Flash Flood

Primary reference(s)

WMO, 2006. Technical Regulations. Volume III: Hydrology, WMO-No. 49. World Meteorological Organization (WMO). Accessed 20 November 2019.

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 a complex approach as more environmental factors must be considered and 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).

Examples of drivers, outcomes and risk management

Drivers of flash flood: 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).

Outcomes and impacts of flash flood: Flash 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). With more than 5000 lives lost to flash flooding each year, flash floods are among the world’s deadliest natural hazards and have significant social, economic and environmental impacts (WMO, 2019).

Control and monitoring measures of flash flood: 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 provides 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).

Health impacts from floods including flash floods: 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 longterm). 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 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).