Flash Flood Risk Management – A Training of Trainers ... - ReliefWeb

Flash Flood Risk Management – A Training of Trainers Manual
Session 14 Resource Materials
RM 14.1: Types of landslides and factors that can trigger landslides
Landslides usually occur as secondary effects of heavy storms, earthquakes, and volcanic eruptions (Box 14).
Landslides can consist of either one of two classes of material: bedrock or soil (earth and organic matter
debris). Landslide are classified by the type of movement that causes them, as shown in Figure 15.
Session 14
Falls. A fall is a mass of rock or other material that moves downward by falling or cascading through the air.
In this category, large individual boulders can cause significant damage. Depending on the type of material
involved, this type of landslide can be a rock fall, earth fall, or debris fall.
Topple. A topple occurs when overturning forces cause rocks to rotate out of their original position. A topple
may not involve much movement, and does not necessarily trigger a rock fall or rock slide.
Slides. Slides result from shear failure (slippage) along one or several surfaces; the slide material may remain
intact or break up. The two major types are rotational and translational slides. Rotational slides occur on
slopes of homogeneous clay or shale and soil, while translational slides are mass movements on a more or
less plane surface.
Lateral spreads. A lateral spread occurs when large blocks of soil spread out horizontally after fracturing off
the original base; these usually occur on gentle slopes of less than 6° and typically spread only 3 to 5 m, but
may move from 30 to 50 m when conditions are favourable. During an earthquake in Alaska, United States,
in 1964, more than 200 bridges were damaged or destroyed by lateral spreading of flood plain deposits
near river channels.
Box 14: Main triggers of landslides
Rainfall
Rainfall is an important landslide trigger. There is a direct correlation between the amount of rainfall and the
incidence of landslides.
• A cumulative rainfall of 50–100 mm in one day or a daily rainfall exceeding 50 mm can cause small-scale and
shallow debris landslides.
• A cumulative two-day rainfall of about 150 mm or a daily rainfall of about 100 mm in a given area greatly
increases the probability and number of landslides.
• A cumulative two-day rainfall exceeding 250 mm, or an average intensity of more than 8 mm per hour in one
day, rapidly increases the occurrence and number of large landslides.
Earthquakes
Earthquakes can cause many large, dam-forming landslides. Seismic accelerations, the duration of the shock, the
focal depth, the angle, and the approach of the seismic waves all contribute to inducing landslides, but environmental
factors such as the geology and landform are the most decisive factors. This is why small earthquakes can sometimes
induce more landslides than large earthquakes.
The type of slope and the slope angle also influence the occurrence of landslides. Landslides rarely occur on slopes
with a grade less than 25°. The large majority of landslides occur on slopes with grades ranging from 30° to 50°.
Source: Shrestha (2008)
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