Flood stage
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Flood stage is the level at which the surface of a river, creek, or other body of water has risen to a sufficient level to cause damage or affects use of man-made structures. When a body of water rises to this level, it is considered a flood event.
[edit] Definition
Flood stage is the water level of a stream as read by a gauge for a particular location, measured from the level at which a flowing body of water threatens lives, property, or commerce.[1] The term "at flood stage" is commonly used to describe the point at which this occurs. "Stream stage" (also referred to as "gage height" or simply "stage") is the level of the water surface above an established zero level at a given location. The zero level can be arbitrary, but is usually close to the bottom of the stream or river. Stream stage was traditionally measured visually using a staff gage, which is a fixed ruler marked in 1/100 and 1/10 foot intervals,[2] however electronic sensors that transmit real-time information to the internet are now used for a vast majority of measurements.[3] The flood stage measurements are given as a height above the zero level.
While usually the flood stage is sometimes set at the elevation of the floodplain, it can be higher (if there are no structures or roads immediately on the floodplain) or lower (if there are structures such as marinas or lakehouses low on the banks of the body of water) depending on the location. Because flood stage is defined by man-made elements, as opposed to the natural topography of the area, flood stages are usually only calculated for bodies of water near communities.
The flood stage can be listed for an entire community, in which case it is set to the lowest man-made structure in the area, or for a specific location ("flood stage is 2 feet on Maple Street at First Avenue" means that the specified intersection will flood when the flood stage reaches 2 feet).
In the United States during flood events, the National Weather Service will issue flood warnings or watches that list the current and predicted flood stages for affected communities as well as the local flood stage. Current stage data is collected by the USGS using a network of streamgages, over 9000 of which transmit real time data via satellite, radio, or telephone.[4] Many communities have inundation maps that provide information on which areas will flood at which stream stages.
There are four levels of flooding[5]:
- Action Stage: typically at this level, the water surface is generally over the top of its banks, but no man-made structures are flooded; typically water overflowing is limited to parkland and marshland.
- Minor Flood Stage: minor flooding is expected at this level, slightly above flood stage. Few, if any, buildings are expected to be inundated, however, roads may be covered with water, parklands and yards may be inundated and water may go under buildings on stilts or higher elevations.
- Moderate Flood Stage: inundation of buildings begins at this stage. Roads are likely to be closed and some areas cut off. Some evacuations may be necessary.
- Major Flood Stage: significant to catastrophic, life-threatening flooding is expected at this stage. Extensive flooding with some low-lying areas completely inundated is likely. Structures may be completely submerged. Large-scale evacuations may be necessary.
