- Floods are high stream flows, which overlap natural or artificial banks of a river or a stream and are markedly higher than the usual as well as inundation of low land.
- Sometimes copious monsoon rains combine with massive flows from the rivers, then the floods indeed become calamitous.
- Through geophysical studies, it has been found that more than one and half billion people on the earth planet reside on riverside or coastal flood plains where they produce 1/3 of world’s food production.
- At least- some fraction of these plains go under flood water one or the other day, hence causing widespread losses to human lives, devastated homes and heads of cattle dead, destroy agricultural crops and disrupt the communication links such as railways, roads as well health hazards (i.e. spread of diseases such as cholera or Gastrointestinal symptoms, etc.).
- Even after the receding of floods, it takes several months or even years for the community to come to the pre-flood status.
- The vulnerability of states or Union Territories of India due to floods was not observed seriously in the past due to low developmental activities and less population pressure.
- However, in the present time, unabated population and high rate of developmental activities have forced the people to occupy the flood plains and making the society highly vulnerable for flood losses.
- Other factors like lack of adequate sites for rescue operations above flood levels, accessed routes for reaching these sites, lack of public information system for escape routes and other appropriate response activities may be rendered to make communities more sensitive.
- The protected area (32 mha) sometimes becomes more vulnerable to floods as the flood control measures mainly the embankments may breach during a severe flood and the protected area may be inundated.
- In India, 25 states and one Union Territory (Andaman & Nicobar) are susceptible to floods. However, the most vulnerable states in India are Uttar Pradesh, Bihar, Assam, West Bengal, Gujarat, Orissa, Andhra Pradesh, Madhya Pradesh, Maharashtra, Punjab and Jammu & Kashmir. District wise, there are 137 districts vulnerable to floods
CAUSES: The primary causes for Floods are-
- Excessive rainfall in river catchments or concentration of runoff from the tributaries and river carrying flows in excess of their capacities.
- Backing water in tributaries at their confluence with the main river.
- Synchronization of flood peaks in the main rivers or their tributaries.
- Intense rainfall when river is flowing full.
- Poor natural drainage system.
- Landslides leading to obstruction of flow and change in the river course.
- Cyclone and very intense rainfall when the EL Nino effect is on a decline.
AREA PRONE TO FLOODS
- Generally, the floods are caused due to the concentrated spells of heavy rains in the upper reaches of river during the monsoon months (June- September).
- The South- west monsoon accounts for 75% to 90% annual rainfall of the country.
- Thus, the irregular and erratic distributions of rains in different parts of the country during monsoon are the reasons for loss of lives, property and agricultural crops in the wake of floods.
- Brahmaputra and the Gangetic Basins are the most flood prone areas.
- The other flood prone areas are the northwest region of west flowing rivers such as the Narmada and Tapti, Central India and the Deccan region with major east flowing rivers like Mahanadi, Krishna and Cavery.
- The area liable to floods is 40 million hectares (98.8 million acres) as assessed by Rashtriya Barh Ayog (RBA) in 1980; the average area affected by floods annually is about 8 million hectares covering Uttar Pradesh with 21.9%, Bihar (12.71%), Assam (9.4%), West Bengal (7.91%), Orissa (4.18%) and other states have 43.9% flood prone areas.
- The heavy rain in the Himalayas at the peak of the South- West Monsoon causes severe floods in the rivers of Uttar Pradesh, Bihar, West Bengal, and Assam, while Central & Southern Rivers get flooded on account of heavy rain produced by depressions in Bay of Bengal during Southwest monsoon season. In most flood prone states, land depression and well marked low pressure/ low-pressure areas are the two most important synoptic systems responsible for devastating floods.
- In case of Bihar, 62% cases of the flood occurred due to well-marked low pressure/low pressure area, while the remaining 38% cases, flood occurred due to Land depression.
- In West-Bengal, the most favorable significant situation for occurrence of flood is either low-pressure area or the cyclonic circulation.
- Similar is the case for Punjab, Gujarat, Rajasthan and Jammu & Kashmir where devastating flood occurred mostly due to Low-pressure area.
- The monsoon depression plays an important role in occurrence of the flood in the states like Orissa and Andhra Pradesh.
- During the period of 1980-99, 53 cases of floods are identified, out of which:
- 25 cases (47%) – due to either low pressure or well-marked Low-pressure area
- 18 cases (34%) – due to either depression over land area
- 7 cases (13%) – due to cyclonic circulation, and
- 3 cases (1%) – due to cyclonic storm
TYPES OF FLOODS
- Such floods that occur within Six hours during heavy rainfall and are usually associated with towering cumulus clouds, severe thunderstorms, and tropical cyclones or during the passage of cold weather fronts.
- This type of flood requires rapid localized warning system and immediate response in favour of affected communities.
- Other causes of flash floods include dam failure or other river obstructions.
River floods –
- Such floods are caused by precipitation over large catchment’s areas or by melting of snow or sometimes both.
- They take place in river systems with tributaries that may cover or drain large geographical area and encompass many independent river basins.
- These floods are normally built up slowly or on seasonl basis and may continue for days or weeks as compared to flash floods.
- Factors such as ground conditions like moisture, vegetation cover, depth of snow, etc. and size of the catchments govern the amount of flood covering the main rivers of India like Ganga, Brahmaputra and Yamuna, etc.
Coastal Floods –
- Some floods are associated with the cyclonic activities like Hurricanes, Tropical cyclones, etc. generating catastrophic flood from rainwater which often aggravate wind-induced storm and water surges along the coast.
- As in river floods, intense rain falling over a large geographic area produces extreme flood situation in coastal river basins.
Approach to flood management
Approaches to dealing with floods may be any one or a combination of the following available options:
- Attempts to modify the flood
- Attempts to modify the sus-ceptibility to flood damage
- Attempts to modify the loss burden
- Bearing the loss.
The main thrust of the flood protection programme undertaken in India so far has been an attempt to modify the flood in the form of physical (structural) measures to prevent the floodwaters from reaching potential damage centres and modify susceptibility to flood damage through early warning systems.
The following structural measures are generally adopted for flood protection:
- Embankments, flood walls, sea walls
- Dams and reservoirs
- Natural detention basins
- Channel improvement
- Drainage improvement
- Diversion of flood waters
Non-structural measures include:
- Flood forecasting and warning
- Floodplain zoning
- Flood fighting
- Flood proofing
- Flood insurance.