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Flood Management


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Flood Management

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  • 1. Flood Management Unit – IV BTCI09007Reference Notes
  • 2. • Indian rivers and floods, Causes of flooding, Alleviation, Leeves and floodwalls, Floodways, Channel improvement, Flood damage analysis. Flood Management
  • 3. Introduction • “Any flow which is relatively high and which overtops the natural or artificial banks in any reach of a river may be called a flood”. • In rainy season, when heavy rainfall occurs in the catchment area, the flow of the river is increased and sometimes it exceeds the normal carrying capacity of the river. Then the surplus water overtops the banks of the river and submerges the surrounding areas consisting of villages, agricultural lands, etc. This phenomenon is known as flood.
  • 4. Introduction • The floods occurs because of heavy rains, or melting of snow or both when the flow in the river is so high that its natural cross-section is unable to contain it. Apart from the overflow of rivers, the floods may be caused by the failure of some dam, with a sudden release of huge amounts of water, causing considerable damage to life and property.
  • 5. Floods
  • 6. Introduction • When the banks are overtopped, water spreads over the flood plain and thus cause damage to crops and property within the flood plain of the river. Since the flood plain is a desirable location for man and his activities, it is important that floods be controlled so that the damage does not exceed an acceptable limit. The damage caused by floods in terms of loss of life, property and economic loss due to disruption of economic activities are all too well-known. It is not to prevent floods but it is possible to prevent or reduce the damage due to floods by controlling the floods. Thus flood control or flood management is defined as the prevention or reduction of the flood damage.
  • 7. Floods
  • 8. Indian Rivers and Floods • Almost all the rivers of India carry heavy floods during the monsoons when their catchment receives intense and heavy rainfall. In the upper reachs, where the river flows through mountainous terrain or undulating area, there is generally no overflow of the banks during high flows. In low reaches especially where the area is flat, the rivers overflow their banks and cause inundation of low low-lying lands, submerge standing crops and property and disrupt communications. In order to understand the availability of surface water in a country, and its related flood problems, it is always necessary to understand the river stream of that country.
  • 9. Indian Rivers and Floods • In India, from the point of view of flood problem, the rivers can be grouped under the four categories: • (1) Brahmaputra Region • (2) Ganga Region • (3) North-West Region • (4) Central India Deccan Region
  • 10. Brahmaputra Region • This region covers the state of Assam, Northern portions of West Bengal, Manipur, Tripura, Nagaland, Arunachal Pradesh and Mizoram. The main rivers in the region are Brahmaputra, barak and their tributaries like Tista, Tarsa, Jaldhaka and Mahananda. The region receives very heavy rainfall from 110 cm to 640 cm per year, which occurs mostly during the monsoon months of June to September every year. Severe and frequent floods, therefore, do occasionally occur in this region. • The main problems in the Brahmaputra region are overspills, drainage congestion, bank erosion, landslides, aggradation and changes in the river course.
  • 11. Brahmaputra Region
  • 12. Brahmaputra Region
  • 13. Ganga Region • This region covers the states of Uttarakhand, U.P. , Bihar, Himachal Pradesh, Rajasthan, Madhya Pradesh, Haryana, west Bengal, and Delhi. The main rivers in the region are the Ganga and its numerous tributaries like Yamuna, Sone, Gomti, Gandak, Kosi and Mahananda. The annual rainfall in the plains varies from 60 cm in the western part to about 175 cm in the eastern. The variation of rainfall in the slopes of the Himalayas is about 125 to 190 cm. More than 80 % rainfall falls during the normal monsoon season, between June to October. The rainfall and Consequently the flood problem increases from west to east and from south to north. The main flood problems in the region are mostly confined to the northern tributaries which bring in a lot of sediment, overspill their banks and change their courses.
  • 14. Ganga Region
  • 15. North-West Region • This Region covers the state of Jammu and kashmir, Punjab, Parts of Himachal Pradesh, Haryana and Rajasthan. The main rivers in the region are the Indus and its tributaries like Sutlej, Beas, Ravi, Chenab and Jhelum. The annual rainfall in hilly areas of Himachal Pradesh is about 175 cm and about 75 cm in Jammu and Kashmir. In the rest of the area, it decreases from about 60 cm in the east to less than 15 cm in extreme western part of Rajasthan.
