NEW ORLEANS — The state’s master plan for coastal restoration may be underestimating how much land Mississippi River diversions could build along the coast, according to computer modeling done through The Water Institute of the Gulf.
The state is investigating the possibilities and challenges of diverting freshwater and sediment from the Mississippi River to rebuild coastal wetlands, like the one being planned near Myrtle Grove in Plaquemines Parish.
The 2012 master plan estimates a 50,000 cubic feet per second diversion near Myrtle Grove could build more than 7,000 acres after 20 years in operation. According to the computer modeling, diversions ranging in size from 25,000 cfs to 75,000 cfs could build from 4,000 acres from a smaller diversion up to 14,000 acres from a larger diversion over 20 years.
“This is refining the master plan. This information should be quite helpful to people as we move forward,” said Ehab Meselhe, director of natural systems modeling with the institute.
The presentation — made Wednesday at the University of New Orleans to the Governor’s Advisory Commission on Coastal Protection, Restoration and Conservation’s diversion subcommittee — contained a few other surprises.
Research has also shown that the impact of a diversion’s angle of approach to the river doesn’t make much difference in how much sediment can be collected from the river, he said. This is useful information for project designers because it means there is now flexibility in where diversions can be built, Meselhe said.
In addition, researchers have found it’s not necessary to build a diversion all the way to the river bottom in order to maximize the amount of sediment that can be captured. At 75 percent of the river bottom depth, the sediment that can be captured is still at a premium, he said.
“But if you design a very shallow diversion, you’ll get very little sand,” Meselhe said. Freshwater diversions currently in operation, like Davis Pond and Caernarvon, are very shallow.
The amount of water flowing through a diversion makes a difference as well, he said. Diversions need at least 50,000 cfs to maximize the capture of sediment. After 100,000 cfs, there is no significant difference in the amount of sediment captured.
“There are considerable differences of opinion on benefits and consequences (of diversions),” said Ted Falgout, subcommittee chairman. The role of the subcommittee, he said, “is to try to separate fact from fiction and educate the public on the best science.”
Falgout noted that some people have claimed the Davis Pond and Caernarvon freshwater diversions have damaged the wetlands. However, he said, in the subcommittee’s visit to Caernarvon earlier this year his impression was of a healthy ecosystem teaming with life unlike other areas of the coast.
As part of that ongoing discussion, Eugene Turner, professor with the School of Coast and Environment at LSU, asked the subcommittee to be careful in applying diversion results found in sediment-based soils of the coast and those that can be expected in plant-based soils.
“There are significant differences,” he said.
If sediment is diverted to build land in one place, he said, it means sediment isn’t available to support other areas.
“In the long run, you’re fine-tuning the gain and loss,” Turner said.
In addition, he pointed to a natural crevasse in the lower Mississippi River that formed in the 1970s and then displayed photos that show continuing land loss in that same area of this natural diversion.
“What I’m trying to encourage is look at the whole system,” he said.
Kirk Rhinehart, who was the chief of planning and research with the state Coastal Protection and Restoration Authority when the latest master plan was developed, said that’s exactly what the state did. What they found, he said, is that river diversions upriver would result in land loss at the mouth of the river; “however, the net gain to the system was positive.”
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