Unit 5
A Brief Overview of Prominent Natural Disasters in India

Learning Outcomes

Prerequisites

Key words

Natural disasters, Man-made disasters, Earthquake, Landslide, Cyclone, Flood

Discussion

Mention some of the natural disasters. How are they caused? Are there any methods to prevent them?

4.5.1 Landslide

A landslide, also called landslip, is defined 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. They can happen suddenly or more slowly over long periods of time. Landslides are caused by disturbances in the natural stability of a slope. They can accompany heavy rains or follow droughts, earthquakes, or volcanic eruptions. Land-slides occur when gravitational and other types of shear stresses within a slope exceed the shear strength (resistance to shearing) of the materials that form the slope.

Shear stresses can be built up within a slope by a number of processes. These include over steepening of the base of the slope, such as by natural erosion or excavation, and loading of the slope, such as by an inflow of water, a rise in the groundwater table, or the accu-mulation of debris on the slope’s surface. Short-term stresses, such as those imposed by earthquakes and rainstorms, can likewise contribute to the activation of landslides. Landslides can also be activated by process-es that weaken the shear strength of a slope’s material. Shear strength is dependent mainly on two factors: frictional strength, which is the resistance to movement between the slope material’s interacting constituent particles, and cohesive strength, which is the bonding between the particles. Coarse particles such as sand grains have high frictional strength but low cohesive strength, whereas the oppo-site is true for clays, which are composed of fine particles. Another factor that affects the shear strength of a slope-forming material is the spatial disposition of the sediment fabric. Some materials with a loose, open sediment fabric will weaken if they are mechanically disturbed or flooded with water. An increase in water content, resulting from either natural causes or human activity, typically weakens sandy materials through the reduction of interparticle friction and weakens clays through the dissolution of interparticle cements, the hydration of clay minerals, and the elimination of interparticle (capillary) tension.

When the force of gravity acting on a slope exceeds the resisting forces of a slope, the slope will fail and a landslide occurs. Exter-nal factors can lead to landslides happening, including:

• heavy rainfall leading to saturation of the ground
• erosion of the base of a slope
• changes to the materials strength through weathering

Why do landslides happen?
A landslide may occur because the strength of the material is weakened. This reduces the power of the ‘glue’ that cements the rock or soil grains together. Located on a slope, the rock is then no longer strong enough to resist the forces of gravity acting upon it.

Several factors can increase a slope’s susceptibility to a landslide event.

• Water: Adding water to the mate-rial on a slope makes a landslide more likely to happen. This is be-cause water adds weight, lowers the strength of the material and reduces friction, making it easier for material to move downslope.
• Erosion processes: If the bottom of a slope is continually eroded, for example by the sea or a river, the slope will eventually become too steep to hold itself up.
• Slope angle (steepness of slope): The slope angle is a key factor as far as landslides are concerned. Any change to this makes it steeper (such as coastal erosion) increases the likelihood of a land-slide.
• Rock type: the type of rocks in the slope, and their combination, can increase the chance of a landslide.
• Grain shape: the shape of the grains that make up a rock can affect the risk of a landslide.
• Jointing and orientation of bed-ding planes.
• Arrangement of the rock layers.
• Weathering processes: eg- freeze-thaw reduces the cohesion (‘stickiness’) between the rock grains.
• Vegetation: Vegetation helps bind material together; removing vegetation increases the chance of a landslide.
• Flooding.
• Volcanoes and earthquake activity nearby.
• Human activity: Mining, traffic vibrations or urbanisation change surface water drainage patterns.

4.5.2 Types of landslides

Landslides are generally classified by type of movement (slides, flows, spreads, topples, or falls) and type of material (rock, debris, or earth). Sometimes more than one type of movement occurs within a single landslide, and, because the temporal and spatial relationships of these movements are often complex, their analysis often requires detailed interpretation of both landforms and geological sections, or cores.

Fig: 4.5.2 Types of landslides

1. Slides– Rockslides and other types of slides involve the displacement of material along one or more discrete shearing surfaces. The sliding can extend downward and outward along a broadly planar surface (a translational slide), or it can be rotational along a concave-upward set of shear surfaces (a slump). A translation-al slide typically takes place along structural features, such as a bedding plane or the inter-face between resistant bedrock and weaker overlying material. If the overlying material moves as a single, little-deformed mass, it is called a block slide. A translational slide is sometimes called a mud slide when it occurs along gently sloping, discrete shear planes in fine-grained rocks (such as fissured clays) and the displaced mass is fluidized by an increase in pore water pressure. In a rotational slide the axis of rotation is roughly parallel to the contours of the slope. The movement near the head of the slide is largely downward, expos-ing a steep head scarp, and movement within the displaced mass takes place along internal slip planes, each tending to tilt backward. Over time, upslope ponding of water by such back-tilted blocks can enlarge the area of instability, so that a stable condition is reached only when the slope is reduced to a very low gradient.
2. Flows– A type of landslide in which the distribution of particle velocities resembles that of a viscous fluid is called a flow. The most important fluidizing agent is water, but trapped air is sometimes involved. Contact between the flowing mass and the underlying material can be distinct, or the contact can be one of diffuse shear. The difference between slides and flows is gradational, with variations in fluid content, mobility, and type of movement, and composite slide movement and flow movement are common.
3. Spreads– A spread is the complex lateral movement of relatively coherent earth mate-rials resting on a weaker substrate that is subject to liquefaction or plastic flow. Coherent blocks of material subside into the weaker substrate, and the slow downslope movement frequently extends long distances as a result of the retrogressive extension from the zone of origin, such as an eroding riverbank or coast-line. Spreads occur as the result of liquefaction caused by water saturation or earthquake shock in such substrates as loess, a weakly cemented wind-lain silt.
4. Toppling – Rotation of a mass of rock, debris, or earth outward from a steep slope face is called toppling. This type of movement can subsequently cause the mass to fall or slide.

