Flood

Flooding is an overflowing of water onto land that is normally dry (NOAA). There are various categories of floods. Coastal flooding is most frequently the result of storm surges and high winds coinciding with high tides (WMO, 2011).

A flash flood is a flood of short duration with a relatively high peak discharge in which the time interval between the observable causative event and the flood is less than four to six hours (WMO, 2006). A fluvial flood is a rise, unusually 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). 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).

Floods affect more people than any other hazard. Worldwide, nearly 200 million live in coastal zones at risk of flooding. Flooding is usually the result of heavy or continuous rain that exceeds the absorptive capacity of the soil and the flow capacity of rivers, streams and coastal areas. Floods can be triggered by thunderstorms, tornadoes, tropical cyclones, monsoons, melting snow and dam breaks. The most common floods are flash floods, snowmelt floods, coastal floods and river floods. Flash floods and sudden floods are the most dangerous, especially when they occur at night.

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 and property. IFM, like Integrated Water Resources Management, should encourage the participation of users, planners and policymakers at all levels (APFM).

This page refers to different types of floods, such as:

  • Coastal flood: Coastal flooding 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.
  • Fluvial flood: A fluvial flood is 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).
  • Flash flood: A flash flood is a flood of short duration with a relatively high peak discharge in which the time interval between the observable causative event and the flood is less than four to six hours (WMO, 2006).
  • 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).

Risk factors

  • Rapid population growth.
  • Rapid urbanization.
  • Environmental degradation: loss of forests and natural flood buffers.
  • Climate change will expose more people to future floods.
  • Melting glaciers and rising sea levels will bring floods to places not previously at risk.

Vulnerable areas

  • Developing countries are most at risk.
  • Although Asia remains the continent most hit by floods, Africa and Latin America are also heavily affected.
  • The poor, with the least means to adapt are often forced to live in high-risk places: slopes, flood plains, ravines, or in crowded, urban low-lying areas in mega-cities.

Risk reduction measures

  • 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.

Latest Flood additions in the Knowledge Base

Aerial Photos of flooding caused by Hurricane Florence
Research briefs
MIT scientists have developed a method that generates satellite imagery from the future to depict how a region would look after a potential flooding event.
Massachussets Institute of Technology
Spring flooding along the Ottawa and Bonnechere Rivers, Ottawa Valley, Ontario Canada
Research briefs
A UBC study finds that conserving just five per cent of watersheds (two per cent of Canada’s land) could shield more than half of urban floodplains, safeguarding millions.
University of British Columbia
Flooded street in the city of York in the United Kingdom. The first story of the red brickstone houses is under water.
Update
Maeve Sherry argues that the best way to defend urban areas in the UK from flood risk is to seek nature-based solutions like green spaces, wetlands and parks and use the biodiversity net gain legislation to achieve this.
London School of Economics and Political Science, the
Tonga drill
Update
Weather Ready Pacific - a major ten-year programme – aims at reducing the human and economic cost of severe weather events, protecting Pacific Island communities and livelihoods on the frontline of climate change.
World Meteorological Organization (WMO)
Aerial picture of a flooded Texan residential area
Research briefs
A new study published in Nature Climate Change estimates that a 1-meter sea level rise by 2100 would affect over 14 million people and $1 trillion worth of property along the Southeast Atlantic coast, from Norfolk, Virginia, to Miami, Florida.
Virginia Tech
Update
The Government of Panama received US$26.7 million from CCRIF SPC within 14 days of a rainfall event that affected that country during October 31 to November 4, 2024.
Caribbean Catastrophe Risk Insurance Facility, the
Student gets his head wet to beat the heat of scorching sun
Update
UNICEF calls for urgent measures as children and young people not included substantively in national climate plans.
United Nations Children's Fund (UNICEF)
Update
AI can predict climate change-fuelled floods but communication gaps and lack of investment in data and systems can hobble response.
Context
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