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Floods as natural hazards
The document discusses floods as significant natural hazards with destructive potential, outlining their causes, types, and environmental impacts. It emphasizes the economic and social ramifications of flooding, particularly in densely populated regions like Asia, and highlights various control methods to mitigate flood damage. Key points include the classification of floods, their effects on ecosystems and human habitation, and the necessity for effective management and preparedness strategies.
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Floods as natural hazards
- 1. 1 FLOODS AS NATURALHAZARDS by Prof. A. Balasubramanian Centre for Advanced Studies in Earth Science University of Mysore India
- 2. 2 Introduction: Floods have thegreatest damage potential when compared to the other natural disasters, over the environment. Floods are also considered to be both social and economic disasters.
- 3. 3 In this episode,the following aspects of floods are discussed: 1. Floods as Natural Disasters 2. Causes of Floods 3. Analysis and Types of Floods 4. Environmental Impacts of Floods and the 5. Methods of controlling Floods.
- 4. 4 1. Floods asNatural Disasters A natural disaster is a major physical event occurring on the environment causing a severe impact over the biotic and abiotic factors. Some of these events may happen suddenly or over a period of time.
- 5. 5 The notable naturaldisasters of the Earth are Earthquakes, Tsunamis, Floods, Cyclones, Tornadoes, Hurricanes, Storms and Volcanic eruptions.
- 6. 6 In addition tothese, Landslides, Forest Fires, droughts and desertification are also causing more devastating effects over the environment. All of these are global phenomena and no part of the world is free from any of these natural hazards.
- 7. 7 The intensity andimpact of a hazard may vary considerably from place to place. The deaths, material loss and environmental damage will severely affect the economy of a country whether it is developed or developing.
- 8. 8 For a developingor an under developed nation, it will have a long-term impact over its growth. Every ecosystem will undergo an unexpected modification with reference to its biotic and abiotic conditions, after a disaster including floods. Earth quakes, Tsunamis and volcanoes are lithosphere induced hazards.
- 9. 9 Floods, cyclones, Hurricanesand storms are induced mainly by the meteorological factors of the atmosphere. Whether they are induced by one or more of the earth’s environmental segments, their effects will be in almost all the spheres of the globe.
- 10. 10 A flood isa body of water overflowing the river channels. Floods are associated with some extreme natural events that happen over a drainage basin. The drainage basin may comprise rural or urban populations. Flooding in rural sector is mainly due to rivers. Floods in urban lands may be due to storm- water drainage. Runoff is that portion of precipitation which reaches the streams.
- 11. 11 When runoff joinsa stream, it becomes a steam flow. It is the total outflow of a catchment or a watershed. Floods are defined as relatively high flows which overtakes the natural channels provided for the runoff. The level at which water flows along streams is called as the river stage.
- 12. 12 During flood, thestage of a river is high. The river water during a flood usually overflows its banks. Floodway is the channel of a water course and those portions of adjoining floodplain which are reasonably required to carry and discharge the floodwaters. A river is a mechanical, ecological and geotechnical system.
- 13. 13 It offers agood habitat to many living organisms. The interaction between the life and the fluvial corridor determines the strength of these ecosystems. As a river water is a carrier of nutrients, every life prefers staying nearer to it.
- 14. 14 A river ischaracterized by : a) a catchment area called as a basin b) a network of streams and drainages c) its channel morphology- which is constituted by its length, depth and width d) extend of banks and flood plains e) Discharge of water represented by volume, duration and velocity of flow f) Movement of suspended and bed-load sediments and pollutants.
- 15. 15 The rain waterwhich is collected by the Catchment zone of a river basin is drained through its network of streams. Cumulative flow of water derived from all tributaries of a river, beyond its channel capacity, will cause the situation of flooding. Most of the time, river stage is below the channel banks. At the time of higher discharge, water overflowing the banks cause the flood.
- 16. 16 At bankfull stage,water fills the channel upto the top of the banks and that is the channel capacity beyond which the river starts flooding. Wider river channels are formed when there is enormous outflow of water over a flat region.
- 17. 17 Over a periodof time, the running water, as a geological agent, develops canyons, gorges, water falls, meanders, flood plains and deltas. The successive river terraces of the Grand canyon of Colarado is a typical example of a widened river course of the olden days. A flood plain of a river is a clearly definable physical feature of its valley. It is the flat area bordering a river.
- 18. 18 Since it containsmainly alluvium, it is also called as alluvial plain. A Flood plain is built up of layers of sediments deposited by the river when it periodically overflows its normal banks. Since they are flat, near-water and desirable places to live, people have long been attracted by the floodplains for settlement.