4.5.3 Landslide mitigation and prevention

Landslides pose a recurrent hazard to human life and livelihood in most parts of the world, especially in some regions that have experienced rapid population and economic growth. Hazards are mitigated mainly through precautionary means, by restricting or even removing populations from areas with a history of landslides, by restricting certain types of land use where slope stability is in question, and by installing early warning systems based on the monitoring of ground conditions such as strain in rocks and soils, slope displacement, and groundwater levels. There are also various direct methods of preventing landslides; these include modifying slope geometry, using chemical agents to reinforce slope material, installing structures such as piles and retaining walls, grouting rock joints and fissures, diverting debris pathways, and rerouting surface and underwater drainage. Such direct methods are constrained by cost, landslide magnitude and frequency, and the size of human settlements at risk.

4.5.4 Flood

Do you remember the Chennai floods of 2016, Kerala floods of 2018 and 2019? Do you know how they were caused?

A flood is an overflow of water that submerges land that is usually dry. Floods can happen during heavy rains, when ocean waves come on shore, when snow melts quickly, or when dams or levees break. Damaging flooding may happen with only a few inches of water, or it may cover a house to the rooftop. Floods can occur within minutes or over a long period, and may last days, weeks, or longer. Floods are the most common and widespread of all weather-related natural disasters.

4.5.4.1 Causes of Flood

Flood is usually a result of natural causes. It may also be caused by man-made factors. It causes huge damage to life and property. There are many different causes leading to flooding.

1. Massive Rainfall
   Drainage systems and the effective infrastructure design aid during heavy rains. They help the drainage of excess water into reservoirs in an easy way. But in cases of heavy rainfall, the systems stop working. Thus flood is caused.
2. Overflowing of the Rivers
   The people living along the river always have a risk of life from the overflowing of the rivers. To prevent such a situation, a string of dams are built. However, if these dams are not managed properly, they may cause flooding and huge damage.
3. Collapsed Dams
   In the event of huge rainfall, the dams built begin to collapse, thus, causing the flood situation to become critical for the people living around.
4. Snowmelt
   At the time of the high melting of snow due to heavy precipitation and other factors, the situation of flooding arises. Adopting sustainable measures for heavy precipitation can help in dealing with the flooding situation.
5. Deforestation
   Trees prevent soil erosion and also the loss of crops. The vegetation is also enriched as a re-sult of more and more trees. This also blocks the massive flow of rain, thus preventing flooding.
6. Climate change
   The climatic changes caused due to human practices also add to the risk of flooding. Human beings cut trees in large numbers, thus affecting the process of photosynthesis. Increased level of carbondioxide in the atmosphere cause changes in climate, posing threats of natural disasters like floods etc.
7. Emission of Greenhouse Gases
   The burning of fossil fuels, industrial influences and pollution are depleting the level of the ozone layer and increasing the level of greenhouse gases, thus becoming a major cause of man-made flooding.
8. Other Factors
   Broken supply lines cause the outflow of water but lead to less damage. There is water flow from the washing machine and overflow from dishwashers worsening the situation. The lack of proper sewage systems adds to the destruction of this natural disaster.

4.5.4.2 Types of Flooding

There are five types of floods. They include:

1. River Flood-A River flood occurs when water levels rise over the top of river banks. This flooding can happen in all rivers and stream channels. This includes everything from small streams to the world’s largest rivers. River floods can happen suddenly or slowly. Sudden river flooding events occur more often on smaller rivers, rivers with steep valleys, rivers that flowmuch of their length over impermeable terrain, and normally dry channels. On the other hand, low-rising river floods typically occur in large rivers with large catchment areas.

Causes of River FloodingRiver flooding typically happens for four reasons. They are:

• Excessive rain from tropical storm systems making landfall
• Persistent thunderstorms over the same area for extended periods
• Combined rainfall and snowmelt

2. Coastal Flooding-This type of flood occurs when land is sub-merged by sea water.

Causes of Coastal Flooding

Coastal flooding is typically a result of a combination of sea tidal surges, high winds, and barometric pressure.These conditions typical-ly come from storms at sea like:

• Tropical cyclones
• Tsunami
• Higher-than-average tides

3. Storm surge– Storm surge is an abnormal rise in water level in coastal areas over and above the regular astronomical tide. Storm surge is an extremely dangerous form of flooding. It can flood large coastal areas at the same time. It can also cause flooding very quickly. Extreme flooding occurs when storm surge happens at the same time as high tide. This can cause storm tides to reach over 20 feet.

