Geomorphic Hazards: Landslides and Avalanches@

Learning Outcomes:

1. Understand the nature and causes of landslides and avalanches as geomorphic hazards.
2. Examine the different types of landslides and avalanches and their underlying mechanisms.
3. Analyze the environmental and human factors contributing to these hazards.
4. Evaluate the impact of landslides and avalanches on landscapes and human settlements.
5. Explore mitigation and management strategies to minimize the risks associated with these hazards.

Geomorphic hazards are natural processes that significantly alter the Earth’s surface, posing risks to human life, infrastructure, and ecosystems. Among these hazards, landslides and avalanches stand out due to their sudden onset and the destructive power they unleash on mountainous and hilly terrains. This chapter provides an in-depth exploration of these hazards, emphasizing their causes, characteristics, and consequences.

Landslides

Landslides involve the downward movement of rock, soil, and debris under the influence of gravity. They occur in various forms and scales, depending on factors like slope angle, material composition, vegetation cover, and human intervention.

Causes of Landslides

Landslides are triggered by a complex interplay of natural and anthropogenic factors, which can be categorized as follows:

1. Geological Factors:

   ▷ Rock Type: Soft and fractured rock formations, like shale or sandstone, are more prone to landslides due to their reduced shear strength.
   ▷ Soil Properties: Loose, unconsolidated soils with high porosity and water content exhibit lower cohesion, making them susceptible to slope failure.
   ▷ Slope Structure: The orientation of rock layers relative to the slope can influence landslide occurrence. Slopes parallel to the dip of bedding planes are more unstable.

2. Morphological Factors:

   ▷ Slope Angle: Steep slopes experience a greater gravitational force, making landslides more likely.
   ▷ Erosion: Natural processes like river erosion, wave action, or glacial retreat can undermine slope bases, reducing their stability.
   ▷ Vegetation Removal: Vegetation stabilizes slopes by binding soil with roots. Deforestation or wildfires reduce this stabilizing effect, increasing landslide risks.

3. Hydrological Factors:

   ▷ Rainfall: Intense or prolonged rainfall increases soil pore water pressure, reducing soil shear strength and triggering landslides.
   ▷ Snowmelt: Rapid snowmelt infiltrates the soil, saturating it and causing slope instability.
   ▷ Water Table Fluctuations: Seasonal changes in groundwater levels affect slope stability by altering the internal pore pressure.

4. Anthropogenic Factors:

   ▷ Construction: Building roads, dams, and other infrastructure on slopes can disturb the natural balance, leading to landslides.
   ▷ Mining Activities: Quarrying and excavation weaken slopes, removing supporting materials and creating conditions for slope failure.
   ▷ Deforestation: Human-driven removal of vegetation disrupts the natural water balance and slope stabilization, exacerbating landslide risks.

Types of Landslides

Landslides are classified based on their movement mechanisms and the materials involved:

1. Falls:

   ▷ Involve the free-fall of detached rock or debris from steep cliffs due to gravitational forces.
   ▷ Typically occur with little to no warning, causing significant damage upon impact.

2. Slides:

   ▷ Rotational Slides: Characterized by the downward movement of a mass along a curved surface, creating a concave shape.
   ▷ Translational Slides: Involve the movement of rock or soil along a planar surface, often triggered by fractures or bedding planes in the rock.

3. Flows:

   ▷ Debris Flows: Mixtures of water, soil, and rock that move downslope as a viscous fluid. They often result from heavy rainfall or sudden snowmelt.
   ▷ Mudflows: A specific type of debris flow with a high concentration of fine particles, such as silt and clay, making them particularly fluid.

4. Creep:

   ▷ A slow, imperceptible movement of soil and rock down a slope, typically driven by cycles of freezing and thawing, wetting and drying.

5. Topples:

   ▷ Involve the forward rotation and movement of a mass of rock or debris around a pivot point, often due to the undercutting of slopes.

Important Note: The speed and impact of landslides vary based on material type, water content, and slope gradient. Rapid movements, such as rockfalls, pose immediate danger, while slow processes like creep alter the landscape gradually.

Impacts of Landslides

The consequences of landslides can be devastating for both the natural environment and human settlements:

1. Human and Economic Losses:

   ▷ Landslides result in loss of life, injury, and displacement of populations.
   ▷ Destruction of infrastructure, such as roads, bridges, and buildings, incurs substantial economic costs.

2. Environmental Changes:

   ▷ Alteration of river courses and valley morphology due to sediment deposition.
   ▷ Landslides can block rivers, creating landslide dams that may eventually breach, causing catastrophic downstream flooding.

3. Soil Degradation:

   ▷ Removal of topsoil and vegetation leads to land degradation and reduced agricultural productivity.

