Debris Flows and Landslides - River Dynamics and Integrated River Management - page 188

4 Debris Flows and Landslides
Abstract
A landslide is a mass movement occurring on steep slopes under the action of gravity. Debris flow is a
distinct type of mass movement commonly triggered by intense rainfall and/or melting snow on steep hill
slopes. It differs from landslide in its “flowing” feature. Flow means relative movement in numerous
layers of the medium, whereas a slide occurs only along one or several interfaces or beds. The main causes
of landslides and avalanches are earthquakes and rainstorms. Disaster chains are initiated by landslides
and avalanches. Great landslides resulted in barrier lakes. The stability of a landslide dam depends on the
development degree of the step-pool system in the spillway channel on the landslide dam and the highest
stream power of flow. Preserved landslide dams may develop into a knickpoint and become a key factor
for river pattern establishment and river stability.
Debris flows have buried towns, villages, highways, railways, and farmland, broken bridges and dammed
rivers; caused casualties and impaired habitats. Debris flow is extremely unsteady, which is initiated on
steep slopes, flows down gullies, and deposits at the mouth of debris flow gullies. Debris flows are classified
into pseudo-one-phase debris flows and two-phase debris flows. There is no obvious relative movement
between the solid particles and liquid in pseudo-one-phase flow and there is relative movement between the
solid phase and liquid phase in two phase debris flow. Pseudo-one-phase debris flows are very non-Newtonian
and are characterized by the striking phenomena of intermittent flow, the “bed-paving process,” low
resistance and drag reduction, extremely high super-elevation at bends, and well-mixed deposit materials.
Two-phase debris flows are composed of stones and gravel as the solid phase and the fluid mixture of water
and low concentrations of clay and sand as the liquid phase. They exhibit high, steep heads consisting of
rolling, colliding, large gravel, which distinguishes two-phase debris flows from a normal torrential flood.
The liquid phase is mostly Newtonian.
Debris flows and landslides are serious challenges in management of mountain rivers. However, they
are especially disastrous in China and Japan where the high population and high percentage of mountainous
land result in a dangerous mixture. This chapter focuses on the description of the basic characteristics,
consequences, and control strategies of debris flows and landslides.
Key words
Landslide, Disaster chains, Landslide dam, Barrier lake, Debris flow control strategies, Hazard mitigation
4.1 Disasters Caused by Landslides and Debris Flows
4.1.1 Landslides
A landslide is a mass movement occurring on steep slopes under the action of gravity. Landslides occur
in two thirds of the mountainous areas of China, especially in northwestern and southwestern China.
More than 120 million dollars were lost due to landslides and avalanches every year before the 1990s
(NFH and IMH, 1994).
Landslides and similar mass movements can be classified according to their movement types, scales,
and material of the sliding body. Four types of mass movements can be recognized: 1) topple; 2) rockfall;
3) avalanche; and 4) landslide. Topple is the phenomenon of rock column detachment due to freezing-
thawing of water in the interstices of rocks. Rockfall is the phenomenon of a few huge rocks falling from
a cliff or steep slope. An avalanche is the collapse of a cliff and slope under the action of gravity
triggered by earthquake or rainstorm. A landslide is the mass movement of rock and soil down a slope