As Hurricane Barry bore down on the Louisiana coast, a unique scenario presented itself: a tropical storm with surge and severe rain threatened the state during a record period of high water in the Mississippi River.

This confluence of events, both early in hurricane season and late in the river's traditional high water period, drew attention to the risk the New Orleans-area levees faced of being overtopped when it looked like Barry was drawing a bead on that city and on Baton Rouge.

But the situation, which southeast Louisiana dodged when Barry shifted farther west than predicted before landfall, also raised an interesting question among some Baton Rouge residents.

How would the future Comite River Diversion Canal, which will draw floodwater from rainfall in the Comite River basin and send it into the Mississippi, be operated when the Mississippi is at flood stage and could see some surge from tropical storms? 

"Would a diversion have even been allowed," asked John Pizer, 66, an LSU language professor whose Baton Rouge neighborhood, Azalea Lakes, flooded in August 2016. "The diversion might have helped lessen flooding in the greater Baton Rouge area, but wouldn't have such a diversion have exacerbated flooding down river?"

According to plans for the 12-mile-long Comite diversion, the canal should be able to move up to 20,000 cubic feet per second of water once it is built in the summer of 2021.

Able to fill an Olympic-sized swimming pool a little more than every four seconds at that maximum capacity, the diversion is expected to cut floodwaters in half in the Upper Comite River basin and have carryover effects in the rest of the Amite River watershed. The Comite joins the Amite near Denham Springs.

Due to heavy rains in the midsection of the country earlier this year, the Mississippi at Baton Rouge had been in flood stage 204 consecutive days as of Sunday and isn't expected to fall below flood stage until around Aug. 5, the National Weather Service said. 

But Army Corps of Engineers officials said the Comite diversion's effect on the Mississippi River, even at flood stage and well above it, would be negligible for communities downstream because of the Mississippi's already huge flow.

"It's a drop in the bucket," said Rene Poche, a Corps spokesman.

Poche pointed out that when the Corps is triggered to the open the Morganza Spillway, the Mississippi must be flowing at 1.5 million cfs and rising at the spillway upriver of Baton Rouge, a point that is well past flood stage on the river.

At those high-water moments, which has happened only twice in the spillway's history, the Comite diversion's maximum flow would represent just 1.3% of what was passing through the Mississippi.

The Morganza structure is operated to ensure Mississippi levels don't surpass 57 feet at the spillway, helping keep flow within the levees past Baton Rouge and New Orleans.

Although Morganza wasn't opened this year, persistent high water in the Mississippi did force the Corps to open the Bonnet Carré Spillway upstream of New Orleans twice in one year, a first in the structure's 88-year history. The agency began closing some of that spillway's bays July 22. 

These bigger spillways dwarf what the Comite diversion will be able send into the Mississippi.

The Bonnet Carré can divert up to 250,000 cfs from the river. The Morganza Spillway can divert up to 600,000 cfs. Their capacity is 11.5 times to 29 times greater at moving water out of the river, respectively, than the Comite diversion is able to dump water into it.

According to the Corps' website, about 10,000 cfs of water simply leaks through the closed Bonnet Carré when spring waters have risen high enough to pass through the wooden timbers that control the closed bays but are not high enough to require the spillway to open. 

But the Comite diversion isn't the only idea proposing the Mississippi as an outlet for community flooding. 

For many years, some residents and officials in Ascension have floated the idea of using large pumps to pull water from Bayou Manchac and dump it in the Mississippi during periods of high water on Manchac and the Amite River. 

Also, shortly after Hurricane Isaac nearly swamped that parish's pump station in the McElroy Swamp in 2012, officials in Ascension and St. James parishes proposed making a planned diversion of fresh Mississippi water into the Maurepas Swamp near Convent reversible. Under that concept, backwater flooding from Lake Maurepas — a major effect from the surge of Isaac — would be sent into the Mississippi through the river diversion.

Neither of those concepts has gotten off the ground, however, due to practical, environmental and cost-effectiveness concerns. Unlike the Comite diversion, which will flow downhill, these diversions farther down the Mississippi would have to get floodwater to go uphill and over levees to reach the river.

But, in December, engineers working for Ascension's eastern drainage district did endorse a $61 million channel and 1,000-cfs pump system to move floodwater from the parish's Panama Canal/Bayou Conway drainage area south of Gonzales into the Mississippi.

It is one of two projects for which the parish is seeking funding from the $1.2 billion that Congress has set aside with the U.S. Department of Housing and Urban Development for flood control projects after the 2016 floods.    

John Dale "Zach" Lea, 72, a retired agricultural economist who spent a career traveling the world to help farmers, said the Comite diversion and other similar ideas are part of a bigger picture nationally, mirroring a continued practice by communities of building in low areas along the river and then sending their rainfall runoff to the Mississippi. 

"The increased flow endangers southern Louisiana and increases the cost of flood protection for Louisiana citizens and the federal government," said Lea, who grew up in Zachary but now lives in Covington.

Lea suggested the Corps use reservoirs in states upriver to hold floodwater temporarily, noting the Corps already uses them to ensure the Mississippi maintains a minimum flow for maritime traffic.

Poche, the corps spokesman, said it can take weeks for water upstream to reach south Louisiana, but he also acknowledged that as more diversions are proposed to put drainage waters into the Mississippi, the agency would have to consider any possible flood effects downstream.


Email David J. Mitchell at dmitchell@theadvocate.com

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