What is a Flood?

What is a Flood?

A flood is an overflowing of water onto land that is normally dry. It can happen during heavy rains, when ocean waves come onshore, when snow melts too fast, or when dams or levees break. Flooding may happen with only a few inches of water, or it may cover a house to the rooftop. The most dangerous is the flash flood, which happens quickly with little or no warning.Other events occur over a long period and may last days, weeks, or longer.

Some occur without problem. Others are devastating, causing large-scale destruction and significant loss of life.  Flooding is experienced all over the world and in some countries such as Bangladesh, flooding occurs regularly.

Both the US Federal Emergency Management Agency (FEMA) and The National Flood Insurance Program define “flood” as:

A general and temporary condition of partial or complete inundation of two or more acres of normally dry land area or of two or more properties (at least one of which is your property) from:

  • Overflow of inland or tidal waters
  • Unusual and rapid accumulation or runoff of surface waters from any source
  • A mudflow
  • Collapse or subsidence of land along the shore of a lake or similar body of water as a result of erosion or undermining caused by waves or currents of water exceeding anticipated cyclical levels that result in a flood.

Types of Floods

River floods

A river flood is also known as fluvial flooding. They occur when water levels rise in a river due to excessive rain from tropical systems making landfall. Thunderstorms over the same area for extended periods of time can be a cause. Combined rainfall and snow-melt, or an ice jam are other causes. Watercourses are more likely to be overwhelmed when rainwater cannot be absorbed into the land onto which it falls. It might be very steep, water logged, or built over. Rapid melting of snow also leads to river flooding in some cases.

Sewer floods

Sewer flooding occur when sewers are overwhelmed by heavy rainfall or when pipes become blocked. Public sewers are designed to protect properties from the risk of flooding in normal wet weather conditions, however, with extreme weather conditions there will always be a  risk. In urban areas, surface water flooding and sewer flooding often combine, thus polluting the floodwater.

Burst water mains

Burst water mains can cause localized disruption to transport links and damage to buildings, particularly properties with a basement.

Surface water floods

Surface water floods occur where high rainfall events exceed the drainage capacity in an area. Such events can also lead to serious flooding of property and possessions where surface water flows and collects, usually in low-lying areas.

Reservoir floods

These occur after the failure of the reservoir’s walls or earth embankments. It may be caused by erosion due to seepage, over-topping of the dam or by accidental damage to the structure..

Coastal floods

These occur when a hurricane, tropical storm, or tropical depression produces a deadly storm surge that overwhelms coastal areas as it makes landfall. Storm surge is water pushed on shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the average water level 15 feet or more. The greatest natural disaster in the United States, in terms of loss of life, was caused by a storm surge and associated coastal flooding from the great Galveston, Texas, hurricane of 1900. At least 8,000 people lost their lives.

Inland floods

When tropical cyclones move inland, they are typically accompanied by torrential rain. If the decaying storm moves slowly over land, it can produce rainfall amounts of 20 to 40 inches over several days.

Flash floods

A flash flood is a rapid rise of water along a stream or low-lying urban area. Flash flooding occurs within six hours of a significant rain event and is usually caused by intense storms that produce heavy rainfall in a short amount of time. Excessive rainfall that causes rivers and streams to swell rapidly and overflow their banks is frequently associated with hurricanes and tropical storms, large clusters of thunderstorms, super-cells, or squall lines. Other types of flash floods can occur from dam or levee failures, or a sudden release of water held by an ice jam. Heavy rainfall in the mountains can cause downstream canyon flooding.