Water Resources Division Management Division
Flood Preparedness & Awareness
Flood Advisory Glossary and Definition List
FLOOD STAGE: A site-specific river level at which flood damage may start to occur; usually at or above the top of the riverbank. Flood heights are often measured relative to the flood stage elevation.
FLOOD WATCH: The first of two basic advisories issued by the National Weather Service. A flood watch is issued when conditions are favorable for flooding. A watch does not mean that flooding will definitely occur, but it does give a community an early notice of potential flooding and allows the community to review flood safety steps.
FLOOD WARNING: The second basic advisory issued by the National Weather Service. A flood warning is issued when flooding conditions are expected to develop. In some cases, the food warning will be river stage or height reading. The National Weather Service tries to issue flood forecasts with an accuracy of plus or minus one foot. But there are many variables that can enter into this forecast. Some of the variables are difficult to predict, yet have great impacts on flood forecasts.
100-YEAR FLOOD: A term used to define an extreme flow rate that has a one percent chance of being exceeded in any given year. The 100-year flood is a flood that is equaled or exceeded once every 100 years on average. The term should not be taken literally as there is no guarantee that the 100-year flood will occur within a 100-year period or that it will not recur several times. Note: While the term "100-year flood" has been commonly used for decades, the misperception that such a flood will only occur once each century led FEMA and others to change the nomenclature to more accurate "1%-annual chance" flood event. While the meaning is the same, the sense of probability is more clearly conveyed. In fact, it is possible to experience multiple 100 year or 1% annual floods in a single 356-day time span.
cfs: The rate of flow past a given point, measured in cubic feet per second. One cubic foot of water equals about 7 ½ gallons.
kcfs: The rate of flow past a given point, measured in thousand cubic feet per second. Used at some gauging stations.
FLOODPLAIN: The area along rivers, streams, lakes or ocean coast that is/may be expected to be inundated by floodwaters in a 1%-annual chance flood.
FLOODWAY: The area carrying the bulk of floodwaters; endangered by waters traveling at the greatest velocity as well as depth of water.
FLOOD INSURANCE: The insurance coverage provided through the National Flood Insurance Program.
FLOOD INSURANCE RATE MAP (FIRM): An official map of a community on which the Federal Emergency Management Agency has delineated both the special hazard areas and the risk premium zoned applicable to the community. FIRMs typically identify the elevation of the one percent annual chance flood and the areas that would be inundated by that level of flooding; they are used to determine flood insurance rates and for floodplain management.
SPECIAL FLOOD HAZARD AREA: The area calculated to be affected by the 1%-annual chance or regulatory floodplain.