Flash Flooding

Unique identifier / Notation

MH0603

Synonyms

Storm-driven flood Freshet Huayco

A flash flood is a flood of short duration with a relatively high peak discharge (WMO, 2021).

Primary reference(s)

WMO, 2021. Technical Regulations. Volume III: Hydrology, WMO-No. 49. World Meteorological Organization (WMO). Accessed 15 May 2025.

Annotations

Additional scientific description

A flash flood is generally characterised by raging torrents after heavy rains, a dam or levee failure or a sudden release of water in a previously stopped passage (i.e., by debris or ice) that rips through riverbeds, urban streets, or mountain canyons, sweeping away everything in its path. Steep terrain tends to concentrate runoff into streams very quickly and is often a contributory factor. Changes in soil properties (e.g., burn areas from wildfires), hydrophobic or impervious soils, removal of surface vegetation, and excess runoff from warm rainfall on significant snowpack can also be important contributors (NOAA, no date a; AMS, 2017).

Metrics and numeric limits

A flash flood is a flood that begins within 6 hours, and often within 3 hours, of a heavy rainfall (NOAA, no date b).

Flash floods are highly localised in space: they are restricted to basins of a few hundred square kilometres or less. They are also restricted in time: response times not exceeding a few hours or even less. This means very little time for warning (NOAA, no date b).

Flash flood hazard measurement and modelling requires considering more environmental factors, regularly monitored: topographic parameters (average slope, slope range and valley density for the catchment), soil and surface parameters (surface runoff, infiltration and interception: soil depth, physical soil type, the ratio of barren/ vegetation-covered surfaces), and hydrological parameters (precipitation, consecutive rainy days, etc.) (Liu and Smedt, 2005).

Key relevant UN convention / multilateral treaty

Sendai Framework for Disaster Risk Reduction 2015-2030.

Drivers

The intensity of the rainfall, the location and distribution of the rainfall, the land use and topography, vegetation types and growth/density, soil type, and soil water-content all determine how quickly flash flooding may occur, and influence where it may occur (NOAA, no date b).

Impacts

lash floods account for approximately 85% of flooding cases and have the highest mortality rate (defined as the number of deaths per number of people affected) among different classes of flooding (e.g., riverine, coastal). Flash floods are among the world’s deadliest natural hazards and have significant social, economic and environmental impacts (WMO, 2019).

Health impacts from floods, including flash floods: As mentioned in the chapeau, the effects of flooding on health are extensive and significant, ranging from mortality and injuries resulting from trauma and drowning to infectious diseases and mental health issues (acute and long- term). Flash flooding is usually associated with trauma and drowning in terms of mortality and injuries. While some of these outcomes are relatively easy to track, ascertaining the human impact of floods is still weak. For example, it has been reported that two-thirds of deaths associated with flooding are from drowning, with the other third from physical trauma, heart attacks, electrocution, carbon monoxide poisoning and fire. Often, only immediate traumatic deaths from flooding are recorded (WHO, 2013).

Morbidity associated with floods is usually due to injuries, infections, chemical hazards (see chemical HIPs) and mental health effects (acute as well as delayed) (WHO, 2013). Hypothermia may also be a problem, particularly in children, if trapped in floodwaters for lengthy periods (WHO, no date). There may also be an increased risk of respiratory tract infections due to exposure (loss of shelter, exposure to flood waters and rain). Power cuts related to floods may disrupt water treatment and supply plants, thereby increasing the risk of water-borne diseases as well as the proper functioning of health facilities, including cold chain (WHO, no date).

Floods can potentially increase the transmission of the following communicable diseases: water-borne diseases (such as typhoid fever, cholera, leptospirosis and hepatitis A) and vector-borne diseases (such as malaria, dengue and dengue haemorrhagic fever, yellow fever, and West Nile Fever) (WHO, no date).

The longer-term health effects associated with a flood are less easily identified. They include effects due to displacement, destruction of homes, delayed recovery and water shortages (WHO, 2013). This also heavily affects the economy and livelihood, disrupting activities or transportation: for example, school flooding could result in them being closed for weeks and students being relocated. Thus, education is affected, and labour is disrupted when parents might have to stay home if their children are unable to attend school.

