I. ENVIRONMENT AND ASSESSMENT OF FLOOD IMPACTS
B. Types of floods
Between one-fifth and one-third of the country is flooded to varying degrees each year during May through September when about two-thirds of the food grain (mainly rice) is produced. The following natural floods are encountered:
- River floods resulting from the bank overflows from the major rivers and their tributaries and distributaries during the monsoon months from June through September. (some 30 per cent of the country is prone to river floods.)
- Rainfall floods in the form of localized floods owing to intense rainfall occurring over long periods of time in the monsoon months and the consequent drainage congestion as the river water level is already very high.
- Flash floods in the piedmont areas in the north-east and south-east parts of the country during the pre-monsoon months of April and May, often causing damage to crops just before or at the time of harvesting and also to towns and other infrastructures.
- Tidal floods characterized by twice-a-day flooding in areas adjacent to estuaries and tidal rivers in the south-west and south-central regions due to astronomical tides from the Bay of Bengal and fortnightly flooding over a vast area due to spring tides.
- Storm surge floods in the coastal area generated by tropical cyclones in the Bay of Bengal from April to June and September to November. (approximately 12,000 km2 of coastal land is prone to storm surge floods).
River floods and rainfall floods are frequently aggravated by spring tides and monsoon winds originating in the Bay of Bengal which slow down drainage and prolong flood duration. Furthermore, simultaneous occurrence of peak flood flows in the Ganges and the Jamuna rivers is liable to cause extreme floods, which sometimes affect as much as half of the country.