The record-breaking storms that battered central and western Tennessee on May 1 and 2 produced massive flooding that left 23 people dead. Two-day rain totals reached upward of 20 inches in some of the hardest-hit regions, with river crests measuring more than 60 feet. How do you measure flood levels?
With a stream gauge. To measure flooding, the National Weather Service checks data from fixed measuring sites that have been set up throughout the country. These were established by the U.S. Geological Survey in locations that are close to population centers or contain features of interest, such as a confluence of two rivers. The most common tool is the stilling well, which measures the height of a float in a hole dug alongside a river. Pipes connect the river water to a cylindrical encasement in the well, known as a “gage house,” so that the water level in the subterranean chamber is the same as that in the river. This sheltered environment permits the relatively delicate reading and recording equipment to be placed directly above the float. These sensors record the water level at regular intervals and transmit the data via satellite to USGS centers. Other types of stream gauge include the pressure transducer, which infers the depth of water in a stilling well by measuring the pressure of the column of water inside. A bubbler gauge checks the pressure required to send bubbles of air or nitrogen gas though a thin tube from an above-ground gage house to a point submerged in the river.
The weather service reports flooding in terms of river crest height or “stage” for a particular site along the water. This value corresponds to the number of feet water has risen above a local reference height known as the “gage 0 datum.” This baseline varies from one gauge location to the next and roughly reflects the elevation of the river bed, measured in units of feet above sea level. (For more on determining sea level, see this 2005 Explainer.) Stage measurements can then be converted into various flood hazard levels that are used to determine whether to close roads or evacuate residents from the area. These categories run from the pre-flood “action stage,” during which the weather service must prepare to combat future flooding, to minor, moderate, and major “flood stages.”
The point at which water levels pose a risk to lives, homes, or commercial activity varies from place to place, so each site has its own set of action and flood stage water levels. As a result, absolute numbers are meaningless when comparing two locations. A river crest of 35 feet could spell disaster for a Tennessee town like Centerville, where flood stage begins at 22 feet, but pose no threat to Nashville, where flooding starts at 40 feet.
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Explainer thanks David Welch and Glenn Carrin of the Lower Mississippi River Forecast Center, and Mike Norris of the U.S. Geological Survey.