Context:
The Chandigarh-Manali highway was blocked on June 26 following flash floods and landslides. Flash floods were witnessed in Khotinallah near Aut (in HP) on the Pandoh–Kullu stretch due to a heavy downpour.
Relevance:
GS I: Geography
Dimensions of the Article:
- Flash Floods: Sudden and Destructive Water Events
- Differences Between Flash Floods and General Floods
- What are Landslides?
- Why are Landslides more frequent in the Himalayas than in the Western Ghats?
Flash Floods: Sudden and Destructive Water Events
- Flash floods are rapid and unexpected floods that occur within a short time, usually hours, following heavy rainfall or intense water accumulation events.
- The National Weather Service in the United States defines flash floods as floods caused by rainfall within a duration of less than 6 hours.
- They are characterized by a swift rise in water levels in rivers, streams, or urban areas, often catching people off guard.
Causes:
- Heavy rainfall is the primary cause of flash floods, but they can also result from dam or levee failures, ice or debris jams, or sudden release of water from natural reservoirs.
- In India, flash floods are often associated with cloudbursts, which are sudden and intense rainfall episodes.
Factors Contributing to Flash Floods:
- Intensity and duration of rainfall, steepness of terrain, soil conditions, and the presence of man-made structures affecting water flow can contribute to flash floods.
Features:
- Flash floods are characterized by their powerful force and velocity, carrying large volumes of water, debris, and sediment.
- Drainage systems can become overwhelmed, rivers may overflow their banks, and low-lying areas can be inundated.
- Flash flooding is more common in narrow and steep river systems, as the water flows rapidly.
- Urban areas located near small rivers are susceptible to flash floods due to the inability of hard surfaces to absorb water.
Differences Between Flash Floods and General Floods:
Flash Floods:
- Rapid onset, occurring within a short span of time (hours or minutes).
- Short-lived events that subside quickly after the intense rainfall or water accumulation ends.
- High intensity with a sudden surge of water, resulting in significant destruction.
- Little to no warning time, happening rapidly and often catching people off guard.
- Localized events, usually occurring in specific areas where intense rainfall or other factors lead to rapid water accumulation.
General Floods:
- Develop gradually over a longer period (days or weeks) due to sustained rainfall or melting snow.
- Longer duration, lasting for days, weeks, or even months.
- Lower peak intensity compared to flash floods due to the slower rise in water levels.
- More advance warning, allowing for evacuation plans and emergency measures to be implemented.
- Can cover larger areas, including river basins, coastal regions, or expansive low-lying areas.
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.
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.
-Source: Indian Express
