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.