A flood is usually defined as overflow of an expanse of water that submerges. Normally, land Floods may be a few inches deep, or may rise to cover whole houses. Flooding is a nationwide and year-round threat to the United States and its territories. Flooding can be a local event, affecting only a small neighborhood, settlement, or community, but it can also be a very large one, affecting whole cities, vast territories, or even multiple states. Flooding can cause extensive flood damage to homes, and sometimes - in the case of the hurricane Katrina, for example - damage to entire communities as well.

Floods are considered disasters due to their effects on life, property, and the disruption of normal activities in and around their affected area. When disasters involving water occur, flood damage is an expected and unwelcome consequence.

Floods and the resulting flood damage may occur due to a variety of causes. The most common cause is heavy rain. Heavy rains often overwhelm urban water drainage systems, and cause rivers and waterways to overflow. Heavy rains may also cause dams and similar water reservoir to overflow. Floods may also occur due to tidal waves brought upon by sea storms. Ice and snow melt can cause floods. Catastrophes such as dam breakage are another cause.

The United States National Weather Service has defined the terms flood and flash flood. The term flood is used when the overflow or inundation of water occurs over a period of more than 6 hours. Floods take more than 6 hours or may take days to occur, and then may take weeks to dissipate. The term flash flood is used when an area becomes submerged under water in less than 6 hours. Flash floods may occur within minutes of heavy rainfall, or may immediately occur when catastrophes such as a dam failure or a tidal wave occur.

Watches and warnings are issued by the U.S. National Weather Service. The National Weather Service issues these Watches and Warnings through its website, the mass media - commercial radio, television, and through the National Oceanic and Atmospheric Administration's (NOAA) Weather Radio. A flood watch is issued when flooding may occur at a particular area. People within the vicinity of the flood watch are advised to listen to the mass media for more information. A flash flood watch is issued when flash flooding is possible - people are therefore advised to move to higher ground, and to listen to the mass media for more information. A flood warning is issued if flooding has already occurred, or would be likely to occur in the area - people advised to evacuate should do so immediately. A flash flood warning is issued when a flash flood is already occurring - people are advised to seek higher ground immediately.

People living in floodplains and other known flood-prone areas are advised to have elevated and reinforced homes in order to avoid flood damage. Electrical systems, furnaces, and the heating systems should be elevated, and additional structures should be built in order to minimize the effects of flooding - barriers such as floodwalls and levees to keep floodwater from entering the building, and increased waterproofing on walls and basements to prevent seepage causing flood damage.

Flood insurance is a must-have, especially for those living in floodplains and similar areas. For those people that are not in floodplains, remember that there are several causes of floods, such as dam failure. Flood insurance will help you claim for flood damage and thus enable you to rebuild your home.

Know your area's flood risk, and act accordingly. The American Red Cross, the local emergency management office, and your city's planning and zoning offices will be very helpful sources.