Floods and flooding and global warming

Floods can have both devastating and positive effects on natural resources and human infrastructure.

Background

Floods happen with any high flow of surface waters that overtop normal confining banks and cover land that is usually dry. Floods occur naturally along most river systems. Low-lying areas and areas downstream of dams are most at risk. Flooding causes loss of human and animal life; structural damage to bridges, buildings, roadbeds, and utilities; soil erosion; destruction of property; and destruction of livestock and crops that provide food for people. As a result, famines may follow floods, with large numbers of people dying from starvation. Floodwaters are typically contaminated with raw sewage, including both human and animal waste, and may contain dangerous levels of bacteria, leading to outbreaks of waterborne illness.

Floods also can have positive impacts. Floods recharge natural ecosystems; provide abundant freshwater for agriculture, health, and sanitation; and deposit nutrient-rich sediment on floodplains, enhancing crop yields. The importance of floods to aquatic ecosystems is demonstrated by the artificial flooding in the Grand Canyon of the Colorado River in the United States.

However, floods are the most devastating of all geological agents, surpassing earthquakes and volcanic eruptions in terms of loss of life and property damage. In developing countries, floods cause large numbers of deaths; in developed countries, floods typically cause more property damage than loss of life, often billions of dollars worth. Between 1995 and 2015, flood disasters killed approximately 157,000 people worldwide and affected another 2.3 billion, costing the world economy a total of $662 billion. Globally, the greatest potential for flooding exists in Asia, which saw an average of 82.8 hydrological events (primarily floods, some landslides and wave actions) each year from 2005 to 2014, creating an annual average of 79.15 million victims. As urbanization increases, particularly in flood-prone areas, the potential for flooding rises because of land-use changes (such as and the covering of once-permeable ground with concrete, asphalt, and buildings). change and sea-level rise also lead to increased flooding. Nearly one billion people, about one-eight of the world’s population, live in areas prone to flooding. Many of these people are among the world’s poorest inhabitants, depending on fertile floodplain soils and wetlands for agriculture and economic opportunity.

Floodplains

Most streams are naturally bordered by flat, low-lying areas known as floodplains. Floodplains have been carved into the landscape by stream erosion and are covered in fine-grained sand, silt, and clay deposited by floodwaters. Some streams have natural levees, moundlike deposits of sediment that border the stream channel. Natural levees form as floodwaters leave the channel and spread onto the floodplain. As rushing water leaves the channel, its velocity drops, and coarser sediment is deposited adjacent to the stream. Man-made levees may be built along streams in an attempt to control flooding. However, if the water in a stream is allowed to spread over its natural floodplain, the impact of downstream flooding is lessened.

Types of Floods

Floods occur when a drainage basin or watershed receives so much water that stream and river channels cannot handle the flow. After a rain, some water infiltrates the soil, some evaporates or is used by plants, and the remainder (about 30 percent) becomes runoff, flowing across the ground surface.

Riverine floods occur when heavy rainfall or spring thaws (melting snow and ice) increase water levels in a drainage basin. Heavy rainfall may be the result of a hurricane, a tropical cyclone, a monsoonal rain, or a prolonged period of unusually wet weather, as in the case of the Great Mississippi River Flood of 1993 in the central United States, which impacted nine states along the Mississippi River and lasted more than four months.

In cold climate areas where rivers freeze in the winter, spring thaws bring ice jams and associated flooding. Rising water levels lift river ice, which breaks into large sheets that float downstream and pile up near narrow passages or against obstructions such as bridges. When the ice stops moving because of a jam, floodwaters rapidly spread over the riverbanks upstream from the jam and may cover vast areas of usually dry land, flooding roads and causing property damage. When the ice jam breaks, a sudden flood of water is released. Ice-jam flooding occurs in Canada, the northern United States, Europe, Russia, Kazakhstan, and China, among other countries.

