Flooding
Flooding is (1) an overflowing by water of the normal confines of a watercourse or other body of water; (2) an accumulation of drainage water over areas which are not normally submerged; (3) a controlled spreading of water for irrigation (WMO and UNESCO, 2012).
Flood
Inundation
High Water
Whereas floods are defined in hydrology as a "(1) rise, usually brief, in the water level of a stream or water body to a peak from which the water level recedes at a slower rate, (2) relatively high flow as measured by stage height or discharge" (WMO and UNESCO, 2012), flooding represents the hazard associated to with these peaks whenever the water overflows the normal confines of the water body. Flooding can include: - Coastal Flood: Coastal flooding occurs from multiple sources, including from storm surges, waves and swell, seiches, riverine and flash floods near the coast, tides, sea-level rise and tsunamis. It is most frequently the result of storm surges and high winds coinciding with high tides. The surge itself is the result of the raising of sea levels due to low atmospheric pressure. In particular conditions, such as major estuaries or confined sea areas, the piling up of water is amplified by a combination of the shallowing of the seabed and retarding of return flow (WMO, 2011; WMO, 2022). - Estuarine (Coastal) Flood: Estuarine flooding is flooding over and near coastal areas caused by storm surges and high winds coincident with high tides, thereby obstructing the seaward river flow. Estuarine flooding can also occur when riverine flooding causes overflow near the estuary or coastal zone, and in specific cases, can also be caused by tsunamis. (WMO, 2011). Therefore, it is challenging to identify a single driver for estuarine flooding. - Flash Flood: A flash flood is a flood of short duration with a relatively high peak discharge (WMO, 2021). - Fluvial (Riverine) Flooding: Overflowing by water of the normal confines of a watercourse or other body of water (WMO, 2012). - Groundwater Flood: A groundwater flood is the emergence of groundwater at the ground surface away from perennial river channels or the rising of groundwater into man-made ground, under conditions where the 'normal' ranges of groundwater level and groundwater flow are exceeded (BGS, 2010). - Ice-Jam Flood including Debris: An ice-jam flood including debris is defined as an accumulation of shuga including ice cakes, below ice cover. It is broken ice in a river which causes a narrowing of the river channel, a rise in water level and local floods (WMO, 2012). - Ponding (Drainage) Flood: A ponding flood is a flood which results from rainwater ponding at or near the point where it falls because it is falling faster than the drainage system (natural or man-made) can carry it away (WMO, 2006). - Snowmelt Flood: A snowmelt flood is a significant flood rise in a river caused by the melting of snowpack accumulated during the winter (WMO, 2012). - Surface Water Flooding: Surface water flooding is that part of the rain which remains on the ground surface during rain and either runs off or infiltrates after the rain ends, not including depression storage (WMO, 2012). - Glacial Lake Outburst Flood: A 'glacial lake outburst flood' is a phrase used to describe a sudden release of a significant amount of water retained in a glacial lake, irrespective of the cause (Emmer, 2017).
Meteorological and Hydrological
Water-related
Sendai Framework for Disaster Risk Reduction 2015-2030.
Floods tend to be caused by a number of natural events such as rain from slow- moving or stationary low-pressure areas, thunderstorms, and tropical cyclones. The amount and duration of the rainfall, soil type and saturation, geography and whether it is an urban area can affect the magnitude and impacts of the flooding. Flooding can occur at the place of a heavy rain event or far downstream away from the causal rain event. Flooding can also be caused by other factors such as storm surges, tsunamis, ice jams and glacial lake outbursts, as well as being human influenced through dam bursts, improper land use planning, etc. (Geoscience Australia, no date).
More people are impacted by floods than by any other hazard. Year on year floods hit various regions around the world, with developing countries most at risk. Almost 2 million people worldwide live in flooding-prone coastal regions. Floods can carry devastating consequences, causing mortality and significant economic loss and damage, with flash floods being the most dangerous due to their speed and destructive power (UNDRR, no date; WMO, 2025; NOAA, no date b). Health impacts from 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 problems (acute and long-term). 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 affecting the proper functioning of health facilities, including cold chain (WHO, no date). Floods can potentially increase the transmission of communicable diseases (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). Economic impacts from floods: According to the updated WMO Atlas of Mortality and Economic Losses from Weather, Climate and Water Extremes, between 1970 and 2021 floods were amongst the costliest hazardous events, after tropical cyclones, with US$1.3 trillion in economic losses attributed to floods - 32% of the total reported economic losses from extremes over the 51-year period (WMO, 2022). Flooding can significantly affect local economies, disrupting transportation networks and agricultural activities, and damaging property and infrastructure. In the Midwestern United States in 2019, for example, the event known as “The Great Flood of 2019” affected more than 13 million people across 11 states, primarily impacting areas along the Missouri River. The flood caused approximately US$6.3 billion in damage to property and agriculture, with the prolonged nature of the event disrupting agricultural production and reducing crop planting by millions of acres. Various industries in the region experienced disruptions, with high water levels hindering barge transportation and other shipping routes (Sun et al., 2022).
Key flooding metrics, such as flood depth and flow velocity, are usually combined with other variables, such as flooding arrival time, flooding duration, sediment load, contamination levels, and so on, to determine flood hazard indicators (Maranzoni et al., 2022). However, these indicators and metrics may vary across different types of flooding. For example: - Coastal Flood: The extent and magnitude of coastal flooding is a function of the elevation inland flood waters penetrate, which is controlled by the topography of the coastal land exposed to flooding and coastal flooding-related drivers (Bell et al., 2017). - Flash Flood: A flash flood is a flood that begins within 6 hours, and often within 3 hours, of a heavy rainfall (NOAA, no date a).
The figure below summarises common interactions between 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.
