What are Landslides & Debris Flows?
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.
How are Landslides caused?
In a landslide, masses of rock, earth or debris move down a slope. Debris and mud flows are rivers of rock, earth and other debris saturated with water. They develop during intense rainfall, runoff, or rapid snowmelt, changing the earth into a flowing river of mud or “slurry.” They can flow rapidly, striking with little or no warning at avalanche speeds (faster than a person can run). They also can travel many miles from their source, growing in size as they pick up trees, boulders, cars and other materials. Debris flows don’t always stay in stream channels and they can flow sideways as well as downhill.
When a wildfire burns a slope, it increases the chance of debris flows for several years. Although some landslides require lengthy rain and saturated slopes, a debris flow can start on a dry slope after only a few minutes of intense rain. “Intense” rain means a burst of rain at a fast rate, about half an inch in an hour. With debris flows, the rate matters more than total rainfall.
How to protect yourself or your property depends on the type of landslide. Land-use zoning, professional inspections, and proper design can reduce many landslide problems but evacuation is often the only way to protect lives from a debris flow or other fast-moving landslide. Never ignore an evacuation order.