Flash floods are associated with intense storms that release large amounts of rain into small drainage basins in a relatively short period of time. Flash floods occur with little or no warning and can reach peak levels within minutes, carrying a deadly cargo of rocks, trees, and other debris. Fifteen centimeters of swiftly moving water can sweep people off their feet, and cars can be swept away by 0.6 meter of water. A notable flash flood occurred July 31, 1976, along the Big Thompson River near Denver, Colorado, after an unusually heavy rainstorm. A wall of water 5.8 meters high roared down a canyon where people were camping. The flood killed 140 people and caused millions of dollars in property damage. Flash floods may even occur in dry streambeds on sunny days when small but heavy rainstorms occur upstream kilometers away.

Storm-surge floods (coastal floods) occur when onshore winds and hurricanes cause the sea level to rise over low-lying coastal areas. If storm surges happen during high tide, leading to a tidal surge, the devastation can be catastrophic. Sometimes during hurricanes coastal areas are affected simultaneously by storm surges and riverine floods. In May 2008, Cyclone Nargis struck Myanmar (Burma) with storm surge, flooding up to four meters in the densely populated Irrawaddy delta region. The death toll was estimated to be more than one hundred thousand.

Coastal flooding can also occur as a result of a tsunami or seismic sea wave following an earthquake. On December 26, 2004, a magnitude 9.3 earthquake off the coast of the Indonesian island of Sumatra produced a tsunami in the Indian Ocean that flooded coastal areas across Southeast Asia, Sri Lanka, India, and other nations bordering the Indian Ocean, including Australia and several African countries. The tsunami, which reached up to twenty-five meters high, killed nearly three hundred thousand people and left more than 1.5 million homeless. Billions of dollars worth of property damage occurred, and several islands were completely submerged.

Floods can also be caused by human interference with a drainage basin. The most obvious example is the bursting of dams or levees. Dam failures represent potentially the worst flooding event in terms of sudden, catastrophic loss of life and destruction of property. Dam failures are primarily caused by neglect, poor design, or structural damage caused by an earthquake or other event. The deadliest flood in US history was the result of a dam failure on the Little Conemaugh River in Johnstown, Pennsylvania, on May 31, 1889. A wall of water twelve meters high killed 2,200 people.

Notable Floods

Near the end of the last ice age, about thirteen thousand years ago, glacial-related ice-jam flooding in the northwestern United States formed prehistoric Lake Missoula along the Clark Fork River in Montana. When the ice jam broke up, the water in the lake, which was about six hundred meters deep with a volume of about 2,500 cubic kilometers, was released catastrophically, flowing westward and both creating the Channeled Scablands and eroding immense channels across the Columbia Plateau.

The worst natural disasters in history, in terms of loss of life, have been floods along Chinese rivers. The Huang River (also known as the Yellow River) in China has killed more people than any other natural feature. According to Chinese history, from around 600 BCE to the early twentieth century, it flooded 1,593 times. The river’s English name derives from the ochre-yellow color of the silt carried by the river. Millions of metric tons of silt deposited on the riverbed choke the channel and displace the water, and, over time, the river level rises. To prevent flooding and to keep the river within its banks, the Chinese built levees or earthen embankments along the sides of the river. As the sediment accumulated in the river channel, the levees had to be built higher and higher. In places, the riverbed is higher than the surrounding countryside, with levees towering nine meters or more above the floodplain. In 1887, heavy rains over a period of months caused the river level to rise. The levees broke catastrophically, spilling floodwaters three meters deep over the surrounding countryside and covering an estimated 129,500 square kilometers. The flooding claimed between 900,000 and 6 million lives (estimates vary widely; the larger figure includes deaths from flood-induced famine). A flood on the same river in 1931 killed nearly 4 million people. The longest river in China, the Chang (also known as the Yangtze), has also flooded numerous times. In 1911, a flood on the Chang River claimed 100,000 lives. In 1931, the river crested at nearly thirty-one meters above its normal level and killed 145,000, but as many as 3,700,000 died as a result of starvation because the flooded area normally produced nearly one-half of China’s grain. Other, more recent floods on the Chang occurred in 1954, killing 30,000, and in 1998. In an effort to control flooding along the Chang, as well as to generate electricity, the Three Gorges Dam was completed in 2006.

