What causes flash floods? Here's how they get so destructive so quickly
Flooding is one of the most common, and most destructive, natural disasters in the world. Triggered by heavy rains, storm surges, or rapid snowmelt, floods can strike with little to no warning. Climate change is fueling more intense storms, leading to heavier, faster rainstorms.
Recent flash floods along the Guadalupe River in Texas killed at least 82 people with many still missing, amid the start of a predicted above-normal hurricane season. From rural towns to urban centers, flooding is reshaping how we live, build, and prepare for natural disasters.
Here's what you need to know about floods, how they differ from flash floods, and how you can prepare to keep your family safe. Officials search for missing people along the banks of the Guadalupe River after recent flooding in Hunt, Texas. Photograph by Rodolfo Gonzalez, AP Photo Officials inspect the banks of the Guadalupe River after a flash flood swept through the area in Hunt, Texas. Photograph by Julio Cortez, AP Photo
A flood occurs when water overruns land that's normally dry, which can happen in many ways.
Excessive rain, a ruptured dam or levee, rapidly melting snow or ice, or even an unfortunately placed beaver dam can overwhelm a river, spreading over the adjacent land, called a floodplain. Coastal flooding occurs when a large storm or tsunami causes the sea to surge inland.
Most floods take hours or even days to develop, giving residents time to prepare or evacuate.
However, flash floods can be extremely dangerous, instantly turning a babbling brook or even a dry wash into rushing rapids that sweep everything in their path downstream. Flash floods can last from just a few minutes to six hours, dumping heavy rainfall over areas and washing out roads, homes, and bridges. Intense rainfall, sloping topography, and rocky soils can make flash floods more likely. The small gulf town Cameron in Louisiana suffered major damage and flooding during Hurricane Laura. Photograph by Kathleen Anderson, Nat Geo Image Collection
Cities are more prone to flash flooding because they have fewer ponds and basins to collect excess rainwater. Urban infrastructure contributes to stormwater accumulation because roads and sidewalks are often impervious, meaning they can't absorb water. The volume of water runoff from an acre of pavement is 10 to 20 times greater than the runoff from an acre of grass.
Dam failures can also lead to flash floods sending a sudden destructive surge of water downstream. In 2020, three days of heavy rain caused the Edenville Dam to collapse, flooding the city of Midland, Michigan, about 130 miles northwest of Detroit. While there were no fatalities or injuries reported, 150 homes were destroyed, and thousands of people were temporarily evacuated from the area.
Flash flooding is also common near mountainous streams and rivers and low-lying areas. The coastal village of Sriwulan in Indonesia is prone to tidal flooding, seen above in 2021. Flooding risks are higher for coastal communities as we see sea level rise due to climate change. Photograph by Aji Styawan, Nat Geo Image Collection A woman holds her great grandchild on a bed in her flooded home in the village of Timbulsloko, Indonesia. The Jakarta area of Indonesia is home to more than 30 million people and is regularly hit by floods. Photograph by Aji Styawan, Nat Geo Image Collection
Floods cause more than $40 billion in damage worldwide annually, according to the Organization for Economic Cooperation and Development. The U.S. losses average close to $8 billion a year, however, a 2022 study shows that number could rise to $41 billion by 2050. U.S. death tolls have increased in recent decades to more than 100 people a year. Some of the world's worst floods have killed millions of people in China's Yellow River Valley.
When floodwaters recede, affected areas are often blanketed in silt and mud. The water and landscape can be contaminated with hazardous materials such as sharp debris, pesticides, fuel, and untreated sewage. Potentially dangerous mold blooms can quickly overwhelm water-soaked structures.
Residents of flooded areas can be left without power and clean drinking water, leading to outbreaks of deadly waterborne diseases like typhoid, hepatitis A, and cholera. An aerial look at Timbulsloko, on the north coast of Java. The village was flooded by rising sea levels in 2020.
Photograph by Aji Styawan, Nat Geo Image Collection
Flooding, particularly in river floodplains, is as natural as rain and has been occurring for millions of years. Famously fertile floodplains such as the Mississippi Valley, the Nile River Valley in Egypt, and the Tigris-Euphrates in the Middle East have supported agriculture for millennia because annual flooding has left tons of nutrient-rich silt deposits behind. Humans have increased the risk of death and damage by increasingly building homes, businesses, and infrastructure in vulnerable floodplains.
(Here's how to stay safe during a flood.)
To try to mitigate the risk, many governments mandate that residents of flood-prone areas purchase flood insurance and set construction requirements aimed at making buildings more flood resistant—with varying degrees of success.
Massive efforts to mitigate and redirect inevitable floods have resulted in some of the most ambitious engineering efforts ever seen, including New Orleans's extensive levee system and massive dikes and dams in the Netherlands. Such efforts continue today as climate change continues to put pressure on vulnerable areas. Some flood-prone cities in the U.S. are even going beyond federal estimates and setting higher local standards for protection.
During extreme flood events, the National Weather Service may issue a flood watch or flood warning. A flood watch alert urges residents to stay alert and prepare in case of high water. A flash flood warning means flooding is already underway or expected soon, alerting communities to move to higher ground immediately. Check your local weather alerts and follow National Weather Service updates, especially as storms grow more intense and unpredictable. This story originally published on April 4, 2019. It has been updated.
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