During the historic Mississippi River floods of 2011, as in all floods, the U.S. Army Corps of Engineers had rules on how and when each flood control structure should be operated.
The problem for south Louisiana in 2011 was that this portion of the river had gotten shallower than when those guidelines were put in place in the 1950s.
On Thursday evening, the Corps presented proposed changes that would better define a water level criteria, as well as a water flow level to determine when the Morganza floodway structure will be operated the next time the river starts to flow wildly.
“We want to anticipate instead of waiting and reaching like we did in 2011,” said Mike Stack, chief of emergency management with the Corps.
The proposed changes also are designed to help eliminate some of the scouring behind the structure after floodwaters recede.
“We ended up with 35-foot-deep holes very close to the structure,” Stack said, and those had to be repaired.
The Morganza flood control structure, located north of Baton Rouge, helps divert water from the Mississippi River into the Morganza floodway, which feeds into the Atchafalaya River basin. The purpose of the structure is to relieve floodwater pressure on the lower river levees, which from Baton Rouge to Bonnet Carré are designed to withstand 1.5 million cubic feet per second water flow.
“The 1.5 million is a very important number,” Stack said.
In the past, the structure was opened when the river reached a flow rate of 1.5 million cfs and rising to take off excess flow for the safety of lower river levees.
However, in 2011, structure operators found that the water level in the river got higher than the structure was designed to hold back before that 1.5 million cfs criteria was reached.
Water levels got to 59.6 feet, but the structure was designed to be operated when the water hits 57 feet.
Because the river had gotten shallower over the years in this area, it now takes less water flow to bring up the water levels than it had when the structure was built in the mid-1950s. At that time, a 1.5 million cfs flow meant the water levels at the structure would be at 56 feet.
Because 2011 was the first time the structure was operated because the river flow trigger was reached — the 1973 opening was done to relieve pressure on the Old River Control structure — the 1.5 million cfs trigger needs to be re-examined, Stack said.
To adapt to those river changes, the Corps is proposing changes to make sure the river water doesn’t get higher than 57 feet when there is also a 10-day forecast of a river discharge of 1.5 million cfs. Looking at the historical record, if that change had been in place since the structure was built, the structure would still have been opened only in 1973 and 2011, Stack said.
Another proposed tweak is that the structure would be initially opened to limit the rise of water in the floodway to 1 foot a day for the three first days, changing the gate opening and closing sequence to minimize scour damage, as well as keeping some of the floodway open to help drain the front bay area quicker.
Of the 20 people at the meeting, most people asked questions about how the new system would work, had praise for how the flood was handled and asked for better landowner notification the next time the structure is opened.
“We thought it was really operated in a way that allowed us to do what we needed to do to protect our property,” said Jim LaGrone, of Jarreau.
Updated interim operating procedures to reflect that the Mississippi River is more shallow near the Morganza structure will be in place in time for the next flood season starting in January, Stack said. The Corps will be undertaking a much larger effort to look at the whole Mississippi River system to see what changes have occurred over time and if that should change how certain flood control structures should be operated. That study will likely take three or four years, he said.
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