Climate variability and change is giving rise to increasingly frequent and intense flood events, with changes in land use, ecosystem degradation and increased population pressure on flood-prone areas exacerbating flood risk worldwide and aggravating flood-related impacts (UNDRR, no date; WMO, 2025). Integrated Flood Management (IFM) encourages “an integrated, rather than fragmented, approach to flood management. It integrates land and water resources development in a river basin, within the context of Integrated Resources Management” WMO, 2009. An example of an initiative promoting an IFM approach is the Associated Programme on Flood Management (APFM), a joint initiative between the World Meteorological Organization (WMO) together with the Global Water Partnership. The APFM “promotes the concept of IFM to maximize net benefits from the use of floodplains and minimize the loss of life and impacts. As such, it provides tools and publications to support flood management” (WMO, 2025). Aside from risk management, there are several measures that can support flood-related risk reduction: - “Integrate flood risk assessment into urban planning strategies. - Avoid building on flood-prone land. - Develop new building codes to reinforce flood resistance. - Create more space for rivers, floodplains and wetlands. - Ensure health of coastal reefs and mangrove plantations. - Maintain early warning systems, backed up by regular drills and evacuation exercises. - Have an evacuation plan for those at risk, including the elderly, disabled and very young. - Catalyse finance and insurance schemes to protect assets and livelihoods. - Protect and evacuate animals” (UNDRR, no date).
The section above and the table below offer an overview of monitoring 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 service tracking rainfall, storms, and weather patterns that contribute to flooding. - Hydrological service monitoring river levels, groundwater saturation, and watershed conditions to assess flood risks. - Oceanographic and marine agencies studying sea-level rise, storm surges, and coastal flooding risks. - Geophysical agencies analysing land stability, soil conditions, and factors like land subsidence that impact flood behaviour. - Health authorities evaluating public health risks from floodwater contamination, waterborne diseases, and emergency medical needs. - Engineers and infrastructure agencies designing and maintaining flood control systems such as levees, dams, drainage networks, and stormwater management systems. - Land use and urban planners integrating flood risk assessments into zoning laws, building regulations, and sustainable development strategies. - Energy and utility companies monitoring flood risks to power plants, water treatment facilities, and communication networks. - Researchers and academics, including hydrologists and climate scientists, studying flood forecasting, climate change impacts, and long-term risk reduction strategies. - Environmental organiszations and conservation groups working on nature-based flood mitigation solutions, such as wetland restoration and watershed management. | How is the Hazard Observed/Monitored/Forecast? | Flooding is monitored using weather radar, rain gauges, river sensors, and satellites that track rainfall, water levels, and soil saturation. Meteorologists and hydrologists analyse these data alongside climate patterns and terrain features to predict when and where floods may occur. Computer models simulate water movement, helping forecasters issue early warnings. By combining real-time observations with historical trends, experts provide alerts that help communities, emergency responders, and planners take action to reduce flood risks and protect lives.
Subsidence and Uplift
Thunderstorm
Depression or Cyclone (Low Pressure Area)
Extra-tropical Cyclone
Sub-Tropical Cyclone
Tropical Cyclone
Rain
Dam Failure
Reservoir Flooding
Lahars
Gravitational Mass Movement (‘Landslide’)
Debris and earth (mud)flows and rock avalanches
Rock, debris and earth (mud) slide
Soil Degradation
Soil Erosion
River Erosion and Accretion
Coastal Erosion and Accretion
Diarrhoeal Diseases
Waterborne Diseases
Cholera
Locust upsurge
Fungal Plant Disease
Human-Wildlife Conflict
Snakebite envenoming
Explosive Ordnance
Building Collapse
Structural Failure
Bridge Failure
Power Outage/ or Blackout
Water Supply Failure
Tunnel Failure
Drain and Sewer Flooding
Pollution
Soil Pollution
Mining Hazards
Rail Accident
Road Traffic Accident
Disaster and Conflict Waste
Landfilling
Waste Treatment Lagoons
Tailings
Runoff / Nonpoint Source Pollution
Land Degradation
2025 update; https://www.undrr.org/publication/2025-update-undrr-isc-hazard-information-profiles-hips
Coastal Flooding
Coastal flooding occurs from multiple sources, including storm surges, waves and swell, seiches, riverine and flash floods near the coast, tides, sea-level rise and tsunamis. It is most frequently the result of storm surges and high winds coinciding with high tides. The surge itself is the result of the raising of sea levels due to low atmospheric pressure. In particular configurations, such as major estuaries or confined sea areas, the piling up of water is amplified by a combination of the shallowing of the seabed and retarding of return flow (WMO, 2011; WMO, 2022).
Estuarine Flooding
Coastal inundation
Major deltas such as the Mississippi and Ganges are prone to coastal flooding when affected by hurricanes (cyclones). Another sensitive area is the southern North Sea in western Europe, as a result of particular tracks of winter depressions. If the surge takes place near the mouth of a river issuing into the sea, the river flow will be obstructed due to the surge, resulting in severe flooding over and near the coastal areas. Tsunamis resulting from sub-seabed earthquakes are a very specific cause of occasionally severe coastal flooding (WMO, 2011). Coastal flooding is largely a natural event, however, human influence on the coastal environment can exacerbate coastal flooding (Dawson et al., 2009). Extraction of water from groundwater reservoirs in the coastal zone can enhance subsidence of the land, increasing the risk of flooding (Nicholls, 2002). Seawater can flood the land via several paths: - Direct flooding: the sea height exceeds the elevation of the land, often where waves have not built up a natural barrier such as a dune system. - Overtopping of a barrier: the barrier may be natural or human- engineered, and overtopping occurs due to swell conditions during storms or high tides, often on open stretches of the coast. The height of the waves exceeds the height of the barrier, and water flows over the top of the barrier to flood the land behind it. Overtopping can result in high velocity flows that can erode significant amounts of the land surface, which can undermine defence structures (Gallien et al., 2011). - Breaching of a barrier: the barrier may be natural (sand dune) or human engineered (sea wall), and breaching occurs on open coasts exposed to large waves. Breaching is where the barrier is broken down or destroyed by waves allowing the seawater to extend inland and flood the area.
United Nations. (1982). United Nations Convention on the Law of the Sea (UNCLOS). Often referred to as the "constitution of the oceans," this convention provides a comprehensive framework for governing ocean space, including provisions to address coastal flooding and protect the marine environment.
