Flood
Water overflows its usual boundaries to submerge land that remains dry under normal conditions. This overflow can involve water or rarely other fluids moving across terrain. Natural types include river flooding, groundwater flooding, coastal flooding, and urban flooding sometimes called flash flooding. Tidal flooding may combine elements of both river and coastal processes in estuary areas. Flooding occurs when water bodies like rivers, lakes, seas, or oceans overtop or break levees. Water escapes its usual channels to flood property or drown domestic animals. Areal flooding happens on flat or low-lying areas where rainfall or snowmelt arrives faster than it can infiltrate the ground. Surface soil becomes saturated which stops infiltration if the water table is shallow. Infiltration slows significantly through frozen ground, rock, concrete, paving, or roofs. Endorheic basins experience areal flooding during periods when precipitation exceeds evaporation rates. River flooding occurs in all channel types from ephemeral streams to world's largest rivers. When overland flow hits tilled fields, sediments pick up as suspended matter or bed load. Slow-rising floods commonly happen in large rivers with large catchment areas. The increase in flow results from sustained rainfall, rapid snow melt, monsoons, or tropical cyclones. Large rivers may have rapid flooding events in arid climates due to intense rainfall in small basin areas. In extremely flat areas like the Red River Valley of the North, hybrid river/areal flooding occurs locally known as overland flooding. This valley was a former glacial lakebed created by Lake Agassiz. Over a length of 400 miles, the river course drops only 1 foot for an average slope of about 5 inches per mile. Spring snowmelt happens at different rates in different places within this vast area. Fast snowmelt pushes water out of tributary banks so it moves overland downstream. Rapid flooding events including flash floods occur on smaller rivers or steep valleys. Flash floods result from localized convective precipitation or sudden release from upstream impoundments behind dams. One instance saw a flash flood kill eight people enjoying water on a Sunday afternoon at a popular waterfall. Without any observed rainfall, the flow rate increased from about 2 cubic meters per second to 300 cubic meters per second in just one minute. Two larger floods occurred at the same site within a week but no one was present during those days. Flash floods are the most common type in normally-dry channels in arid zones called arroyos in the southwest United States.
Human changes to the environment often increase the intensity and frequency of flooding globally. Examples include land use changes such as deforestation and removal of wetlands that naturally store large amounts of water. Changes in waterway courses or flood controls like levees alter natural drainage patterns. Global environmental issues influence causes of floods namely climate change which intensifies the water cycle. Sea level rise contributes to more intense floods and increased flood risk for coastal populations. Climate change makes extreme weather events more frequent and stronger leading to intensified flooding. Flooding can be exacerbated by increased amounts of impervious surface from building paved roads and parking lots. Other natural hazards like wildfires reduce vegetation supply that absorbs rainfall effectively. About 30 percent of all precipitation becomes runoff according to MSN Encarta data from 2008. This amount might increase further with water from melting snowpacks. Upslope factors determine flood magnitude including land area of watersheds upstream of interest areas. Rainfall intensity is the second most important factor for smaller watersheds under approximately 10 square kilometers. Channel slope becomes the third most important factor for larger watersheds over that size. Time of concentration defines critical duration of peak rainfall for specific areas of interest. The fraction of incident precipitation reaching a drainage channel ranges from nil for light rain on dry ground to as high as 170 percent for warm rain on accumulated snow. Convective precipitation events produce shorter duration storms than orographic precipitation. Duration, intensity, and frequency of rainfall events are crucial for accurate flood prediction. Short duration precipitation matters significantly within small drainage basins where rapid response is needed. Downslope factors encounter conditions slowing movement ultimately before reaching oceans or coastal bars. Elevation changes such as tidal fluctuations determine coastal and estuarine flooding in low lands. Less predictable events like tsunamis and storm surges cause elevation changes in large bodies of water. Effective flood channel geometry changes through vegetation growth, ice accumulation, debris buildup, or construction projects. Bridges, buildings, and levees within channels restrict flow and control water elevation above restrictions. Climate change intensifies these dynamics by raising sea levels and increasing extreme weather event frequency.