Management and Mitigation of Landslides

Efforts to reduce landslide risks involve a combination of engineering, environmental management, and community planning:

1. Slope Stabilization:

   ▷ Retaining Walls: Constructing retaining walls helps support and stabilize slopes.
   ▷ Soil Nailing: Involves inserting steel bars into the slope to reinforce the soil structure.

2. Drainage Control:

   ▷ Surface Drainage: Channels and gutters are installed to direct water flow away from vulnerable slopes.
   ▷ Subsurface Drainage: Drains are placed within the slope to lower the pore water pressure and increase stability.

3. Reforestation and Vegetation:

   ▷ Planting deep-rooted vegetation to anchor the soil and absorb excess water, reducing landslide likelihood.

4. Land Use Planning:

   ▷ Restricting construction and human activities in landslide-prone areas through zoning regulations and hazard mapping.

Avalanches

Avalanches refer to the rapid downslope movement of snow, ice, and debris. They typically occur in mountainous regions, posing significant hazards to life and infrastructure.

Causes of Avalanches

Avalanches result from a combination of factors that disturb the equilibrium of snowpacks on slopes:

1. Snowpack Conditions:

   ▷ Layering: A snowpack consists of various layers formed during different snowfall events. Weak layers, such as hoar frost or freshly fallen snow, can become sliding surfaces.
   ▷ Density: Snow with low density, like powder snow, has less cohesion, making it more prone to sliding.

2. Slope Angle:

   ▷ Avalanches typically occur on slopes between 30° and 45°. Slopes steeper than 60° shed snow frequently, preventing large accumulations, while slopes below 30° are usually too gentle to sustain avalanches.

3. Weather Conditions:

   ▷ Snowfall: Heavy snowfall within a short period adds weight to the snowpack, increasing the stress on weak layers.
   ▷ Temperature Fluctuations: Rapid warming causes snow to melt and refreeze, creating hard crusts and weak layers within the snowpack.
   ▷ Wind: Wind can redistribute snow, forming unstable snowdrifts on leeward slopes.

4. Human Activity:

   ▷ Skiing, snowboarding, or other recreational activities can disturb the snowpack, triggering avalanches.

Types of Avalanches

Avalanches vary in their mechanisms of movement and the materials involved:

1. Slab Avalanches:

   ▷ Occur when a cohesive slab of snow breaks away from the underlying layers, sliding down the slope. They are the most dangerous type due to their speed and mass.

2. Loose Snow Avalanches:

   ▷ Begin at a single point and gather snow as they descend, forming a fan-shaped pattern. They typically occur with dry, powdery snow.

3. Wet Avalanches:

   ▷ Involve snow saturated with water, usually occurring during warm weather or rain-on-snow events. They move more slowly but are dense and destructive.

Important Note: The avalanche danger scale ranges from 1 (low) to 5 (extreme), indicating the likelihood and potential severity of avalanches. Understanding this scale is critical for safe navigation in avalanche-prone regions.

Impacts of Avalanches

Avalanches can have severe and far-reaching impacts, including:

1. Loss of Life:

   ▷ Avalanches pose a direct threat to skiers, climbers, and residents of mountain communities. Burial under snow can lead to asphyxiation within minutes.

2. Infrastructure Damage:

   ▷ Destruction of ski resorts, mountain roads, and communication lines, resulting in economic losses and isolation of communities.

3. Alteration of Terrain:

   ▷ Avalanches can clear vast areas of vegetation, exposing bare soil and leading to increased erosion and landslide risks.

Management and Mitigation of Avalanches

To minimize the risks associated with avalanches, a range of strategies are employed:

1. Snowpack Monitoring:

   ▷ Avalanche Forecasting: Utilizing weather data, snowpack analysis, and field observations to assess avalanche risks.
   ▷ Remote Sensing: Using satellite imagery and LIDAR to monitor snow accumulation and stability.

2. Structural Measures:

   ▷ Snow Fences: Erected on slopes to alter wind patterns and prevent snow accumulation.
   ▷ Avalanche Barriers: Rigid structures designed to intercept and slow down avalanches before they reach vulnerable areas.

3. Controlled Triggering:

   ▷ Artificial Avalanche Release: Using explosives or other methods to trigger controlled avalanches in a safe manner, reducing snowpack instability.

4. Public Education:

   ▷ Educating recreational users about avalanche safety, proper use of rescue equipment, and hazard assessment techniques.

Comparative Overview of Landslides and Avalanches

Aspect	Landslides	Avalanches
Material Involved	Rock, soil, debris	Snow, ice, debris
Main Trigger Factors	Rainfall, slope angle, human activities	Snowpack conditions, weather, human activities
Speed of Movement	Varies (slow to rapid)	Typically rapid
Risk Areas	Hillslopes, mountainous terrain	Snow-covered mountain slopes

MCQ: Which slope angle is most conducive to avalanches?

1. 10°
2. 25°
3. 35°
4. 60°
   Correct Answer: 3. 35°