  • 16. North-West Region • Compared to Ganga and Brahmaputra river regions, the flood problems in this region are generally mild. However, large scale drainage congestion is felt in this region, particularly in the states of Haryana and Punjab. In the Punjab, However, choes (i.e. Hill torrents) are a problem; cascading down steep slopes, they cause flash floods and extensive sand casting over agricultural lands.
  • 17. North-West Region
  • 18. North-West Region
  • 19. Central India Deccan Region • This region covers States like: Andra Pradesh, Karnataka, Tamilnadu, Kerala, Orissa, Maharashtra, Gujarat and parts of Madhya Pradesh. The main rivers in the region are: The Narmada, the Tapi, the Mahanadi, the Godavari, the Krishna and the Cauvery. The area receives its rainfall mainly during the south-west monsoon. The area receives its rainfall mainly during the south-west monsoon. Coastal areas of Andra Pradesh and Tamilnadu receive north-east monsoon also. The annual rainfall over the Western Ghats can be as high as 500 cm, in the remaining areas it varies from about 75 cm to 125 cm. The above rivers are generally stable, having sufficient capacities to carry flood waters, except in the delta areas.
  • 20. Central India Deccan Region • The region, as a whole, does not have a serious flood problem, except that on some rivers of Orissa State. The river Narmada and Tapi occasionally face high floods, adversely affecting areas in the lower reaches in Gujarat State. The small rivers streams of Kerala state, when happen to be in high floods, also cause occasional flood problems. In the delta areas of this region, there is the usual problem of sediment deposition, raising of flood levels, drainage congestion and synchronization of river floods with sea tides. • The most flood-prone area in the Brahamaputra basin and the Northern sub-basins in the Ganga Basin. Thus, the problem of floods varies from basin to basin.
  • 21. Rivers of India
  • 22. South and Central Indian Rivers.
  • 23. Causes of Flooding (Causes of Floods) • Intensity of Rainfall in Catchment area: • Whenever there is heavy precipitation over the catchment in terms of intensity, duration and spread, the river will carry high flow and thus this is the main reason of a rivers to be in flood. • The intensity of rainfall in the catchment area is the main cause of flood. If the rainfall is normal and the storm duration is short, the surface run-off will flow smoothly through the tributaries and rivers will not create any problem to the downstream area. But if the rainfall is very heavy and the storm duration is longer, then the surface run-off will be increased and it may exceed the normal carrying capacity of the river and hence overtopping of the river banks may occur and the surrounding area may get flooded.
  • 24. Rainfall in Catchment area
  • 25. Causes of Flooding (Causes of Floods) • Topography of the Catchment: • It is obvious that larger the size of the catchment more will be the flood. • The catchment area with steep slope increases the run-off and increase the sediment inflow due to the high velocity of flow. While the catchment area with flatter slope reduces the run-off and reduces the sediment inflow due to the velocity of flow. • The Shape of the basin also definitely affects floods. For a fan- shapped catchment, the time of concentration will be less and hence the storm hydrograph base period will be less and the peak flow will be more. As against this, in the case of an elongated-shaped catchment of equal area and the same storm, the time of concentration will be more. Thus, the storm hydrograph base period will be more and the peak flow will be comparatively less. • Thus, the size, slope and shape of the catchment area directly affects the flow of the river.
  • 26. Causes of Flooding (Causes of Floods) • Sedimentation of Rivers and Reservoirs: Reduction in flood control capacity of reservoir due to unabated heavy silting up will affect the flow of the river. • If the tributaries of a river carry heavy sediment load the river bed goes on silting up gradually every year. It will affect the carrying capacity of the river. Ultimately the cross-section of the river will be shallow and it will not be able to carry the high flood flow. Thus, the sedimentation of rivers and reservoirs are also responsible for the flood.
  • 27. Topography of the Catchment:
  • 28. Causes of Flooding (Causes of Floods) Obstruction in the River flow: • Whenever there is a heavy landslide in the river, it may cause flood on the u/s side due to arrest of flow and consequent rise in water level. Due to heavy rainfall, when the water pressure reaches a maximum value, then suddenly then obstruction may be removed and a high column of water may rush downstream area. • Obstruction in or aggradation of river bed, changes in river course and convulsions resulting in flooding by the river flowing over land outside its channel.