Hurricane Katrina, which struck the southeastern United States in August 2005, caused flooding along the coast of the Gulf of Mexico from Florida to Texas. Federal disaster declarations covered an area of 233,000 square kilometers. Much of the damage was caused by the highest storm surge in US history (8.2 meters) as the hurricane approached the Mississippi coast. However, the most severe damage was in New Orleans, Louisiana, where the man-made levees and floodwalls along the Mississippi River failed in more than fifty places, flooding 80 percent of the city. Floodwaters covered the area for weeks, and at least 1,836 people were killed and 705 were missing. At the time, this was the costliest natural disaster in US history, with damage estimates near $100 billion.

In August 2017, Hurricane Harvey made landfall on the Gulf Coast of Texas, shortly after developing into a category 4 storm, and brought with it the largest-scale flood in US history. A total of thirty-three trillion gallons of precipitation fell on Texas, Louisiana, Tennessee, and Kentucky, the majority of it concentrated on the Houston area. An area of 3,643 square miles around Houston and the nearby city of Beaumont received more than forty inches of rain between August 23 and August 30, and another 11,492 square miles received thirty or more inches. The flood was calculated to be a thousand-year storm—the first in recorded US history—meaning that a flood of its size has a 0.1 percent chance of occurring in any given year. Estimates of the damage caused by Harvey ranged from $65 billion and $190 billion, potentially surpassing Katrina as the costliest natural disaster in US history.

The state of Louisiana was once again the target of natural disaster when Hurricane Ida made landfall in August 2021. The storm, which previously caused wide scale flooding in Venezuela, Cuba, and other countries on the Caribbean Sea, was the second-most intense hurricane to affect Louisiana since Katrina. The remnants of Ida, which traveled up the eastern seaboard of North America, led to catastrophic flooding in the northeastern United States and particularly affected the states of New York, New Jersey, and Pennsylvania. By the time the storm dissipated, it had caused an estimated $75 billion in damage. Other areas within the US continued to experience heavily flooding, including Yellowstone National Park and parts of Montana when heavy rainfall caused a record flooding of the Yellowstone River in June 2022. The aftermath of the flood and resulting mudslides, which washed away roads, bridges, and homes, was estimated to have caused over $1 billion in damage.

Human Influences on Flooding

Human activities along waterways can increase flooding inadvertently. Paving and building on floodplains and surrounding areas decrease infiltration of rainwater into the soil and, as a consequence, increase runoff. Runoff also increases when forests are cleared or when wetlands are destroyed by construction or infilling. Agriculture decreases the ability of soil to retain water and therefore increases runoff. Rapid runoff causes soil erosion. Sediment-clogged streams cannot support normal levels of aquatic life, and wildlife habitats are destroyed. Sediment in stream channels also leaves little room for water and leads to the likelihood of flooding.

Effects of Flooding

People are attracted to floodplains because floods deposit nutrient-rich topsoil, eroded from upstream, producing fertile land for agriculture. In Egypt, for example, floods and deposition of nutrient-rich sediment from the Nile River have increased agricultural yields for perhaps five thousand years. Floodplains tend to be flat, making them easy to cultivate, and near water, making them easy to irrigate. In addition, the nearby source of water is useful for transportation of agricultural products. Flooding is beneficial to streams as well: It serves to maintain both local and regional environmental balance, affecting water quality and aquatic life. Floods also recharge groundwater supplies.

Floods can be considered human-caused disasters in that people build on floodplains without properly taking into account the risk. Dangers of flooding include losses of both human and animal lives; structural damage to bridges, buildings, roadbeds, dams, and utilities; agricultural losses; severe soil erosion (sometimes even unearthing coffins in cemeteries and washing them downstream); and property destruction. Most flood deaths are attributable to drowning, and in the United States, more than one-half of them are associated with motor vehicles being driven into areas covered by water.

