Landslides - earth moving downhill

Remember the horrendous landslides that took place recently in Sri Lanka? These landslides occurred mostly in Walapane, and in other places around the Nuwara Eliya district. Many people lost their lives, houses as well as other belongings. Today, we'll tell you what a landslide is, how it happens and what can be done to prevent it.

Landslides are rock, earth or debris flowing down along slopes due to gravity. They can occur on any environment given the right conditions of soil, moisture, and the angle of slope there. Essential to the natural process of the earth's surface geology, landslides restructure soil and deposits in a process that can be in quick collapses or in slow gradual slides. That is how the nature of the earth's surface activates.

Also known as mud flows, debris flows, earth failures and slope failures, these can be started by rains, floods, earthquakes, and other natural causes as well as human-made causes, such as sloping, land cutting

 and filling and excessive development.

Because the factors affecting landslides can be geophysical or human-made, they can occur in developed areas, undeveloped areas or any area where the ground has been changed for roads, houses, utilities, buildings, and even for lawns in one's backyard.

A landslide is capable of generating enough force and momentum to wipe anything in its path. The best such example is the devastating landslide which wiped entire towns and villages in Colombia in 1985, claiming 20,000 lives. After a landslide, the ground changes permanently. In the Great Plains, heavy rains combined with loss of vegetation due to wildfires trigger landslides in clay-rich rocky areas.

Earthquakes can also be a cause for landslides. Although the term landslide is often used somewhat loosely to mean any fairly rapid movement of rocks and sediment down slopes, it is actually more accurate to use the term 'mass wasting' to refer to the wide variety of mass movement processes that wear away at the earth's surface.

There are three main factors that control the type and rate of landslides that might occur at the Earth's surface. They are, Slope gradient; the steeper the slope of the land, the more likely that landslides will occur: Slope consolidation; sediments and fractured or poorly cemented rocks and sediments are weak, and more prone to a landslide: Water, if slope materials are saturated (completely absorbed) with water, they may lose cohesion (sticking together) and flow easily.

There are three basic types of landslides. They are Falls - rocks fall or bounce through the air; Slides - rock and/or sediment slides along earth's surface; and Flows - sediment flows across the earth's surface.

Falls

While travelling through the hill country such as Bandarawela, you may have noticed signs such as 'WARNING! ROCK FALL HAZARD'. Because weathering is an ongoing process, steep mountain slopes are constantly wasting away, often in the form of rocks falling and bouncing down slopes.

Such falls can be triggered by rain water, the growth of plants (and their roots), earthquakes, or by people hiking on the slope. Rock falls occur in just a matter of seconds, so they are difficult (though fun) to observe. But, you can predict where rock falls could occur on a mountain slope by looking for talus, a build-up of loose, angular rocks at the base of a steep slope.

Slides

Whenever a mass of slope material moves as a sound block, we say that a slide has taken place. There are several types of slides, but one of the most common is a slump. A slump occurs when a portion of hillside moves down a slope under the influence of gravity.

A slump has a characteristic shape, with a scarp (steep slope) or cliff at the top of the slump, and a bulge of material (often called the toe of the slump) at the base of the slump.

Flows

We say that a flow has occurred if the material moving down slope is being transported as a very thick fluid (like a river of debris, rock and/or soil), rather than as a coherent unit. Often, water is the primary transport agent for the flow.

Lahars

For sheer drama and destruction, you can't beat the mud flows that can occur when a volcano erupts. These flows are called lahars. After the eruption of Mount St. Helens in the USA, the heat of the volcanic products melted the snow cap.

The resulting liquid water rushed down the volcano flanks (sides), incorporating debris as it progressed. The damage to forests and humans was extensive.

According to geologists, existing sites of old or recent landslides, on or at the base or top of slopes, in or at the base of minor drainage hollows, at the base or top of an old fill slope and at the base or top of a steep cut slope are places that are highly prone to landslides.

There's a lot to talk about landslides. We'll tell you more about them in our next 'Geo Facts' page.

Compiled by

Janani Amarasekara