Flood And Drought Occurence

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Flood and Drought Occurrence

Drought

  • Drought is a complex, slow-onset phenomenon of ecological challenge that affects people more than any other natural hazards by causing serious economic, social and environmental losses in both developing and developed countries.
  • The period of unusual dryness (i.e. drought) is a normal feature of the climate and weather system in semi-arid and arid regions of the tropics, which covers more than one-third of the land surface and is vulnerable to drought and desertification.
  • A drought is an extended period where water availability falls below the statistical requirements for a region.
  • Drought is not a purely physical phenomenon, but instead is interplay between natural water availability and human demands for water supply.
  • There is no universally accepted definition of drought. It is generally considered to be occurring when the principal monsoons, i.e. southwest monsoon and northeast monsoon, fail or are deficient or scanty.
  • Monsoon failure causing crop failure, drying up ecosystems and shortage of drinking water results in undue hardship to the rural and urban communities.
  • Although droughts are still largely unpredictable; they are a recurring feature of the climate. Drought varies with regard to the time of occurrence, duration, intensity and extent of the area affected from year to year.
  • Land abuse during periods of good rains and its continuation during periods of deficient rainfall is the combination that contributes to desertification.
  • Dry regions in India include about 94 mha and about 300 million people (one-third of India’s population) live in these areas; more than 50% of the region is affected by drought once every four years.
  • Different countries and states have developed codes, manuals, procedures, processes and policies for monitoring and management of drought with varying understanding.
  • Over the years, India has developed a fairly elaborate governance system of institutionalized drought monitoring, declaration and mitigation at different levels.
  • India’s response to the need for enhanced drought management has contributed to overall development. For example, the drought of 1965–1967 encouraged the ‘green revolution’, after the 1972 drought employment generation programmes were developed for the rural poor; the 1987–1988 drought relief effort focused on preserving the quality of life.

Drought classification systems

Classification by British Rainfall Organization (BRO, 1936)

  • Absolute drought -When there are at least 15 consecutive days with less than 0.01 inch of rainfall per day.
  • Partial drought -When there are at least 29 days having mean rainfall of 0.01 inch or less.
  • Dry spell -When 15 consecutive days receive less than 0.04 inch of rain per day.

Thornthwaite (1947) classification

  • Permanent drought -Characteristics of the desert climate, possibility of vegetation and agriculture only by irrigation.
  • Seasonal drought -Planting dates and crop duration should be synchronized with rainy season and residual moisture storage.
  • Contingent drought -Irregular occurrence and there is no regular season of occurrence.
  • Invisible drought -Occurs even when there is frequent rainfall and occurs in humid region.

Classification based on physical aspects

  • Agricultural drought – When soil moisture is inadequate to support healthy growth of crops, resulting in very low yield.
  • Hydrological drought – Associated with shortfalls in surface or subsurface water supply (stream flow, reservoir and lake levels, and groundwater) on a watershed or river basin-scale.
  • Meteorological drought -Related to the deficiency of rainfall compared to the average mean annual rainfall of an area.

Indian National Commission on Agriculture (1978)

  • Meteorological drought –Normal precipitation below 25%.
  • Hydrological drought – Prolonged meteorological drought and drying of reservoirs, lakes, streams and rivers, cessation of spring flows and fall in groundwater levels.
  • Agricultural drought -Depletion of soil moisture during the growing season. A dry situation with 20% probability and rainfall deficiency of more than 25% in drought-prone states of India

Occurrence and effects of droughts in India during 1900–2002

  • July 2002 —–Uttar Pradesh, Madhya Pradesh, Rajasthan, Punjab, Haryana, Delhi, Karnataka, Kerala, Nagaland, Orissa, Chhattisgarh, Himachal Pradesh Gujarat, Maharashtra, Andhra Pradesh and Tamil Nadu —–300,000,000 affected; damage – US$ 910,721,000
  • May 2001 —–New Delhi, Rajasthan, Gujarat, Orissa ——-20 deaths
  • November 2000 —–Mahasamund, Raipur, Kawardha, Rajnandgaon and Durg districts in Chhattisgarh region
  • April 2000 —–Gujarat, Rajasthan, Madhya Pradesh, Orissa, Andhra Pradesh and Maharashtra —–90,000,000 affected; damage – US$ 588,000,000
  • March 1996 —–Rajasthan
  • March 1993 —–Bihar, Orissa, Andhra Pradesh, Maharashtra, Gujarat, Madhya Pradesh, Uttar Pradesh and Karnataka —–1,175,000 affected
  • July 1987 —–Orissa —–110 deaths
  • 1987 —–Gujarat, Rajasthan, Orissa, Madhya Pradesh, Andhra Pradesh, Maharashtra and four Union Territories —–300 deaths, 300,000,000 affected
  • 11 April 1983 —–Kerala, Tamil Nadu and Rajasthan —–100,000,000 affected
  • 1972 —–Central India 100,000,000 affected; damage – US$ 50,000,000
  • August 1964 —–Mysore —–166,000,000 affected
  • 1964 —–Rajasthan, center —–500,000 affected
  • 1942 —–Kolkatta, Bengal region —–1,500,000 deaths
  • 1900 —–West Bengal —–1,250,000 deaths

Preventive measures and preparedness plan for drought mitigation

  • Dams/reservoirs and wetlands to store water
  • Improvement in agriculture through modifying cropping patterns and introducing drought-resistant varieties of crops
    • Watershed management
    • Water rationing
  • Management of rangeland with improvement of grazing patterns, introduction of feed and protection of shrubs and trees
    • Cattle management
    • Proper selection of crop for drought-affected areas
    • Development of water resource system with improved irrigation, development of improved storage facilities, protection of surface water from evaporation and introduction of drop irrigation system
    • Levelling, soil-conservation techniques
    • Reducing deforestation and fire-wood cutting in the affected areas
  • Alternative land-use models for water sustainability
    • Checking of migration and providing alternate employment
  • Animal husbandry activities can help in mitigation with use of improved and scientific methods
    • Education and training to the people
    • Participatory community programmes

 

 

 

Floods

  • 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-

  1. Excessive rainfall in river catchments or concentration of runoff from the tributaries and river carrying flows in excess of their capacities.
  2. Backing water in tributaries at their confluence with the main river.
  3. Synchronization of flood peaks in the main rivers or their tributaries.
  4. Intense rainfall when river is flowing full.
  5. Poor natural drainage system.
  6. Landslides leading to obstruction of flow and change in the river course.
  7. 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

Flash 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.

Structural measures

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

Non-structural measures include:

  • Flood forecasting and warning
  • Floodplain zoning
  • Flood fighting
  • Flood proofing
  • Flood insurance.

 


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