A new study suggests more land has been lost in the path of freshwater diversions rather than built.
Within the study, Louisiana State University researchers Eugene Turner, Erick Swenson and Michael Layne and University of Maryland researcher Yu Mo reviewed satellite imagery between 1985 and 2015 to look at the percentage of land before and after the diversions took place.
Turner said that while there were gains in the immediate area of the freshwater diversions, there were negative impacts beyond it.
"The three diversions so far have caused more land loss, and not just a trivial amount," said Turner. "So, we should be very careful."
He criticized the state's models for designing the proposed sediment diversions for large-scale coastal restoration, saying they needed to account for the what was happening on the ground at the Davis Pond and Caernarvon freshwater diversions.
"The state is using these models to predict the land gain from a future diversion," he said. "They need to use the existing evidence of what these current ones did and calibrate the model with on-the-ground data."
Turner said the state also needs to consider the effect of the additional water on area beyond where the sediment is expected to be deposited. He said the higher water in the marsh can flood the plants and bring in nutrients that weakens the roots, making them easier to wash away.
The study said implementing the proposed river diversions within the state's Coastal Master Plan are "premature."
"The modeling is incomplete, and the empirical results from three nearby diversions are not replicated in model outputs," according to the study.
Neither the Davis Pond nor Caernarvon diversions were designed to build land. Their primary function is to address saltwater intrusion with fresh water from the Mississippi River to adjust salinity levels. Davis Pond is labeled as a flood control structure.
The proposed sediment diversions -- two of which are in the engineering and design phases -- will likely be about seven to eight times the size of the two freshwater diversions.
Unlike the smaller freshwater diversions that take water closer to the surface of the river, the sediment diversions will pull water from lower depth to pull in heavier sediment and be placed where sediment naturally deposits along the river's banks.
After reading the study, Louisiana Universities Marine Consortium scientist Alex Kolker said he doesn't think the study was "well-designed" to look at the diversions' land-building as it looked at a wide area.
"There is small-scale land building at the mouth of both diversions," he said. "Most of the sediment settles out relatively close to the diversion."
Kolker said larger diversions would grow at a different rate than the freshwater diversions, and he didn't think the math represented in the study properly extrapolated the data to the extent required.
Opposition to the large-scale sediment diversions has been raised by the commercial fishing industry, worried about how the fresh water effects could impact the fisheries.
Proponents said the diversions mimic the river's natural functions and are the most sustainable way to restore the coast by rebuilding land.
In a statewide poll released earlier this week, 82 percent of the 554 participants who were familiar with sediment diversions supported the project.
Staff Writer Halle Parker can be reached at firstname.lastname@example.org or 857-2204. Follow her on Twitter, @_thehalparker.