Multi-hazard context

The figure below summarises common interactions between flash flooding and other hazards. This information should be used with caution and not be solely relied upon in Disaster Risk Management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify, or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.

Multi-hazard diagram

Risk Management

Flash floods are a major natural hazard throughout the world. Flash floods are the number one killer among all weather-related hazards. A vast majority of deaths from flash floods, as high as 90% in tropical countries, are due to drowning from victims being caught by rapidly rising waters (Smith, 1992).

Predicting flash floods requires accurate detection and estimation of rainfall events, which are typically intense and very localised. Operational prediction methods include flash flood monitoring and prediction algorithms (FFMPA) used in Europe and the United States. FFMPA alerts forecasters when flash flooding is imminent based on radar-estimated rainfall amounts compared to hydrologic model-based rainfall thresholds. Advances in forecasting convective rainstorms help to improve the performance of FFMPA by providing a longer lead time of impending flash floods (Hong et al., 2013).

Reducing societal exposure to flash floods is key to any mitigation measure. The combination of non-structural measures and small-scale structural measures could be more effective in managing flash flood risk. There are many non-structural measures that can reduce the impact of floods, such as land-use planning, building construction codes, soil management, acquisition policies, insurance and risk transfer, awareness raising, public information, emergency system, and recovery plans. Structural activities, including property protection such as relocation and reinforcement, and structural engineering projects such as levees, diversions, and channel improvements, are some of the actions that mitigate the societal impacts of flash floods (Colombo et al., 2002).

Flash floods differ from river floods in various ways. Notably they manifest and dissipate in less time and occur in more condensed spatial areas. These factors make their forecasting a unique challenge compared to traditional flood forecasting approaches. In this regard, the World Meteorological Organization has launched the Flash Flood Guidance System (FFGS) with global coverage. A system such as the FFGS is an important tool for providing operational forecasters and disaster management agencies with real-time informational guidance products pertaining to the threat of small-scale flash flooding. The FFGS utiliszes remotely sensed precipitation estimates (e.g., radar and satellite-based rainfall estimates) and allows product adjustments based on forecaster experience with local conditions, incorporation of other information (e.g., Numerical Weather Prediction output) and any last-minute local observations (e.g., non-traditional rain gauge data) (WMO, 2019).

Monitoring

The section above and the table below offer an overview of monitoring flash flooding. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.

Which institution(s) produce(s) Disaster Risk Data/Information?

Meteorological and Hydrological services track heavy rainfall, storm systems, and atmospheric conditions that contribute to flash floods. Hydrological organiszations monitor river levels, soil moisture, and runoff to assess flash flood potential.
National, subnational, and local disaster management agencies responsible for monitoring flash flood risks, issuing alerts, and coordinating emergency response efforts.
Geophysical agencies studying land stability, terrain characteristics, and changes that affect water flow during extreme rainfall events.
Health authorities evaluating risks to public safety, including waterborne diseases, infrastructure failure, and emergency medical response.
Engineers designing flood control systems, stormwater drainage infrastructure, and protective barriers to mitigate flash flood impacts.
Land use and urban planners integrating flash flood risk assessments into zoning laws, watershed management, and urban development planning.
Energy and utility companies monitoring risks to power stations, water supply systems, and communication networks in flood-prone areas.
Researchers and academics, including hydrologists and climate scientists, studying rainfall-runoff relationships, flash flood forecasting models, and climate change impacts.
Organiszations and community representatives engaged in flash flood preparedness, public education, and local early warning dissemination.

How is the Hazard Observed/Monitored/Forecast?

Flash floods are monitored using weather radar, rain gauges, and satellite data to track intense rainfall, while river sensors and soil moisture measurements help detect a sudden water buildup. Meteorologists and hydrologists analysze storm patterns, terrain, and runoff behaviour to predict potential flooding. Hydrological models simulate how rainwater moves through landscapes, improving early warning systems. Because flash floods develop quickly, real-time data and rapid forecasting are crucial for issuing timely alerts to protect lives, infrastructure, and communities.