The Indian sub-continent has seen world’s worst flood related destruction over the years. The Ganga and the Brahmaputra have annually flooded vast tracks of land for millennia. Bloated by heavy monsoons and the Himalayan snowmelt Subcontinent Rivers, big and small cause much havoc. Their intensity and fury has been increasing mainly due to deforestation. Denuded forests and other vegetative cover no longer absorb the heavy monsoon rains.
Reasons for Floods:
1. Heavy Downpours:
Areas which are subject to serious floods are mainly in the Plains of Northern India. It is estimated that over 90 per cent of the total damage done to property and crops in India is done in the Plains of Northern India. The Lower Narmada and the Lower Tapti valleys, the deltas of the Mahanadi, the Godavari and the Krishna are also inundated from time to time.
Heavy downpour of rain in the catchment areas of rivers and in the poorly drained areas is the major cause of floods. Spells of unusually heavy rains occur during the summer monsoon rainy season except in Tamil Nadu where some areas may, however, be flooded during winter months.
In the Punjab-Haryana Plain, rivers sometimes overflow their banks and inundate the neighbouring low-level areas. Away from the rivers incessant rainfall, particularly in the waterlogged and poorly drained areas, causes flood havoc. The choked courses of the Ghaggar and the Markanda, which are rivers only during the rainy months, inundate large areas. The poor drainage in some parts of Jind, Rohtak, and Hissar and Gurgaon districts of Haryana State is the main cause of floods.
2. Poor Drainage and Congestion:
In the plains of eastern Uttar Pradesh and Bihar, large areas are inundated where the Ganga receives its left-bank tributaries namely the Ghaghara, the Gandak, the Burhi Gandak, and the Kosi, and the right-bank tributary, the some during the rainy season. Near the confluences of these tributaries with the Ganga the free flow of their water is obstructed when the Ganga itself is in flood.
Owing to this congestion of water, the water-level rises in the tributaries, sometimes causing floods in the neighbouring areas. The interfluves between the left-bank tributaries of the Ganga are drained by rivulets which also increase the intensity of floods.
3. Annual Deposition of Silt:
The Kosi, the Tista, the Torsa and a number of other rivers drain large catchment areas in the Himalayas. Laden with heavy charge of sand and silt they emerge from the Himalayan gorges and enter the level plains where they lose speed. Slackening of the speed of water results in the deposition of the load and spreading of water over large areas. The annual deposition of silt and sand raises the bed and thus reduces the capacity of the river to accommodate flood water.
Large areas once fertile become wastelands after the deposition of course sands. Millions of people have been subjected to misery from time to time by the vagaries of the Kosi in northern Bihar and those of the Tista and the Torsa in northern West Bengal. Annual destruction of property and crops by the Kosi is rightly called “India’s River of Sorrow”. One the average, 33% of the flood damages in India are accounted for by Uttar Pradesh and 27 per cent by Bihar.
4. Land Depression:
The Assam Valley is another fertile belt which is affected sometimes seriously by flood havocs. Heavy rainfall amounting to over 250 cm during the rainy season in north-east Assam is the chief cause of the floods in the Assam Valley. The Brahmaputra which drains this valley receives from its tributaries, the Dibang and the Luhit, a large amount of water heavily laden with silt. The silt which is deposited in the river channel makes it shallow.
Thus, the capacity of the rivers to carry large volume of flood-water is reduced. Besides, landslides are frequent in north-eastern Assam. Huge landslides which obstruct the flow of water for sometime yield later to the pressure of the accumulated water. Water devastates large areas downstream.
Earthquakes which are not uncommon in Assam, cause floods in a number of ways. Earthquakes cause changes in the level of the land. Some areas are depressed and others raised by a few decimetres. The change in the level of the land also shifts the course of a river. South West Bengal suffers from floods which are caused by incessant rains and poor drainage.
Flood situation worsens if tidal bores are unusually high at the time when the Hooghly is in flood. A large number of small rivers such as the Ajay, the Damodar, etc., drain the deforested Bihar Plateau and after flowing eastward join the right bank of the Hooghly. Torrential rain in the deforested plateau is a major contributor cause of floods in the West Bengal Plain.
5. Deforested Catchment Areas:
Floods are almost a regular feature in the coastal lowlands of Odisha. Devastating inundations occur in this belt almost every other year. Here the major causes of floods are (a) the deforested catchment areas of the rivers and (b) the off-shore bars which choke the mouths of the rivers and obstruct the free flow of water into the sea. Owing to the deposition of silt in the deltas, rivers have a tendency to change their courses.
In addition to these causes, high tide at the time of flood aggravates the flood situation. Though the Mahanadi has been dammed at Hirakud for storing excess water, the littoral of Odisha is not free from floods. The main cause of the floods was heavy rainfall in the catchment areas of the rivers Brahmani, Baitarani and Sulandi, which resulted in simultaneous flooding of all rivers.
6. Cyclonic Storms:
The deltas of the Godavari and the Krishna have also suffered from floods time and again. Lower courses of the Narmada and the Tapti have to face a similar fate. Floods in these areas also result from incessant rains and the occurrence of high tides at the time when the rivers are in spate. In the Plateau of Peninsular India south of the Satpura Range, floods are not only far less destructive but also far less recurrent than in the other parts of the country.
This is due to the fact that here the rainfall is relatively less and load carried by the rivers small. The organs of modern civilization viz., road embankments, railways and canals also obstruct the free, flow of water and thus cause floods. Near the coastal areas, devastating floods have been caused by the cyclonic storms, particularly when they visit the coastal areas at the time of high tides.