Flooding constitutes a particular challenge in low-lying areas such as deltas and coastal plains, and land subsidence caused by sediment deficits or ground water extraction can further exacerbate the problem.
Coastal flooding can result in considerable economic losses and casualties. Moreover, coastal flooding can result in a wide range of environmental impacts on different spatial and temporal scales. Flooding can destroy coastal habitats such as coastal wetlands and estuaries and can erode dune systems (Hunt and Watkiss, 2011).
The extent and magnitude of coastal flooding is a function of the elevation inland flood waters penetrate, which is controlled by the topography of the coastal land exposed to flooding; storm surge conditions; and the broader bathymetry of the coastal area (Bell et al., 2017).
The figure below summarises common interactions between coastal 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.
The governance structure of coastal zones may be strengthened in different ways (WMO, 2013): - Promoting better co-operation between different levels and sectors of government, taking into account trends of decentralisation and the need for (national) coordination. - Facilitating cooperation between the government and the private sector, taking into account trends of privatisation but also the need to safeguard the public interest. - Better involvement between stakeholders and citizens in development and management issues, to promote societal acceptance of development projects as well the long-term sustainability of development projects (arrangements and incentives for maintenance). - Creating arrangements for dealing with uncertainties and sharing of risks (insurance). There are many ways in which humans are trying to prevent the flooding of coastal environments. Technical measures for flood control include dike or seawall or levee construction (hard engineering structures) (Short and Masselink, 1999), maintenance of natural dune systems, protection of coastal ecosystems (natural defence) (Alongi, 2008) and different flood- proofing and accommodation activities. Engineered protection structures along the coast, such as sea walls, if not well planned, can alter the natural processes of the beach, often leading to erosion on adjacent stretches of the coast, which also increases the risk of flooding (Pope, 1997). Non-structural mechanisms also exist to mitigate coastal flooding: building regulations; coastal hazard zoning; urban development planning; spreading risk through insurance; and enhancing public awareness (Snoussi et al., 2008; Dawson et al., 2011). Generally, even moderate flooding hazards should be taken very seriously due to the potentially disastrous consequences, and flood protection is a key aspect in coastal disaster risk reduction. 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). 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 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).
The section above and the table below offer an overview of monitoring coastal 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 storm surges, extreme weather events, and tidal patterns that contribute to coastal flooding. - Oceanographic and marine agencies monitoring sea level rise, wave action, and coastal erosion to assess flood risks. - National, subnational, and local disaster management agencies responsible for monitoring coastal flood risks, issuing warnings, and coordinating emergency response. - Geophysical agencies studying land subsidence, tectonic activity, and coastal geomorphology that influence flooding. - Health authorities assessing public health risks from saltwater contamination, disease outbreaks, and emergency response capabilities. - Engineers designing coastal defences such as seawalls, flood barriers, drainage systems, and resilient infrastructure. - Land use and urban planners integrating coastal flood risk assessments into zoning laws, building codes, and climate adaptation strategies. - Energy and utility companies monitoring risks to power plants, water treatment facilities, and communication networks in coastal areas. - Researchers and academics, including climate scientists, studying sea-level rise, storm surge impacts, and long-term flood mitigation strategies. - Organiszations and community representatives engaged in coastal resilience efforts, early warning dissemination, and flood preparedness initiatives. | How is the Hazard Observed/Monitored/Forecast? | Coastal flooding is monitored using tide gauges, satellite observations, and weather radar to track rising sea levels, storm surges, and extreme weather events. Meteorologists, hydrologists, and oceanographers analyse wind speeds, atmospheric pressure, and wave heights to predict potential flooding, while hydrological models simulate how water will move inland. Forecasts combine real-time data with historical trends to provide early warnings, helping communities, emergency responders, and infrastructure planners prepare for and mitigate coastal flooding impacts.
Loss of Mangroves
Fluvial (Riverine) Flooding
Rogue Wave
Seiche
Storm Surge
Storm Tides
Tsunami
Surface Rupture and Fissuring
Estuarine (Coastal) Flooding
Critical Infrastructure Failure
Leaks and Spills
Safety Hazards Associated with Oil and Gas
Wastewater
Marine Debris
Estuarine flooding is flooding over and near coastal areas caused by storm surges and high winds coincident with high tides, thereby obstructing the seaward river flow. Estuarine flooding can be caused by tsunamis in specific cases (WMO, 2011).
Coastal flooding
Estuaries are inlet areas of the coastline where the coastal tide meets a concentrated seaward flow of fresh water in a river. The interaction between the seaward flow of river water and landward flow of saline water during high tides may cause a build-up of water or an inland-moving tidal bore. Frequently, the funnel shape characteristic of many estuaries causes an increase in high water levels in the upper, narrowing reaches of the associated river. These types of floods are mostly experienced in deltaic areas of rivers along the coasts, for example, the mouths of the Ganges. They are more frequent and less severe in terms of inundated depth and area than flooding caused by storm surges (WMO, 2011).
Storm surges and high winds coinciding with high tides are the most frequent cause of estuarine/coastal flooding. The surge itself is the result of the rising of sea levels due to low atmospheric pressure. In particular configurations, such as major estuaries or confined sea areas, the piling up of water is amplified by a combination of the shallowing of the seabed and retarding of return flow. Major deltas such as the Mississippi and Ganges are prone to this type of flooding when affected by hurricanes (cyclones). Another sensitive area is the southern North Sea in western Europe, owing to particular tracks of winter depressions. If the surge takes place near the mouth of a river issuing into the sea, the river flow will be obstructed due to the surge, resulting in severe flooding over and near the coastal areas. Tsunamis resulting from sub-seabed earthquakes are a very specific cause of occasionally severe coastal flooding (WMO, 2011). Also, estuarine flooding can happen in cases when riverine flooding (i.e. when the river is experiencing a significant freshwater flow) produces an overflow near the estuary/coastal zone. It is therefore difficult, as in the case of coastal flooding, to identify a single driver for estuarine flooding.
Floods are one of the most common hazards. 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). 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 floods and related hazards, such as mould in homes, are less easily identified. They include effects due to displacement, destruction of homes, delayed recovery, and water shortages., (WHO, 2013).
