High river in New Orleans could prompt May opening of Bonnet Carre Spillway
Published: Monday, April 25, 2011, 1:03 PM Updated: Monday, April 25, 2011, 1:06 PM
Mark Schleifstein, The Times-Picayune By Mark Schleifstein, The Times-Picayune NOLA.com
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The Mississippi River at New Orleans will crest at 17.5 feet on May 17, the highest level in more than a dozen years, and a level that could require the opening of the Bonnet Carre Spillway to reduce the rate of water flowing past the city's levees.
Bonnet Carre TestBrett Duke, The Times-PicayuneA crew of Army Corps of Engineers employees work on top the Bonnet Carre Spillway structure in Norco practicing removing and replacing wooden needles in one of the 350 bays earlier this month.
The spillway, 28 miles above New Orleans, was last opened April 11-29, 2008, with a maximum 160 individual bays having wooden pins removed to allow water to flow into the spillway and then into Lake Pontchartrain. The river that year crested on April 26 at 16.96 feet.
The record water level at the Carrollton Gage was 21.27 feet on April 25, 1922. The river reached 21 feet on the same day during the historic 1927 flood.
The unusually high river has been triggered by heavy rains in the Midwest, including more than 11 inches in Cincinnati and almost 7 inches in tornado-hit St. Louis, during the past three weeks, according to hydrologists with the National Weather Service's Lower Mississippi River Forecast Office.
"A small part of this is snow melt, but the primary driver of this is the ongoing heavy rain stretching from St. Louis to the west and in the Ohio Valley, where they've had seven to 10 inches of rain over the last several days," said David Reed, hydrologist in charge of the river forecast center in Slidell.
Computer models indicate the cold front moving over the Midwest and Ohio Valley could drop another 10 to 15 inches over the next three days, although official forecasts at a number of locations indicate only 2 to 4 inches of rain are expected.
In New Orleans, the official flood stage at the Carrollton Gage is 17 feet, but the city is protected from water rising to at least 20 feet by a combination of levees and floodwalls.
A high river also is likely to require restrictions by the Coast Guard on shipping moving through New Orleans because of the increased speed of river water and its potential to cause or exacerbate steering problems.
Officials with the Army Corps of Engineers, who operate the spillway and regulate water flow in the river, are watching the rising water levels carefully, said Mike Stack, chief of emergency management for the corps' New Orleans District office. Water begins leaking through spaces between the wooden pins in the 350 bays of the concrete spillway when the river reaches about 12 1/5 feet.
Corps officials already have begun inspecting levees along the river, in part because of an earlier high river event about a month ago, he said. The first stage inspections are conducted twice a week by corps officials looking for seepage, sand boils, bank erosion or other threats to the levees, such as ships or debris in the water. Similar inspections also are being conducted by local levee district employees.
When the river reaches 15 feet, the inspections will be conducted once a day.
The decision to open the spillway is based on a combination of concerns, including whether the amount of water flowing south of the spillway is expected to climb above 1.25 million cubic feet per second and how long the upper portions of the levees have been saturated with water.
It takes about 10 days to prepare for an opening, he said. The long, wooden pins that keep the bays closed to water are removed by a crane that moves on a track atop the spillway structure.
"We'll monitor it for the next week and see where we are," Stack said. "If we see that we are likely to reach 1.25 million cubic feet per second, we'll start the process of notifying stakeholders, who will give us their input and then send their recommendations to the Mississippi River Commission."
The commission, chaired by the commander of the corps office overseeing the entire Mississippi River, is based in Vicksburg.
While the southern end of one cold front storm system now dumping rainfall on the Midwest is expected to pass through New Orleans on Wednesday, local rainfall generally does not increase the height of the river, as most of it is drained into area lakes and wetlands, and not over the levees and into the river.
Breaks in levees upstream from Louisiana caused by what's expected to be record high water levels also could result in lowered forecasts during the next three weeks for the river in New Orleans.
Related topics: bonnet carre spillway, corps of engineers, mississippi river
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veefox April 25, 2011 at 1:25PM
I am curious and maybe someone can answer this. What is with the Morganza Spillway above Baton Rouge? I don't ever remember that spillway being opened in my lifetime. What triggers that spillway to open? If the river is expected to crest above 17' in NOLA, why not open it for relief of some of the water before it reaches LaPlace?
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