  • 29. Obstruction in the River flow
  • 30. Causes of Flooding (Causes of Floods) • Contraction of River Section: • Inadequate waterways at rail and road crossings will affect river flow. While constructing road or railway bridges across a river, the approach works are done on both banks which reduce cross-section of river. Again, the waterways provided by constructing piers may not be sufficient for let outlet of the high flood flow. In this case, the water rises on the u/s side due to insufficient passage and thus the upstream area may get flooded.
  • 31. Causes of Flooding (Causes of Floods) • Inadequate Cross Drainage Works: • Construction of cross drainage works reduce the depth of flow for the high flood flow. • In cross drainage works like aqueduct the river passes below the canal. Here, the structure which is constructed for the smooth running of the river flow may be inadequate for the high flood flow and water level may rise on the u/s side may submerge the surrounding area.
  • 32. Cross Drainage Works
  • 33. Causes of Flooding (Causes of Floods) • Other Causes: • Floods may be caused due to heavy melting of snow and ice. • In Case there is breach of a dam, the reservoir water may rush towards the downstream side causing heavy flood for a short period. • Sometimes, because of earthquake, it may so happen that the river bed is raised. This may cause flood on the upstream due to the arrest of flow as well as on the downstream, for some distance for a short period.
  • 34. Flood due to breach of a dam
  • 35. Alleviation (Flood Control or Flood Mitigation) • The various methods of flood control (or flood mitigation) and reduction of flood damage can be classified under different categories depending on the attempts made to solve this problem. • The flood control measures can be classified broadly as structural and non-structural measures.
  • 36. Alleviation (Flood Control or Flood Mitigation) • Flood control aims at attempts to • (i) modify the flood, • (ii) modify the susceptibility to flood damage • (iii) modify the loss of burden and bearing the loss or living with floods. • The various flood control measures aim at avoiding damages from floods • (i) by the construction of protective works, • (ii) through the reduction of flood flows by storage, change in land use or similar methods, • (iii) by modifying the susceptibility to flood damage and • (iv) by soil concentration in catchment area,
  • 37. Alleviation (Flood Control or Flood Mitigation)
  • 38. Structural Mitigation Measures • They are of following types: • (i) Storage Reservoirs: • It is the most effective measure of flood disaster mitigation measures. The modern reservoir are mostly multipurpose. The aim of reservoir is to store excess water during flood period and release it when flood subsides. • Generally, the reservoirs are formed on the upstream of the area to be protected or on the head reach of the river.
  • 39. Storage Reservoirs
  • 40. Structural Mitigation Measures • The flood control reservoir may be of two types: • (a) Detention Reservoirs • (b) Retarding Reservoirs
  • 41. Classification of Reservoir • Detention Reservoir In this type of reservoir, the spill ways with adjustable gates provided with the dam so that the flood water may be detained for sometime and then released according to the situation of the downstream area by operating the gates of the spillways. • Retarding Reservoir In this type of reservoir spillways are provided with the dam at such a level an capacity so that the flood discharge is retarded and it takes long time for the flood water to flow completely towards the downstream area. The discharge stops when the water level falls below the crest of the spillways.
  • 42. Detention Reservoir
  • 43. Retarding Reservoir
  • 44. Structural Mitigation Measures • Confining river flow by embankments (Levees): • Earthen embankments have been the principle methods of controlling flood as a short-term measure. • The leaves are earthen embankments constructed parallel to the river bank to continue the river water within a specified section. Thus the surrounding area may be protected from being flooded with surplus water which flow through the river during the heavy rainfall in catchment area.
  • 45. Confining river flow by Embankments (Levees)
  • 46. Structural Mitigation Measures • Channel Improvement Works: • Work performed to increase the discharge or velocity of stream or to decrease the stage and duration of flood is known as channel improvement works. • It Includes, • (a) Increase in size of cross section by widening and excavation of stream bed. • (b) Increasing velocity of flow in the channel by smoothing the river bed and sides, and removing roughness offered by sand bed, weed growth, etc.
