Posted on May 22, 2023
Another “flood fight” season has come and gone for the U.S. Army Corps of Engineers, New Orleans District – the Corps ceased inspections on April 24, 2023, nearly 47 days after the Mississippi River at the Carrollton Gage in New Orleans, La., reached its trigger, and dropped back below the 11-foot mark.
Flood fight for the river began March 9, 2023, when the water level at the Carrollton Gage in New Orleans first rose above 11 feet this year.
According to Sarah Stone, civil engineer and flood fight planner, New Orleans District Emergency Operations Center, the Corps only entered Phase I for the 2023 flood fight season since the readings at the Carrollton Gage didn’t rise above the 15-foot trigger that would have sent the Corps into Phase II.
Flood fight normally takes place during the spring months each year, when precipitation events and snow melt flows into the Mississippi River and through the New Orleans District on its way to the Gulf of Mexico.
“Lower Mississippi River floods are mainly caused by rain that occurs in the Upper Mississippi and Ohio River Basins, so anything that occurs north of Cairo, Illinois,” said Stone. “This point is called the ‘confluence,’ where the Ohio River and the Mississippi River meet. Most of the water we see through New Orleans is already in the Mississippi River at that point – about 90 percent of the water is in the basin, and then the other 10 percent gets added to the river south of Cairo.”
According to Stone, the Corps activates Phase I of flood fight when the Mississippi River stage reaches 11 feet and rising at the Carrollton Gage and the Atchafalaya River reaches 5 feet and rising at the Morgan City Gage.
Corps personnel inspect the levees within the New Orleans District’s area of responsibility, looking for instances of seepage, sliding, encroachments and anything else that could negatively impact the levee system.
Depending on the levee sector, Phase I inspections may happen weekly or twice weekly.
“In Phase I, that’s when we start inspecting levees either once or twice a week looking for areas where water may be seeping through. In addition, we’re looking for levee slides, where material is sliding the off either side of the levee, and we’re looking for encroachment issues, such as vehicles parked on the levee or barges pushed too close to the levee. Barges are supposed to be no closer than 180 feet away from the levee, so they’re not pushed against the bank and potentially causing stability issues,” said Stone. “We’re also looking for any unauthorized work being performed near the levees – whether it’s subsurface digging, pile driving – things that would need a permit in order for the work to be performed.”
Corps inspectors submit their findings to engineers at the USACE New Orleans District, who take the inspectors’ information and provide recommended actions.
“It could something as simple as ‘continue to monitor’ or it could be to go place sandbag rings around a sand boil or check for permit stipulations to make sure any work being done around the levee is still authorized,” said Stone. “Then they send a copy of all the engineering comments to the levee districts after each inspection so they know what we’re looking at and can see what we’re recommending to do for each point.”
Since subsurface work along the Mississippi River can affect the integrity of the levee system, the Corps and the State of Louisiana set guidelines for halting work within 1,500 feet of the levees during flood fight.
Normally all work must cease once the Corps enters Phase I, but waivers can be issued on a case-by-case basis after the Corps has provided a “Letter of No Objection” to the levee districts for the work requested, Stone noted.
Sometimes when water levels increase even further in the Mississippi and Atchafalaya Rivers, the Corps can enter what is known as Phase II of flood fight. The Corps activates Phase II when the Mississippi River reaches 15 feet and rising at the Carrollton Gage and the Atchafalaya River reaches 7 feet and rising at the Morgan City Gage. In Phase II, the Corps increases the frequency of levee inspections and inspections of flood risk management structures within the Mississippi River & Tributaries (MR&T) system.
“Subsurface work within 1,500 feet of the levees must stop and no waivers are granted once levels reach 15 feet at the Carrollton Gage,” said Stone. “15 feet is also when we start seeing flow rates that could potentially trigger a structure operation.”
The structures Stone referenced are the Bonnet Carré Spillway and Morganza Control Structure, which are used for flood control operations. The Old River Control Complex (ORCC) is also used to control flows in the Mississippi and Atchafalaya Rivers, but not for flood risk management.
The ORCC is operated daily to maintain the 70/30 flow distribution between the Mississippi and Atchafalaya Rivers. During high water events, ORCC continues its daily operation of the 70/30 split with increased levels based on river conditions.
The Bonnet Carré Spillway is located about 33 river miles above New Orleans on the east bank of St. Charles Parish. It can open any number of its 350 bays to divert a portion of the Mississippi River’s floodwaters into Lake Pontchartrain, which then finds its way into the Gulf of Mexico. This allows high water to bypass New Orleans and other downstream river communities.
The Morganza Spillway is located 186 river miles above New Orleans on the west bank of Pointe Coupee Parish. It’s designed to divert a portion of the Mississippi River’s floodwaters southward, guided by the East Atchafalaya River levee, before joining the Atchafalaya River Basin Floodway near Krotz Springs, La.
“The trigger to operate the Bonnet Carré Spillway is when the Mississippi River flow reaches 1.25 million cubic feet per second (cfs) at the structure, while the trigger for operating the Morganza Spillway is when it reaches 1.5 cfs at Red River Landing,” said Stone.
Stone mentioned that this was a relatively benign flood fight season compared to others in more recent years.
“This year has been pretty low-key. The highest we had at the Carrollton Gage was about 13 and a half feet – which is still high, but not enough to push us into Phase II,” said Stone.
Stone also noted that while the Corps and levee districts work together during the flood fight season, anybody who comes across areas of concerns – from leaks in flood walls to sand boils at the base of the levees – can call in to report their findings.
“We always like to say, ‘if you see something, say something.’ Feel free to call your local levee districts or the Corps of Engineers itself at 504-862-1102 to report any issues you may see and be concerned about,” said Stone.