Combining diversions, dredging could increase land-building efficiency

Computer models show tandem approach also could slow effects of subsidence

For several years, people in the coastal restoration community have debated whether the state should rebuild its sinking, crumbling wetlands with massive river diversions or by pumping mud from the river into the marsh.

People in favor of diversions contend they’re the most cost-efficient way to rebuild wetlands over decades. Proponents of the dredge-and-fill method, called “marsh creation,” argue the flood offreshwater from diversions would displace some fisheries, harming the communities and industries that depend on them.

A ceasefire may be in the works, thanks to new research. It shows that the two methods can be used in tandem at the same site, improving the efficiency of land-building and reducing the harm to fisheries, according to a report in The Lens.

The Water Institute of the Gulf and the state Coastal Protection and Restoration Authority ran computer models to see what would happen if sediment were pumped to the Myrtle Grove Diversion, the first of the massive structures that would send river water into nearby wetlands.

It produced three positive results: The sediment and nutrients from the diversion extended the life ofmarshes created by the pipeline from 20 years to as much 50 years by reducing the effect of subsidence. Marshes built by the pipeline helped the diversion build wetlands more rapidly by slowing down the water after it passed through its gates. This caused sediment to settle to the bottom more quickly, and shrimp and oyster fisheries would be less affected.

Because wetlands grew more quickly near the diversion, less freshwater flowed into thecritical fishing grounds of the lower Barataria Basin.

“We kind of expected the first two results,” said Rudy Simoneaux, engineer manager at the restoration authority, because it fits with prior research into diversions and marsh creation projects.“But thelast one was really surprising – it was lagniappe.”

Simoneaux said the agency hopes to confirm those results with more definitive computer modeling over the next few months. If the results hold, this could influence future editions of the Coastal Master Plan, the 50-year, $91.6 billion plan to rebuild coastal Louisiana.

Experts say the plan is the state’s best chance to prevent the southern third of the state from being consumed by the Gulf of Mexico over the next 70 years. The cause: subsidence and accelerating sea level rise caused by global warming.

The head of a group opposed to diversions welcomed the findings.

“I’m happy to see them paying attention to the impacts of diversions on the fisheries,” said George Ricks, a charter boat captain and head of the Save Louisiana Coalition. “But they can’t get around the fact that the pipelines can do the job quicker and cheaper.”

“They say we’re running out of time and lack funding,” Ricks said. “But it takes 10 years and a billion dollars to build one diversion…. You could build many thousands of acres of wetlands for a lot lessmoney with the pipeline during that period.”

Research shows that pipelines build land more quickly, but it too starts to sink. Diversions take longer to start building land, but they end up creating more in the end because they last longer.

However, no one has ever built one.

The state has considered building eight river diversions, but just one is in the engineering and design phase: the Myrtle Grove project, about 30 miles south of New Orleans.

Due to the complicated construction and federal permitting process, it won’t go into operation for at least 10 years.

So far, the state has relied mostly on dredge-and-fill projects to rebuild land. More than 30 projects have been completed, are under construction or are about to get underway. Coastal authority officials say they get too expensive if they’re too far from the river because there aren’t many places to dredge sediment and because the projects have such a short lifespan.

About Adam Jaynes 14 Articles
Capt. Adam Jaynes grew up fishing both Sabine Lake and Lake Calcasieu and their surrounding bayous, marshes and rivers. He specializes in light tackle with artificial lures in the year-around pursuit of speckled trout and redfish.