Causes of floods

FLOODS

 

Among all the disasters that occur in India, floods are the most commonly occurring natural disasters due to the irregularities of the Indian monsoon. About 75% of the annual rainfall in India is concentrated in 3-4 months of the monsoon season. As a result there is very heavy discharge from rivers during the period causing widespread floods.

Flood is a state of higher water level along a river channel or on coast leading to inundation of land that is not normally submerge. Flood therefore is a natural disaster which causes considerable damage to the crops, livestock and human life.

The major floods are mainly caused in the Ganges-Brahmaputra-Meghna basin which carries 60% of the total river flow of the country. According to National Flood Commission about 40 million hectares of land area is prone to flood in the country. On an average, the area affected by floods annually is about 8 million hectare, out of which the cropped area affected is about 3.7 million hectare.

Floods are caused by natural, ecological or anthropogenic factors either individually or as a combined result. The various causes of floods in India are as follows:

• Heavy precipitation: Rainfall of about 15 cm or more in single day may be beyond the carrying capacity of the river and this causes the spilling of river over natural banks. Areas affected include west coast of Western Ghats, Assam and sub-Himalayan West Bengal and Indo-Gangetic plains

• Rise in river bed: Due to large gradients the Himalayan rivers carry a large amount of silt and sand which are ultimately deposited in the catchment area, and on the river bed. Siltation reduces the carrying capacity of river.

• Meandering tendency of river-flow: In the flat terrain rivers have the tendency to meander or change the course within a specific boundary. Lower reaches of Gangetic plains and Brahmaputra.
   
• Cyclones: Tropical cyclones accompanied by strong winds, high tidal bores causing inundation of coastal regions. Floods due to cyclone are common in the East coast of Tamil Nadu, Andhra Pradesh, Odisha and West Bengal.
   
• Silting in Delta areas: Sea tides deposit silt on the river-mouths and discharge channels leading to steady deterioration of their discharge capacity.
   
• Obstruction of free-flow of rivers: Embankments, railways, canals etc. obstruct the free flow of rivers leading to flood.
   
• Inadequate drainage arrangement: After introduction of irrigation in some areas, the sub-soil water table rises fast unless adequate arrangement are simultaneously made for both surface and sub-surface drainage. Punjab, Haryana and Uttar Pradesh.
   
• Earthquake and Landslide: These natural disaster change the river course and consequently cause flood.
   
• Deforestation: Deforestation causes the acceleration of runoff and lowering of infiltration. Deforestation of hill slopes leads to greater run-off which raises the water level in rivers. Western Ghats, Siwaliks and Chotanagpur plateau region.
   
• Cloud burst: Cloud bursts leads to high amount of rainfall within a short time leading to flash floods. Flash flood generally occurs in Himalayan region.
  
  
  A sudden violent flood caused by exceptionally heavy rain in a normally dry valley in a semi-arid area, the torrential stream sometimes being laden with debris. Flash floods are often caused by:
  
  
  1. Cloud burst / heavy downpour in the mountain areas 
  2. Rapid melting of snow and ice in mountain areas. 
  3. Glacial lake outburst in the high Himalayas and 
  4.  Failure of landslide / debris flow dams in high and rugged mountain areas.
   

The flood prone regions of India are listed below:

• The basin of the Himalayan rivers covering a part of Punjab, Haryana, Himachal Pradesh, Delhi, Rajasthan, Uttar Pradesh, Bihar and West Bengal. The Kosi and the Damodar are the main rivers causing floods.
• The North-Western river basin covering the states of Jammu and Kashmir, parts of Punjab, Haryana, Western Uttar Pradesh and Himachal Pradesh. The Jhelum, the Sutluj, the Beas the Ravi and the Chenab are the rivers causing floods in this region. 
• The Central and Peninsular river basins covering Madhya Pradesh, Odisha, Andhra Pradesh and Maharashtra, contain the Narmada the Tapi, the Chambal and the Mahanadi. Heavy floods occur in the Godavari, the Krishna the Pennar and the Cauvery at long intervals and flood problem is generally serious.