A flood is an overflow of water that submerges land. The European Union (EU) Floods Directive defines a flood as a covering by water of land not normally covered by water. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows or breaks levees, with the result that some of the water escapes its usual boundaries, or may be due to accumulation of rainwater on saturated ground in an areal flood.

While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless such escapes of water endanger land areas used by man like a village, city or other inhabited area.

Floods can also occur in rivers, when flow exceeds the capacity of the river channel, particularly at bends or meanders. Floods often cause damage to homes and businesses if they are placed in natural flood plains of rivers. While flood damage can be virtually eliminated by moving away from rivers and other bodies of water, since time out of mind, people have lived and worked by the water to seek sustenance and capitalize on the gains of cheap and easy travel and commerce by being near water. That humans continue to inhabit areas threatened by flood damage is evidence that the perceived value of living near the water exceeds the cost of repeated periodic flooding.

The word "flood" comes from the Old English flod, a word common to Germanic languages (compare German Flut, Dutch vloed from the same root as is seen in flow, float; also compare with Latin fluctus, flumen). Deluge myths are mythical stories of a great flood sent by a deity or deities to destroy civilization as an act of divine retribution, and are featured in the mythology of many

Principal types and causes

Areal

Floods often happen over flat or low-lying areas when the ground is saturated and water either cannot run off, or cannot run off quickly enough to stop accumulating. This may be later followed by a river flood as water moves away from the areal floodplain into local rivers and streams.

Floods can occur if water accumulates across an impermeable surface (e.g. from rainfall) and cannot rapidly dissipate (i.e. gentle orientation or low evaporation).

A series of storms moving over the same area can cause areal flash flooding. A muddy flood is produced by an accumulation of runoff generated on cropland.

Sediments are then detached by runoff and carried as suspended matter or bed load. Muddy runoff is more likely detected when it reaches inhabited areas. Muddy floods are therefore a hill slope process, and confusion with mudflows produced by mass movements should be avoided.

Riverine

Slow kinds: Runoff from sustained rainfall or rapid snow melt exceeding the capacity of a river's channel. Causes include heavy rains from monsoons, hurricanes and tropical depressions, foreign winds and warm rain affecting snow pack. Unexpected drainage obstructions such as landslides, ice, or debris can cause slow flooding upstream of the obstruction.

Fast kinds: include river flash floods resulting from convective precipitation (intense thunderstorms) or sudden release from an upstream impoundment created behind a dam, landslide, or glacier.

Dam -building beavers can flood low-lying urban and rural areas, often causing significant damage.

Estuarine

Commonly caused by a combination of sea tidal surges caused by storm-force winds and high river stages due to heavy rain.

Coastal

Caused by severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane). A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

Catastrophic

Caused by a significant and unexpected event e.g. dam breakage, or as a result of another hazard (e.g. earthquake or volcanic eruption)

Human-induced

Accidental damage by workmen to tunnels or pipes.

Effects

Primary effects

Physical damage – damage to structures, including bridges, buildings, sewerage systems, roadways, and canals.

Secondary effects

Water supplies – Contamination of water. Clean drinking water will become scarce. Diseases – Unhygienic conditions. Spread of water-borne diseases. Crops and food supplies – Shortage of food crops can be caused due to loss of entire

harvest. However, lowlands near rivers depend upon river silt deposited by floods in order to add nutrients to the local soil.

Trees – Non-tolerant species can die from suffocation. Transport – Transport links destroyed, so hard to get emergency aid to those who need it.

Tertiary and long-term effects

Economic – economic hardship due to temporary decline in tourism, rebuilding costs, food shortage leading to price increase, etc.

Psychological – flooding can be highly traumatic for individuals, in particular where deaths, serious injuries and loss of property occurs.

In many countries across the world, rivers prone to floods are often carefully managed. Defenses such as levees,[6] bunds , reservoirs, and weirs are used to prevent rivers from bursting their banks. When these defenses fail, emergency measures such as sandbags or portable inflatable tubes are used. Coastal flooding has been addressed in Europe and the Americas with coastal

defences, such as sea walls, beach nourishment, and barrier islands.

Clean-up safety

Clean-up activities following floods often pose hazards to workers and volunteers involved in the effort. Potential dangers include: water polluted by mixing with and causing overflows from sanitary sewers, electrical hazards, carbon monoxide exposure, musculoskeletal hazards, heat or cold stress, motor vehicle-related dangers, fire, drowning, and exposure to hazardous materials.[12] Because flooded disaster sites are unstable, clean-up workers might encounter sharp jagged debris, biological hazards in the flood water, exposed electrical lines, blood or other body fluids, and animal and human remains. In planning for and reacting to flood disasters, managers provide workers with hard hats, goggles, heavy work gloves, life jackets, and watertight boots with steel toes and insoles. Benefits

There are many disruptive effects of flooding on human settlements and economic activities. However, floods (in particular the more frequent/smaller floods) can also bring many benefits, such as recharging ground water, making soil more fertile and providing nutrients in which it is deficient. Flood waters provide

much needed water resources in particular in arid and semi-arid regions where precipitation events can be very unevenly distributed throughout the year. Freshwater floods, particularly play an important role in maintaining ecosystems in river corridors and are a key factor in maintaining floodplain biodiversity.[14] Flooding adds a lot of nutrients to lakes and rivers which leads to improved fisheries for a few years, also because of the suitability of a floodplain for spawning (little predation and a lot of nutrients).[15] Fish like the weather fish make use of floods to reach new habitats. Together with fish also birds profit from the boost in production caused by flooding.[16]

Periodic flooding was essential to the well-being of ancient communities along the Tigris-Euphrates Rivers, the Nile River, the Indus River, the Ganges and the Yellow River, among others. The viability for hydrological based renewable sources of energy is higher in flood prone regions.

