Flood

This Copernicus Sentinel-1 image combines two acquisitions over the same area of eastern Iraq, one from 14 November 2018 before heavy rains fell and one from 26 November 2018 after the storms. The image reveals the extent of flash flooding in red, near the town of Kut. Image: modified Copernicus Sentinel data (2018), processed by ESA, CC BY-SA 3.0 IGO.

Definition

Flood is usually used as a general term to describe the overflow of water from a stream channel into normally dry land in the floodplain (riverine flooding), higher-than–normal levels along the coast and in lakes or reservoirs (coastal flooding) as well as ponding of water at or near the point where the rain fell (flash floods) (IRDR Glossary).

Facts and figures

Floods are the natural hazard with the highest frequency and the widest geographical distribution worldwide. According to the Organization for Economic Cooperation and Development (OECD)  flooding is one of the most common, widespread and destructive natural perils, affecting approximately 250 million people worldwide and causing more than $40 billion in damage and losses on an annual basis (OECD).

Flooding occurs most commonly from heavy rainfall when natural watercourses lack the capacity to convey excess water. It can also result from other phenomena, particularly in coastal areas, by a storm surge associated with a tropical cyclone, a tsunami or a high tide. Dam failure, triggered by an earthquake, for instance, will lead to flooding of the downstream area, even in dry weather conditions.

Various climatic and non-climatic processes can result in different types of floods: riverine floods, flash floods, urban floods, glacial lake outburst floods and coastal floods.

Flood magnitude depends on precipitation intensity, volume, timing and phase, from the antecedent conditions of rivers and the drainage basins (frozen or not or saturated soil moisture or unsaturated) and status. Climatological parameters that are likely to be affected by climate change are precipitation, windstorms, storm surges and sea-level rise (UNDRR).

When floodwaters recede, affected areas are often blanketed in silt and mud. The water and landscape can be contaminated with hazardous materials such as sharp debris, pesticides, fuel, and untreated sewage. Potentially dangerous mold blooms can quickly overwhelm water-soaked structures. Residents of flooded areas can be left without power and clean drinking water, leading to outbreaks of deadly waterborne diseases like typhoid, hepatitis A, and cholera (UNDRR).

Further information

UN Guidelines for Reducing Flood Losses
OECD Report on Financial Management of Flood Risk
PreventionWeb Floods

UN-SPIDER Regional Support Offices with hazard-specific expertise

Argentina Regional Support Office
Asian Disaster Reduction Center (ADRC) Regional Support Office
Brazil Regional Support Office
Germany Regional Support Office
Greece Regional Support Office
Indonesia Regional Support Office
International Centre for Integrated Mountain Development (ICIMOD)
International Water Management Institute (IWMI)
Nigeria Regional Support Office
Pakistan Regional Support Office
Romania Regional Support Office
United States of America Regional Support Office

Related content on the Knowledge Portal

News

International Charter activated for floods in Yemen
Logo of the International Charter "Space and Major Disasters"

The International Charter Space and Major Disasters has been activated for flooding in Yemen on 2 June.

Cyclone Mekunu brought severe rainfall and winds causing flooding in areas of Yemen as it made landfall on 25 May. Local reports suggest that at least seven people have been killed and over 1,000 families have been displaced due to the flooding. The cyclone and floods have also damaged infrastructure and agricultural equipment in many districts of the Yemeni mainland. Some 120 fishing ships have reportedly been lost to the cyclone and a search is ongoing for missing fishermen.

One of the worst affected areas is the Yemeni island of Socotra in the Arabian Sea. Residents on the island are fleeing from torrential floods and more than 30 people are reported missing. Smaller islands off the coast of Socotra have also been heavily impacted and there is growing concern for the 2,500 families living on the islands of Abd Al Quri and Samhah as... read more

Publishing date: 04/06/2018
International Charter activated for floods in Djibouti and Sri Lanka

The International Charter Space and Major Disasters has been activated for flooding in Djibouti on 20 May and Sri Lanka on 21 May 2018.

In Djibouti, tropical cyclone Sagar caused flash floods across the country, impacting up to 30,000 people according to authorities. The United Nations Institute for Training and Research's (UNITAR) Operational Satellite Applications Programme (UNOSAT) requested the activation on behalf of the United Nations Office for the Coordination of Humanitarian Affairs (UNOCHA). UNOSAT is acting as the project manager for the activation.

In Sri Lanka, monsoon rains have claimed the lives of eight people and have left a further 172 requiring emergency assistance. Rain and winds have closed roads, interrupted power lines and displaced many people. With the continuation of heavy rains, landslides also became an issue.

