External Landform Processes and Related Features

Lessons

Senior 6 · Geography

External Landform Processes and Related Features

5 lessons

Lesson 5 of 5

Mass Wasting and Weathering Landforms in Human Activities

25 min

When the hillside lets go

On the evening of 2 May 2023, very heavy rain fell on north-western Rwanda. Through the night floods and landslides killed at least 135 people; the worst-hit districts were Rutsiro, Nyabihu, Rubavu and Ngororero, and about 20,000 people were displaced. You studied that disaster in S2. This lesson asks the A-level questions behind it: what holds a slope up, what makes it fail, how the different kinds of movement differ, and how people can live safely with steep, deeply weathered land. It ends by drawing together the links between weathering landforms and human activities across the whole unit.

Definition

Mass wasting (mass movement): the downslope movement of rock, regolith and soil under the direct pull of gravity, without being carried by a river, glacier or the wind (although water in the material often helps it move).

Shear stress: the force pulling material down the slope. Shear strength: the resistance of the material, from friction between grains and the cohesion that binds them. A slope fails when shear stress exceeds shear strength.

Why slopes fail: a balance of forces

Figure 10: the forces on material resting on a slope of angle θ.

The part of the weight acting down the slope, $W\sin\theta$ , drives movement. The part pressing into the slope, $W\cos\theta$ , creates friction that resists it. For loose, dry granular material the friction is $W\cos\theta\tan\phi$ , where $\phi$ is the material's angle of internal friction, typically about 30° to 40° for sand and gravel. Engineers compare the two with a factor of safety, $F$ :

F = \frac{\text{resisting force}}{\text{driving force}} = \frac{W\cos\theta\tan\phi}{W\sin\theta} = \frac{\tan\phi}{\tan\theta}

If $F > 1$ the slope stands; if $F < 1$ it fails. This simple form applies to a dry, cohesionless material on a long, straight slope.

Worked example 1: a dry slope

Problem. Weathered material with $\phi = 35^\circ$ lies on a slope of 25°. Is it stable? What happens if a road cutting steepens the slope to 38°?

Solution.

F = \frac{\tan 35^\circ}{\tan 25^\circ} = \frac{0.700}{0.466} = 1.50 \quad (\text{stable})

F = \frac{\tan 35^\circ}{\tan 38^\circ} = \frac{0.700}{0.781} = 0.90 \quad (\text{fails})

Steepening the slope beyond the angle $\phi$ makes failure certain.

Worked example 2: the same slope after heavy rain

Problem. When the slope is saturated, with water seeping down parallel to it, the factor of safety becomes

F = \frac{\gamma_{\text{sat}} - \gamma_w}{\gamma_{\text{sat}}}\times\frac{\tan\phi}{\tan\theta}

where $\gamma_{\text{sat}} \approx 20\ \mathrm{kN\,m^{-3}}$ is the unit weight of the saturated soil and $\gamma_w = 9.81\ \mathrm{kN\,m^{-3}}$ that of water. Find $F$ for the 25° slope, and the steepest slope that is stable when saturated.

Solution.

\frac{20 - 9.81}{20} = 0.51, \qquad F = 0.51 \times 1.50 = 0.77 \quad (\text{fails})

\tan\theta_{\max} = 0.51\times\tan 35^\circ = 0.357, \qquad \theta_{\max} = 19.6^\circ

Water pressure in the pores pushes the grains apart and roughly halves the friction. A slope that is safe up to 35° when dry can fail at about 20° when saturated, which is why prolonged heavy rain triggers so many landslides on steep, deeply weathered hillsides.

Types of mass movement

Figure 11: four types of mass movement.

Type	Speed	Water	Description
Soil creep	Very slow (mm to cm a year)	Moist	Gradual movement of soil particles, by wetting and drying, heating and cooling; shown by tilted posts and trees, bulging walls and small steps (terracettes)
Solifluction	Slow	Saturated	Flow of a waterlogged surface layer over frozen ground in cold climates
Earthflow and mudflow	Moderate to very fast	High	Wet soil flows as a tongue; mudflows on volcanoes are called lahars
Rotational slump	Slow to fast	Often wet	A block slides on a curved surface and tilts back, leaving a scarp; common in clays and deep regolith
Translational slide (landslide)	Fast	Variable	A slab slides along a flat plane such as a bedding plane or the weathering front
Rockfall	Very fast	Dry	Blocks fall freely from a cliff and collect as scree

Terracettes on a steep grassy slope below Morgan's Hill, England: rows of small steps running across the slope. They are associated with slow soil creep, often helped by animals walking along the slope. Photo: Derek Harper, CC BY-SA 2.0, via Wikimedia Commons.

Causes, effects and control

Preparatory causes (make a slope weaker)	Triggers (set off the failure)
Steep slopes; deep, clay-rich regolith from chemical weathering; joints or bedding dipping out of the slope	Heavy or prolonged rain that saturates the ground
Removal of trees, whose roots bind the soil and draw out water	Earthquakes and volcanic activity
Undercutting of the slope foot by rivers or road cuttings	Vibration from blasting or heavy traffic
Extra weight of buildings, fills or water tanks on the slope	Sudden release of water from a leaking pipe or blocked drain

Effects: loss of life; destruction of houses, roads and farmland; rivers blocked by debris, which can later burst and flood; mud and sediment filling rivers and reservoirs. Control works on one side of the balance or the other:

Reduce the water: drainage channels across and down slopes, protected gutters, no leaking pipes.
Increase the resistance: trees and deep-rooted grasses, retaining walls and gabions at the foot, rock bolts and steel netting on cliffs.
Reduce the driving force: terracing and regrading steep slopes, no heavy buildings near the crest.
Reduce exposure: hazard maps, no building on the steepest slopes and in old landslide scars, rainfall warnings and evacuation plans.

