Man-made disasters

TSUNAMI

TSUNAMI

The term ‘Tsunami’ has been coined from the Japanese term Tsu meaning ‘harbour’ and nami meaning ‘waves’. Tsunamis are waves generated by earthquakes, volcanic eruptions, or underwater landslides and can reach 15m or more in height devastating coastal communities. Tsunamis caused by nearby earthquakes may reach the coast within minutes. When the waves enter shallow water, they may rise to several feet or, in rare cases, tens of feet, striking the coast with devastating force. The Tsunami danger period can continue for many hours after a major earthquake.

 

Tsunamis are nearly always created by movement of the sea floor associated with earthquakes which occur beneath the sea floor or near the ocean. Tsunamis may also be generated by very large earthquakes far away in other areas of the Ocean. Waves caused by these travel at hundreds of kilometers per hour, reaching the coast several hours after the earthquake. Unlike ordinary tides, which are short, frequent and surface level, tsunami, are barely noticeable in their deep-sea formation stage. At this point despite a wavelength up to 100 km, they are shallow in depth and move at hundreds of
 Kilometer per hour.

Tsunamis can be very large. In coastal areas their height can be as great as 10m or more (30m in extreme cases), and they can move inland several hundred meters.All low-lying coastal areas can be struck by tsunamis.

A tsunami consists of a series of waves. Often the first wave may not be the largest. The danger from subsequent tsunami waves can last for several hours after the arrival of the first wave.

Sometimes a tsunami causes the water near the shore to recede, exposing the ocean floor. This is nature’s Tsunami warning.

The force of some tsunamis is enormous. Large rocks weighing several tons along with boats and other debris can be moved inland several meters by tsunami wave activity. Homes and other buildings are destroyed. All floating material and water move with great force and causing mortality or injuries to people.

Tsunamis can occur at any time of day or night.

Tsunamis can travel up rivers and streams that lead to the ocean thereby polluting them.

Tsunamis may also be generated by very large earthquakes far away in other areas of the Ocean. Waves caused by these travel at hundreds of kilometers per hour, reaching the coast several hours after the earthquake. Unlike ordinary tides, which are short, frequent and surface level, tsunami, are barely noticeable in their deep-sea formation stage. At this point despite a wavelength up to 100 km, they are shallow in depth and move at hundreds of kilometer per hour.

landslides

LANDSIDES

Landslides
Landslides are among the major natural disasters or calamities in the world. In hilly terrains of India, including Himalayan mountains landslides have been a major and widely spread natural disasters that strike life and property almost perennially and occupy a position of major concern. These landslides, year after year, bring about untold misery to human settlements apart from causing devastating damages to transportation and communication network. Landslides, debris fall, debris slide, debris flow, rock toppling etc. cause destruction of slope and ground surface, initiating the change of uncontrolled erosion in the mountain terrains.

FACTORS THAT CAUSE LANDSLIDES
Landslides occur because of the interplay of several factors.
Natural factors
★ Intensity of rainfall
★ Steep slopes
★ Stiffness of slopes
★ Highly weathered rock layers
★ Soil layers formed under gravity
★ Seismic activity
★ Poor drainage
Man made factors
★ Deforestation leading to soil erosion
★ Non-engineered excavation
★ Mining and quarrying
★ Non-engineered construction
★ Land use pattern

Vulnerable houses are those which are situated on:

• Existing landslides area.
• Steep natural slopes.
• Areas in or at the mouths of drainages (such as canyons).
• Houses constructed near foothills.

PROTECTION MEASURES FROM DAMAGE TO BUILDINGS
Site Selection
Landslides generally happen where they have occurred in the past, and in identifiable hazard locations. Areas that are typically considered safe from landslides include areas that have not moved in the past; relatively flat areas away from sudden changes in slope; and areas at the top of or along ridges. Houses built at the toe of steep slopes are often vulnerable to slides and debris flows.
Signs and Warnings
A house located on a hill can detect possible slope failure by watching for these signs:
Doors or windows stick or jam for the first time.
New Cracks appear on plaster, tile, brick or foundations.
Outside walls, walks or stairs begin pulling away from the building.
Slowly developing, widening cracks appear on the ground or on paved areas such as streets or driveways.
Underground utility lines break.
Fences, retaining walls, utility poles or trees tilt or move.
Water or bulging ground appears at the base of a slope.

The potential for landslides and destructive erosion can be greatly reduced or prevented with proper development, sound construction techniques, seasonal inspections and regular maintenance of drainage facilities.

Earthquake

 EARTHQUAKE

"An earthquake is a sudden motion or trembling of the ground produced by the abrupt displacement of rock masses".

• Most earthquakes result from the movement of one rock mass past another in response to tectonic forces.
• The focus is the point where the earthquake's motion starts.
• The epicenter is the point on the earth's surface that is directly above the focus.
• Earthquake Magnitude is a measure of the strength of an earthquake as calculated from records of the event made on a calibrated seismograph.
• In 1935, Charles Richter first defined local magnitude, and the Richter scale is commonly used today to describe an earthquake's magnitude.
• Earthquake Intensity.  is a measure of the effects of an earthquake at a particular place. It is determined from observations of the earthquake's effects on people, structures and the earth's surface.
• Among the many existing scales, the Modified Mercalli Intensity Scale of 12 degrees, symbolized as MM, is frequently used
  

Earthquake Hazards
Earthquake hazards can be categorized as either direct hazards or indirect hazards.
Direct Hazards

• Ground shaking
• Differential ground settlement
• Soil liquefaction
• Immediate landslides or mud slides, ground lurching and avalanches
• Permanent ground displacement along faults
• Floods from tidal waves, Sea Surges & Tsunamis
  

Indirect Hazards

• Dam failures
• Pollution from damage to industrial plants
• Delayed landslides.

Most of the damage due to earthquakes is the result of strong ground shaking. For large magnitude events, trembling has been felt over more than 5 million sq. km.

Site Risks
Some common site risks are:

• Slope Risks - Slope instability, triggered by strong shaking may cause landslides. Rocks or boulders can roll considerable distances.
• Natural Dams - Landslides in irregular topographic areas may create natural dams which may collapse when they are filled. This can lead to potentially catastrophic avalanches after strong seismic shaking.
• Volcanic Activity - Earthquakes may be associated with potential volcanic activity and may occasionally be considered as precursory phenomena.
• Explosive eruptions are normally followed by ash falls and/or pyroclastic flows, volcanic lava or mud flows, and volcanic gases.