Landslides: Causes, Types & Effects of Landslides
The term landslide includes a wide variety of mass movements that result in the downward and outward movement of slope forming materials like rocks, soils, artificial fills or any combination of these, under the influence of gravity. It is a geographical phenomenon, including wide range of ground movement, when the shear stress exceeds the shear strength of the materials. Some landslides are rapid occurring in seconds, while some may take hours, weeks or years to develop.
Significantly, the landslides are associated with mountainous terrains, but can also occur in areas, where open pit mines and surface excavations for buildings and highways takes place. The materials may move by falling, sliding, toppling, spreading or flowing. Annually, landslides occur worldwide and cause thousands of casualties and billions of monetary losses.
What are Mudflows?
Debris flows or mudflows are fluid masses of rock, earth and other debris saturated with water. They are the characteristics of steep, vegetated slopes, on which heavy rainfall initiates the movement. They develop due to the rapid accumulation of water in the ground during heavy rainfall, changing the earth into slurry. Slurry can travel miles, growing in size, as it picks up the trees, cars and anything along the way.
Types of landslides:
Landslides can be differentiated on the basis of the type of the movement and the material involved. The type of movement describes the internal mechanics of the displacement of the landslide mass.
They occur by the detachment of the soil, rock or both from a steep slope, by bouncing falling or rolling.
Occurrence: Common on the steep slopes, in coastal areas and along the banks of rivers and streams.
Predictability: The falls can be predicted from the mapping of hazardous rock falls, done all around the world. Rock bounce calculations and estimation methods can also help.
Effects: Falls can be life-threatening, may cause severe damage to the property and can block roads and highways.
Mitigation: Slope covers and protective covers can be used over the roadways. Explosive blasts of the predicted hazardous areas can be proposed. Warning signs can be recommended in hazardous areas for awareness.
It is occurred by the forward rotation of the soil or rock, around a point, below the centre of gravity of the displaced mass. They can be very complex and dangerous.
Occurrence: They occur in volcanic terrains and along the banks of the rivers and streams, with steep slopes.
Predictability: Warning systems based on tiltmeters (used to record changes in slope inclination and areas of vertical movements) can help in predicting topples.
Effects: When the velocity is rapid or sudden failures can create extreme destruction.
Mitigation: As seepage contributes to the instability of the rocks, it should be prevented. Drainage should be considered and addressed as corrective measure.
It is the down slope movement of soil or rock mass on thin zones of intense shear strain. They can be rotational or translational slides.
Occurrence: Rotational slides occur frequently in homogenous materials, while translational slides can be found in any kind of environment.
Predictability: Cracks at tops of slopes are good indicators of the initiation of failures.
Effects: They can be extremely damaging to structures and buildings, but are not usually life threatening, if movement is slow.
Mitigation: Instrumental monitoring can be done to predict the slides. Proper grading and engineering of the slopes would reduce the probability of the hazard. Translational slides on steep slopes are difficult to stabilize, permanently.
They result from the flow of the soft underlying material, formed from cohesive soil or the rock mass.
Occurrence: They are known to occur worldwide in the areas where there are liquefiable soils.
Predictability: Most prevalent in the areas with probability of extreme earth quake hazard, as well as liquefiable soils.
Effects: Can spread slowly or quickly depending on the water saturation extent and can cause extensive property damage. Lateral spreads may be precursors to earth flows.
Mitigation: Liquefaction potential maps can be used, if available to predict the spreads. Areas with liquefiable oils are to be avoided for construction purposes.
They include many types of mass movements such as creek, lahar, mudflow.
Occurrence: They are common in volcanic areas with weak soil.
Predictability: Maps of potential flows are available and hence can be predicted using them.
Effects: They can move objects as large as houses and hence they can be lethal.
Mitigation: They are difficult to prevent and hence homes should not be built in such areas, where flows are predicted.
Lahars(Volcanic debris flows):
They are known as volcanic mud flows and originate on the slopes of volcanoes.
Occurrence: found in the volcanic areas around the world.
Predictability: Maps based on past occurrences of lahars can be constructed to predict their occurrence.
Effects: The flows can bury human settlements on the surroundings and can also dam rivers and cause flooding.
Mitigation: They cannot be prevented except preventing the damage by not constructing anything on the volcanic slopes.
A variety of very rapid to extreme rapid debris flow
Occurrence: They occur in steep terrain environments around the world.
Predictability: during volcanic eruptions, there is more probability of debris avalanche to occur and hence proper measures are to be taken.
Effects: they can inundate towns and cities and can be extremely devastating, as they are rapid and unpredictable.
Mitigation: they can be stopped or prevented by the engineering means and hence avoidance of construction in valleys or volcanoes may lessen damages.