Floods have caused devastating loss of life throughout recorded history with death tolls exceeding hundreds of thousands. The deadliest floods worldwide include events with death tolls at or above 100,000 individuals. The 1931 China floods killed between 2,500,000 and 3,700,000 people making them the worst natural disaster in history. The 1887 Yellow River flood claimed between 900,000 and 2,000,000 lives across China. The 1938 Yellow River flood resulted in 500,000 to 700,000 deaths during that year. The Banqiao Dam failure resulting from Typhoon Nina in 1975 caused approximately 86,000 flooding deaths plus another 145,000 disease-related fatalities. The 2004 Indian Ocean tsunami killed around 230,000 people in Indonesia alone. The 1935 Yangtze river flood took between 100,000 and 145,000 lives in China. St. Felix's storm surge in the Netherlands in 1530 killed over 100,000 people. Hanoi and Red River Delta flood in North Vietnam in 1971 caused more than 100,000 deaths. The 1911 Yangtze river flood also exceeded 100,000 fatalities in China. Fatalities connected directly to floods usually result from drowning since flood waters are very deep with strong currents. Deaths occur not just from drowning but from dehydration, heat stroke, heart attacks, or illness requiring medical supplies unable to be delivered. Injuries lead to excessive morbidity when floods strike affecting rescue teams and supply deliverers too. During floods accidents happen with falling debris or fast-moving objects in water. After floods large numbers of injuries occur during rescue attempts. Communicable diseases increase due to pathogens and bacteria transported by floodwater including cholera, hepatitis A, hepatitis E, and diarrheal diseases. Gastrointestinal disease and diarrheal diseases become common due to lack of clean water during flooding events. Clean water supplies often get contaminated when flooding occurs raising risks of typhoid, giardia, cryptosporidium, and many other location-specific diseases.
Floods cause billions of dollars' worth of damage annually threatening livelihoods worldwide. Every year flooding causes countries billions of dollars in economic destruction that threatens individual survival. In Bangladesh in 2007 a single flood destroyed more than one million houses across the region. Yearly in the United States floods cause over $7 billion in property and infrastructure damage. Floodwaters typically inundate farmland making soil unworkable preventing crops from being planted or harvested. Entire harvests for entire countries can be lost in extreme flood circumstances destroying food security. Some tree species do not survive prolonged flooding of their root systems leading to forest loss. Wet basements lower property values by 10, 25 percent according to industry experts in the United States. Almost 40 percent of small businesses never reopen their doors following a flooding disaster according to FEMA data. Economic hardship results from temporary tourism declines rebuilding costs or food shortages driving price increases. Psychological damage affects those experiencing deaths serious injuries and property losses during disasters. Fatalities connected directly to floods usually result from drowning since flood waters are very deep with strong currents. Deaths occur not just from drowning but from dehydration heat stroke heart attacks or illness requiring medical supplies unable to be delivered. Injuries lead to excessive morbidity when floods strike affecting rescue teams and supply deliverers too. During floods accidents happen with falling debris or fast-moving objects in water. After floods large numbers of injuries occur during rescue attempts. Communicable diseases increase due to pathogens and bacteria transported by floodwater including cholera hepatitis A hepatitis E and diarrheal diseases. Gastrointestinal disease and diarrheal diseases become common due to lack of clean water during flooding events. Clean water supplies often get contaminated when flooding occurs raising risks of typhoid giardia cryptosporidium and many other location-specific diseases. Floods frequently damage power transmission sometimes generation causing knock-on effects from lost electricity. Loss includes drinking water treatment facilities sewage disposal systems leading to severe contamination risks. Damage to roads and transport infrastructure makes it difficult to mobilize aid or provide emergency health treatment. Chronically wet houses lead to indoor mold growth resulting in adverse respiratory symptoms for residents. Research suggests 30, 50% increase in adverse respiratory health outcomes caused by dampness and mold exposure. Vector-borne diseases like malaria dengue West Nile and yellow fever increase due to residual still water after floods settle.
