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The Corps is closing some of the Morganza Bays: Where didn’t the water go?n Where could it go if the Atchafalaya were managed differently?

Floodwater from the Atchafalaya Floodway spreading east and west through the Coastal Marshes

A week ago I wrote about the interplay between the Lower Atchafalaya River and the GIWW, the Intracoastal Waterway. I included an image of the backwater flooding that could occur throughout the coastal wetlands. It was one of several the Corps issued when it opened the Morganza on May 14. structure. The Daily Comet in Lafourche Parish has an article about where the water didn’t go. The worry over flooding in the Atchafalaya Basin isn’t over, but it is easing.

A key phrase in the article: “backwater flooding from a complex network of bayous, lakes and canals was not materializing.” And the sinking of a barge in Bayou Chene prevented flooding in Terrebonne Parish.

Bayou Chene at Avoca Island

When the Mississippi and Atchafalaya are not in flood, fresh water is funneled into the western Terrebonne marshes in a most indirect way:  When the Atchafalaya runs 3 to 4 feet about flood stage, water works its way around Avoca Island through the Avoca Island Cutoff to Bayou Chene and into Bayou Penchant and the fresh marches of the western Terrebonne basin.

Crab Boat on Bayou Penchant

The delta that is forming at the mouth of the Atchafalaya River and the Wax Lake Outlet broke the surface of Atchafalaya Bay in 1973. Only 10% of the sediment in the Atchafalaya River is actually used to build land at its mouth. The rest is wasted in the Gulf of Mexico, much like the sediment shooting out of the mouth of the Mississippi.

The Kemp/Hyfield proposal to divert water and sediment from the Atchfalaya to the Penchant Marshes. From the Multiple Lines of Defense Proposal

In 2006 Paul Kemp and Emily Hyfield (see page 11 in Multiple Lines of Defense Proposal) proposed redirecting all that freshwater and sediment into the western Terrebonne marshes by breaking through the East Protection Levee of the Atchafalaya Floodway and running a spillway through Lake Palourde north of Morgan City and into the Penchant Basin. Doing so would strengthen the Penchant Landbridge of freshwater marshes and flotant. The spillway would be designed to operate at 140,000 cubic feet per second, but would normally operate at 20,000 cubic feet per second and would take some of the pressure off Morgan City when the Atchafalaya is flooding.

An alternative would be to short cut what is already happening and run a spillway across Avoca Island to Bayou Penchant.

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