Flash flood
Flash floods are rapid and intense flooding events that can occur with little warning, often within six hours of heavy rainfall or other contributing factors. They are considered one of the deadliest natural disasters, with a high mortality rate and an estimated annual death toll of around five thousand. Common causes of flash floods include heavy rains, melting snow, dam failures, and urban drainage issues, particularly in densely populated areas where infrastructure can impede water absorption. Due to their speed and unpredictability, flash floods pose significant dangers to life and property, often leading to debris flows that further exacerbate hazards. The risk of flash flooding has increased in recent years, attributed to climate change and urban development. Various warning systems have been established to mitigate risks, although their effectiveness can be limited by the short response time available. Historically, significant flood events, such as the failure of levees during Hurricane Katrina, demonstrate the potential for catastrophic consequences. Global efforts have been initiated to improve forecasting and disaster management related to flash floods, aiming to enhance community resilience against these sudden and dangerous events.
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Flash flood
Flash floods happen rapidly and leave little time for those in the path of the surge to react. They are one of the deadliest types of natural disasters and have the highest mortality rate of all floods, with an annual death toll of around five thousand. Possible causes of flash floods include heavy rainfall, melting snow or ice, dam or levee breaks, ice jams in rivers, and surface drainage problems in urban areas. Flash floods may lead to debris flows, which are also quite dangerous. In order to reduce the risk to life and property, flash-flood warning systems are used, but the effectiveness of these warning systems can be hampered by the short window of time available in which to act.
Background
Flooding is defined as the overflowing of water on usually dry land. While floods are commonly caused by heavy rains, they may also result from ocean phenomena such as tsunamis or storm surges, the melting of ice or snow at higher elevations, or the breaking of a dam or levee. Floods can happen quickly, or they can last days and weeks. More lives are lost due to floods than to lightning, hurricanes, or tornadoes.
Flash floods are characterized by their rapid onset, typically occurring within six hours or less of the precipitating event. They are the most dangerous type of flood because of their speed and unpredictability. Water can rise rapidly in a short period of time without warning. Certain areas are at higher risk than others for flash floods. According to the US National Oceanic and Atmospheric Administration (NOAA), densely populated areas are at high risk for flash floods, because structures such as highways, driveways, parking lots, and buildings reduce the amount of rainfall that the ground can absorb. While streams that run through populated areas are often routed to underground storm drains, heavy rains can fill these drains too quickly, causing them to overflow and flood roads.
Areas near rivers are particularly prone to flash floods. Although dikes or levees are often built along rivers to prevent rising water levels from overflowing, if these barriers fail or if the water level is too high to be contained, the results can be catastrophic. In 2005, levees around the city of New Orleans failed in the wake of Hurricane Katrina, resulting in severe flooding. Dam failures also pose a serious risk, especially if the dam is holding back a reservoir or artificial lake. The failure of the South Fork Dam in 1889 released twenty million tons of water into Pennsylvania’s Little Conemaugh River; the resulting flood in nearby Johnstown, Pennsylvania, killed more than 2,200 people and destroyed 1,600 homes.
Flash Floods Today
Since the late twentieth century, flash floods have become more frequent and more serious. This is due to several factors, including climate change, which has resulted in heavier and more frequent downpours, and human activities such as construction that affect the ground’s capacity to absorb water. In addition, the dangers posed to the human population by flash flooding has increased simply because the human population itself has increased. Greater population means a higher population density and an expansion of infrastructure into vulnerable and potentially hazardous areas. Similarly, the increasing dependence of communities on the continuous operation of local businesses has meant that property damage to businesses leads to compounded economic losses in the community until those businesses can become operational again.
In her article "Impacts of Flash Floods," published in Coping with Flash Floods (2001), Margaret S. Petersen of the University of Arizona describes how damage due to flood is classified as either direct damages, indirect damages, secondary damages, or intangible damages:
- Direct damages are losses due to exposure of property to floodwater, including replacing and repairing private property and infrastructure, and income loss resulting from crop damage.
- Indirect damages include the value of lost business and services, measures to safeguard health, traffic delays, etc.
- Secondary effects include adverse impacts on people who depend on output produced by damaged property or services.
- Intangible effects include impacts on environmental quality, social well-being (including loss of life), and aesthetic values.
According to the US Flood Control Act of 1936, the federal government should "improve or participate in the improvement of" flood-control measures "if the benefits to whomsoever they may accrue are in excess of the estimated costs, and if the lives and social security of people are otherwise adversely affected." As a result of this act, the government instituted procedures for estimating the economic impacts of flooding and also charged federal agencies, primarily the US Army Corps of Engineers, with erecting and maintaining civil-engineering measures such as dams, dikes, and levees throughout the United States for the purpose of flood control.
In 2007, at the Fifteenth World Meteorological Congress in Geneva, Switzerland, the World Meteorological Organization (WMO) approved the rollout of a global flash-flood guidance system (FFGS) project, intended to enable better and faster forecasting of potential flash floods. The system was envisioned as a global network of regional FFGS headquarters covering areas of concern. By 2024, six regional systems—Africa; Asia; South America; North America, Central America, and the Caribbean; South-West Pacific; and Europe—had been implemented in various stages of operation, covering more than three billion people in seventy-two countries. The FFGS works by providing local forecasters and disaster-management agencies with real-time information about rainfall events and hydrological models, which allows them to input local information in order to determine the threat of a flash flood in their area.
Bibliography
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"Flash Flooding: Be Ready to Act." FEMA, 7 Sept. 2022, www.fema.gov/fact-sheet/flash-flooding-be-ready-act. Accessed 20 Nov. 2024.
Gruntfest, Eve, and John Handmer, eds. Coping with Flash Floods. Norwell: Kluwer, 2001. Print.
"HRC Participates in the Second Flash Flood Guidance System (FFGS) Global Workshop." Hydrologic Research Center, 2024, www.hrcwater.org/all-news/hrc-participates-in-the-second-flash-flood-guidance-system-ffgs-global-workshop/. Accessed 26 Nov. 2024.
National Disaster Education Coalition. "Floods and Flash Floods." Talking about Disaster: Guide for Standard Messages. Washington: Author, 2004. National Disaster Education Coalition. Web. 12 Aug. 2016.
NWS Flood Safety Home Page. Natl. Weather Service, 2015. Web. 12 Aug. 2016.
Schneider, Bonnie. Extreme Weather: A Guide to Surviving Flash Floods, Tornadoes, Hurricanes, Heat Waves, Snowstorms, Tsunamis, and Other Natural Disasters. New York: Palgrave, 2012. Print.
Sene, Kevin. Flash Floods: Forecasting and Warning. New York: Springer, 2013. Print.
"Severe Weather 101: Floods." The National Severe Storms Laboratory. NOAA, 18 Feb. 2016. Web. 12 Aug. 2016.