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).

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

Flash flooding has been described as the main environmental risk to people living in the capital by the Greater London Authority (GLA). One solution put forward is that London should become a "sponge city".
British Broadcasting Corporation (BBC)
As Kazakhstan and parts of Russia grapple with severe flooding due to rapid and unusually early snowmelt and heavy rainfall, the stark reality of climate change impacts becomes increasingly undeniable.
Modern Diplomacy
This study proposes a high tide flood (HTF) thresholding system that leverages machine learning (ML) techniques to estimate SLR and HTF thresholds at a relatively fine spatial resolution (10 km) along the United States’ coastlines.
A man treks the banks of a lake high up Nepal near Mt. Everest
Despite earlier mitigation measures, the ‘critically dangerous’ Tsho Rolpa glacial lake has expanded further, with 6,000 households facing an imminent threat.
Dialogue Earth
Officials in Indonesia have begun cloud seeding as they try to prevent further rain and flash flooding after a deluge hit the country's Sumatra Island over the weekend and left at least 67 people dead and another 20 missing.
Australian Broadcasting Corporation
Arial view of rainforest jungle in Brasil
In Rio Grande do Sul - an area larger than the UK - is currently affected by unprecedented floods. The flying river has acted like a firehose, fuelling five months of rainfall in just two weeks.
Conversation Media Group, the
SEI has been working with local partners to improve early warning systems on the Thailand-Myanmar border, and to build resilience to drought and climate change.
Stockholm Environment Institute
Research published this week estimated that the record floods could reduce Britain's ability to feed itself by nearly 10% due to a reduced crop area and poor yields.

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