- 19. 19 As they arevery fertile lands, farmers love to cultivate crops along floodplains. The world's largest alluvial plain is the region of the valleys of the Brahmaputra, Ganges (Ganga), and Indus rivers in India and Bangladesh. Large and thickly populated cities, industrial establishments and capitals are located along river courses.
- 20. 20 When there issevere flood, all these human activities along it path are destroyed. A flood is characterized by its volume, duration and velocity of water flowing along these courses. Several things happen to a river when it undergoes flooding.
- 21. 21 Most of thework performed by streams takes place during floods. As the flood subsides, the river drops its sediment load and the channel aggrades back to its previous level. The obstructions to flow in a stream channel cause local pockets of turbulence leading to rapid downward Scour.
- 22. 22 Such obstructions includeboulders and bridge tiers, which can be completely undermined by the scour during a flood. 2. Causes of Floods Flooding of rivers come due to both meteorological and physiographic factors.
- 23. 23 The major climaticfactors are: a) Form, intensity and duration of Precipitation b) Presence of a large catchment zone c) Direction of a storm movement d) Antecedent Precipitation Index e) High Humidity and low temperature f) Variation of atmospheric pressure
- 24. 24 The notable physiographicfactors are: a) Type of soil b) Landuse and land cover c) Area and Shape of the basin d) Slope of the catchment e) Natural and artificial drainage f) Storage facilities of surface water Flood is mainly caused by the high intensity short duration rainfall.
- 25. 25 Even low intensityrainfall for a longer period also may induce extended flows and flooding. If the catchment zone is large, definitely the outflow will be more causing floods. A very unique factor causing floods is the direction of movement of the storm. If the storm moves in the downstream direction, then the magnitude of river discharge is aggravated by the storm.
- 26. 26 Antecedent Precipitation Index(API) is a measure of the soil moisture condition existing before the occurrence of the storm. The runoff will be more, if the API is high and the soil is quite wet. Any variation in the atmospheric pressure is responsible for the movement of storms. High humidity and low temperature reduces the rate of evapotranspiration of the environment.
- 27. 27 When the rateof rainfall is more than the rate of infiltration of the soil, overland flow and runoff will be more. Infiltration is controlled by the texture and type of soil. Infiltration rates of sandy and alluvial soils will be more than silt and clayey soils. An area covered by fine grained and compacted soils like the clayey loam will have more surface runoff than infiltration.
- 28. 28 Coarse grained andloose soils promote more infiltration than runoff. The landuse and Landcover of an area also control the runoff. Built environment and barren land always encourage overland flow. Human changes of land by farming, deforestation and urbanization all try to increase the runoff from rains.
- 29. 29 Improper landuse andchannelisation of natural water ways and blockage of drains by structures are some of the contributing factors. If the shape of the basin is oval and large, more water will be collected by the catchment and will lead to floods in rivers. Lengthy and elongated basins may show flooding at the downstream ends.
- 30. 30 If the basinhas adequate network of natural or artificial drainage systems, runoff will be highly dissipated to the points of confluence. A steep slope always encourages more overland flow. Natural depressions, trenches, pools, lakes and ponds can hold much amount of water and control floods. Open ditches, tile drains, storm sewers and paved surfaces can encourage runoff.
- 31. 31 When all otherfactors remain static in a big basin, floods are mainly caused by heavy rainfall. The maximum flood discharge usually occurs over a relatively short period when the intensity of rainfall is high and the meteorological and other conditions are favourable for producing maximum discharge.
- 32. 32 3. Analysis andTypes of Floods Floods often cause the channel a stream to straighten and change its position within the flood plain. Over a long period of time, changes in base level, annual discharge, or bed-load can cause a stream to erode rapidly downward to a new level that put the old flood plain out of reach of flooding.
- 33. 33 A new floodplainwill form leaving the old floodplain standing above as a terrace. It must be emphasized that flooding is a normal part of the long range dynamics of a river and that the floodplain is a feature that is constructed and maintained by flooding. Flooding may be an annual event to some streams. The recurrence of floods is directly related to the occurrence of heavy rainfall.
- 34. 34 A Flood riskis the chance of experiencing flood, expressed in terms of the return period. Flood discharge is usually expressed in cu.m/s. Floods differ in their types based on their occurrence.
- 35. 35 The major typesof floods are: a) Flash Floods b) Single event floods c) Multiple event floods d) Seasonal floods e) Coastal floods f) Estuarine floods g) Floods caused by Dam failures h) Floods due to sudden melting of snow and glacier.
