Environmental Engineering Reference
In-Depth Information
Chapter 1
Debris Flows
1.1. Introduction
Debris flows are a major natural hazard, claiming thousands of lives and millions
of dollars in property loss each year in almost all mountainous areas on the Earth.
After a catastrophic eruption of Mount St. Helen in the United States in May 1980,
water from melting snow, torrential rains from the eruption cloud, and water displaced
from Spirit Lake mixed with deposited ash and debris produced very large debris flows
and caused extensive damage and loss of life [SCO 88]. During the 1985 eruption of
Nevado del Ruiz in Colombia, more than 20,000 people perished when a large debris
flow triggered by the rapid melting of snow and ice at the volcano summit swept
through the town of Armero [VOI 90]. In 1991, the eruption of Pinatubo volcano in the
Philippines dispersed more than 5 km
3
of volcanic ash into surrounding valleys. Much
of that sediment has subsequently been mobilized as debris flows by typhoon rains and
has devastated more than 300 km
2
of agricultural land. Even in European countries,
recent events of torrential floods may have very destructive effects (Sarno and Quindici
in southern Italy in May 1998, where approximately 200 people were killed).
The catastrophic character of these floods in mountainous watersheds is a
consequence of significant transport of materials associated with water flows. Two
limiting flow regimes can be distinguished.
Bed load
and
suspension
refer to dilute
transport of sediments within water. This means that water is the main agent
in the flow dynamics and that the particle concentration does not exceed a few
percent. Such flows are typically two-phase flows. In contrast,
debris flows
are
mass movements of concentrated slurries of water, fine solids, rocks and boulders.