Not identified.
The figure below summarises common interactions between estuarine 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.
Effective risk management for estuarine flooding requires integrated approaches that combine structural measures, such as tidal barriers, embankments, and floodgates, with nature-based solutions like wetland restoration and mangrove protection, which buffer surge impacts. Accurate flood modelling, early warning systems, and land-use planning are essential to reduce vulnerability in densely populated estuarine zones. Long-term resilience also depends on adaptive governance frameworks that account for sea-level rise, climate change, and evolving socio-economic pressures. (UK Department of Environment, Food & Rural Affairs and Environment Agency, 2023)
The section above and the table below offer an overview of monitoring estuarine 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 tracking rainfall, storm surges, river discharge, and tidal influences that contribute to estuarine flooding. - Oceanographic and marine agencies monitoring sea-level rise, tidal patterns, and sediment transport affecting estuarine flood risks. - National, subnational, and local disaster management agencies responsible for monitoring estuarine flood risks, issuing alerts, and coordinating emergency response. - Geophysical agencies studying land subsidence, tectonic activity, and coastal geomorphology that impact estuarine flood behaviour. - Health authorities assessing public health risks from saltwater intrusion, water contamination, and flood-related disease outbreaks. - Engineers designing flood defences such as tidal barriers, levees, drainage systems, and resilient infrastructure to mitigate estuarine flooding. - Land use and urban planners integrating estuarine flood risk assessments into zoning laws, wetland conservation, and urban development strategies. - Energy and utility companies monitoring risks to power plants, water supply systems, and communication networks in estuarine areas. - Researchers and academics, including hydrologists and climate scientists, studying the interactions between river flow, tides, and storm surges to improve flood forecasting and long-term adaptation strategies. | How is the Hazard Observed/Monitored/Forecast? | Estuarine (coastal) flooding is monitored using tide gauges, river flow sensors, satellite imagery, and weather radar to track tidal movements, storm surges, and river discharge. Scientists analyse interactions between sea level, river flow, and extreme weather events to predict flooding risks. Advanced hydrodynamic models simulate how water moves through estuaries, helping forecasters issue advisories and early warnings. By combining real-time data with historical trends, experts provide timely alerts to protect communities, ecosystems, and critical infrastructure from estuarine flooding.
Eutrophication
Flash Flooding
A flash flood is a flood of short duration with a relatively high peak discharge (WMO, 2021).
Storm-driven flood
Freshet
Huayco
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).
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).
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.
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).
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.
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).
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.
Glacial Lake Outburst Flooding
Ice-Jam Flooding Including Debris
Insect Pest Infestations
Surface Water Flooding
Rock, debris and earth spreads (including landscape creep)
Supply Chain Failure
Overflowing by water of the normal confines of a watercourse or other body of water (WMO, 2012).
Whereas floods are defined in hydrology as “a rise, usually brief, in the water level of a stream or water body to a peak from which the water level recedes at a slower rate” (WMO, 2012), flooding represents the hazard associated with these peaks whenever the water overflows the normal confines of the water body. Fluvial flooding occurs over a wide range of river and catchment systems. Floods in river valleys occur mostly on flood plains or wash lands as a result of flow exceeding the capacity of the stream channels and spilling over the natural banks or artificial embankments (Fernandez, 2015).
Fluvial (riverine) flooding primarily results from an extended precipitation event that occurs at, or upstream from, the affected area. It can also occur when traditional flood-control structures, such as levees and dikes, are overtopped or fail (NOAA, no date).
Whereas the general impacts of flooding have already been mentioned in the chapeau, riverine flooding is mostly associated with socio-economic impacts (such as disruption of activities, property and livelihood losses) rather than with direct mortality (drowning, traumatic events). Health impacts are generally more long-term, associated with contamination of freshwater resources, water-borne diseases, etc. Flooding of areas used for socio-economic activities produces a variety of negative impacts. The magnitude of adverse impacts depends on the vulnerability of the activities and population and the frequency, intensity and extent of flooding. Some of these factors include loss of lives and property, loss of livelihoods, decreased purchasing power and production power, mass migration, psychosocial effects, hindering of economic growth and development, and political implications (WMO, 2009). Control and monitoring measures of fluvial flood: Floods are important components of the natural hydrological regime. They are a major source of water; they flush pollutants and sediment from river networks. It is also natural for rivers to overtop their banks and occupy their flood plains. As a result, flooding can cause property damage and bring death and injury to many communities. While there is no evidence as yet that the frequency or magnitude of flooding has increased world-wide, flood-prone areas are becoming increasingly densely populated and thus more vulnerable. Consequently, a series of major flood disasters has occurred in recent years, with death and destruction being caused by such events on every continent (GWP, 2013). There is a need for an approach to flood management that improves the functioning of the river basin as a whole, recognising that floods have beneficial impacts and can never be fully controlled. Such an approach seeks to maximisze the net benefits from the use of floodplains and to minimise loss of life, subordinating flood loss reduction to the overall goal of maximising the efficient use of the floodplain (APFM, 2020). Integrated Flood Management (IFM) is a process that promotes an integrated, rather than fragmented, approach to flood management. It integrates land and water resources development in a river basin, within the context of Integrated Resources Management, with a view to maximising the efficient use of floodplains and to minimising loss of life . IFM, like Integrated Water Resources Management, should encourage the participation of users, planners and policymakers at all levels. The approach should be open, transparent, inclusive and communicative; should require the decentralisation of decision- making; and should include public consultation and the involvement of stakeholders in planning and implementation. IFM calls for a paradigm shift from the traditional fragmented approach and encourages the efficient use of the resources of the river basin, employing strategies to maintain or augment the productivity of floodplains, while at the same time providing protective measures against losses due to flooding (WMO, 2009). Health impacts of floods, including fluvial (riverine) floods: Floods are one of the most common hazards. 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 problems (acute and long-term). While some of these outcomes are relatively easy to track, ascertaining the human impact of floods is still challenging. 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). 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 affecting 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).
The figure below summarises common interactions between fluvial 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.