  • 47. Channel Improvement Works
  • 48. Structural Mitigation Measures • (iv) Diversion Works: • From the upstream side of the flood affected area a diversion channel is excavated to connect the river at the d/s area. A diversion channel with a regulator upstream of the important area is constructed. The important area is placed on convex side of the river where inundation and erosion are possible. The diversion channel decreases the stage of flood near the important area, and therefore possible flood disaster may be reduced.
  • 49. Diversion Works
  • 50. Structural Mitigation Measures Flood Wall: • When no space is available for the construction of levee or when it is not suitable to construct the levee due to local site condition, then flood wall is constructed as local flood hazard prevention for some important area situated at low level. • There are masonry or concrete walls constructed just on the river bank. These are trapezoidal in section and act as retaining wall.
  • 51. Flood Wall
  • 52. Structural Mitigation Measures • Flood Ways: • The low lying areas along the course of the river are known as floodways. Floodways are the vast depressions into which a portion of the flood water diverted from a river through a natural or artificial channel is temporarily stored during the rising flood. When the river flow recedes the water from the flood ways returns back to river. The floodway is ordinarily used only during major floods. Floodway may be used for agriculture at other times.
  • 53. Flood Ways
  • 54. Structural Mitigation Measures • Construction of Cut-off • In case of sharp bends in the course of a river, the velocity of flow and the rate of discharge is reduced. During heavy rainfall when large flood discharge approaches the sharp bend of the river, it overflows its banks and submerges the surrounding area. So cut-off may be constructed in the meandering type of river to reduce travel time and the water can flow with high velocity along a straight path.
  • 55. Construction of Cut-off
  • 56. Structural Mitigation Measures • Runoff reduction by watershed management: • It is an indirect method applied to the watershed, which has long- term effect on flood disaster mitigation. Different watersheds management are • (i) Afforestation • (ii) Contour farming • (iii) Contour bunds • (iv) Check bunds • (v) Gullying • (vi) Bank Protection • (vii) Diversion drains • (viii) Strip cropping etc.
  • 57. Runoff reduction by watershed management:
  • 58. Non-Structural Mitigation Measures • The non-structural measures include, modifying the susceptibility to flood damage by • (i) flood plain management • (ii)Flood proofing including disaster preparedness • (iii) response planning • (iv) flood forecasting and warning. • Modifying loss burden by • (i) Disaster relief, • (ii) flood fighting including public awareness, and • (iii) flood insurance.
  • 59. Non-Structural Mitigation Measures • Flood plain Zoning: • Area near the river are the most vulnerable for the flood hazards provided the areas are not a highland. Therefore, people should not be allowed to those flood prone areas for dwelling houses. Places below high flood level should not be recommended for inhabitation. These areas may be used as parks, recreation ground, etc. so that inundation of such areas may result in saving life and properties
  • 60. Flood plain Zoning
  • 61. Non-Structural Mitigation Measures • Flood Forecasting: • The emergency evacuation of the threatened area is one of the most effective means of reducing damage due to flood under certain circumstances. Once the flood occurs, the normal activities of the society are badly disrupted with immense losses. Therefore, flood forecasting is a real necessity to minimized to a great extent. Temporary evacuation of persons and shifting the important property to safer paces could be done before the flood arrives.
  • 62. Flood Forecasting
  • 63. Non-Structural Mitigation Measures • Flood Proofing: • In cases where certain isolated of high value are threatened by flooding, they may sometimes be individually flood proofed. It essentially consists of a combination of structural change and emergency action. Structural change includes construction of the building wall with some water proofing material, closure of lower level windows and providing some means of watertight closure for the doors. Thus, even through the building may be surrounded by water, the property within it is protected from damage and many normal functions can be carried on. In Case of an industrial plant comprising building, storage yards, roads, etc. may be protected by a ring levee or flood wall.
  • 64. Flood Proofing
  • 65. Non-Structural Mitigation Measures • Mathematical Modeling: • In the flood disaster mitigation measures, mathematical model by computer can predict flood intensity and area of inundation whether it is flood in river or a flash flood- due to dam break or dike failure. • When natural flood upstream of a river is known, mathematical model of flood routing can predict the flood intensity at some downstream point of the river. Thus, mathematical modeling techniques can alleviate the flood disaster in the river valley in case of both natural and dam- break flood situation. Mathematical model has become a very powerful tool in this age by the use of modern high- speed computer in different fields of science and technology.