Causes of Storm Surge
Storm surge is always a result meteorological storms that cause higher than normal tides on the coast. There are three parts of a storm that create this surge. They are:

• Wind
• Waves
• Low atmospheric pressure

4. Inland Flood– An inland flood is flooding that occurs inland or not in a coastal area. Therefore, coastal flooding and storm surge are not inland floods.
Causes of Inland Flooding

• Rainfall is almost always to blame for inland floods. Rain causes in-land flooding in two ways. It can happen with steady rainfall over several days or it can happen because of a short and intense period of rainfall.
• Snowmelt also causes inland floods, although rainfall is a more common cause.
• Another way inland flooding happens is when water ways get blocked by debris, ice, or dams.

Inland floods are often worse in urban areas because there isn’t anywhere for the water to go. The following urban features can create urban flooding or make inland floods worse:

• Paved roads and streets
• Low-capacity drainage equipment
• Dense buildings
• Low amounts of green space

5. Flash Flood– A flash flood is flooding that begins within 6 hours, and often within 3 hours, of heavy rainfall (or other cause).
Causes of Flash Floods
Flash floods can happen for several reasons. Most flash floods happen after extremely intense rainfall from severe thunderstorms over a short period of time (normally 6 hours or less). Flash floods also happen when dams break, when levees fail, or when an ice jam re-leases a large amount of water. There are two key elements to determine flash flooding is

1. Rainfall rate
2. Rainfall duration

4.5.5 Cyclones

Cyclones are caused by atmospheric disturbances around a low-pressure area distinguished by swift and often destructive air circulation. Cyclones are usually accompanied by violent storms and bad weather. Or in other words, Cyclones are large revolving tropical storms caused by winds blowing around a central area of low atmospheric pressure. In the southern hemisphere these tropical storms are called cyclones and rotate in a clockwise direction, while in the northern hemisphere cyclones are called hurricanes or typhoons and rotate in an anti-clockwise direction.
Cyclones are classified as:

1. tropical cyclones.
2. extra tropical cyclones (also called temperate cyclones)

Tropical Cyclones – Cyclones that form closer to the Equator (i.e., at latitudes 10° to 25° north and south over the oceans) are known as tropical cyclones, typhoons or hurricanes and are much smaller in diameter. They form an intense circular storm that originates over warm tropical oceans and is characterized by low atmospheric pressure, high winds, and heavy rain. Tropical cyclones are the progeny of ocean and atmosphere, powered by the heat from the sea; and driven by easterly trades and temperate westerlies, high planetary winds and their own fierce energy. They develop in the regions between the Tropics of Capricorn and Cancer. Tropical cyclones are large-scale weather systems developing over tropical or subtropical waters, where they get organized into surface wind circulation. Drawing energy from the sea surface and maintaining its strength as long as it remains over warm water, a tropical cyclone generates winds that exceed 119 km (74 miles) per hour.

Extra tropical Cyclones– Extratropical cyclone, also called wave cyclone or mid-latitude cyclone, a type of storm system formed in middle or high latitudes, in regions of large horizontal temperature variations called frontal zones. Extratropical cyclones present a contrast to the more violent cyclones or hurricanes of the tropics. In high and middle latitudes a number of extratropical cyclones normally exist around the globe at any given time. These storms tend to form in preferred locations and follow typical paths, although exceptions to these typical patterns often occur.

4.5.6 Earthquake

Earth quake refers to any sudden shaking of the ground caused by the passage of seismic waves through Earth’s rocks. Seismic waves are produced when some form of energy stored in Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and “slip.” The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocentre, and the location directly above it on the surface of the earth is called the epicentre.

While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally over-comes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground.

Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead, the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded. Scientists then use a method called triangulation to determine exactly where the earthquake was (see image below). It is called triangulation because a triangle has three sides, and it takes three seismographs to locate an earthquake.

4.5.7 Earthquake waves

When an earthquake occurs, it releases waves of energy, which are known as Seismic waves. It is like the ripples created in water when you throw a stone in it. Seismic waves are like those ripples which can travel through the in-side of the earth, as well as on the surface.

P waves, S waves, and Surface waves

Body waves are those waves that travel through the earth. They originate at the epicentre of the earthquake and travel through the earth at amazing speeds. There are two types of body waves namely,

• P waves
• S waves

Surface waves are those waves that travel on the surface of the earth. The destruction caused by earthquakes is primarily done by these waves.

P waves and S waves

P waves or Primary waves are the first waves to hit the seismographs when an earthquake strikes. They are longitudinal waves which means that the direction of motion and propagation are the same.

• S waves also called secondary waves and shear waves, are the second waves to hit the seismographs. They are transverse waves, which means that the motion is perpendicular to the direction of wave propagation. S waves can only travel through solids and scientists have been successful in mapping the interior of the earth by studying the routes of these waves.

Recap

Objective type questions

Answer to Objective type questions

Assignments

Suggested Reading

Reference