When water filtration facilities are inundated, floods spread waters polluted by industrial contaminants and human waste. Polluted floodwater can also contaminate wells and water supplies. Wild animals, including poisonous snakes, often come into homes with rising floodwater. Disease spread by waterborne pathogens and insects such as mosquitoes, in addition to famine due to crop damage and loss of food supplies, can cause great loss of life. Additional long-term problems include homelessness and losses to commerce, employment, and education.

Flood Control

Floods can be controlled in two ways: by controlling the waters or by controlling floodplain development. To minimize the effects of flooding, engineers build dams, levees, and floodwalls along rivers. Dams can store water during periods of heavy runoff and release it gradually during periods of low flow. Artificial levees and floodwalls are built along streams to confine floodwaters and to keep them from covering the floodplain. As more communities build levees, however, river levels rise because floodwaters cannot spread out. The river deposits its sediment in the channel instead of on the floodplain, raising the riverbed and displacing the water. Artificial levees must be heightened because of rising water levels over time.

Levees are by no means a foolproof solution to flood prevention. Floodwaters occasionally overflow levees, burst through them, or go around their upstream ends. Where levees or floodwalls are built on only one side of a river, towns on the other side experience higher flood levels than normal. Other methods of flood control include restoring vegetation, instituting soil conservation measures, constructing floodways to divert floodwaters, widening rivers to accommodate more water, and purposely flooding certain areas to prevent flooding in others.

Flood Frequency

Flood frequencies are described in statistical terms to estimate the chance of a particular flood level. For example, the term “one-hundred-year flood” means that a flood of a particular level will have a 1 percent chance of occurring within a given year. It does not mean that a flood of this level would happen only once in one hundred years. A one-hundred-year flood can occur any time. Similarly, a “ten-year flood” has a 10 percent chance of occurring in a given year. In some cases, the difference between a ten-year and a one-hundred-year flood is only a few centimeters.

In the twenty-first century, floods have become increasingly frequent and deadly. A report by the United Nations Office for Disaster Risk Reduction (UNISDR) and the Centre for Research on the Epidemiology of Disasters (CRED) found that in South America, for example, during the period 1995–2004, an average of 560,000 people were affected by floods each year; in 2005–14, the average annual number almost quadrupled, to 2.2 million. More people in the region—820,000—were affected by floods in the first eight months of 2015 than in a single year from 1995 to 2004.

Around the world, the average number of floods per year during 2005–14 was 34.6 percent greater than in the previous decade. This was attributed in part to socioeconomic reasons such as population growth and changes in land use, including increased building on floodplains, and in part to rising global temperatures that result in greater amounts of precipitation.

Floods and Global Climate Change

With global climate change and predictions about increases in temperature, the potential exists for the to intensify, leading to more extremes in climate. For every 1 degree Celsius in temperature rise, the capacity of the atmosphere to hold water increases by 7 percent. This creates the potential for more intense precipitation and, as a consequence, more intense flooding. In recent years, changes also have occurred in the timing of floods, including a decrease in the number of ice-jam floods in Europe. In 2024, the National Weather Service announced that there was a record-breaking 91 flash flood warnings issued that year. This dramatic increase has been attributed to global climate change. Scientists expect these numbers to steadily increase over the following decades.

During the twentieth century, sea level rose between ten and twenty centimeters. It is expected to rise another nine to eighty-eight centimeters by the end of the twenty-first century, suggesting that coastal flooding will become more widespread. This will have a significant impact on coastal inhabitants; more than 70 percent of the world’s population lives on coastal plains. Increasingly, islands are affected by sea-level rise. The Pacific island nation of Kiribati, which has already lost two islands to rising seas, is a prime example. In early 2005, several other islands in Kiribati were flooded by high spring tides that damaged buildings, contaminated wells with salt water, and eroded farmland. A sea-level rise of one meter would have a devastating effect on some of the world’s poorest countries, displacing tens of millions of people and flooding low-lying areas used for growing rice and other food crops.

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Space-Based Measurement of Surface Water for Research, Educational, and Humanitarian Applications.

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Flood!

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