Can sediment diversions from river keep up with land loss, rising Gulf?

In 2009 LSU researchers Harry Roberts and Mike Blum concluded the sediment load in the Mississippi River was too little to prevent the scenario shown here from happening by 2100.

GeoCover Data / NASA

In 2009 LSU researchers Harry Roberts and Mike Blum concluded the sediment load in the Mississippi River was too little to prevent the scenario shown here from happening by 2100.


As the state prepares a required five-year update to its long-term Coastal Master Plan, new research has revived three questions that some experts have had for years. The Lens is looking at each question individually this week. Today:

Is there enough sediment in the river keep up with the new, higher projections for sea-level rise and land subsidence?

The claims for rebuilding wetlands included in the 2012 master plan were based on sea-level gains between 10 and 17 inches through 2061. But the latest estimates are much higher because global warming has continued unabated.

The range for sea-level rise used in the 2017 plan has tentatively been set at 39 to 70.8 inches through 2100.

And some researchers think that could be too optimistic because the world has yet to address the major driver of sea-level rise — global warming caused by greenhouse-gas emissions, primarily fossil fuels.

Water expands when heated, and the rising air temperatures have warmed the oceans. This is called thermal expansion. Meanwhile water stored as ice on land is melting and flowing into the oceans, further increasing their volumes.

In 2014 Intergovernmental Panel on Climate Change, a worldwide consortium of scientists studying the issue, increased its projections for sea-level rise by 60 percent, pushing its high-end level to 4 feet and even that is a figure many climatologists consider too conservative. More recent studies show as much as 6 feet,.

But southeast Louisiana faces an additional problem: Its sediment-starved delta land base is sinking. In many places, subsidence is faster than sea level rise.

In 2013 NOAA said relative sea-level rise — which accounts for subsidence — in southeast Louisiana could be as high as 5 feet by 2100. Indeed, the combination of worst-case figures for both factors could result in 11 feet of sea level rise at the mouth of the Mississippi River, where subsidence has been measured at up to five feet a century.

So is there enough sediment in the river to build land in the face of this steeper curve?

The consensus among plan critics as well as its designers: Yes – but those shifting figures have already forced some projects to be taken out of the plan, and may force more changes in the future.

“If you turn the river loose in one place, I don’t care what the subsidence rate and sea-level rise is, you’re going to have enough material to build a delta in that spot,” said LSU’s Harry Roberts, one of the state’s most distinguished coastal researchers.

“Now, your chances of building a larger delta that you can maintain will improve as you move that diversion further north on the river.”

Both subsidence and sea-level rise generally decrease the further north one travels from the river’s mouth for two reasons: The land elevation increases and the ground becomes more solid.

Subsidence declines northward because the layer of soft, delta soils becomes thinner as bedrock pushes closer to the surface. At the mouth of the river, where there is almost 400 feet of those wet soils before bedrock, subsidence has averaged about five feet a century. Around New Orleans, where the layer is about 100 feet thick, subsidence runs about 2 feet a century, according to the latest report by the Coastal Protection and Restoration Authority.

But subsidence also varies horizontally across the landscape depending on the composition of the delta soils in each place, as well as the activity of subsurface faults. And any combination of those factors can result in a sudden increase or slowing of sinking at any spot.

The wide range of variables makes it impossible for geologists to give a meaningful “average”  for subsidence rates for entire the southeastern corner of the state. The coastal agency runs its projections based on subsidence map that shows averages at specific locations.

“Subsidence is the real wild card in all of this,” said Bren Haase, in the Planning and Research Division at the agency.

Roberts, who served on one of the winning Changing Course teams, has long been concerned the river lacks the sediment supply to achieve the ambitious goals in the master plan, which aims to save much of what is now outside the levees south of New Orleans. A 2009 paper he co-authored with colleague Mike Blum included a now-famous illustration showing the bottom of Louisiana’s “boot” all but covered by the Gulf.

Roberts has said the diversions should be moved further north, even suggesting some be built between Baton Rouge and New Orleans. He also believes the projects should be many times larger than currently planned.

“I think at their current locations, the diversions can have a positive result,” he said, “but you can’t have the large positive result that will last into the 21st century without much bigger diversions and further north.”

The CPRA staff agrees the challenge is growing due to the changing sea-level rise and subsidence numbers.

Those changes prompted the agency to mothball two sediment diversions in the 2012 plan that were scheduled around Empire and Port Sulphur, about 35 miles north of the river mouth.

“When we ran the models [with the new figures] as well as changes in the wetlands down there, we didn’t get the results we showed before,” Haase said. “So, they’re not on the board any more.”

The new numbers didn’t change plans for the Mid-Barataria and Mid-Breton Sound diversions 30 miles further north. Hasse said computers models showed the Mid-Barataria project could still stay ahead of as much as 6 feet of subsidence and sea level rise through 2061.  One model showed Mid-Barataria could build between 6 and 22 square miles of wetlands in 20 years and as much as 38 square miles in 50 years.

Haase said the agency also plans to run the models with the less optimistic predictions that include 5 feet of sea level rise and as much as 50 inches of subsidence. Those results won’t be finished until mid-year.

With almost each new report showing the challenges deepening and the river with a finite amount of sediment, it would seem logical the agency had found a break point – the point at which the volume to be filled exceeded the sediment supply.

But Jim Pahl, also of the agency’s Planning and Research Division, said modeling such an event is difficult because once the diversions are open and building land the break point would be constantly changing.

An increase in relative sea level that could swamp the area today would not have that effect in 20 years because the elevation would have been raised by the projects, he said.

Roberts agreed.

“The [master plan] project life is 50 years, but the diversions will continue to operate after that,” he said. “I think it’s reasonable to expect they will continue to build land as long as they are being operated.

“But once again, the question is: are these the best locations and best use of the sediment supply beyond this century, especially given what the projections are now for sea level rise?”

Tomorrow: Should the state be planning for the worst-case scenarios of sea level rise and be looking ahead for 100 years instead of 50?

This story is the second part of a series looking at three questions facing state officials as they update the Coastal Master Plan. The first story is here, with links to the rest at the bottom of that story.


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