How To Plan for a Flood

Develop a Family Disaster Plan. Please see the "Family Disaster Plan" section for general family planning information. Develop flood- specific planning. Learn about your area's flood risk and elevation above flood stage.

Contact your local Red Cross chapter, emergency management office, local National Weather Service office, or planning and zoning department about your area's flood risk.

Knowing the elevation of your property in relation to nearby streams and dams will let you know if forecasted flood levels will affect your home.

If you are at risk from floods:

Talk to your insurance agent. Homeowners' policies do not cover flooding. Ask about the National Flood Insurance Program (NFIP).

Use a NOAA Weather Radio with a tone-alert feature, or a portable, battery-powered radio (or television) for updated emergency information.

Develop an evacuation plan. (See "Evacuation" in the "Family Disaster Plan" section.) Everyone in your family should know where to go if they have to leave. Trying to make plans at the last minute can be upsetting and create confusion.

Discuss floods with your family. Everyone should know what to do in case all family members are not together. Discussing floods ahead of time helps reduce fear and anxiety and lets everyone know how to respond.

What to Tell Children

If you come upon flood waters, stop, turn around, and go another way. Climb to higher ground. If it is moving swiftly, even water six inches deep can knock you off your feet. Many people are swept away wading through flood waters, resulting in injury or death.

Stay away from flooded areas. Even if it seems safe, flood waters may still be rising. Never try to walk, swim, drive, or play in flood water. You may not be able to see on the

surface how fast flood water is moving or see holes and submerged debris. If you are in a vehicle and become surrounded by water, if you can get out safely, do so

immediately and move to higher ground. Vehicles can be swept away in two feet of water. Watch out for snakes in areas that were flooded. Flood waters flush snakes from their homes. Stay away from creek and stream banks in flooded and recently flooded areas. The soaked

banks often become unstable due to heavy rainfall and can suddenly give way, tossing you into rapidly moving water.

Never play around high water, storm drains, ditches, ravines, or culverts. It is very easy to be swept away by fast moving water.

Throw away all food that has come into contact with flood waters. Contaminated flood water contains bacteria and germs. Eating foods exposed to flood waters can make you very sick.

How to Protect Your Property

Keep insurance policies, documents, and other valuables in a safe-deposit box. You may need quick, easy access to these documents. Keep them in a safe place less likely to be damaged during a flood.

Avoid building in a floodplain unless you elevate and reinforce your home. Some communities do not permit building in known floodplains. If there are no restrictions, and you are building in a floodplain, take precautions, making it less likely your home will be damaged during a flood.

Raise your furnace, water heater, and electric panel to higher floors or the attic if they are in areas of your home that may be flooded. Raising this equipment will prevent damage. An undamaged water heater may be your best source of fresh water after a flood.

Install check valves in building sewer traps to prevent flood water from backing up into the drains of your home. As a last resort, when floods threaten, use large corks or stoppers to plug showers, tubs, or basins.

Construct barriers such as levees, berms, and flood walls to stop flood water from entering the building. Permission to construct such barriers may be required by local building codes. Check local building codes and ordinances for safety requirements.

Seal walls in basements with waterproofing compounds to avoid seepage through cracks.

Consult with a construction professional for further information if these and other damage reduction measures can be taken. Check local building codes and ordinances for safety requirements.

Contact your local emergency management office for more information on mitigation options to further reduce potential flood damage. Your local emergency management office may be able to provide additional resources and information on ways to reduce potential damage.

Computer modelling

While flood modelling is a fairly recent practice, attempts to understand and manage the mechanisms at work in floodplains have been made for at least six millennia.[17] The recent development in computational flood modelling has enabled engineers to step away from the tried and tested "hold or break" approach and its tendency to promote overly engineered structures. Various computational flood models have been developed in recent years; either 1D models (flood levels measured in the channel) and 2D models (variable flood depth measured across the extent of the floodplain). HEC-RAS,[18] the Hydraulic Engineering Centre model, is currently among the most popular if only because it is available free of charge. Other models such as

TUFLOW combine 1D and 2D components to derive flood depth across the river channel and floodplain. To date the focus has primarily been on mapping tidal and fluvial flood events, but the 2007 flood events in the UK have shifted the emphasis there onto the impact of surface water flooding.

Deadliest floods

Below is a list of the deadliest floods worldwide, showing events with death tolls at or above 100,000 individuals.

Death toll Event Location Date

2,500,000–3,700,000[21] 1931 China floods China 1931

900,000–2,000,000 1887 Yellow River (Huang He) flood China 1887

500,000–700,000 1938 Yellow River (Huang He) flood China 1938

231,000

Banqiao Dam failure, result of Typhoon Nina. Approximately 86,000 people died from flooding and another 145,000 died during subsequent disease.

China 1975

230,000 Indian Ocean tsunami Indonesia 2004

145,000 1935 Yangtze river flood China 1935

100,000+ St. Felix's Flood, storm surge Netherlands 1530

100,000 Hanoi and Red River Delta flood North Vietnam 1971

100,000 1911 Yangtze river flood China 1911