It is the informal name for the slow earth flow and occurs with slow and steady down slope movement of soil or rock.
Occurrence: It is the most common type of landslide and occurs in most parts of the world.
Predictability: indicated by curved tree trunks, tilted poles or fences.
Effects: Creep can slowly damage pipelines, roads, highways and fences.
Mitigation: The best measure is to ensure proper drainage of water. Slope modification like flattening can also be done.
Common Causes of Landslides:
Many factors contribute to landslides and some of them are:
- Gravity: works more effectively on the steeper slopes.
- Earth quakes: Plate tectonics move the soil and when earth quakes occur on areas with steep slopes, the soil and rock slips causes landslides.
- Geological Factors: Many slides occur at the geographically weak areas (known as zone of weakness), with permeable sands and gravels above bedrocks.
- Heavy Rainfall: Storm water runoffs results in the rise in the level of the ground water level, making some slopes unstable and hence slides occur.
- Forest Fires: They cause soil erosion and induce floods, which may result in the slides.
- Volcanoes: Strato volcanoes, prone to sudden collapse in wet conditions may lead to landslides.
- Deforestation, improper land use practices in agriculture and irregular settlement patterns trigger landslides.
- In appropriate drainage system, increase the landslide vulnerability.
- Deep excavations on slopes for buildings, roads, canals and mining make the critical slopes vulnerable.
Areas at risk:
The following areas are likely to get effected from the landslides.
- Channels along a stream or a river.
- Areas, where landslides have occurred before.
- Steep slopes and the areas at the bottom of the canyons.
- Areas where vegetation has been destroyed.
- Areas where surface runoff is directed.
- Slopes that have been modified or altered for construction purposes.
- Areas with improper drainage system.
Relationship of Landslides with other natural hazards: The Multiple Hazard Effect
Natural hazards like floods, volcanic eruptions, earth quakes or landslides can occur simultaneously or trigger one or more of the others. Landslides occur often due to the earth quakes, floods and volcanic activities and in turn may lead to subsequent hazards. For example, an earthquake induced landslide may lead to a deadly Tsunami, if sufficient landslide material slides into the water body, displacing the water present.
Landslides and Volcanic activity:
Landslides due to the volcanic activities are the most devastating types. The lava may melt the snow at a rapid rate and hence rapid acceleration may be caused, devastating anything in its path.
The 1980 eruption of Mt. St. Helens, in Washington triggered a massive landslide to the north of the volcano, which became the largest landslide in the recorded times.
Recent landslides that shook the world:
- Rio de Janeiro 2011: The floods, mudslides and landslides in several towns of the mountainous region in the Brazilian state of the Rio de Janeiro, caused at least 903 deaths.
- Gansu mudslide 2010: It was a deadly mudslide in Zhugqu country in China that occurred at 12 midnight on 8 August 2010. The mudslides killed more than 1,400 people and more than 1200 people were rescued.
- 2008 Cairo landslide: It happened on September 6, 2008 in Al-Duwayqa settlement in east Cairo, resulting in the deaths of more than 100 people in the rockslide.
- 2007 Chittagong mudslides: occurred in the port city of Chittagong in Bangladesh on 11 June 2007, killing at least 128 and causing one third of the city to come under water due to heavy rainfall and floods.
- 2006 Philippines rockslides: Heavy rainfall and rockslides resulted over 1100 deaths and many schools and homes were destroyed in Philippines, due to the rockslides and debris avalanche.
- 2005 India–Pakistan: Landslides and rock falls caused deaths of at least 25,000, damming two tributaries of Jhelum river and burying a village by the Hattian Bala rock avalanche.
Extra terrestrial landslides:
Landslides has been detected on many bodies of the solar system in the past, but very few observations are made in the recent times and hence the planet bodies are considered to be inactive. Significant landslides are observed on both the planets of Venus and Mars and hence they are subjected to long term mapping by the satellites to detect any abnormal changes on the planets.
How to protect yourself from Landslides?
Before intense storms and landslides:
- Collect data of the previous occurrences of the landslides, by contacting local authorities or geologists nearby.
- Learn about emergency and evacuation plans from the local authorities.
- Develop an emergency communication plan for the safety of the family.
- Leave areas, vulnerable to landslides.
- Listen to radio or watch TV for warnings.
- Listen for rumbling sounds that may indicate approaching landslides.
- Be alert while driving as the roads may get blocked due to landslides.
After a landslide or debris flow:
- Flooding may occur after landslides and hence stay away from the site.
- Check for the injured or trapped victims, with emergency backup.
- Report broken utility and communication lines to the local authorities.
- Consult a geotechnical expert, if possible for advice on additional risk.