No Information Available
The section above and the table below offer an overview of monitoring fluvial 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 services tracks rainfall patterns, storm systems, and atmospheric conditions that influence river flooding. - Hydrological services monitors river levels, streamflow, soil moisture, and watershed conditions to assess flood potential. - Geophysical agencies studying terrain characteristics, sediment transport, and land subsidence that impact river flooding. - National, subnational, and local disaster management agencies responsible for monitoring river flood risks, issuing warnings, and coordinating emergency responses. - Health authorities assessing public health risks from waterborne diseases, contamination, and emergency medical response needs. - Engineers designing flood control infrastructure such as dams, levees, embankments, and drainage systems to mitigate river flooding. - Land use and urban planners integrating river flood risk assessments into zoning laws, watershed management, and sustainable development strategies. - Energy and utility companies monitoring flood risks to power plants, water treatment facilities, and critical infrastructure along rivers. - Researchers and academics, including hydrologists and climate scientists, studying river flow dynamics, climate change impacts, and flood forecasting models. | How is the Hazard Observed/Monitored/Forecast? | Fluvial (riverine) floods are monitored using river gauges, satellite imagery, and weather radar to track water levels, rainfall, and upstream flow conditions. Hydrologists analysze watershed characteristics, soil moisture, and precipitation trends to predict how rivers will respond to heavy rainfall or snowmelt. Advanced flood models simulate river behaviour, helping forecasters issue warnings days or even weeks in advance. By combining real-time data with historical trends, experts provide early alerts to protect communities, infrastructure, and ecosystems from river flooding.
Snowmelt Flooding
A ‘glacial lake outburst flood’ is a phrase used to describe a sudden release of a significant amount of water retained in a glacial lake, irrespective of the cause (Emmer, 2017).
GLOF
The term glacial lake outburst flood (GLOF) is used here to refer to the catastrophic release of a water reservoir that has formed either at the side, in front, within, beneath or on the surface of a glacier. Dam structures that impound the water reservoir may be composed primarily of glacial ice, morainic debris, or bedrock (GAPHAZ, 2017). GLOFs are characterised by extreme peak discharges, often several times in excess of the maximum discharges of hydrometeorological induced floods, with an exceptional erosion/transport potential. They can therefore turn into flow-type movements, such as GLOF-induced debris flow (Emmer, 2017; UN-SPIDER, no date).
Key overarching determinants of GLOF susceptibility and the resulting event magnitude are the size of the glacier lake, the outburst mechanism (and related hydrograph), and the characteristics of the downstream torrent (determined by channel inclination and debris availability). Ice-dammed lakes can develop at the margin of an advancing (or surging) glacier, when side -valleys or depressions at the side of the glacier become truncated and blocked. Many such lakes formed in high mountain regions during the Last Glacial Maximum and more recently during and following the Little Ice Age. Over time, as the glaciers retreat, the support of the ice dam is removed, and the lake may drain catastrophically or remain trapped behind lateral moraines of the former glacier (GAPHAZ, 2017; UN-SPIDER, no date). Additionally, as glaciers retreat, the surface areas of glacial lakes increase. Debuttressing effects and landslides, even ice avalanches, are frequent drivers (Cook et al, 2018).
Notwithstanding the general considerations on flooding impacts contained in the chapeau, GLOFs impacts are similar to those of 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 problems (acute and long-term). 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 affecting 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).
The figure below summarises common interactions between glacial lake outburst 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.
Various schemes have been proposed for assessing the susceptibility of glacial lakes to an outburst flood, mostly drawing on remotely sensed information to characterise, semi-quantitatively, the cryospheric environment, lake and dam area, and other geotechnical and geomorphic characteristics of the upstream catchment area of the lake. The potential for unstable rock and/or ice to impact into a lake can be determined using worst-case runout distances. Assessment approaches have mostly been developed and tailored towards regional implementation, especially for moraine- dammed lakes, for which McKillop and Clague (2007) gave a comprehensive overview of many of the relevant susceptibility factors that may condition or trigger an outburst event (GAPHAZ, 2017).
The section above and the table below offer an overview of monitoring glacial lake outburst 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 services tracking temperature fluctuations, precipitation, and climate patterns influencing glacial melt and lake stability. - Hydrological services monitoring glacial lake water levels, river discharge, and downstream flood risks. - National, subnational, and local disaster management agencies responsible for monitoring GLOF risks, issuing alerts, and coordinating emergency response efforts. - Geological and geophysical agencies studying glacial moraine stability, terrain changes, and seismic activity that could trigger a GLOF. - Glaciological research institutions analysing glacier retreat, ice dynamics, and lake formation to assess long-term flood risks. - Remote sensing and space agencies using satellite imagery, LiDAR, and drones to monitor glacial lakes, detect changes, and map flood-prone areas. - Health authorities assessing public safety risks from waterborne diseases, infrastructure collapse, and emergency medical needs. - Engineers and infrastructure agencies designing early warning systems, flood barriers, and drainage channels to mitigate GLOF impacts. - Land use and urban planners incorporating GLOF risk assessments into zoning laws, infrastructure placement, and disaster preparedness strategies. - Energy and utility companies monitoring risks to hydropower stations, water supply systems, and other critical infrastructure in glacial regions. - Researchers and academics, including hydrologists and climate scientists, studying glacial dynamics, climate change impacts, and GLOF forecasting models. - Environmental organiszations and conservation groups promoting watershed management, reforestation, and ecosystem-based flood mitigation strategies. | How is the Hazard Observed/Monitored/Forecast? | Glacial Lake Outburst Floods (GLOFs) are monitored using satellite imagery, drones, and on-the-ground sensors to track glacial lake levels, ice stability, and terrain changes. Scientists analyse temperature trends, rainfall, and seismic activity to assess the risk of sudden lake bursts. Hydrological models simulate potential flood paths, helping forecasters predict downstream impacts. Early warning systems, including water level sensors and community alerts, help protect lives, infrastructure, and ecosystems from sudden glacial floods.
