Flooding occurs when the water level in a stream, river, bay, or lake rises above the level of the surrounding dry terrain.
Floods date back millions of years, and most of the historic floods occurred because glacial-age ice dams failed. These floods have been studied using archaeological records. Other large floods that occurred once there was documentation of floods happened because of rainfall, landslides, snowmelt, and volcanic eruptions. Debris from volcanic eruptions and landslides can block natural drainage systems, causing debris dams to form and then breach, sending out massive rushes of water.
Most of the largest floods in the past 100,000 years occurred because natural dams failed. Ice-jam floods have accounted for three of the world's largest 27 recorded flows. They occur when dislodged ice in rivers accumulates in limited areas or at bends in the river, causing water to pond upstream from the ice jam. When the ice jam breaks, the water flows rapidly out.
Spring and winter floods occur with some frequency primarily in the mid-latitude regions of the earth and particularly where continental climate is the norm. Five climatic features contribute to the spring and winter flooding potential of any year or region: 1) heavy winter snow cover; 2) saturated soils or soils at least near their field capacity for storing water; 3) rapid melting of the winter's snow pack; 4) frozen soil conditions that limit infiltration; and 5) somewhat heavy rains, usually from large-scale cyclonic storms. Any combination of three of these five climatic features usually leads to some type of flooding. This type of flooding can cause hundreds of millions of dollars in property damage, but it can usually be predicted well in advance, allowing for evacuation and other protective action to be taken, sandbagging, for instance.
On a smaller scale, flash floods occur in remote areas and in cities because of extremely heavy precipitation occurring over a short period of time. The water can flood streams, creeks, arroyos, and low-lying areas in a matter of a few hours. People often have warning but might not be aware of weather conditions upstream. Flash flooding is generally caused by violent weather, such as severe thunderstorms and hurricanes. This type of flooding more frequently occurs during the warm season when convective thunderstorms develop more frequently. Rainfall intensity is so great that the carrying capacity of streams and channels is rapidly exceeded, usually within hours, resulting in sometimes life-threatening flooding.
Not many areas are at high risk for flooding, but some are at more risk than others. Rainfall is not the only risk factor for flooding. People living in regions with low precipitation can experience flash floods when a sudden, heavy rainfall occurs. Communities along ocean shorelines, river banks and deltas, along with some lakes, usually are at more risk than those located inland or on higher ground. Geologists and county planners can let homeowners know if they are located in a flood plain.
In some highly developed urban areas, the risk of flash flooding has increased over time as the native vegetation and soils have been replaced by buildings and pavement, which produce much higher amounts of surface runoff. In addition, the increased use of parks and recreational facilities that lie along stream and river channels has exposed the public to greater risk.
Although the causes of floods are rooted in natural weather phenomena or even natural disasters, humans can contribute to flooding. Flooding is a complex process that occurs over time, however, and it is difficult to pinpoint a single cause for many floods. For example, when a dam or levee fails to hold rushing water, the engineers who designed the system might be blamed. But if the flood occurs as a result of several strange weather phenomena coming together or unexpectedly high rainfall for an area, it is more difficult to say whether humans or human error contributed. The storm surge that accompanied Hurricane Katrina in 2005 was extremely unusual, but New Orleans' system of walls and levees had been built in pieces over a 40-year period, with no real planning for an event close to the magnitude of Katrina. In this case and many others, the flooding was caused by extreme weather and likely worsened by humans. Experts predict that if global warming continues, sea levels will keep rising and risk of flooding in low-lying coastal areas of the United States and elsewhere in the world will increase. As of summer 2017, at least 90 coastal communities in the United States already were battling ongoing problems with flooding.
Nearly every year severe flooding occurs around the world. In 2017, severe floods hit China twice, killing more than 140 people. Flooding in Peru the same year was attributed to warming Pacific waters. The flooding killed more than 150 people and cost more than $9 billion in reconstruction. More than 600 people died in Sierra Leone when flooding sparked a huge mudslide. Developing countries can suffer even more severe damage from flooding because they lack infrastructure for storm draining and buildings.
Hurricane Maria pummeled Puerto Rico in September 2017 and resulted in deaths, displacements, and problems with water and power. Even in the continental United States, where advanced systems are in place to help lessen the impact of floods, the destructive effects of moving water can cause major damage. The National Weather Service estimates that on average, 126 people in the United States die each year from flooding.
Flooding water might seem harmless, but it can cause plenty of health problems. There is the initial danger of injuries and deaths caused by people being pulled away in rapidly rushing water in tsunamis, dam breaches, or flash floods. Further, if individuals are wounded during a flood or already have an open wound or rash, the floodwater can cause the wound to become infected. Other health problems occur in more slowly rising waters or following floods. Flooding can contaminate drinking water supplies, causing numerous gastrointestinal problems such as diarrhea.
Malaria is a common health problem in some parts of the world following floods, because mosquitoes breed in the standing water and can spread the disease. People also can be more vulnerable to infectious diseases following a flood. Mold can affect people's health after flooding if they continue to live inside a moldy home for a period of time. People also can be harmed by downed electrical power lines immediately after a flood, or they can be bitten by an animal, insect, or reptile displaced by the floodwaters. People also might become ill with carbon monoxide poisoning from running generators in or under their homes.
The problem with floodwater is that it rushes or rises over areas and picks up sediment, sewage, or debris not typically contained in the water source. The floodwater can quickly become contaminated and, because it is not controllable, can enter or damage wells, pumps, and other drinking water systems. People usually are cautioned to obtain bottled water or boil their water following floods until the safety of the water supply is determined and can return to normal. Floodwater even can pick up chemicals and solvents as it passes through areas where chemicals normally would be contained. Food contaminated by floodwater also should be avoided.
Caring for people who are displaced presents additional public health challenges. It is more difficult to help people maintain personal hygiene when water and sanitation systems are scarce. Some infectious diseases spread more easily because people are displaced and crowded into shelters. Diseases might be more likely to break out, but only if they naturally occur in the area affected by the flood.
Public health professionals examine access to public or private drinking water and the potential waterborne infectious diseases that might occur, along with helping educate the public about purifying water or disinfecting wells. Other areas of public health priorities are food safety, sanitation and personal hygiene, communicable diseases and vaccinations, mosquito control, and monitoring of other potential hazards posed by such as chemicals, animals, and rapidly flowing water. Monitoring actual diseases also helps prevent rumors and fear that are unwarranted and can cause people to panic.
In some situations flood control measures such as stream or channel diversions, dams, and levees can greatly reduce the risk of flooding. This is more often done in floodplain areas with histories of damaging floods. In addition, land use regulations, encroachment statutes and building codes are often intended to protect the public from the risk of flooding. Many government weather services provide the public with flash flood watches and warnings to prevent loss of life. Many flash floods occur as the result of afternoon and evening thundershowers that produce rainfall intensities ranging from a few tenths of an inch per hour to several inches per hour.
In the case of Hurricane Katrina, more than $14 billion was spent on civil systems designed to stop a surge like the one that caused such massive flooding in 2005. Massive gates that can be lowered, flood walls, pumps, and a 133-mile chain of levees subsequently were built to help protect New Orleans against another hurricane and storm surge of Katrina's magnitude.
Early disease surveillance has proven to help in floods and other natural disaster emergencies. Careful tracking and surveillance enables an early public health response, before a disease spreads. Mass vaccinations to prevent infectious disease spread is another recommended response in areas of the world more susceptible to infectious diseases such as measles or cholera.
See also Waterborne illness.
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Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC, 20004, https://www.epa.gov .
Mark W. Seeley
Revised by Teresa G. Odle