What this lesson teaches
I can explain how mass movement and water erosion move material down slopes, and what controls them.
Syllabus 9173, Topic 2.2, 1. Geomorphic Processes. What influences the movement of materials on slopes?
Make a guess
After a week of steady rain, a hillside collapses even though no river is cutting its base. How did rain cause this?
- Rain lubricated the slope like oil.
- A river is needed to make a slope fail.
- It made the slope heavier and weaker.
- The rain dissolved the hill.
Show the answer
It made the slope heavier and weaker.
Water adds weight and raises water pressure between particles, so they press together less and friction falls. Stress then beats strength.
Mass movement happens when gravity's downslope pull is greater than the strength holding the slope together, and rain can do both.
Mass movement is soil and rock moving downslope under gravity. Gravity creates stress down the slope. Friction, cohesion and roots give the slope strength to resist. Movement starts when stress beats strength. Steep slopes, extra weight and loss of support make failure more likely.
A fall is rock dropping, bouncing or rolling from a steep face. A slide moves along a clear surface: flat in a translational slide, curved in a rotational slide. In a slide, the mass stays fairly intact.
In a flow, the particles move past each other like a liquid, as in a wet debris flow. Creep is very slow, steady downhill movement of soil. Bent tree trunks or tilted posts show it over time.
Water makes slopes heavier. It also raises water pressure between particles, so they press together less and friction falls. In deeply weathered tropical slopes, long rain can trigger failure once water reaches a weak layer. Saturation also removes the suction that helps hold drier soil together.
Natural controls include earthquakes, weak rock layers and rivers undercutting slopes. People add risk by cutting roads, building on slopes, leaking pipes and clearing vegetation. How much vegetation clearance matters also depends on drainage and how deep the failure surface is.
- Fall
- Rock drops from a steep face.
- Slide and flow
- Moves as an intact mass, or like a liquid.
- Creep
- Very slow downhill movement of soil.
Step by step
Driving force
Gravity pulls harder on steep or loaded slopes.
Resistance drops
Water, weak layers or lost roots cut strength.
Movement begins
It falls, slides, flows or creeps.
Worked example: A road cut after heavy rain
A road cut removes support at the foot of a tropical hillside. Rain then raises water pressure along a weak clay layer, and the slope slides. Both the road cut and the rain explain it. Blaming only the rain misses the weakened slope.
Watch out for this
Every movement of wet soil is a mudflow.
A wet mass can also slide along a clear surface. Identify how it moves.
Check your understanding
Why can rising water pressure in the soil cause a slope to fail?
- It weakens gravity's pull on the slope.
- It reduces how firmly particles press together.
- It increases friction between the soil particles.
Show the answer
It reduces how firmly particles press together.
Yes. Less friction means less strength.