Overflow situation Unusually high stage in a river River overflow its banks and inundates the adjoining area. Significance: damage in terms of life, property and economic loss. Thousands of crores of rupees are spent every year in flood control and forecasting.
Death Toll
1)2,500,0003,700,000 2)900,000– 2,000,000 3)500,000– 700,000
Event
China floods
Location
Date
China
1931
China
1887
China
1938
China
1975
China
1935
St. Felix's Flood, Flood, storm surge
Netherlands
1530
Hanoi and Red River Delta flood
North Vietnam
1971
Yangtze river flood
China
1911
Yellow River (Huang He) flood
1938 Yellow River (Huang He) flood
4)231,000 Banqiao Dam failure, result of Typhoon of Typhoon Nina. Nina. Approximately 86,000 people died from flooding and another 145,000 died during subsequent disease.
5)145,000
Yangtze river flood
6)More than 100,000 7)100,000 8)100,000
Heavy rainfall Huge snow melting Failures of dams, barrages etc., (koshi) Landslides causing blockage of river
Riverine floods Slow kinds: Runoff from sustained rainfall or rapid snow melt exceeding exceeding the capacity of a river's channel. Causes include heavy rains from monsoons , Unexpected drainage obstructions such as landslides,, ice landslides ice,, or debris can cause slow flooding upstream of the obstruction. Fast kinds: include Flash floods which are much more dangerous dangerous and flow much faster than regular floods. Result from tropical storms, dam failures or excessive rain and snow. Estuarine floods Commonly caused by a combination of sea tidal surges caused by storm-force winds. •
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Coastal floods Caused by severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane). Catastrophic floods Caused by a significant and unexpected event e.g. dam breakage, or as a result of another hazard (e.g. earthquake or volcanic eruption). Muddy floods A muddy flood is generated by run off on crop land. •
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EFFECTS •
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The damage due to flood may vary with respect to the magnitude of the flood. Thus we can classify the effects as:
Primary effects Physical damage - Can range anywhere from bridges,cars, buildings, sewer systems, roadways, canals and any other type of structure. Casualties - People and livestock die due to drowning. It can also lead to epidemics and diseases. Secondary effects Water supplies - Contamination Contaminatio n of water. water. Clean drinking d rinking water becomes scarce. Diseases - Unhygienic conditions. Spread of water-borne diseases •
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Crops and food supplies - Shortage of food crops can be caused due to loss of entire harvest. Trees - Non-tolerant species can die from suffocation Tertiary/ ertiary/long-term long-term effects Economic -, rebuilding costs, food shortage leading to price increase, temporary decline in tourism etc. •
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Greater losses due to flood Need for control, Measures to be taken No complete control of flood to zero level So Flood Management rather than Flood Control Classification of control measures ›
Structural measures and 2. Non-structural measures
Storage Reservoir Detentio Reservoir Levees Floodways Channel Improveme Improvement nt Watershed Management Flood Plain zoning Flood forecasting and Warning Evacuation and relocation
STORAGE RESERVOIR RE SERVOIR:: Most reliable and effective flood control method Storage reservoir to absorb incoming flood Release in controlled way so that downstream channels do not get flooded. Several reservoirs to be placed in a river for complete flood control. Graph Kheichera rakhnu hai………
DETENTION RESER RESERVIORS VIORS -Consist of an obstruction to a river with uncontrolled outlet. -For small structures and temporary storage LEVEES -Also called dikes or flood embankments -Earthen structures parallel to the course of river.
Height higher than the design flood level. One of the oldest and common methods of flood protection Considerable care and maintenance required since earthen
FLOODWAYS -Channels into which part of the flood will be diverted during high stages. -Natural or man-made CHANNEL IMPROVEMENT -Widening or deepening of channel -Reduction of channel roughness (clearance of vegetation)
WATERSHED MANAGEMENT -Aims at cutting down and delaying the runoff before it gets into the river. -Check dams, contour bonding, terraces etc. -Improve soil infiltration capacity hence reduce soil erosion
FLOOD PLAIN ZONING Identifies the flood prone areas of a river and regulates the land use to restrict the damage due to flood. Development plans are prepared FLOOD FORECASTING AND WARNING WARNING Enables civil authority to take appropriate control measures. Orbiting satellites—Pictures—Cloud growth —rainfall prediction—flood prediction
-historical records of flooding with study on the parameters: season, river channel topography, soil conditions, rainfall intensity and weather. Study of FLOOD HYDROGRAPH
Hydrograph: Study of the hydrological characteristics of a catchment. Responses of a given catchment to a rainfall input. USES Use in the design of hydraulic structures Developmentt of flood forecasting and warming systems Developmen based on rainfall Extension of flood flow records based on rainfall records (components of hydrograph: \ 1.Rising limb 2. Crest segment 3. Recession limb) Factors affecti affecting ng flood hydrograp hydrograph: h: Physiographic and climatic factors Limitations Precipitation must be from rainfall only Non uniform ppt—doesn’t give good results (unit hydrograph)
EVACUATION AND RELOCATION Evacuation of communities along with their live stocks and other valuables. Tempora T emporarily—No rily—Nonstructu nstructural ral measure, Permanent resettlement—Struct resettlement—Structural ural measure Decrease loss burden
-Flood is on of deadliest natural disaster. -Preventive measures to be taken in time. -Better engineering structures to prevent flood control -Stringent government steps. -Awareness on flood disaster is required.
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