Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 241

C HAPTER D2
Mass Wasting
1 WATER AND INSTABILITY
Mass wasting is a natural process, which has a great infl uence, along with
erosion, on the shaping and evolution of landscapes. They can generate
signifi cant risks for humans and the built environment, which has resulted,
over the last few decades, in different prevention studies, fi rst ZERMOS
maps (zones exposed to risks linked to movement of the soil and bedrock),
then PERs (risk exposure plans), and fi nally, more recently, PPRs (foreseeable
natural risk prevention plans).
There are generally several factors governing these risks, but two of
these play a primary role in the mechanisms of destabilization:
• gravity, which leads to mass wasting sometimes being termed gravity-
driven movement;
• water, which often acts as a trigger for instability.
Human interventions can also worsen the situation in sensitive
areas, or generate truly anthropogenic instabilities due to ill-adapted
construction, which most often fails to adequately take into account the
various components of the natural environment.
The most common types of mass wasting can be grouped into the
following families:
• gullying, which cuts more or less deeply into slopes under the effects
of surface runoff;
• rockfalls, which generally affect massive rock units;
• landslides, which shape unconsolidated slopes;
• collapses and subsidence, which are caused by the dissolution of
soluble rock or the washing away of fi ne particles due to groundwater
circulations.
Only the last two types will be described in further detail, due to the
importance of groundwater's role in their genesis and evolution.