  • 66. Mathematical Modeling
  • 67. Levees and Flood walls • Levees and flood walls are one of the oldest and most widely used methods of protecting land from floodwater. These are essentially longitudinal dams constructed along the river banks roughly parallel to the direction of flow of the river. These serve as artificial high bank of the river and thus prevent the flood water from spilling over to the adjoining land. The flood water is confined between the levees or flood walls and is made to flow down the river without causing any damage to the country side of the levees or flood walls and is made to flow down the river without causing any damage to the country side of the levees or flood walls.
  • 68. Levees and Flood walls
  • 69. Levees • A Levee is an earth dyke or embankment. Levees are most frequently used for flood control because they can be built at relatively low or cost of material available at the site. • It is a sort of an earthen dam constructed along the river. Levees are usually built of material excavated from borrow pits parallel to the levee line. The material should be placed in layers and compacted, with the least previous material being placed along the river side of the levee.
  • 70. Levees • Levee cross section must be adjusted to fit the site and the available materials. The top width of levee is usually governed by the requirements of the movement of maintenance equipment. An adequate space is left between the inner toe of the levee and the top of the borrow ditch to avoid collapse of the ditch bank.
  • 71. Levees
  • 72. Location of Levees • Levees should be located on both the banks such that sufficient area of flow is provided between them to transmit the design flow with a reasonable free board against wave action. • It is often cheaper to place the levees along the high ground. Further in any case, full advantage should be taken of ridges, which permit lower levees and often provide better foundation condition conditions. When a city or agricultural district is to be protected a ring levee which completely encircle the area may be provided.
  • 73. Levees
  • 74. Flood Walls • A levee is an earth dyke or embankment, while a flood wall is usually of masonry or concrete. Flood walls are used in developed area where it is difficult to obtain to obtain enough land for the construction of levees. Because of flat slopes, levees require very large width. If the land is costly or limited, it is more economical to construct flood walls. These are masonry or concrete walls constructed just on the river bank. A flood wall is a sort of gravity dam constructed along the river, proper foundation should be provided and all precautions should be taken against scouring. If there is a back fill on the land side of the wall, it acts as an earth retaining wall.
  • 75. Flood Wall
  • 76. Floodways • The low lying area along the course of the river are known as floodways. Floodways are the large depressions into which a portion of the flood water diverted from a river, the water diverted from a river through a natural or artificial channel is temporarily stored during the rising flood. After the flood receeds in the river, the water stored in these depressions is permitted to flow back to the river. • The flood way serves two function • (i) They create large, shallow reservoirs which store a portion of the flood water and hence decrease the flow in the main channel below the diversion. • (ii) They provide an additional outlet from water and hence increase velocity and decrease stage for some distance above the point of diversion.
  • 77. Floodways • The contraction of floodways is limited by the topography of the valley and the availability of low-value land which can be used for the floodway. As the floodways are normally used only during the period of major floods, the land can be used for agriculture in the rest of the period. The diversion channel carrying the water from the river to the floodways may be naturally or artificial. Water may be admitted to floodways in a number of ways. The water may be diverted over a low spot in the natural bank of the river or through a gap left in the levee line. In some cases a fuse plug levee is provided.
  • 78. Floodways
  • 79. Channel Improvement • Channel improvement as a flood control measure can be used to reduce the river stage at a specific point in the reach. A river can be made to carry higher discharges at lower levels by merely improving its hydraulic conditions.
  • 80. Channel Improvement
  • 81. Channel Improvement • Various methods of channel improvement can be summarized as follows: • Increase in Size of cross-section of the channel • Realignment of the channel along a shorter route. • Increasing velocity of flow in the channel. • Increasing waterways at crossing.
  • 82. Channel Improvement • Increasing in size of cross-section of the channel: • The increase in size of cross section-to cater for a high designed flood without spilling over the bank is economically practicable only for small streams. In cases of larger rivers, such a method is very expensive. Moreover, there is problem to dispose of excessive excavated material.
  • 83. Increasing in size of cross-section of the channel
  • 84. Channel Improvement • Realignment of the channel along a shorter route: • Realignment of the channel along a shorter route is practicable only in the case of smaller streams and the discharge capacity of streams can be increased. However, this method is economically practicable for small streams with manageable low discharge.