The activation... read more

Publishing date: 22/05/2018
GPS sensors used to aid the forecasting of monsoon flash floods
Highway 78 in San Diego State flooded by monsoon rains in 2013. Image: NASA

The forecasting of torrential monsoon downpours has become more accurate due to improved GPS and geodetic sensors. The system, developed by meteorologists at the National Oceanic and Atmospheric Administration (NOAA) Weather Forecast Offices in the United States of America has been in use since 2012. It features next-generation, real-time geodetic modules that have been added to existing GPS stations across California, enabling more accurate rain and weather forecasting.

More than half the annual rainfall in the American Southwest falls in the form of the unpredictable downpours during the North American monsoon season. Whilst monsoon precipitation is essential for the ecosystem of the region, the torrential rain often causes flash flooding and hence poses a risk to life and property. Technology that aids the accurate forecasting of these rains is therefore of paramount importance.

The “Next-Generation Real-Time Geodetic Station Sensor... read more

Publishing date: 15/05/2018
Unrecorded Himalayan floods discovered though satellite data
Runoff water of melting glaciers in the Himalayas. Image: Suket Dedhia.

Researchers at the University of Potsdam in Germany have analysed satellite data of nearly 30 years to retrace glacial lake outburst floods across Bhutan, China, India and Nepal. As a result of the survey, the scientists were able to reveal 10 previously undocumented floods in Bhutan, China and Nepal which had been caused by glacial runoff water suddenly breaking through moraine dams.

As global temperatures rise, runoff water of melting glaciers creates unstable lakes. Sudden breaches of those lakes can lead to natural disasters with devastating social, economic and environmental consequences as the lakes’ water bulks deluge villages, farmland, and important infrastructure. Because the Himalayas are home to a quarter of all glaciers world-wide, countries such as Bhutan, China, India and Nepal are particularly exposed to flooding.

Gathering in situ data in an area as vast as the Himalayas can be challenging, especially due to mountainous... read more

Publishing date: 14/05/2018
International Charter activation for flood in Somalia
Homes flooded after river Shabelle ran over in Belet Weyne Capital City of Hiran, Somalia. Image: Ahmed Qeys.

The International Charter Space and Major Disasters has been activated for flooding in Somalia on 7 May, caused by weeks of heavy rainfall.

The rainwater has caused the Shabelle and Juba rivers to run over and flood parts of HirShabelle, Jubbaland and South West state. As a result, more than 200,000 people had to be evacuated and emergency settlements are being built up for the victims. However, limited access to clean water might increase infections of water-borne diseases.

The activation has been requested by UNITAR-UNOSAT on behalf of United Nations Office for the Coordination of Humanitarian Affairs.

Publishing date: 11/05/2018
Using new techniques in Greece to track the effects of flooding
The Sentinel-1 RGB stack showing (in red) flooding on February 23, 2018. Image: Sentinel Playground

Greek Geographic Information Systems company Geospatial Enabling Technologies (GET) has tested a new crisis management solution based on services rendered through the Copernicus Sentinel Hub after flooding in Farkadona, Greece.

The Copernicus Sentinel Hub, set up by the European Commission and European Space Agency, is a service that provides quick and up-to-date satellite information to be used by authorities. Through the Sentinel-1 satellite in particular, they use radar to track differences between flooded and dry land through before and after images.

In late February, flooding and landslides caused by heavy rainfall affected the Farkadona municipality in central Greece, leading to evacuations and loss of farmland in the region. In response, GET used imagery produced by the Sentinel-1 satellite in a prototypical algorithm that uses the Sentinel Application Platform (SNAP) and Open Geospatial Consortium (OGC) to produce data sets. This... read more

Publishing date: 30/04/2018

Data Source

MI-SAFE
Screenshot of MI-SAFE
Publishing institution: Foreshore Assessment using Space Technology (FAST)
DEM, LiDAR, Satellite data
South African Risk and Vulnerability Atlas (SARVA)
Publishing institution: South African Environmental Observation Network (SAEON)
ETOPO1 Global Relief Model
Screenshot of ETOPO1 Global Relief Model
Publishing institution: National Oceanic and Atmospheric Administration (NOAA)

UN-SPIDER Training Activity

Sri Lanka - National Training Course
Training workshop on the UN-SPIDER Knowledge Portal and the UN-SPIDER Recommended Practices on floods and droughts. | Date of training: 26/04/2017 to 28/04/2017

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