Thaw
Groundwater Flooding
A groundwater flood is the emergence of groundwater at the ground surface away from perennial river channels or the rising of groundwater into man-made ground, under conditions where the ‘normal’ ranges of groundwater level and groundwater flow are exceeded (BGS, 2010).
ponding
Groundwater flooding is a different type of hazard than river or surface water flooding in that the onset is not an immediate process where water suddenly encroaches areas not normally inundated. In contrast, the water levels within permeable strata will gradually rise over time giving rise to the formation of springs and ephemeral streams, areas of ponding, surcharging of wells and boreholes, and water ingress in basements, tunnels, or other below- ground structures. Water levels can remain high for many weeks or even months, depending on the nature of the underlying strata and meteorological and hydrological conditions (WMO, 2007). Groundwater flooding occurs when the natural underground drainage system cannot drain rainfall away quickly enough, causing the water table to rise above the ground surface. It can pose a significant flood hazard for many rural communities, and its increased frequency in recent years highlights the need for further research (Geological Survey Ireland, 2021). Both perched groundwater and periodic springs can be causes of floods: - Perched groundwater: a groundwater body, generally of moderate dimensions, supported by a relatively impermeable stratum and which is located between a water table and the ground surface (WMO, 2012). - Periodic spring: a spring flowing irregularly in relation to the hydrological regime of groundwater or in connection with karstic conditions is generally known as a periodic or seasonal spring (WMO, 2012).
European Union Floods Directive (2007/60/EC): The Directive on the assessment and management of flood risks entered into force on 26 November 2007. It requires member states to assess if all water courses and coastlines are at risk from flooding, to map the flood extent and assets and humans at risk in these areas, and to take adequate and coordinated measures to reduce this flood risk. The Directive also reinforces the rights of the public to access this information and to have a say in the planning process (European Commission, 2007).
Floods are one of the most common hazards. Due to their usually slow onset, groundwater flooding’s main health effects are related to water-borne and vector-borne diseases. 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 problems (acute and long- term). 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 affecting 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).
The figure below summarises common interactions between groundwater 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.
Effective risk management involves continuous groundwater monitoring, hydrological modelling, and restricting development in high-risk zones. Sustainable drainage systems and early warning mechanisms can help mitigate impacts, particularly for infrastructure, basements, and agricultural land. Long-term resilience depends on coordinated planning among local authorities, environmental agencies, and water management institutions. (Environment Agency, 2012)
The section above and the table below offer an overview of monitoring groundwater 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 services tracking prolonged rainfall, snowmelt, and climate patterns that contribute to rising groundwater levels. - Hydrological services monitoring groundwater tables, soil moisture levels, and aquifer recharge rates to assess flood potential. - Geological and geophysical agencies studying land subsidence, soil permeability, and underground water flow dynamics that influence groundwater flooding. - National, subnational, and local disaster management agencies responsible for monitoring groundwater flooding risks and coordinating emergency response. - Health authorities assessing public health risks from contaminated groundwater, waterborne diseases, and sanitation issues. - Engineers and water resource management agencies designing drainage systems, flood mitigation structures, and sustainable groundwater management strategies. - Land use and urban planners integrating groundwater flood risk assessments into zoning laws, infrastructure planning, and water conservation strategies. - Energy and utility companies monitoring groundwater flooding risks to underground infrastructure, including power stations, tunnels, and water treatment plants. - Researchers and academics, including hydrologists and environmental scientists, studying groundwater behaviour, long-term aquifer changes, and climate impacts on subsurface flooding. - Environmental organisations and conservation groups working on natural groundwater management solutions, wetland restoration, and sustainable water usage. | How is the Hazard Observed/Monitored/Forecast? | Groundwater floods are monitored using boreholes, groundwater sensors, and satellite data to track rising water tables and soil moisture levels. Hydrologists analyse prolonged rainfall, snowmelt, and aquifer recharge rates to assess flood risks. Forecasting groundwater floods involves computer models that simulate underground water movement based on weather patterns and geological conditions. Since groundwater flooding develops slowly but lasts longer than surface floods, early warnings help communities, infrastructure managers, and planners prepare for prolonged waterlogging and its impacts.
Sinkhole
Ponding (Drainage) Flooding
An ice-jam flood including debris is defined as an accumulation of shuga including ice cakes, below ice cover. It is broken ice in a river which causes a narrowing of the river channel, a rise in water level and local floods (WMO, 2012). Shuga is defined as the accumulation of spongy white ice lumps, a few centimetres across, formed from grease ice or slush, and sometimes from anchor ice rising to the surface (WMO, 2012).
Flash Flood
Ice-jam flooding is caused by an accumulation of ice in a river, stream or other flooding source that reduces the cross-sectional area available to carry the flow and forces an increase in water-surface elevation (WMO, 2012).
In rivers/streams that experience seasonal ice formation and melt, if the melting is more rapid upstream than downstream, ice floes can accumulate in rivers, forming constrictions and damming flows, causing river levels to rise upstream of the ice jam. A sudden release of the ‘ice jam’ can cause a flood wave similar to that caused by a dam break, to move downstream. Both meltwater and heavy rainfall in steep areas can cause landslips and debris flows. As the debris picked up by rivers moves downstream, major constrictions can build up. When these build-ups collapse or are breached, severe flooding can result. These phenomena are difficult to predict (WMO, 2011; FEMA, 2018).
Floods are one of the most common hazards. In line with the general impacts defined in the chapeau, ice-jam flooding can have both a slow onset (when flooding occurs upstream to the ice jam) and an abrupt occurrence (similar to a dam break) when the ice jam collapses, leading to a flash-flood-like impact downstream. 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 problems (acute and long- term). 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 affecting 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).
Not applicable.
The figure below summarises common interactions between ice-jam flooding, including debris, 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.
