Flood Terms

Terms defined for each forecast point which describe or categorize the severity of flood impacts in the corresponding river/stream reach. Each flood category is bounded by an upper and lower stage (see Example 1). The severity of flooding at a given stage is not necessarily the same at all locations along a river reach due to varying channel/bank characteristics or presence of levees on portions of the reach. Therefore, the upper and lower stages for a given flood category are usually associated with water levels corresponding to the most significant flood impacts somewhere in the reach.

minimal or no property damage, but possibly some public threat

some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations

extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations

flooding which equals or exceeds the highest stage or discharge at a given site during the period of record keeping

A rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam). However, the actual time threshold may vary in different parts of the country. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters.

Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.

Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the occurrence is neither certain or imminent.

(FLW) In hydrologic terms, a release by the NWS to inform the public of flooding along larger streams in which there is a serious threat to life or property. A flood warning will usually contain river stage (level) forecasts.

Issued to inform the public and cooperating agencies that current and developing hydrometeorological conditions are such that there is a threat of flooding, but the occurrence is neither certain nor imminent.

is normally issued for flooding that develops more gradually, usually from prolonged and persistent moderate to heavy rainfall. This results in a gradual ponding or buildup of water in low-lying, flood prone areas, as well as small creeks and streams. The flooding normally occurs more than six hours after the rainfall begins, and may cover a large area. However, even though this type of flooding develops more slowly than flash flooding, it can still be a threat to life and property.

The one hundred-year flood, also called the base flood, is the flood that has a one percent chance of being equaled or exceeded in any given year.

A floodplain is any land susceptible to being inundated by water.

A floodway is a channel of a river or other watercourse and adjacent land that must be reserved in order to pass the 100-year flood without increasing the elevation more than a designated height.

The Flood Insurance Rate Map (FIRM) is designed to illustrate special flood hazard areas, e.g., the limits of the floodway, floodplain, and 100-year flood.

The Flood Insurance Study (FIS) provides detailed information for a watercourse that has had a detailed study performed on it. Your local flood insurance study can provide base flood elevations.