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Pre-Monsoon Devastation in Assam

Context:

While the Monsoons are yet to arrive, Assam has already been beset by Floods and Landslides that have left 15 people dead and more than 7 lakh affected.

  • The hill district of Dima Hasao, in particular, has been ravaged by flash floods and landslides, with connectivity to the rest of the state snapped.

Relevance:

GS III- Disaster management

Dimensions of the Article:

  1. Factors behind this Unprecedented Devastation
  2. What is Causing the Landslides During Pre-monsoon?
  3. What are Landslides?
  4. Two Primary varieties of Landslides in India
  5. Why are Landslides more frequent in the Himalayas than in the Western Ghats?
  6. Geographical distribution of floods in India

Factors behind this Unprecedented Devastation

  • The average rainfall in Assam from March 1 to May 20 is 434.5 mm, however the equivalent figure for this year is 719 mm, a 65 percent increase.
    • In the neighbouring state of Meghalaya, the excess was considerably higher, at 137 percent.
  • Climate Change is responsible for the timing and scale of rainfall.
    • There are more and more concentrated rain and heavy rainfall episodes as a result of climate change.

What is Causing the Landslides During Pre-monsoon?

  • “Undesirable, unpragmatic, unplanned structural intervention on the sensitive terrain of hills” is to blame.
  • Not only has there been significant deforestation for the construction of the railway line and the four-line highway over the years, but there has also been rampant riverbed mining, which is frequently done in conjunction with district authorities.
  • Many highways are being built across streams and springs, and hastily completed infrastructure development work in Assam and neighbouring states has resulted in a surge in landslides in recent years.

What are Landslides?

Landslides are physical mass movement of soil, rocks and debris down the mountain slope because of heavy rainfall, earthquake, gravity and other factors.

Why do Landslides Occur?
  • Base of the huge mountains eroded by rivers or due to mining activities or erosion agents resulting in steep slopes.
  • Increased industrialisation leading to climate change and weather disturbances.
  • Change in river flow due to construction of dams, barriers, etc.
  • Loose soil cover and sloping terrain.

Two Primary varieties of Landslides in India

I- Himalayas

  • India has the highest mountain chain on earth, the Himalayas, which are formed due to collision of Indian and Eurasian plate, the northward movement of the Indian plate towards China causes continuous stress on the rocks rendering them friable, weak and prone to landslides and earthquakes.
  • The Northeastern region is badly affected by landslide problems causing recurring economic losses worth billions of rupees.

II- Western Ghats

  • A different variety of landslides, characterized by a lateritic cap (Laterite is a soil and rock type rich in iron and aluminium , and is commonly considered to have formed in hot and wet tropical areas), pose constant threat to the Western Ghats in the South, along the steep slopes overlooking the Konkan coast besides Nilgiris, which is highly landslide prone.

The problem needs to be tackled for mitigation and management for which hazard zones have to be identified and specific slides to be stabilized and managed in addition to monitoring and early warning systems to be placed at selected sites.

Zone Map

Himalayas of Northwest and Northeast India and the Western Ghats are two regions of high vulnerability and are landslide prone.

Why are Landslides more frequent in the Himalayas than in the Western Ghats?

In the Himalayas, Landslides are very frequent because:

  • Heavy snowfall in winter and melting in summer induces debris flow, which is carried in large quantity by numerous streams and rivers – which results in increases chances of Landslides.
  • Himalayas are made of sedimentary rocks which can easily be eroded – hence, erosions contribute to more landslides.
  • Drifting of Indian plate causes frequent earthquakes and resultant instability in the region.
  • Man-made activities like grazing, construction and cultivation abet soil erosion and risks of landslides.
  • Himalayas not yet reached its isostatic equilibrium which destabilizes the slopes causing landslides.
  • Diurnal changes of temperature are much more in northern India than in southern slopes – weakening the rocks and increasing mass wasting and erosion.

In the Wester Ghats, Landslides are comparatively less frequent because:

  • Western Ghats are eroded, denuded, aged, mature, worn out by exogenic forces and have a much lower height – hence, occurrence of Landslides is lesser.
  • The Western Ghats are on more stable part of Indian plate, hence, there is a lesser occurrence of earthquakes and landslides.
  • While steep slope on western side with high rainfall creates idea condition for landslide but gentle eastern slope with low rainfall and rivers in senile stage, counters the condition.
  • Moving of Indian plates doesn’t affect the Western Ghats much (as they are old block mountains), hence the reduced number of landslides.
  • Small & swift flowing streams of western side and big matured rivers on eastern side (like Krishna, Godavari, etc) cannot carry large amount of debris.

Geographical distribution of floods in India

  • Rashtriya Barh Ayog (National Flood Commission) identified 40 million hectares of land as flood-prone in India.
  • Historically, Bihar has been known to be India’s most flood-prone State. The Flood Management Improvement Support Centre (FMISC), Department of Water Resources, Government of Bihar estimates that 76% of the population in north Bihar faces the recurring threat of flood devastation.
  • Assam, West Bengal and Bihar are among the high flood-prone states of India.
  • Most of the rivers in the northern states like Punjab and Uttar Pradesh, are also vulnerable to occasional floods.
  • States like Rajasthan, Gujarat, Haryana and Punjab are also getting inundated in recent decades due to flash floods. This is partly because of the pattern of the monsoon and partly because of blocking of most of the streams and river channels by human activities.
  • Sometimes, Tamil Nadu experiences flooding during November – January due to the retreating monsoon.

-Source: Down to Earth


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