  • 85. Realignment of the channel along a shorter route
  • 86. Increasing velocity of flow in the Channel • The velocity of flow in the channel can be increased by deepening the river, reducing width or by shortening the channel lengths at bends by artificial cutoffs, removing barriers in the channels section and lining the channel to improve its coefficient of rugosity. All these methods are adopted for increasing the velocity of flow if they are economical and practicable
  • 87. Increasing Waterways at Crossing
  • 88. Increasing Waterways at Crossing • Generally, there are a number of crossing on the river, such as bridges, culverts, and aqueducts. The inadequacy in the waterways of bridges, culverts, canal cross-drainage works etc. may result in aggravating the flood situation. To avoid this problem, adequate waterways should be provided at crossing after proper evaluation of the ultimate drainage requirement of the basin a the time of any proposed crossing across the drainage line.
  • 89. Increasing Waterways at Crossing
  • 90. Channel Improvement • The overall effect of the various methods of the channel improvement is to reduce the stage of river at the time of flood. After adopting the measure of channel improvement, the banks of the channel are not overtopped or less frequently overtopped during the high flood discharge.
  • 91. Flood Damage Analysis • There are two types of flood damages: • Direct Damage: • Indirect Damage: • (i) Direct Damage: Direct Damage results due to physical contact of flood water, for example, damage to crop, damage to houses, damage to human lives and live-stock, damage to public utilities, roadways, rail road, etc. • (ii) Indirect Damage: Indirect Damage result to property or services which are not touched by flood water, but is a loss or damage as a result of interrupted trade or division of rail or roadway traffic or other effect. For example, factories, shops, and business centre in flood affected area may be closed, resulting in loss of their owners, stoppage of industrial production and temporarily unemployment of workers.
  • 92. Flood Damage Analysis
  • 93. There are two types of losses due to Flood Disaster. • Tangible Losses • Intangible Losses • Tangible Losses: • The tangible losses are those which can be estimated in terms of some money value.
  • 94. There are two types of losses due to Flood Disaster. • The following are tangible losses: • (i) Damage of Personal properties like buildings, furnitures, electronic goods etc. • (ii) Loss of Crops • (iii) Loss due to disruption of trade, business etc. • (iv) Loss due to disruption of road and railway communications • (v) Cost of relief measures.
  • 95. Tangible Losses • Damage to properties such as buildings, public utilities, roadways, bridges, canals, embankments and telephone lines and electric lines bear significant part of flood damages. These damage are assessed on the basis of estimated cost of repairs.
  • 96. Tangible Losses • Damage to the crops constitute more than 60 % of the total damages in India and hence greater care is needed for its assessment. Thus the correct method of estimating the flood damage of crop is the preliminary estimate prepared just after the flood less the return obtained from replacing after the flood. • Rail road communication in affected areas may be interrupted resulting in reduced earning of railways and transport operations. • Daily wage earnings and smaller peddler working in the affected areas may suffer loss in their daily wages or earnings. • Cost of relief measures should be estimated.
  • 97. Tangible Losses
  • 98. Intangible Losses • The Intangiable losses are those which cannot be estimated in money values. The following are the intangiable losses. • (i) Loss of Human life and Cattle • (ii) Loss of health due to disease caused by flood • (iii) Loss Caused by social distress • (iv) Loss due to hindrance in development works of towns or cities • (v) Snake bites and other physical ailments and hardships.
  • 99. Intangible Losses • Loss of human lives in flood may be due to capsize boats, house collapse, etc. Loss of human lives is assessed only in number not in monetary values. But in case of loss of cattle, monetary value can be estimated. • Importance of the analysis of the flood damages is aimed to find realistic assessment of benefit-cost ratio of the flood control schemes.
  • 100. Intangible Losses
  • 101. Questions • Explain Various Causes of Floods. • Discuss the use of levees and flood walls for the flood control. What are their relative advantages and Disadvantages. • Explain Structural and non-structural approaches of controlling damages due to floods. • Write a detail note on flood management.
  • 102. References • Irrigation Engineering & Hydraulic Structures: By: Prof S.K.Garg Khanna Publishers • Internet Websites
  • 103. Thanks..