Measuring river’s depth not like it used to be
Published: September 18,2012
MISSISSIPPI RIVER — Reading the depth of the Mississippi River is high-tech stuff compared to decades ago, when engineers used sticks and pressure lines to measure water levels.
Still, a clear measure of water in the river when levels are low can be confusing even when updated hourly on the Internet. To get the amount, elevation above sea level comes into play.
“You go ask a farmer and tell them Vicksburg is going to about 103.5 (feet) and he would say, ‘No idea what that means.’ No clue,” said Drew Smith, a civil engineer with the U.S. Army Corps of Engineers. “That’s about what the elevation was last year at the bridge. But, if you tell them Vicksburg’s going to 57 feet … he knows what that means.”
The river stage was 1.7 feet in Vicksburg on Saturday, but that doesn’t mean the river is dried out. Most of the Corps’ Vicksburg District’s 170 river gauges use zero as a base to record depths — which current and former hydraulics engineers readily admit aren’t accurate measures of water in the river.
“The gauge has nothing to do with depth of water,” said Wayland Hill, a retired civil engineer who worked in the district’s water control division for 41 years. “What it tells you is the surface elevation.”
At Vicksburg, a “zero” mark measures out to 46.23 feet deep, the depth of the local gauge above mean sea level. Elevations on the river vary from one station to the next. Any depth below zero is recorded as a negative number.
The best way to measure water in the river during low times is to add the official stage to the elevation. Today’s amount would be about 47 feet when elevation is added and last spring’s record flood was 103.33 feet when elevation is added to the official 57.1-foot crest.
Hill, who compiled historic data for the water control division, said the way depths are recorded has its roots in the 19th century and were arbitrary.
“Around 1900, it was decided that zero was the lowest anyone had ever seen the river,” Hill said. “And they use zero up and down the river.”
For decades, engineers checked levels on wooden gauges in the water and adjusted readings for changes in water pressure detected with bubbler lines.
“I used to come out here with the (Mississippi River) Division and read all these,” Hill said from the spot where gauges stood near the mouth of the Yazoo Diversion Canal from 1871 to 1995.
“We had two sets of them going during the 1973 flood, here and at the bridge.” The official crest in Vicksburg that year was 51.6 feet, but gauges at the canal exceeded 53 feet.
“We published both at the time, but the ones at the bridge are the ones recorded,” Hill said.
In 1934, gauges were placed near the old U.S. 80 bridge, where measuring sticks still mark spots on the bank beneath the old bridge and Interstate 20. Since about 1976, official stages have been taken from the bridge, Hill said.
Since the 1990s, measuring the river has gone wireless. Levels on major U.S. river systems are updated on the Internet and historic marks, high and low, are a click away. Gauges off the river’s main stem, such as the one at Steele Bayou Control Structure, take into account elevation on official readings.
Statistics on river levels shown on Corps and National Weather Service websites are compiled in Vicksburg from a 15-by-8-inch, box-shaped, solar-powered radar gauge bolted to a steel beam between piers 2 and 3 on the old U.S. 80 bridge.
Radio waves are sent from a cone-shaped attachment to the water surface, then bounced off a receiver on the riverbank to a satellite in space that sends data back to the water control division’s offices.
Installed in 2005, the current gauge has been in the shop just once, in 2011, to change the battery, Smith said. Unlike the Natchez gauge, which is moved higher when stages rise, the gauge in Vicksburg stays in position, shooting about 120 feet of signals hourly.
“This is probably the farthest that any radar gauge on the planet is shooting, given the elevation,” Smith said. “We’re testing the limits of this device.”
Short of re-calibrating all the district’s gauges, negative numbers to express especially low water on the Mississippi River will endure, said Col. Jeffrey Eckstein, commander of the district.
“That’s what everybody has records on for 100-plus years,” Eckstein said. “Our folks know what it is.”
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