- 36. 36 Flash floods arefrequently associated with violent, convectional storms of short duration. They are floods of great volume and short duration. These results from cloudburst on relatively small and widely- dispersed streams. Heavy rainfall or slow moving thunderstorms may create flash floods.
- 37. 37 Single event floodis the most common type of flooding. It is caused due to widespread heavy rains of longer duration of 2 to 3 days. These are associated with cyclonic disturbances like storms and depressions. Multiple event floods are caused due to successive weather disturbances. Floods in Indo-Gangetic plains are of this type.
- 38. 38 Seasonal floods arefloods occurring during different seasons. Floods in some parts come during summer monsoons and in some other parts during winter monsoon. The southern half of the Indian Peninsula experiences floods mostly during winter seasons.
- 39. 39 Coastal floods comedue to the action of high tides and waves. Storm surges can create extensive floods along a coast. Hurricanes and tropical storms can produce heavy rains or derive ocean water onto the land. Beaches and coastal houses can be swept away by the water. Coastal flooding can also be produced by killer sea waves called tsunamis.
- 40. 40 Failure of damsand check dams, embankments and earthen bunds in village tanks will also create a flood. Sudden melting of snow and glaciers will induce severe flooding along the river courses of a mountainous catchment. Floods are also classified as Urban floods and Rural floods. Flood is relatively a random phenomena.
- 41. 41 It is thereforedifficult to predict the exact maximum flood which can occur in future. The estimation of flood is also required for its relief works. It can be estimated only with a specified probability. The following are the criteria of recognizing a flood and its characteristics: a) Depth of water b) Duration of rain c) Velocity of flow d) Rate of rise in river level and discharge
- 42. 42 e) Frequency ofoccurrence f) Seasonality. The estimation of maximum flood is another aspect of its analysis. It involves a) analysis of past flood records correlating with the basin yield b) Empirical methods using appropriate equations c) Preparation of unit hydrographs d) Flood frequency charts
- 43. 43 4. Environmental Impactsof Floods Floods are natural behavior of streams. Most floods are harmful. They may destroy homes and other property and even carry off the topsoil, leaving the land barren. When people are not prepared, sudden and violent floods may bring huge losses. Rivers, lakes, or seas may flood the land.
- 44. 44 River floods aremore common, though lake and seacoast floods can be more serious. Large boulders in river beds are moved during floods, and human structures such as bridge pilings and concrete barriers can be swept away by the rushing floodwaters. The water of flash floods move very fast. They have the power to move boulders, tear out trees, destroy buildings and obliterate bridges. Huge amounts of debris are carried away by these waters.
- 45. 45 The environmental impactsof flood are of many kinds. The effects of floods may be geomorphological, sedimentological, demographic, social and economical. A flood may change the configuration of a normal and natural stream. It can create new creeks and rills.
- 46. 46 The erosive powerof the floods may change the slope and remove the top soil which is loose. It can also roll away the boulders from one place to the other. Sediment transport is severe during floods. Floods destroy settlements and kill thousands of people depending upon their location.
- 47. 47 Loss of humanlife and housing, Destroyal of commercial and industrial properties, inflow of storm water into sanitary sewers, combined sewer overflows and promoting disease vectors, spilling of sewers over the streetsand delays in public transportation are the major effects.
- 48. 48 In addition tothese there will be adverse effects on aesthetics, disturbance of wildlife habitat, economic losses due to washing away of crop lands, pollution on water ways and receiving water bodies. Floods can devastate the land by removing human population and their establishments.
- 49. 49 Damage to civilengineering structures, broken mains, stoppage of transport services and wired communication systems are the impacts on urban lands. In many places, impacts of floods are seen over human health. Breaking epidemics and spreading of water- borne diseases are the severe environmental impacts to life.
- 50. 50 Flood waters arethe carriers of disease vectors. The impact will be from upstream end to the downstream end. The diseases that are to be feared of are various diarrhoea, acute respiratory infections, measles, tuberculosis and malnutrition. Sudden emergence of communicable diseases are also warranted.
- 51. 51 During a majorflood many deaths, caused by drowning, may occur. It happens to mainly the weaker sections of the population, such as infants, elderly and the sick. Among the survivors, the most common medical problem is hypothermia, meaning loss of body temperature due to long immersions in water and exposure to wind.
- 52. 52 Floods can causesevere food and food grain shortage. Water is a major solvent and carrier of organic, inorganic and toxic pollutants. Flood waters can increase the mobility of these elements over land and water resources by reducing their residence time in a matrix.
- 53. 53 The sources ofstorm water pollution in urban environments are diffuse and highly variable. Pollutants come from roads, parking slots, residential, commercial and industrial activities within municipal boundaries. Fertility of soil will be severely affected by floods.
