Slope-stability or mass-movement problems occur where either
sediment and/or rock and/or snow move downslope in response to gravity. Potential
slope-stability problems exist wherever development has taken place at the base of
Rockfall at Upper Island Cove
An eight tonne block of stone sits on top of a car parked beside a house after falling from the top of a 100 m slope.
Reproduced by permission of the Government of Newfoundland and
Labrador © 1999.
Downslope movement is a natural process, but can be accentuated by
undercutting of the base of slopes, clearance of stabilizing vegetation, or
diversion of natural drainage. Types of downslope movement include landslide,
avalanche, rockfall, rock slip, and
rotational slumps. The first three are rapid events, and generally the most
dangerous to life and property.
Mass Movement Types.
Some main types of mass movement. Variations in water content and rates
of movement produce a variety of forms. A rockfall is simply a volume of
rock made up of individual pieces that fall independently through the air
and hit a surface. A debris avalanche is a mass of falling and tumbling
rock, debris, and soil. It is differentiated from a slower landslide by
the tremendous velocity of onrushing material. The extreme danger of a
debris avalanche results from its high speed and consequent lack of
warning. A landslide is a sudden rapid movement of a cohesive mass of
material (soil, rock, etc) that is not saturated with moisture. It
involves a large amount of material failing simultaneously. A common type
of slide is the rotational slide or slump which occurs when
surface material moves along a concave surface. Frequently water is
present along this movement plane and acts as a lubricant. The simplest
form of rotational slump is when a small block of land shifts downward.
The upper surface of the slide appears to rotate backward and often
remains intact. When the moisture content of moving material is high, the
term flow is used. Flows include earthflows and more fluid
Adapted by Tina Riche, 2000.