The section above and the table below offer an overview of monitoring ice-jam flooding, including debris. 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 services tracking temperature fluctuations, precipitation, and freeze-thaw cycles that contribute to ice jam formation. - Hydrological services monitoring river levels, ice thickness, flow velocity, and debris accumulation in water channels. - Geological and geophysical agencies studying riverbed morphology, sediment transport, and terrain conditions affecting ice jam development. - National, subnational, and local disaster management agencies responsible for monitoring ice-jam flood risks, issuing warnings, and coordinating emergency response. - Health authorities assessing public safety risks, including waterborne diseases, hypothermia exposure, and emergency medical needs. - Engineers and water resource management agencies designing ice control structures, flood defences, and debris removal strategies. - Land use and urban planners incorporating ice-jam flood risk assessments into zoning laws, infrastructure siting, and watershed management. - Energy and utility companies monitoring risks to hydropower plants, bridges, pipelines, and other infrastructure vulnerable to ice and debris blockages. - Researchers and academics, including hydrologists and climate scientists, studying ice jam formation, flood forecasting, and climate change impacts on freeze-thaw dynamics. - Environmental organiszations and conservation groups working on river ecosystem management, ice jam mitigation, and debris control strategies. | How is the Hazard Observed/Monitored/Forecast? | Ice-jam flooding, including debris, is monitored using river gauges, aerial and satellite imagery, and on-the-ground observations to track ice buildup and water levels. Hydrologists analyse temperature fluctuations, river flow rates, and freeze-thaw cycles to predict when and where ice jams may form. Models simulate ice movement and water flow, helping forecasters issue early warnings. By combining real-time data with historical patterns, experts provide alerts to protect communities, infrastructure, and ecosystems from sudden flooding and debris blockages.
A ponding flood is a flood that results from rainwater ponding at or near the point where it falls because it is falling faster than the drainage system (natural or man-made) can carry it away (WMO, 2006).
Pluvial Flood
Drainage flood
Surface retention
Waterlogging
The Federal Emergency Management Agency (FEMA) refers to the term “ponding hazard”, defined as “a flood hazard that occurs in flat areas when there are depressions in the ground that collect "ponds" of water” (FEMA, 2020).
Ponding flooding, also known as waterlogging, is primarily driven by a combination of intense or prolonged rainfall that exceeds both the soil’s infiltration capacity and the limits of urban drainage infrastructure. In cities, the widespread replacement of permeable land with impervious surfaces such as asphalt and concrete significantly increases surface runoff, leading to rapid accumulation of water during storms (Environment Agency, 2011). This effect is particularly severe in topographic depressions—natural or human-made—where water tends to collect, including in areas of land subsidence or reclaimed terrain. Inadequate or ageing drainage systems, often designed using outdated precipitation data, struggle to cope with current rainfall extremes and are prone to failure (UN-Habitat, 2020). Additionally, human-induced changes such as wetland loss, which eliminates natural water retention, and modifications to landforms from agriculture or construction can disrupt historical drainage pathways and increase vulnerability (BGS, 2023). These drivers, taken together, contribute to the rising incidence and severity of ponding flooding, particularly in rapidly urbanising regions.
Floods are one of the most common hazards. Similar to groundwater flooding, and notwithstanding the general impacts of flooding provided in the chapeau, due to their usually slow onset, ponding flood’s main health effects are related to water-borne and vector-borne diseases. 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 problems (acute and long- term). 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 affecting 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).
The figure below summarises common interactions between ponding 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.
The section above and the table below offer an overview of monitoring ponding 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 services tracking heavy rainfall, storm intensity, and weather conditions that contribute to drainage system overloads. - Hydrological services monitoring soil moisture, runoff patterns, and water accumulation in urban and rural areas. - Geological and geophysical agencies studying land permeability, topography, and ground stability that affect water drainage. - Municipal and urban drainage authorities managing stormwater infrastructure, sewer systems, and flood mitigation efforts. - National, subnational, and local disaster management agencies responsible for monitoring drainage-related flood risks, issuing warnings, and coordinating emergency response. - Health authorities assessing public health risks related to stagnant water, mosquito-borne diseases, and water contamination. - Engineers and infrastructure agencies designing and maintaining storm drains, retention basins, and permeable surfaces to manage excess water. - Land use and urban planners integrating drainage flood risk assessments into zoning laws, building regulations, and sustainable urban development. - Energy and utility companies monitoring flood risks to underground electrical grids, sewage treatment plants, and water supply systems. - Researchers and academics, including hydrologists and urban planners, studying urban flooding, drainage efficiency, and climate change impacts on stormwater management. - Environmental organisations and conservation groups promoting green infrastructure, wetland restoration, and sustainable drainage solutions. | How is the Hazard Observed/Monitored/Forecast? | Ponding (drainage) floods are monitored using rain gauges, weather radar, and drainage system sensors to track heavy rainfall and water accumulation in low-lying areas. Hydrologists and urban planners assess soil saturation, stormwater capacity, and runoff patterns to predict potential flooding. Computer models simulate drainage efficiency, helping forecasters issue early warnings. By combining real-time data with historical trends, experts provide alerts to help cities, infrastructure managers, and communities prevent and manage localised flooding.
A snowmelt flood is a significant flood rise in a river caused by the melting of snowpack accumulated during the winter (WMO, 2012).
Melt-induced flooding
Snowmelt-runoff floods
In upland and high-latitude areas where extensive snow accumulates over winter, the spring thaw produces meltwater runoff. If temperature increases are rapid, the rate of melting may produce floods, which can extend to lower parts of the river systems. The severity of meltwater floods will increase if the thaw is accompanied by heavy rainfall and can be further exacerbated if the subsoil remains frozen. Although a seasonal occurrence, where major snowfields exist in headwaters, which may produce beneficial flooding in downstream areas, severe effects can occur on smaller scales, especially in areas subject to changes between cold and warmer rainy winter weather (USGS, no date) and steep slopes.
High soil moisture conditions prior to snowmelt, frozen ground, heavy snow cover, widespread heavy rain during the melt period, and rapid snowmelt (unseasonably warm temperatures, high humidity, rainfall, etc.) (NOAA, no date).
Notwithstanding the general considerations provided in the chapeau about floods, snowmelt floods can have similar impacts to flash floods (in the upper part of the catchment, combined with a fast thaw) or riverine floods (downstream to the melting area). The effect of snowmelt on potential flooding, mainly during the spring, causes concern for many people around the world. Besides flooding, rapid snowmelt can trigger landslides and debris flows. In combination with specific weather conditions, such as excessive rainfall on melting snow, for example, it may even be a major cause of floods (USGS, no date). Health impacts from floods: Floods are one of the most common hazards. 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 problems (acute and long-term). 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 affecting the proper functioning of health facilities, including cold chain (WHO, no date). Floods can potentially increase the transmission of communicable diseases (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).