- 54. 54 Manures, fertilizers andsoil humus will be leached away from their localities. Soil erosion will lead to huge siltation in tanks, lakes and ponds. Flood is one of the major causes of reservoir sedimentation. It controls and limits the space available for holding water and increase outflow and induce flooding.
- 55. 55 Floods can destroythe cattle, crops, and homes of millions of items. Flooding has been recognised as one of the most recurring, widespread and disastrous natural hazards in the densely populated regions of Asia. In many parts of the Indian Sub-continent, flooding reaches catastrophic proportions during monsoons.
- 56. 56 A historically notableexample was the Harappan (Indus) Civilization which was eroded due to a series of large floods in Indus river. Catastropes of outstanding proportions have occurred in the past and there is no end to the hazards created by floods.
- 57. 57 It is acontinuous and destructive phenomena in the Ganga and Brahmaputra River basins in India. Several lakh hectares of land was affected by the ravages of floods causing a sever loss of economy. In India, the average annual damage is of the Order of 7500 millions of Indian rupees and the annual human loss is accounted to be approximately 1400.
- 58. 58 Floods occur duringsouthwest monsoon. It is observed from the records that during the period from 1953 to 1987, about 50,000 people died in India due to flood hazards. Global statistics show that over 5 million people were affected so by internationally recognised floods every year. The rate also increases year after year. The expenditure on flood relief also gets increased proportionately.
- 59. 59 5. Methods ofcontrolling Floods. The study of causes and effects of floods, management and protection of flood are carried out by experts of meteorology, hydrology, geomorphology and engineering.
- 60. 60 Several methods areadopted to control the floods and minimize the impacts of flooding. They are mainly attempted a. To Modify the flood by construction of reservoirs, river channels, natural detention basins, inter-basin water transfer, construction of levees and flood walls, diversion canals, bank stabilization, best agronomic practices, afforestation and catchment zone management
- 61. 61 b. Flood controlmeasures are done to modify the susceptibility of flood damage through flood plain management, structural changes, flood proofing, disaster preparedness, flood forecasting and flood warning c. The methods are also adopted to reduce the loss though emergency evacuation, flood fighting, public health, disaster relief, flood insurance and tax remission.
- 62. 62 It is possibleto lessen the impact of a disaster. It is necessary to understand the nature of hazard in order to reduce the harmful effects. In flood controls, the main aspect lies in the identification of risk areas or flood-prone areas.
- 63. 63 Mapping of flood-pronezones of a river basin can be done through an integrated hydrological and meteorological analysis. Flood Management is to be attempted with specific goals, objectives and methodologies, depending upon the location, area and drainage systems of basins and the frequency of floods. Assessment of past floods is of primary importance in controlling future floods.
- 64. 64 The first stepin assessing flood hazard is estimating the probable future runoff rates and associated characteristics( depths of water and velocities) on the basis of rainfall patterns, topography, hydrology of soils, landuse pattern and the morphology of the natural and artificial water courses.
- 65. 65 This will resultin flood extend maps showing the boundaries of flood damage. These hazard zonation maps will certainly help the people to plan for controlling and managing the flood events. It helps in redistributing waters, providing gravel beds, growing shelter belts, flood proofing, land treatment, tree management, fencing and construction of flood walls.
- 66. 66 Flood controls cannever eliminate floods. But it can minimize the loss and save all life. It needs political and scientific attention in order to mitigate the effects of natural hazards world-wide. International bodies like the UNESCO and the WMO emphasized the needs of computing flood intensities right from 1967. The period of 1990s was declared as the International Decade of Natural Disaster Reduction.
- 67. 67 In India about40 Mha of land is flood- prone, which is about 12% of the total geographical area. The National Flood commission was setup in the year 1976. For flood controls a large number of measures such as construction of storage reservoirs, levees and flood walls are adopted.
- 68. 68 Estimation of riverdischarge is done in almost all major rivers having potential threats. Creating public awareness about the floods and their impacts and methods of minimizing their impacts are very much needed in flood disaster management. Computation of the recurring interval is also an attempt to arrange for flood preparedness.
- 69. 69 Flood warning systemsare established to inform the people of flood plains well in advance. Floods are natural hazards. Their occurrence can be predicted based on scientific computations. Flood controls are well within the hands of human activities.
- 70. 70 It is ourduty to plan the landscape and human settlements to mitigate the loss due to floods. Conclusion: Disasters are continuing natural hazards in the earth and hence, it is necessary to study the causes and effects and understand their processes.
- 71. 71 This is needednot only to mitigate their effects but also to save the life and the environment.