Explainer: How a landslide happens

Monday November 25 2019

A man leaps over a section of land affected by mudslides at Nyarkulian in West Pokot County on November 24, 2019 following heavy rains the previous night. PHOTO | JARED NYATAYA | NATION MEDIA GROUP


On Saturday, Kenyans woke up to news that dozens of men, women and children were buried alive in more than three remote villages of West Pokot County.

By Sunday, close to 50 villagers had been confirmed dead and more than 400 displaced by the wet soil and rocks set in motion by heavy rain.

But what is this phenomenon called a landslide? How does it happen? And how can you be safe?

•What is a landslide?

In his paper, Charles Maina Gichaba from the University of Nairobi’s Geology Department defines 'landslide' as many types of downhill earth movements.

These vary from catastrophic rock avalanches and debris flows in mountainous regions to more slow-moving earth slides.


The US Geological Survey (USGS) defines a landslide as “the movement of a mass of rock, debris, or earth down a slope.”

“Landslides are a type of "mass wasting," which denotes any down-slope movement of soil and rock under the direct influence of gravity,” USGS says.


Sina village in West Pokot County, as pictured following mudslides after heavy rains on November 24, 2019. PHOTO | JARED NYATAYA | NATION MEDIA GROUP

•There are other terms for landslides. Are they the same thing?

A landslide is movement of earth materials.

And when the movement is falling, toppling, sliding, spreading or flowing due to gravity, then the landslide acquires a new name.

Toppling is a term for when a rock peels itself from a larger piece of block and falls below the entire block due to gravity and forces exerted by adjacent units or by fluids in cracks.

There are also flows, which come in many forms. Debris flow is a rapid mass movement in which a loose soil, rock, organic matter, air, and water mobilises as a slurry that flows downslope and this is mainly of a large proportion of silt-and-sand-sized material.

When the movement of the debris is rapid, it is called an avalanche.


Tecla Cheboi (L) and Damaris Nasia with her children, inside a classroom at Lolwei Primary School on November 24, 2019, where they camped after they were displaced by mudslides from Nyarkulian in West Pokot County. PHOTO | JARED NYATAYA | NATION MEDIA GROUP

•What's the difference between a landslide and a mudslide?

According to an article on National Public Radio (NPR), landslides occur when masses of rock, earth or debris move down a slope.

Debris flows, also known as mudslides, are a common type of fast-moving landslide that tends to flow in channels, according to a Centers for Disease Control and Prevention fact sheet.

•What causes landslides?

There are three causes of landslides, which are natural so man has no control over them, such as the movement of tectonic plates, weather that causes freezing and thawing at the same time.

However, studies show that majority of, if not all, the landslides in Kenya are triggered or caused by water or human activities or a combination of both.

Slope saturation by water is a primary cause of landslides.

The human activities include clearing of vegetation, farming, undercutting, building, quarrying, blasting, construction and other engineering works. 


People displaced by mudslides following heavy rains in Nyarkulian, West Pokot County, camp at Lolwei Primary School on November 24, 2019. PHOTO | JARED NYATAYA | NATION MEDIA GROUP

•Are there areas in Kenya that are prone to landslides?


The Departments of Mines and Geology released a map on the occurrence of landslides in Kenya in 2012.

It showed that central highlands of Kenya, the Rift Valley, and the western region are more susceptible.

There are also studies that have pointed to landslides in Yatta in Machakos County

•Why are these areas so vulnerable?

The Rift Valley is most prone to landslides. It experiences earth tremors and when the area is steep, it is likely that there will be a landslide.

When the ground shakes, or the soils dilate, water rushes to fill the spaces left.

These areas usually have heavy downpours, more than 1,200 mm annually, a high population density and steep slopes of up to 80 metres.

Profiles show the areas have volcanic soils that become saturated with water when it rains heavily.

With such weather and the population, people put pressure on the land trying to take advantage of the good environment and this leads to land degradation, which, in turn, renders the land prone to landslides.


A home in the mudslide-prone Nyarkulian in West Pokot County as pictured on November 24, 2019. PHOTO | JARED NYATAYA | NATION MEDIA GROUP

•So is there a way landslides can be prevented?


Land use and care is key.

People crowded on one portion tend to over-cultivate their pieces of land.

To prevent slides, allow the soil to ‘rest and recover’.

Also, some crops have a better soil-holding capacity than others due to the size of their roots. Farmers in such areas should focus on crops that help prevent slides.

Such crops can be intercropped with other plants, including trees.

Human activities that can trigger landslides, such as undercutting, building, quarrying, blasting, construction and other engineering works, should be strictly controlled in areas prone to slides.

•Are there other methods of preventing landslides?


Communities should be discouraged from construction on steep slopes and existing landslides.

Steep slopes and existing landslides can be stabilised by covering them with an impermeable membrane; directing surface water away, draining ground water away from the landslide and minimising surface irrigation.

Communities should also plant deep-rooted trees on steep slopes, instead of cultivating the land or settling on it.

They should also construct support structures on vulnerable areas, as well conservation areas.

Parts of this article were extracted from "Overview of Landslide Occurrences in Kenya: Causes, Mitigation, and Challenges" [PDF].