Waterways prone to floods are carefully managed through defenses such as detention basins levees bunds reservoirs and weirs. These structures prevent waterways from overflowing their banks into surrounding communities. When these defenses fail emergency measures include sandbags or portable inflatable tubes used to stem flooding temporarily. Coastal flooding has been addressed in portions of Europe and the Americas with sea walls beach nourishment and barrier islands. In riparian zones near rivers erosion control measures slow down or reverse natural forces causing meandering over long periods. Dams can be built and maintained over time to reduce occurrence and severity of floods globally. The U.S. Army Corps of Engineers maintains a network of flood control dams across American territories. Urban flooding solutions involve repairing and expanding human-made sewer systems and stormwater infrastructure nationwide. Another strategy reduces impervious surfaces in streets parking lots and buildings through natural drainage channels porous paving and wetlands collectively known as green infrastructure. Sustainable urban drainage systems allow areas identified as flood-prone to convert into parks and playgrounds tolerating occasional flooding. Ordinances require developers to retain stormwater on site and elevate buildings protected by floodwalls and levees. Property owners invest in re-landscaping flow away from buildings installing rain barrels sump pumps and check valves. Critical community-safety facilities like hospitals emergency-operation centers police fire and rescue services should build in least-risk areas. Structures such as bridges unavoidably located in flood hazard areas must withstand flooding through engineering design. Areas most at risk could be put to valuable uses abandoned temporarily when people retreat to safer zones during imminent floods. Planning involves observation of previous and present flood heights inundated areas statistical hydrologic hydraulic model analyses mapping future scenarios. Long-term land use planning regulation engineering design construction intermediate monitoring forecasting short-term warning response operations form comprehensive strategies. Attempts to understand mechanisms at work in floodplains have been made for at least six millennia according to historical records. The Association of State Floodplain Managers promotes education policies activities mitigating current and future losses costs human suffering without adverse impacts. A portfolio of best practice examples for disaster mitigation exists available from Federal Emergency Management Agency in the United States.
Floods bring many benefits including recharging groundwater making soil more fertile increasing nutrients in some soils. Periodic flooding was essential to well-being of ancient peoples along Tigris-Euphrates Rivers Nile River Indus River Ganges Yellow River among others. Flood waters provide much needed water resources in arid semi-arid regions where precipitation unevenly distributed throughout year kills pests farming land. Freshwater floods maintain ecosystems in river corridors key factor maintaining floodplain biodiversity globally. Flooding spreads nutrients to lakes rivers leading increased biomass improved fisheries for few years following events. For fish species an inundated floodplain forms highly suitable spawning location with few predators enhanced nutrient food levels. Weather fish use floods to reach new habitats expanding their range across landscapes. Bird populations profit from boost in food production caused by flooding events occurring regularly. Viability of hydropower renewable energy source higher in flood-prone regions providing sustainable electricity generation. Flood pulses support diverse aquatic life cycles enabling reproduction migration feeding patterns critical ecosystem functions. Sediment deposition enriches agricultural lands naturally fertilizing crops without chemical inputs required by modern farming methods. Natural floodplains absorb excess water reducing downstream damage while creating wetland habitats supporting amphibians reptiles birds mammals. These ecological benefits demonstrate how periodic flooding sustains biodiversity and supports human civilization development historically.
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Common questions
What is the definition of flooding?
Floods occur when water overflows its usual boundaries to submerge land that remains dry under normal conditions. This process involves water or rarely other fluids moving across terrain and escaping channels to flood property.
Which floods caused the highest death tolls in recorded history?
The 1931 China floods killed between 2,500,000 and 3,700,000 people making them the worst natural disaster in history. The 1887 Yellow River flood claimed between 900,000 and 2,000,000 lives while the 1938 Yellow River flood resulted in 500,000 to 700,000 deaths during that year.
How does climate change affect flood intensity and frequency globally?
Climate change intensifies the water cycle leading to more frequent and stronger extreme weather events that cause intensified flooding. Sea level rise contributes to more intense floods and increased flood risk for coastal populations while global environmental issues influence causes of floods.
What are the economic impacts of flooding on communities and businesses?
Every year flooding causes countries billions of dollars in economic destruction that threatens individual survival. Almost 40 percent of small businesses never reopen their doors following a flooding disaster according to FEMA data and yearly in the United States floods cause over $7 billion in property and infrastructure damage.
How do humans manage waterways prone to floods through defenses and engineering?
Waterways prone to floods are carefully managed through defenses such as detention basins levees bunds reservoirs and weirs. These structures prevent waterways from overflowing their banks into surrounding communities and dams can be built and maintained over time to reduce occurrence and severity of floods globally.