock falls). The speed of these movements varies significantly, from less than a millimeter per year for soil creep to over four kilometers per hour for rock falls and avalanches.
  • The mechanism of mass wasting involves a balance between gravitational forces pulling materials down and shear resistance holding them in place. When the forces become unbalanced, slippage occurs, leading to mass wasting events.
  • Soil thickness and composition play crucial roles in slope stability; thicker soils, especially clay, can become slippery when saturated with water, increasing the likelihood of mass wasting.
  • Vegetation contributes to slope stability by anchoring soil with roots. The removal of vegetation can destabilize slopes, as seen in previous mudslide events where heavy rainfall followed deforestation.
  • Research at the U.S. Geological Survey's landslide flume in Oregon involves simulating landslides by adding water to loosely packed soil to study how different soil types respond to rainfall and the conditions that lead to liquefaction.
  • In experiments, adding five inches of simulated rain can cause a slope to liquefy within 92 minutes, demonstrating how quickly conditions can change and lead to landslides.
  • The study of mass wasting is enhanced by using electronic sensors to measure various factors, such as water pressure and surface displacement, providing valuable data to understand and predict landslide behavior.

20:57

Landslide Dynamics and Stability Challenges

  • A shallow slope failure occurred initially, typical in mountainous regions, which was later exacerbated by added water, leading to a deeper failure where the material liquefied, resulting in a significant debris flow.
  • The process of adding water to the slope mimicked conditions of prolonged rainfall, which can destabilize soil and trigger landslides, as demonstrated in flume experiments aimed at refining predictive models for landslides.
  • Loose, compacted materials are more easily moved than denser materials because tightly packed grains have less space for water infiltration, which is crucial for movement; this concept relates to sedimentary rock formation and weathering.
  • The impact of increased water on slope stability was highlighted, emphasizing that destabilization can occur through various means, including earthquakes, added weight, or changes in moisture content.
  • An example from Athens County, Ohio, illustrated the consequences of building on unstable slopes, where a million-dollar home was condemned due to instability caused by overloading the slope, leading to visible cracks and structural failure within six months.
  • Road cuts and vegetation removal can destabilize slopes, as seen in areas with signs warning of falling rocks; these practices can lead to oversaturation of soil during heavy rainfall, increasing the risk of landslides.
  • Stabilization techniques for slopes include creating gradual cuts instead of steep ones, using steel plates and cables for reinforcement, and installing nets to prevent rockfalls, which are essential for maintaining road safety in hilly areas.
  • Different types of mass wasting were described, with creep being the slowest form, leading to gradual soil movement, while faster movements include slumps, debris flows, and avalanches, each characterized by their speed and moisture content.
  • The 2014 Oso mudslide in Washington resulted in 42 fatalities, demonstrating the rapid and unpredictable nature of deep-seated landslides, which can bury homes and disrupt river flows, often without sufficient warning for evacuation.
  • Landslides are classified based on their speed and depth, with shallow landslides typically involving the top meter or two of soil, while deep-seated landslides can involve entire valley walls, leaving behind a scarp, which is a steep slope formed after the event.

42:05

Landslide Dynamics and Prevention Strategies

  • The Oso landslide involved a significant failure of a hillside that transitioned into a debris flow, which spread laterally across the valley bottom, indicating a rapid and extensive movement of material.
  • To assess the potential for future landslides, researchers utilize Lidar (Light Detection and Ranging) technology, which involves flying a laser-equipped plane to scan the topography and measure the time it takes for the laser to return, revealing past landslide scars.
  • Lidar images of the Oso Valley illustrate the contours before and after the March 22nd landslide, showing a history of landslides with color-coded representations: yellow for the oldest, tan and orange for intermediate, and red for the most recent, including the March 22nd event.
  • David Montgomery's team carbon-dated buried logs from an ancient landslide deposit to approximately 5,300 years old, using three carbon samples taken from the bark to establish a timeline of past landslide activity.
  • Aerial photos and satellite images since 1933 have documented the Oso slope's repeated sliding events, which altered the river's course and concealed the slope's unstable history with regrowth of vegetation.
  • The 2014 landslide in Minneapolis, Minnesota, temporarily closed a hospital due to instability near buildings, highlighting the dangers of landslides and the need for monitoring and assessment of slope stability.
  • The text distinguishes between types of slope movements: slides, which involve rotational movement (like a box sliding down stairs), and flows, which are more fluid and lack distinct sections, as demonstrated by video clips of ground movement.
  • Rock falls occur when rocks break off and fall freely due to gravity, often creating dust clouds upon impact, while rock slides involve saturated ground conditions that allow larger rock masses to slide down slopes, as seen in historical events in Wyoming and Yosemite.
  • The Hebgen Lake landslide in Montana, triggered by a 7.3 magnitude earthquake in August 1959, resulted in 28 fatalities and $11 million in damages, primarily due to flooding caused by a dam failure.
  • Recommendations from the U.S. Geological Survey (USGS) for landslide prevention include avoiding construction near steep slopes, conducting ground assessments, ensuring proper drainage systems, and planting ground cover on slopes to stabilize soil.

01:02:25

Flood Risks and Landslide Research Assignment

  • Floods are a significant concern in regions such as New York, Massachusetts, Pennsylvania, Ohio, and Indiana, which have a higher-than-average risk for flooding, while Texas and Florida are below average. When considering moving to an area, it is crucial to assess the proximity to rivers, as homes near floodplains may experience frequent flooding, despite these areas being beneficial for farming. Changes in climate can also influence rainfall patterns, affecting flood risks.
  • For the assignment, students are required to find one to two recent events of landslides or mudslides that occurred between 2018 and 2020. They should search for terms like "landslide," "mudslide," "mud flow," or "avalanche," and post a link to a relevant video or news story, along with a two to three-sentence summary of what they learned about the event. Students are encouraged to reach out with any questions regarding the assignment.
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