Reintroducing fresh water from the Atchafalaya River using the Gulf Intracoastal Waterway has long been discussed as a partial solution to help push back salt water moving farther north into the fresh marshes of Terrebonne Parish.
That solution is getting closer to becoming a reality with a project in the works to push more water eastward through a diversion channel near Avoca Island.
The project is all about land loss prevention, not land building.
“The real story is that it prevents the loss of 15,000 acres,” said Simone Theriot Maloz, executive director of the coastal group Restore or Retreat. “We are not even talking about creating land; we’re talking about holding the line.”
Terrebonne is a notoriously difficult area for land-building projects because of the parish’s distance from river sources of fresh water and sediment and the spongy, old-delta soil.
“This one, it’s feasible. We can do it,” said Austin Feldbaum, coastal resources scientist and project manager with the state Coastal Protection and Restoration Authority.
The reintroduction project would be adjacent to a lock system on Bayou Boeuf, which is part of the Mississippi River flood system. The lock allows large barges to continue traveling on the Intracoastal Waterway by equalizing the water levels and helping to lower the speed of the water as it flows through the area.
To get around the lock system on Bayou Boeuf, the state first looked at going to the north of the lock and using an existing channel, but it’s narrow and there are docks and industry along that bank that could complicate matters.
So they turned their attention to the southern bank, where the privately held Avoca Island is located. Using computer models, state officials looked at a curved canal that is wider and deeper at each end than it is in the middle — like an hourglass — to keep the water moving through the channel while also making sure the water doesn’t move too fast when it enters Bayou Boeuf again in order to minimize the impact on navigation.
The model used for the beginning phases of the work were general in nature, but future computer modeling will be much more specific to give better results on potential impacts, including additional floodwater. CPRA staff members are working on getting information that will be fed into the more-specific computer modeling effort.
At this point, the diversion canal would be 375 feet wide and could be closed off if water levels rise too high in Amelia. The water would flow through the diversion and continue east through the Intracoastal Waterway. Improvements in existing channels that flow south would be made at Bayou Copasaw and Minor’s Canal.
Computer modeling so far shows that the diversion could be open 60 percent of the time or more to help benefit wetlands in central Terrebonne Parish.
“We’re very much in support of it,” said Michel Claudet, Terrebonne Parish president. “That gets more fresh water to the eastern parts of our parish.”
The project will help push back salt water coming in from the Gulf of Mexico through the tattered coastal wetlands, but there could be a small amount of land building, as well.
“We’re going to have a lot of dredged material from the GIWW and it would cost a lot more to put it anywhere else than Avoca Island,” Feldbaum said.
However, the state isn’t far enough into engineering and design to say how much land could be built with the dredged material.
“Avoca is examining the project,” said Paul Hogan II, general manager of Avoca Inc., which owns the island. He declined to comment any further.
Flooding concerns, operation of the structure and other details will be addressed in the future as the project planning moves forward, Feldbaum, said.
“We’ve made that commitment over and over, so we’re not going to make it worse,” he said.
CPRA has been operating its computer models under the assumption that they would close the diversion if water reached a 3-foot mark at Amelia. To see how the diversion would act in the future as the landscape changes, they modeled from 2020 to 2070 to see what impact global sea level rise would have on the function of the diversion.
In the first 30 years, the diversion works as it is intended, but if global sea level rise continues as expected, the water would hit the 3-foot mark more often, and the diversion would have to be shut down when that happened, Feldbaum said.
In a perfect world, and assuming money is found to build the project, engineering and design of the project could be far enough along to start construction in about five years.
However, there are a number of hurdles, including possible pipelines in the area and the need to get a permit from the U.S. Army Corps of Engineers to do a modification of a Mississippi River and Tributaries levee, which would need to be cut through to create the diversion.
Follow Amy Wold on Twitter, @awold10.