The figure below summarises common interactions between snowmelt 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.
Cryosphere observation is needed to monitor the snowmelt rate, combined with traditional streamflow observation and flood forecasting techniques downstream to the area experiencing snowmelt. The Flash Flood Guidance System (FFGS) of the World Meteorological Organization takes into account estimated precipitation from several sources, such as satellites, radar as available, and gauges as available to be input into a snow model (SNOW -17) which estimates snow water equivalent (SWE) and melt that are input into the Sacramento-soil moisture accounting model (SAC-SMA) to estimate upper soil moisture (soil water deficit). SNOW-17 uses air temperature as an index to determine energy exchange across the snow-air interface. In addition to temperature, the only other input variable needed to run the model is precipitation. Air temperature is also used to estimate snowmelt. SWE is referred to as the depth of water produced if a snow cover is completely melted on a horizontal surface. The SWE product generated by FFGS is a direct output of the SNOW-17 accumulation and ablation model. Melt is the estimate of ablation due to melt processes and is the direct output of the SNOW-17 model (WMO, 1999).
The section above and the table below offer an overview of monitoring snowmelt 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 services tracking temperature fluctuations, precipitation, and weather patterns influencing snowmelt rates. - Hydrological services monitoring snowpack levels, river flows, soil moisture, and groundwater recharge to assess flood risks. - Geological and geophysical agencies studying terrain characteristics, soil permeability, and land stability that affect snowmelt runoff. - National, subnational, and local disaster management agencies responsible for monitoring snowmelt flood risks, issuing alerts, and coordinating emergency response efforts. - Health authorities assessing public health risks from floodwater contamination, infrastructure failures, and emergency medical needs. - Engineers and water resource management agencies designing and maintaining flood control structures, reservoirs, and drainage systems to manage excess runoff. - Land use and urban planners incorporating snowmelt flood risk assessments into zoning laws, watershed management, and urban development strategies. - Energy and utility companies monitoring flood risks to hydropower stations, water treatment facilities, and critical infrastructure in snowmelt-prone regions. - Researchers and academics, including hydrologists and climate scientists, studying snowmelt dynamics, climate change impacts, and flood forecasting models. - Environmental organisations and conservation groups working on watershed management, reforestation, and ecosystem-based flood mitigation strategies. | How is the Hazard Observed/Monitored/Forecast? | Snowmelt floods are monitored using satellite imagery, snowpack sensors, river gauges, and weather stations that track temperature, snow depth, and water content in the snow. Hydrologists analyse how quickly snow is melting and how much water will flow into rivers and streams. Forecast models combine temperature trends, soil moisture, and precipitation data to predict flooding risks. Early warnings help communities, emergency responders, and infrastructure managers prepare for rising water levels and potential flooding.
Surface water flooding is that part of the rain which remains on the ground surface during rain and either runs off or infiltrates after the rain ends, not including depression storage (WMO, 2012).
Surface detention
Depression storage
Surface water flooding is caused when the volume of rainwater falling does not drain away through the existing drainage systems or soak into the ground but lies on or flows over the ground instead. This type of flooding is usually short-lived and associated with heavy downpours of rain, thunderstorms etc. (NFU, 2019). The UK Government provides a real time flood information service which is easily accessible (UK Government, no date).
Floods tend to be caused by a number of natural events such as rain from slow- moving or stationary low-pressure areas, thunderstorms, and tropical cyclones. The amount and duration of the rainfall, soil type and saturation, geography and whether it is an urban area can affect the magnitude and impacts of the flooding. Flooding can occur at the place of a heavy rain event or far downstream away from the causal rain event. Flooding can also be caused by other factors such as storm surges, tsunamis, ice jams, glacial lake outbursts, as well as being human influences such as dam bursts, improper land use planning, etc. (Geoscience Australia, no date).
This type of flooding’s impacts are very similar to those of groundwater flooding or ponding. Floods are one of the most common hazards. 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 problems (acute and long- term). 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 affecting 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).
The figure below summarises common interactions between surface water 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.
The section above and the table below offer an overview of monitoring surface water 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 services tracking intense rainfall, storm systems, and weather conditions that contribute to surface water flooding. - Hydrological services monitoring runoff patterns, soil moisture levels, and drainage system capacity to assess flood risks. - Geological and geophysical agencies studying land characteristics, permeability, and topography that influence water pooling and flood extent. - National, subnational, and local disaster management agencies responsible for monitoring surface water flooding, issuing warnings, and coordinating emergency response. - Municipal and urban drainage authorities managing stormwater infrastructure, retention basins, and drainage networks to prevent flooding. - Health authorities assessing public health risks from stagnant water, waterborne diseases, and sanitation issues caused by surface water flooding. - Engineers and infrastructure agencies designing flood mitigation measures such as permeable pavements, stormwater management systems, and urban drainage improvements. - Land use and urban planners incorporating surface water flood risk assessments into zoning regulations, sustainable development plans, and watershed management. - Energy and utility companies monitoring flood risks to underground electrical grids, water treatment plants, and transportation networks. - Researchers and academics, including hydrologists and urban planners, studying urban flooding patterns, climate change impacts, and flood forecasting techniques. - Environmental organiszations and conservation groups working on natural water retention solutions, wetland conservation, and infrastructure projects. | How is the Hazard Observed/Monitored/Forecast? | Surface water flooding is monitored using weather radar, rain gauges, and drainage system sensors to track heavy rainfall and water accumulation in streets, fields, and low-lying areas. Hydrologists analyse soil saturation, runoff patterns, and stormwater capacity to predict where flooding might occur. Computer models simulate water flow and drainage performance, helping forecasters issue early warnings. By combining real-time data with historical flood patterns, experts provide alerts that enable cities, infrastructure managers, and communities to prepare for and mitigate surface water flooding.