Steel mesh rockfall netting fixed over a large cliff face above a road beside Pettycur Harbour, Scotland: the netting holds loosened blocks against the rock so that they cannot fall onto the road. Photo: Simon Johnston, CC BY-SA 2.0, via Wikimedia Commons.

Weathering landforms and human activities

This table draws together the whole unit.

Landform or product	Uses and opportunities	Problems
Deep regolith and red tropical soils	Farming; easy to dig for foundations and bricks	Weak and unstable when wet; landslides; soil erosion
Laterite	Murram for roads and building blocks; bauxite (aluminium ore)	Hard crusts limit roots and ploughing
Kaolin (clay from hydrolysis of feldspar)	Pottery, ceramics, paper, paint	Clay-rich slopes slump easily
Inselbergs, tors and exfoliation domes	Building stone and aggregate; tourism; water collected from bare rock	Rockfalls; sheets of rock peeling off
Karst	Limestone for cement and lime; caves for tourism; spring water	Water shortage, groundwater pollution, collapse
Glacial landforms	Hydroelectric power, water supply, tourism, fertile till	Avalanches, glacial floods, steep slopes
Scree and landslide scars	Loose rock for building	Unstable ground for roads and houses

Warning

Common mistakes

"Rain causes landslides." Rain is usually the trigger; the slope was made vulnerable by its steepness, its deep, weak regolith, and often by people (cutting, clearing, building). A full answer names both.
Confusing a slump with a slide. A slump rotates on a curved surface; a slide moves along a flat plane.
Forgetting the units of the factor of safety. $F$ is a ratio with no units; only whether it is above or below 1 matters.

Tip

Exam technique. Organise any mass-movement answer around the balance: what increased the shear stress (steeper, heavier, undercut) and what reduced the shear strength (saturation, weathering, loss of roots). Management measures then follow from the same balance.

Crust to Coast: "Types of Mass Wasting". A summary of the main kinds of mass wasting, from creep to falls and slides. Match each one to a panel of Figure 11.

✦

Practice

Define mass wasting and distinguish between shear stress and shear strength.
Weathered material with $\phi = 30^\circ$ rests on a slope of 22°. Find the factor of safety when dry, and say whether the slope is stable.
Using the saturation factor 0.51, find the factor of safety of the same slope when saturated, and the steepest saturated slope that would be stable.
Distinguish between soil creep, a rotational slump and a rockfall.
Explain how deforestation can make a slope more likely to fail.
Suggest three ways of reducing landslide risk on a steep, farmed hillside, and explain how each works.
"Landslides in Rwanda are natural disasters that people cannot prevent." Evaluate this statement.

Show answersHide answers

Mass wasting is the downslope movement of rock and soil under gravity. Shear stress is the force pulling material down the slope ( $W\sin\theta$ ); shear strength is the resistance from friction and cohesion.
$F = \frac{\tan 30^\circ}{\tan 22^\circ} = \frac{0.577}{0.404} = 1.43$ : stable.
$F = 0.51 \times 1.43 = 0.73$ : it fails. $\tan\theta_{\max} = 0.51\times 0.577 = 0.294$ , so $\theta_{\max} = 16.4^\circ$ .
Creep is a very slow, continuous movement of soil particles; a rotational slump is a block sliding and tilting back on a curved surface; a rockfall is the free fall of blocks from a cliff, very fast.
Roots bind the soil and add strength; trees draw water out of the soil and intercept rain. Without them the soil is weaker and wetter, so shear strength falls and pore-water pressure rises.
Any three, for example: terraces (reduce the slope angle and slow runoff); drainage channels (stop the soil becoming saturated, keeping friction high); planting trees and deep-rooted grasses (bind the soil and dry it); not cutting into the foot of the slope (keeps the support).
Partly true: the triggers (intense rain, earthquakes) cannot be prevented. But much of the risk is reduced by people: hazard mapping, keeping houses off the steepest slopes and old scars, drainage, terracing, tree cover and early warning all lower the chance of failure or of loss. Landslides are natural events; disasters depend largely on human choices.

Summary

Slopes fail when shear stress exceeds shear strength: $F = \frac{\tan\phi}{\tan\theta}$ for dry granular material; saturation roughly halves the friction.
Types range from slow creep to slumps, slides and very fast rockfalls; heavy rain is the usual trigger on steep, weathered slopes.
Control: drain, strengthen, unload and keep people out of danger. Weathering landforms bring resources (soil, laterite, kaolin, stone, cement) and hazards.

1When the hillside lets go

2Why slopes fail: a balance of forces

3Types of mass movement

4Causes, effects and control

5Weathering landforms and human activities

When the hillside lets go

Why slopes fail: a balance of forces

Types of mass movement

Causes, effects and control

Weathering landforms and human activities