Unit ii flood
- 1. FLOODS
- 2. What is Flood ? Flood is a state of high water level along a river channel or on the coast that leads to inundation of land, which is not usually submerged. Floods can form where there is no stream, as for example when abnormally heavy precipitation falls on flat terrain at such a rate that the soil cannot absorb the water or the water cannot run off as fast as it falls. 2
- 3. Can floods be manmade ? Floods are caused not only by rain but also by human changes to the surface of the earth. Farming, deforestation, and urbanization increase the runoff from rains; thus storms that previously would have caused no flooding today inundate vast areas. Not only do we contribute to the causes of floods, but reckless building in vulnerable areas, poor watershed management, and failure to control the flooding also create the disaster condition 3
- 4. Distributional Pattern of floods in India Floods occur in almost all the river basins of the country. Around 12 per cent (40 million hectare) of land in India is prone to floods. 4
- 5. Distributional Pattern of floods in India Most of the flood affected areas lie in the Ganga basin (Ganga and Yamuna) Brahmaputra basin (comprising of Barak, Tista, Torsa, Subansiri, Sankosh, Dihang and Luhit) Northwestern river basin (comprising Jhelum, Chenab, Ravi, Sutlej, Beas and the Ghagra) Peninsular river basin (Tapti, Narmada, Mahanadi, Baitarani, Godavari, Krishna, Pennar and the Kaveri) coastal regions of Andhra Pradesh, Tamilnadu, orissa and Kerela. 5
- 7. Distributional Pattern of floods in India Assam, Uttar Pradesh, Bihar and Orissa are some of the states who have been severely prone to floods. Our country receives an annual rainfall of 1200 mm, 85% of which is concentrated in 3-4 months i.e June to September. Due to the intense and periodic rain, most of the rivers of the country are fed with huge quantity of water, much beyond their carrying capacity. 7
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- 9. Types of Floods 9 1. Flash Floods1. Flash Floods 2. Riverine floods 3. Storm Surge3. Storm Surge
- 10. Flash floods Flash floods are local floods of great volume and short duration. A flash flood generally results from a torrential rain or “cloudburst” on relatively small and widely-dispersed streams. Runoff from the intense rainfall results in high flood waves quickly reach the maximum and diminish almost as rapidly. Flood flows frequently contain large concentrations of sediment and debris. Flash floods also result from the failure of a dam. Flash floods are particularly common in mountainous areas and desert regions but are a potential threat in any area where the terrain is steep, surface runoff rates are high, streams flow in narrow canyons, and severe thunderstorms prevail. 10
- 11. Riverine floods Riverine floods are caused by precipitation over large areas or by melting of the winter’s accumulation of snow, or by both. These floods differ from flash floods in their extent and duration. Flash floods are of short duration in small streams whereas riverine floods take place in river systems whose tributaries may drain large geographic areas and encompass many independent river basins Floods on large river systems may continue for periods ranging from a few hours to many days. The condition of the ground (amount of soil moisture, seasonal variations in vegetation, depth of snow cover, imperviousness due to urbanization, etc.) directly affects runoff. 11
- 12. Storm surge Storm surge or tidal surge is an offshore rise of water associated with a low pressure weather system, typically a tropical cyclone. Storm surge is caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea level. It is this combined effect of low pressure and persistent wind over a shallow water body which is the most common cause of storm surge flooding problems. 12
- 13. Causes of Floods Some of the major causes are: ♦ Heavy rainfall ♦ Heavy siltation of the river bed reduces the water carrying capacity of the rivers/stream. ♦ Blockage in the drains lead to flooding of the area. ♦ Landslides blocking the flow of the stream. ♦ Construction of dams and reservoirs ♦ In areas prone to cyclone, strong winds accompanied by heavy down pour along with storm surge leads to flooding 13
- 15. Impact on Built and Natural Environments Flooding can have any of the following effects on housing or other small buildings. Houses washed away due to the impact of the water under high stream velocity. The houses are commonly destroyed or dislocated so severly that their reconstruction is not feasible. Flotation of houses caused by rising waters. This occurs when light-weight, typically wood houses are not securely anchored. They can be removed too far from their foundations for relocation and repair. Damage caused by inundation of house. The house may remain intact and on its foundation, but the water damage to materials may be severe. Repair is often feasible but may require special procedures to dry out properly. Undercutting of house. The velocity of the water may scour and erode the house’s foundation or the earth under the foundation. This may result in the collapse of the house or require substantial repair. Damage caused by debris. Massive floating objects such as trees and other houses may impact on standing houses and cause significant damage. 15
- 16. Impact on Built and Natural Environments Health-Related Effects Floods may create conditions that promote secondary threats of waterborne and vectorborne diseases. 16
- 17. Impact on Agriculture In most flood prone countries where economies are based on agriculture, the largest economic flood-related losses are in the agricultural sector. Even in the industrialized United States 48 percent of flood losses in 1975 were in agriculture. Obviously most losses to agriculture result from the drowning of crops. Susceptibility to drowning depends on the type of crop and duration of flooding. Some are quickly killed by a relatively small amount of superfluous water. Others can resist as much as a few days of submersion. Even crops that thrive on large amounts of standing water will be killed if the water stagnates. Other agricultural losses occur in the submersion of crop storage facilities. Grains and other crops will quickly spoil if saturated with water, even for a short time. 17
- 18. Impact on Development An additional negative impact on the agricultural sector is the erosion of topsoil by the floods. Here the impact is indeed long term, resulting in the reduced productivity of the land and possibly eventual abandonment. Flooding, however, is not all bad. For some agricultural areas flooding is a positive and necessary event. These lands depend on the periodic silt deposits for added nutrients to the soil. Flooding also serves other advantages including the filtering or dilution of pollutants that enter the waterways, flushing of nutrients in river systems, preserving of wetlands, recharging of groundwater, and maintaining of river ecosystems by providing breeding, nesting, feeding and nursery areas for fish, shell fish, migrating waterfowl, and others. Widespread floods can have a significant effect on the long-term economic growth of the affected region. Indirect and secondary effects on the local and national economy may include reduction in family income, decline in the production of business and industrial enterprises, inflation, unemployment, increase in income disparities, and decline in national income. 18
- 19. In addition, relief and reconstruction efforts often compete with development programs for available funds. In countries where flooding occurs frequently, floods can create an enormous financial burden. The loss of crops and the need to find alternate sources of income have often caused small-scale migrations of farmers and skilled workers from rural areas to cities. Once established in a city, few return to their homes or farms. Small marginal farms usually cannot survive economically following a major flood. Farmers are often forced to sell their land because they cannot afford to rehabilitate it. This may result in a substantial increase in the number of people migrating to urban areas, and thus a related housing shortage. 19
- 20. Typical adverse effects The most important consequence of floods is the loss of life and property. Structures like houses, bridges; roads etc. get damaged by the gushing water. Boats and fishing nets also get damaged. There is huge loss to life and livestock caused by drowning. Lack of proper drinking water facilities, contamination of water (well, ground water, piped water supply) leads to outbreak of epidemics, diarrhea, viral infection, malaria and many other infectious diseases. Flooding also leads to a large area of agricultural land getting inundated as a Result there is a huge crop loss. This results in shortage of food, and animal fodder. Floods may also affect the soil characteristics. The land may be rendered infertile due to erosion of top layer or may turn saline if sea water floods the area. 20
- 21. ‘Komen’ Flood (2015) Effect in W.Bengal, Bangladesh and NE States of India