Geography (9696) | Slope processes

Welcome to Slope Processes! Your Guide to Landscape Movement

Hello Geographers! This chapter, Slope Processes, is where we learn how landscapes move—from the incredibly slow crawl of soil to terrifying, fast-moving landslides. Understanding slopes is crucial because they cover most of the Earth's surface, and their stability directly impacts where we can build and live safely.

Don't worry if this feels like physics at first! We’ll break down the forces at work using simple examples. By the end, you will be able to explain exactly why that patch of soil on a steep hill might suddenly decide to move, and what engineers do to stop it!

Section 1: The Physics of Slopes – Why Does Stuff Move?

Every slope, from a gentle hill to a steep cliff, is constantly fighting a battle between two forces: the force pulling the material down, and the force resisting that pull.

Forces Influencing Slope Stability

For any material (rock, soil, debris) on a slope, stability is governed by the balance between two main forces:

1. Shear Stress (The Driving Force)

This is the force trying to pull the material down the slope. It is primarily caused by gravity.

• Analogy: If you put a heavy backpack on a smooth, steep slide, the weight of the backpack trying to pull itself down is the shear stress.
• A steeper slope angle means higher shear stress.
• More mass (e.g., heavy rain saturating the ground) increases shear stress.

2. Shear Strength (The Resisting Force)

This is the internal strength of the slope material that resists movement. It keeps the material together and anchored to the hillside.

Shear strength comes from two main sources:

• Internal Friction: The resistance between individual soil grains or rock particles rubbing against each other.
• Cohesion: The 'stickiness' of the material (like clay or wet soil) that glues particles together. Vegetation roots also dramatically increase cohesion.

The Threshold of Movement

Movement (or failure) happens when:

\[ \text{Shear Stress} > \text{Shear Strength} \]

When the pulling force exceeds the internal strength, the material gives way. This is known as mass movement or slope failure.

Section 2: The Types of Mass Movement

Mass movement is the large-scale movement of material down slope under the influence of gravity. The syllabus requires you to know four main categories, based largely on their speed and water content.

Classification of Mass Movements

We classify mass movements as Heaves, Flows, Slides, and Falls.

Memory Trick: Think of a downhill race. The slowest competitor is the H-eave, and the fastest (most dramatic) is the F-all.

1. Heaves (Creep)

Heaves are the slowest form of mass movement, often moving only a few millimetres per year. They involve the repeated expansion and contraction of soil or regolith.

• Process: The most common type is soil creep.
  1. Expansion: Soil expands (often due to freezing (frost heave) or wetting). Particles move outwards and slightly upward, perpendicular to the slope surface.
  2. Contraction: Soil contracts (due to thawing or drying). Particles fall straight down under gravity.
• Effect on Slopes: Causes noticeable features over time, such as tilted fences, curved tree trunks (called ‘pistol butt’ trees), or minor ripples on the surface.
• Conditions: Common in humid environments where temperature or moisture changes frequently.

2. Flows

Flows occur when the material is highly saturated with water, behaving like a viscous liquid. There is no clear failure surface; the material mixes and deforms as it moves.

• Solifluction: A specific type of flow common in cold environments (periglacial zones). The upper layer of soil thaws and slides over a permanently frozen, impermeable layer below (permafrost).
• Mudflows/Earthflows: Rapid movement of highly fluid, water-saturated fine material (clay, silt). Often triggered by intense rainfall or volcanic activity (lahars). These are incredibly dangerous due to their speed and bulk.
• Conditions: High water content and generally impermeable sub-strata.

3. Slides

Slides involve the movement of an intact mass of rock or soil along a well-defined failure surface. They can be sudden and dramatic.

• Rotational Slides (Slumps): The mass moves along a curved or concave failure surface. This often results in the land surface at the top tilting backward into the slope, forming a shallow depression.
• Translational Slides: The mass moves along a straight or planar failure surface (e.g., a bedding plane in sedimentary rock or a fault). They travel long distances very quickly.
• Conditions: Often occur after intense rainfall lubricates the failure surface, significantly reducing shear strength.

4. Falls

Falls are the fastest form of mass movement, involving the free-fall of rock from a steep cliff or vertical face.

• Process: Rock breaks away due to weathering (like freeze-thaw or pressure release) and moves through the air, hitting the slope surface repeatedly before settling at the bottom.
• Effect on Slopes: Forms a cone-shaped pile of debris at the base called a scree slope or talus cone.
• Conditions: Steep, jointed rock faces, often in areas experiencing intense physical weathering.

Section 3: Water and Sediment Movement on Slopes

While mass movement involves large volumes of material moving under gravity, water and sediment movement on slopes relate to surface erosion by running water and impacts from rainfall.

1. Rainsplash

This is the first type of water erosion. It is caused by the kinetic energy of raindrops hitting the ground.

• Process: A raindrop hits bare soil, dislodging a particle and throwing it into the air. If the particle lands downhill, erosion has occurred.
• Effect: Rainsplash erosion is most effective on bare ground (no vegetation) and tends to move particles short distances downhill.

2. Surface Runoff

When the rate of precipitation exceeds the rate of infiltration (water soaking into the soil), water flows over the surface. This is known as surface runoff or overland flow.

Sheetwash

This is the initial stage of surface runoff. It is a thin, continuous film of water moving across the slope. It acts like a ‘sheet’ of water, removing fine, unconsolidated sediment relatively uniformly.

