Geography (9696) | Hazards resulting from mass movements

HAZARDS RESULTING FROM MASS MOVEMENTS (9696 Advanced Physical Geography)

Hello Geographers! Welcome to a fascinating, yet sometimes scary, topic. This chapter looks at how the ground itself can suddenly become a hazard. Mass movements, often called landslides, are not just slow, boring geological processes—they are rapid, destructive events that cause huge loss of life and property worldwide.

Understanding the factors that make a slope fail is vital for managing risk and saving lives. Don't worry if the vocabulary seems tough at first; we will break down the causes and management strategies step by step!

1. Defining Mass Movements and Resultant Hazards

1.1 What is Mass Movement?

Mass movement (or slope failure) is the downslope movement of material (rock, soil, debris) under the direct influence of gravity. It does not require a transporting agent like running water or ice, although water plays a critical role in triggering the event.

Think of it this way: Gravity is always pulling material down. Mass movement occurs when the pulling force (stress) becomes greater than the material’s ability to resist the movement (strength).

1.2 Key Forces: Stress vs. Strength

• Shear Stress: This is the force pulling the slope material downwards, parallel to the slope. It is mainly driven by gravity.
• Shear Strength: This is the internal resistance of the material to movement. It is provided by factors like friction, internal cohesion (how well particles stick together), and interlocking of rock grains.

A mass movement hazard occurs when Shear Stress > Shear Strength. The slope fails.

Quick Analogy: Imagine trying to hold a wet, heavy sack of sand (high Shear Stress). If your muscles (Shear Strength) aren't strong enough, you drop it—the sack fails.

2. Nature and Causes of Slope Failure

The causes of mass movements can be broken down into factors that *reduce* shear strength (making the slope weaker) and factors that *increase* shear stress (making the pull stronger).

2.1 Factors Reducing Shear Strength (Internal Weakening)

• Weathering: Physical and chemical weathering break down rock material, reducing internal cohesion. For example, repeated freeze-thaw action cracks rocks, making them unstable.
• Increased Water Content (Saturation): This is the single biggest trigger. Water has two main effects:
  1. It adds weight, significantly increasing Shear Stress.
  2. It fills pore spaces, increasing pore water pressure, which acts to push particles apart, reducing Shear Strength (like lubricating the material).
• Geological Structure: Slopes are weak if rock layers (strata) dip in the same direction as the slope face (known as dip slopes).
• Removal of Vegetation: Tree roots help bind soil together. Deforestation removes this natural binding, decreasing strength.

2.2 Factors Increasing Shear Stress (External Triggers)

• Vibrations/Shaking: Earthquakes (Syllabus 9.1), volcanic eruptions, or even heavy blasting in quarries can shake material loose and drastically reduce friction between layers.
• Steepening the Slope: Natural erosion at the base of a slope (e.g., wave action or river undercutting a bank) removes supporting material, making the slope steeper and less stable. This is called toe erosion.
• Human Activity: Excavation for roads or buildings at the base of a slope (known as undercutting) increases the angle and therefore the stress. Piling waste material or buildings on the top of the slope (surcharge) adds weight and stress.

3. Types of Mass Movements and Resultant Hazards

Mass movements are categorized primarily by the speed of movement and the water content (as noted in Core Physical Geography 3.3). Each type results in a specific hazard.

3.1 Falls (Fastest and Driest)

• Nature: Free-fall of rock or debris from a vertical or near-vertical face. Movement is extremely fast.
• Resultant Hazard: Rockfalls or topples. These can destroy anything directly beneath them, often without warning.
• Example: Rockfalls along coastal cliffs or steep mountain roads.

3.2 Slides (Movement along a Plane)

• Nature: Movement of a cohesive block of material along a distinct plane of weakness (shear plane).
• Types:
  • Translational Slide: Movement along a relatively flat or straight plane.
  • Rotational Slide (Slump): Movement along a curved shear plane, resulting in the material rotating backwards and leaving a curved scar (a 'slump head') at the top.
• Resultant Hazard: Destruction of infrastructure built on the moving block and burial of anything below the toe of the slide.

3.3 Flows (Saturated and Highly Fluid)

• Nature: Material mixes with a large amount of water (or air) and moves like a viscous fluid. These are often triggered by prolonged, intense rainfall or rapid snowmelt.
• Resultant Hazards:
  • Mudflows/Debris Flows: Fast-moving mixtures of soil, rock, and water, capable of travelling long distances, destroying everything in their path.
  • Lahars: A specific and highly dangerous type of mudflow involving saturated volcanic ash and debris (a secondary hazard of volcanic eruptions). These can be extremely hot and rapid.
• Example: The 1970 disaster in Peru, where an earthquake-triggered debris avalanche from Mount Huascarán buried the town of Yungay.

