The U.S. Army Corps of Engineers continues to monitor Mississippi River levels throughout the Mississippi River Drainage Basin to ensure the recent opening of the Bonnet Carré Spillway will be sufficient to divert the river’s flow past New Orleans. On April 11, the Corps opened 38 of the Bonnet Carré’s 350 bays, diverting an estimated 37,000 cu ft per second of the river’s flow as it approached 1.25 million cfs in the New Orleans area. The following day, the Corps opened an additional 46 bays, for a total of 84 bays and an estimated diversion of 83,000 cfs.
“The Bonnet Carré Spillway is only operated when the river’s flow reaches 1.25 million cfs on a rising hydrograph or we have a desired freeboard,” says Nancy Powell, chief of hydraulics and hydrologic engineering for the Corps’ New Orleans District (NOD).
Mississippi River Commission President Brig. Gen. Michael J. Walsh, commander of the Corps’ Mississippi Valley Division in Vicksburg, Miss., signed the order to open the spillway on April 10. Col. Alvin Lee, NOD commander, made the recommendation based on public safety, predicted crest levels, overall river and levee conditions and navigational traffic.
Located 28 miles north of New Orleans, the Bonnet Carré is the southernmost structure in the Mississippi River and Tributaries Project, which provides flood control to the alluvial Mississippi Valley from Cape Girardeau, Mo., to the river’s mouth. The Bonnet Carré structure consists of an 8,000-acre floodway and a concrete, leaky weir control structure that parallels the river for a mile and a half. The structure has a design capacity of 250,000 cfs.
The Corps has been monitoring water levels throughout the MR&T system and watching a crest moving southward from Cairo, Ill. On April 14, the National Weather Service forecasted it would pass Baton Rouge by April 19. “This is a very dynamic system,” says Lt. Col. Murray Starkel, NOD deputy commander. “We take into account the flow and rain events throughout the entire system, all the way up to the New Madrid Floodway, and the status of levee systems along floodways and tributaries.”
NOD continues to monitor river levees for sand boils and seepage on a 12-hour daily schedule, which it has done since April 1, says Mike Lowe, the district’s emergency manager. “We are adding more people to inspect because sometimes sand boils occur away from levees, as much as 1,000 ft,” Lowe says. “We drive the 973 miles of levees to make visual inspections. We also talk to locals, farmers who are familiar with the areas and may know of wet spots.”
Opening Bonnet Carré spills fresh water from the Mississippi into brackish Lake Pontchartrain. The Corps will offer support services to several agencies that monitor the lake to measure the environmental effects, says Richard Boe, NOD environmental resource specialist.
Between those agencies and the Lake Pontchartrain Basin Foundation, 30 parameters, including temperature, salinity and turbidity, and 36 contaminants will be monitored, Boe says. “The data that we gather will be useful in the design and implementation of future proposed diversion projects from the river for coastal restoration.”
Locals are primarily concerned about short-term effects of a diversion, including the effects on fish populations and crabs. The Corps also is anticipating some sort of algae bloom in the lake by late summer. “By the fall, things will have pretty much returned to normal, with the possible exception of oyster reefs, which have seen considerable damage in some past openings,” Boe says. “If we have damage, it takes oysters from 18 to 24 months to reach maturity, so we could be looking at two more years of nonproduction from those areas.”