Rills

As sheetwash concentrates, it begins to carve small, temporary channels called rills. Rills are typically only a few centimetres deep. If these rills grow larger and deeper, they become gullies, which represent more severe erosion.

Key Takeaway: Rainsplash and runoff are crucial because they destabilise the very surface layer, making the slope more vulnerable to larger mass movements.

Section 4: The Human Impact on Slope Stability (3.4)

Human activities can dramatically influence whether a slope is stable or unstable. Geographers must be able to analyse the ways in which we increase and decrease slope stability.

Decreasing Slope Stability (Making it Dangerous)

Activities that often reduce the shear strength or increase the shear stress, making the slope more prone to failure:

1. Removing Vegetation (Deforestation)

• Tree roots bind the soil together, acting like natural reinforcing bars (increasing cohesion).
• Removing trees eliminates this binding, significantly decreasing shear strength.
• Trees also intercept rainfall and draw up soil water (transpiration), keeping the soil drier. Removing them leads to increased soil saturation and weight (higher stress).

2. Adding Mass (Overloading)

• Building houses, roads, or piling waste material on the top or middle of a slope adds significant weight.
• This directly increases shear stress, pushing the material downwards.

3. Changing the Slope Geometry (Undercutting)

• Activities like road construction, mining, or river channel straightening often involve cutting away the base (toe) of a slope.
• This increases the slope angle and removes the supporting material, significantly increasing shear stress and likelihood of failure.

4. Improper Drainage

• Poor irrigation or leaky pipes can saturate the sub-surface material, adding weight and reducing internal friction.

Increasing Slope Stability (Making it Safe)

Activities designed to reinforce the slope, increasing shear strength or reducing shear stress:

• Improved Drainage: Installing drainage channels or piping to remove excess water, thereby reducing weight and maintaining friction.
• Terracing: Creating steps on the hillside to reduce the overall angle of the slope and catch moving debris.
• Retaining Walls: Building structures at the base of the slope to hold the material back (increasing shear strength).

Section 5: Strategies to Modify Slopes and Reduce Mass Movement

Engineers use a mix of “Hard” (structural, concrete, costly) and “Soft” (natural, ecological) techniques to manage unstable slopes.

Hard Engineering Techniques

These methods physically change the slope to increase shear strength.

1. Pinning (Rock Bolts or Anchors)

• What it is: Long steel rods or cables are drilled deep into the unstable rock mass and anchored into stable rock behind it.
• How it works: Essentially staples the unstable layers to the stable deeper layers, dramatically increasing the overall shear strength of the rock mass.

2. Netting (Meshing)

• What it is: Steel mesh or wire netting is draped and secured over the rock face.
• How it works: It prevents smaller pieces of rock (that might otherwise fall onto roads or infrastructure) from scattering, instead guiding the material safely down into a catchment ditch at the base. It doesn’t stop the failure, but manages the debris.

3. Grading (Benching)

• What it is: The process of excavating and reshaping a steep slope to create a gentler angle or a series of stepped, less-steep sections (benches).
• How it works: Directly reduces the shear stress by making the angle less steep. This is a very common technique in large infrastructure projects like motorways.

Soft Engineering Techniques

These methods work with nature, focusing on ecological processes.

1. Afforestation

• What it is: Planting trees and other deep-rooted vegetation on the slope.
• How it works:
  • Roots act as anchors, increasing soil cohesion (shear strength).
  • Leaves and branches intercept rainfall, reducing saturation and surface runoff (reducing stress).
• Benefit: Often cheaper and more environmentally friendly than hard engineering, but takes time to become fully effective.

Section 6: Case Study Focus (Mandatory Component)

The syllabus requires you to study a case study on the impacts of human activity on slope stability and evaluate the attempts to reduce mass movement. Your notes should include:

Honing Your Case Study: Landslides and Human Activity

Choose a recent, specific example of a major landslide or mass movement event (e.g., in Seattle, USA; or areas impacted by deforestation in the Philippines or Nepal) and structure your study around these three points:

1. Cause and Impact:

• How did human activity decrease stability? (e.g., removal of key support, road construction, poor water management).
• What was the primary natural trigger? (e.g., intense, prolonged rainfall or an earthquake).
• What were the impacts on people (lives lost, property damage) and the environment?

2. Strategies Implemented:

• Which modification techniques were used? (e.g., Was the slope graded? Were rock bolts used? Was replanting necessary?).

3. Evaluation of Attempts:

• Assess the success of the strategies. Were they effective in preventing future movement?
• Evaluate the sustainability and cost. Was hard engineering (like expensive pinning) necessary, or could soft engineering (like afforestation) have been sufficient?
• Consider conflicting viewpoints: Were local residents happy with the visual appearance of the new structures?

By preparing a detailed case study, you ensure you can answer the higher-level ‘evaluate’ questions in the examination.

Chapter Summary: Key Takeaways

Understanding slope processes is fundamentally about the balance between Shear Stress (gravity/driving force) and Shear Strength (cohesion/resisting force).

• The four main types of mass movement – Heaves, Flows, Slides, and Falls – vary widely in speed and water content.
• Surface water movement (rainsplash, sheetwash, rills) is a precursor to larger failure.
• Humans often destabilise slopes through deforestation and undercutting, but we can stabilise them using engineering techniques like pinning, netting, grading, and afforestation.

Keep practising diagrams of these processes—a well-labelled sketch can earn you crucial marks!