3.4 Heaves (Slowest)

• Nature: Extremely slow, imperceptible downslope movement caused by the expansion and contraction of soil particles (e.g., due to freezing/thawing or wetting/drying).
• Resultant Hazard: Soil Creep. While not immediately life-threatening, it causes long-term structural damage (tilted fence posts, broken retaining walls) and affects property value.

4. Impacts on Lives and Property

The scale of impact depends entirely on the speed, volume, and location of the mass movement.

4.1 Primary Impacts (Directly caused by the movement)

• Loss of Life and Injury: Especially in fast events like flows and falls, which give little warning. In LICs/MICs, housing built on unstable slopes (often due to poverty and land pressure) are highly vulnerable.
• Destruction of Property: Homes, schools, and essential buildings in the path of the movement are destroyed or rendered uninhabitable.
• Infrastructure Damage: Roads, railways, pipelines, and electricity cables are cut off or buried, isolating communities.

4.2 Secondary Impacts (Resulting from the primary event)

• Economic Losses: Costs associated with rescue, relief, clean-up, and rebuilding infrastructure. Loss of agricultural land or industrial sites.
• Damming of Rivers: A large landslide can block a river valley, creating a temporary lake. If this natural dam fails, it releases a devastating flash flood downstream. This happened in the 1925 Gros Ventre landslide in Wyoming, USA.
• Displacement and Social Disruption: People may lose their homes and livelihoods, leading to temporary or permanent migration.

Did you know? Even extremely slow movements like soil creep cost millions annually because they constantly damage foundations, roads, and buried pipes that require regular repair.

5. Prediction, Preparedness, and Risk Management (Syllabus 9.2)

Because mass movements are often secondary hazards (triggered by rain, earthquakes, etc.), management focuses heavily on assessment and early warning.

5.1 Hazard Mapping and Zoning

Hazard Mapping is the process of identifying areas prone to mass movement based on geological surveys, historical data, slope angle, and hydrological conditions.

1. Geologists identify high-risk features (steep slopes, weak rock joints, areas of historical failure).
2. Maps are created showing different risk levels (e.g., high, moderate, low).
3. Land-use Zoning is then implemented: High-risk zones are prohibited from development, or only low-impact uses (like forestry) are allowed.

This is a proactive strategy to reduce vulnerability.

5.2 Prediction and Monitoring

Prediction is difficult because the exact moment of failure is hard to pinpoint, but monitoring provides warnings.

• Ground Movement Monitoring: Instruments like tiltmeters measure slight changes in the slope angle, and extensometers measure cracks or widening fissures on the slope surface.
• Hydrological Monitoring: Tracking groundwater levels and pore water pressure (using piezometers). A rapid rise in water pressure is a strong indicator of imminent failure.
• Remote Sensing: Satellite imagery (especially InSAR—Interferometric Synthetic Aperture Radar) can detect ground movement of just a few millimeters over large areas.

5.3 Preparedness and Mitigation

Preparedness involves actions taken *before* the event to reduce the impact.

• Early Warning Systems: Linking monitoring data to alerts. If rainfall exceeds a threshold, authorities issue evacuation orders.
• Education and Evacuation Drills: Ensuring populations living in risk areas know the signs of instability and where to go during an emergency.
• Structural Mitigation (Slope Stabilisation - Links to Core Geo 3.4): Although not purely hazard management, these physical works reduce the likelihood of failure:
  • Pinning/Rock Bolts: Long steel rods drilled into unstable rock masses to hold them together.
  • Netting/Meshing: Wire mesh placed over loose rock faces to contain small falls and direct debris to controlled catchment areas.
  • Grading/Terracing: Changing the angle of the slope to make it less steep (reducing shear stress).
  • Afforestation: Planting trees to increase shear strength through root binding.

5.4 Perception of Risk

How people living in hazardous areas perceive the risk directly affects their willingness to prepare or move.

• Fatalism: The belief that the hazard is "God’s will" and nothing can be done to stop it.
• Optimistic Bias: The belief that "it won't happen to me," especially if the last major landslide was decades ago.
• Economic Constraints: For many in low-income areas, the high cost of safer land means they are forced to live on cheaper, unstable slopes. They often perceive the *risk of poverty* as greater than the *risk of the landslide*.

Effective hazard management requires understanding these perceptions, as structural solutions alone may fail if the community is unwilling or unable to participate in preparedness or relocate.