Mass Movement and Hillslope Geomorphology

Hillslope geomorphology is the study of the shape and evolution of slopes, and mass movement is the downslope movement of soil, rock and debris under gravity; a slope fails when the driving shear stress exceeds the shear strength of the material, a balance that is described by the factor of safety. In brief: slopes are shaped by a mix of weathering (which produces loose material) and transport (which carries it away). Slow processes such as soil creep gradually round the hilltops, while fast ones such as landslides and debris flows shape slopes in sudden events. For loose, dry material, the maximum stable angle is the angle of repose, about 30–35° for scree and sand.

Hillslopes cover most of the land surface, and their stability affects roads, settlements and water supply. This guide explains slope form, the processes of mass movement and their classification, the factor of safety and triggers, compares slope types in a table, works through stability and diffusion calculations, outlines the history and corrects common mistakes.

What do hillslopes look like?

A typical slope profile from the hilltop to the valley bottom has a convex upper part (the crest), where soil creep and rainsplash move material slowly, a straight (rectilinear) middle part at a roughly constant angle, sometimes with a steep rock face (free face) above it and a debris slope (talus) below, and a concave lower part, where material accumulates, and wash by water is important. J. A. Dalrymple and colleagues proposed a nine-unit hillslope model in 1968, which divides the slope into units such as the interfluve, seepage slope, convex creep slope, fall face, transportational midslope, colluvial footslope and alluvial toeslope, each linked to a dominant process.

Slopes are often described as weathering-limited, where the rate of removal is faster than the production of loose material, which leaves bare rock and thin soil, as in steep mountains and deserts, or transport-limited, where there is a thick soil or regolith cover because the production of material exceeds the removal, as in humid and gentler landscapes.

What are the main mass movement processes?

The widely used classification by D. J. Varnes (1978) groups movements by type of motion and material.

• Falls: detachment of rock or soil from steep slopes, falling through the air and bouncing, such as rockfalls from cliffs.
• Topples: forward rotation of blocks about a pivot, common in jointed rock.
• Slides: movement of a mass along a surface of rupture. Rotational slides (slumps) occur on a curved surface, in clay, and translational slides on a planar surface, such as a bedding plane.
• Flows: movement of material as a viscous fluid: debris flows (mixtures of water, soil and rock that can move at tens of km/h), mudflows, earthflows and lahars.
• Creep: very slow, continuous downslope movement of soil, typically 1–10 mm per year, caused by wetting and drying, freeze-thaw, and the activity of organisms.
• Spreads and complex movements: lateral spreading in liquefied soils and movements that combine several types.

What controls slope stability?

The stability of a slope is the balance between the shear stress (the driving force of gravity along a potential failure surface) and the shear strength (the resistance from cohesion and friction). The factor of safety (FS) is the strength divided by the stress. If FS is above 1, the slope is stable, and if it falls below 1, failure occurs. For a thin layer of cohesionless soil parallel to the slope, FS = tan φ ÷ tan β, where φ is the angle of internal friction and β is the slope angle.

Factors that reduce stability include steep slopes, water (rain and snowmelt that add weight and raise pore-water pressure, which reduces friction), undercutting by rivers, waves or excavation, loss of vegetation roots, weak materials such as clay and weathered rock, earthquake shaking, loading by buildings or dumps, and structure, such as bedding planes dipping out of the slope. Rainfall thresholds of intensity and duration are used to forecast landslides in many regions.

How do slopes evolve?

Models of slope evolution have changed over time. Davis thought that slopes decline with time, Walther Penck proposed that slopes retreat parallel to themselves at a constant angle (parallel retreat), and Lester King applied this to the pediments of arid regions. Today, diffusion models are widely used for soil-mantled slopes: the downslope soil flux is proportional to the slope gradient: q = −K × (dz/dx), where K is a diffusivity of the order of 0.001 to 0.01 m² per year. This explains the rounded, convex form of the hilltops. Where slopes are steep and landslides occur, a threshold angle appears, so that the slope tends to stay near the angle at which failure begins.

Mass movement types compared

Worked example: factor of safety and creep

1. Dry cohesionless slope: a soil has an angle of internal friction φ = 35°, and the slope angle is β = 30°. FS = tan 35° ÷ tan 30° = 0.7002 ÷ 0.5774 = 1.21, so the slope is stable.

2. A steeper slope: at β = 38°, FS = 0.7002 ÷ tan 38° = 0.7002 ÷ 0.7813 = 0.90, which is below 1, so the slope fails. The limit is the angle of repose, where β = φ = 35°.
3. Effect of saturation: if the soil becomes saturated with water flowing parallel to the slope, the strength falls approximately by the ratio of buoyant to saturated unit weight. With a saturated unit weight of 20 kN/m³ and water of 9.81 kN/m³, the ratio is (20 − 9.81) ÷ 20 = 0.51. For the 30° slope, FS = 0.51 × 1.21 = 0.62, which is below 1, so the same slope that was stable when dry now fails.
4. Slope angle from height and length: a slope drops 120 m over a horizontal distance of 200 m. β = arctan(120 ÷ 200) = arctan(0.6) = 31.0°.

5. Soil creep: a creep rate of 5 mm per year moves a fence line 5 × 200 = 1,000 mm = 1 m in 200 years.
6. Diffusion flux: with a diffusivity K = 0.005 m² per year and a slope gradient of 0.2, the soil flux is q = 0.005 × 0.2 = 0.001 m³ per meter of slope width per year, which is 1 liter per meter per year.

How did slope studies develop?

In the 19th century, Davis and Gilbert discussed the shape of hillslopes. In the 1940s and 1950s, studies by Strahler, Kirkby and others began to measure slope processes and angles quantitatively. Terzaghi’s work on soil mechanics in the 1920s and 1930s supplied the engineering theory for slope stability. In the 1970s and 1980s, geomorphologists combined process measurements with mathematical models of slope evolution. Since the 2000s, high-resolution lidar mapping and satellite radar have made it possible to detect landslides and measure slow movements, and the use of rainfall thresholds in warning systems has increased. Today, climate change is expected to alter the frequency of landslides through changes in rainfall, snowmelt and permafrost.

Which mistakes are common?

Mistake 1: Thinking that slope failure needs a very steep slope

Failures occur on gentle slopes in weak, saturated clay. The material and water conditions matter as much as the angle.

Mistake 2: Assuming that a stable slope will always remain stable

Rain, undercutting, earthquakes or the loss of vegetation can lower the factor of safety below 1, sometimes years after the change.

Mistake 3: Confusing creep with landslides

Creep is slow and continuous, and is visible in tilted fences and curved trees. Landslides are sudden failures along a distinct surface.

Mistake 4: Ignoring the role of water in dry-looking slopes

Water pressure inside the slope is often the final trigger, and a slope that is stable in dry weather may fail after intense or prolonged rain.