Early results from study of river sediment show enough to make diversions work

Early results from an ongoing state-federal research project indicate the Mississippi River carries enough sediment to meet the  wetlands-building projections made in the 2012 Master Plan for the Coast.  The diversions are designed to divert the sediment seen exiting the mouth of the river in this NASA satellite photo into the sinking basins south of New Orleans


Early results from an ongoing state-federal research project indicate the Mississippi River carries enough sediment to meet the wetlands-building projections made in the 2012 Master Plan for the Coast. The diversions are designed to divert the sediment seen exiting the mouth of the river in this NASA satellite photo into the sinking basins south of New Orleans

Since 2007,  Louisiana coastal experts have claimed they can prevent the state’s bottom third from sinking into the Gulf of Mexico based on this assertion: The Mississippi River carries enough sand and sediment not just to repair some of the damage but actually begin rebuilding wetlands.

Now they finally may have the research to prove it.

Preliminary findings from the first comprehensive study in more than 50 years on the amount of material the river hauls south of New Orleans appear to support the land-building conclusions in the 2012 Master Plan, researchers said in several recent interviews.

Among the key findings from the continuing Louisiana Coastal Area Hydrodynamic and Delta Management Study:

  • Enough sand and sediment move through the river to meet the land-building projections for sediment diversions and dredge-and-fill projects in the 2012 plan.
  • The proposed sites for sediment diversions are in the right locations to achieve the projected results.
  • Sediment loads in the river have stabilized over the past two decades after steady declines.
  • Deposits of sediment at critical locations are deep enough to supply dredge-and-fill marsh-building projects.
  • Effects expected from climate change and coastal subsidence will not prevent the river from delivering enough material for the projects to meet goals.

Bren Haase, heading the project for the state Coastal Protection and Restoration Authority, said while those findings are encouraging, more data collection and analysis are needed to refine conclusions of what the river carries and how to effectively capture it for rebuilding wetlands.

“Sediment is flowing down river continuously as if it were on a conveyor belt,” he said. “Our focus is on finding strategies like sediment diversions and dredging to efficiently harvest that sediment as it goes by on the conveyor belt.”

All of south Louisiana rests on deltas built by the Mississippi River. But almost 2,000 square miles of its coastal landscape has converted to open water in the last 80 years due to river levees that have starved the deltas of sediment, as well as erosion caused by thousands of miles of canals dredged for oil, gas and shipping. The National Oceanic and Atmospheric Administration has projected at current rates of subsidence and sea level rise, the entire southeast corner of the state will be part of the Gulf by the end of the century.

The 2012 Master Plan drew national attention and buoyed the spirits of coastal advocates with the claim the state could be gaining more land than it was losing if its $50 billion suite of projects was implemented by 2060. But skeptics of the plan quickly pointed out some of the information used in those models dated from the 1960s.

Current data would have been available had Congress funded a study it authorized in 2004. But the money didn’t come until 2011.

Since then, the Army Corps of Engineers has partnered with the coastal authority on the $25 million research effort, which includes projecting the effect projects would have on their receiving basins.

Past studies had shown sediment load dropping steadily over the past few decades as farming conservation methods have reduced soil runoff into the Mississippi River drainage. But Hasse said the new study shows the river is back to a form not seen in centuries.

“We hear a lot of talk about sediment load in the river but found that sediment loads have stabilized in the lower river since those declines,” Haase said. “The surprising thing is, it looks like it’s stabilized at a level not seen since European settlement came to the river,” based on soil borings.

The river also appears to carry enough water to move large enough loads of sand and sediment needed to rebuild the sinking basins south of New Orleans. That became a concern after 2011 research found the Mississippi loses 54 percent of its volume between the Old River Control Structure north of Baton Rouge and its mouth at the Gulf of Mexico.

“The water volume flow down the river has been stable since about 2000 and is projected to remain stable,” said Ehab Meselhe of The Water Institute of the Gulf, which is working for the coastal authority on the research.

Meselhe said those results included running numerous scenarios for subsidence, as well as various levels of sea level rise and rainfall projected by climate change models.

And the researchers said nature provided unexpected help with the 2011 river flood. The Bonnet Carre Spillway opening provided a real-world test for the accuracy of the models being used to predict the effects withdrawing large volumes of water from the river will have on shipping as well as sediment transport.

“We did model periods right before, during and right after the flood and a couple years after the flood, and that gave us a lot of confidence this model can give us reliable information on how diversion openings might impact the river,” Hasse said.

But they still have plenty of work to do, he said.

“We’re encouraged but we still have a lot of questions that have to be answered,” he said.

Even those answers might not be the final approval for sediment diversions.  Kyle Graham, head of the coastal authority, has said the agency was still determining a cost-benefit analysis for the diversions.

The key question: Will the projected benefits in storm surge reduction and fisheries production gained by wetlands restoration outweigh the costs in construction and operation of the diversions, any possible added flood risk to surrounding communities, as well as any disruption to current fishing industries.

Some fishing groups oppose the diversions because they are concerned reduction in salinity levels caused by the river water could displace their target species.

Graham said the first decision of that kind likely will come this fall when research is final on the impacts of the planned Myrtle Grove diversion in Plaquemines Parish.

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About Bob Marshall

From 2013 to 2017, Bob Marshall covered environmental issues for The Lens, with a special focus on coastal restoration and wetlands. While at The Times-Picayune, his work chronicling the people, stories and issues of Louisiana’s wetlands was recognized with two Pulitzer Prizes and other awards. In 2012 Marshall was a member of the inaugural class inducted into the Loyola University School of Communications Den of Distinction.

  • JWarren

    There’s never been any question that the river carries enough sediment to build land. The issue is, and always be, the cost. Dredging is expensive and will move a sediment/water slurry containing at most 15% solids. Diversions typically can’t be designed to divert bed load, so they typically will divert than 1% solids. The “excess” freshwater can raise havoc with oyster beds. A dilemma.

  • Chris McLindon

    It’s impossible to tell from the content of this article, but one could infer that the “numerous scenarios for subsidence” run by the Water Institute of the Gulf center around the value derived from the Grand Isle tidal gauge in the NOAA reference that you did provide. The current rate relative sea level rise measured by the Grand Isle gauge is 9.24 millimeters per year in this report. Given that the IPCC value for global (absolute) sea level rise is 3.1 mm/yr the subsidence rate measured at Grand Isle is 6.14 mm/yr. This is actually toward the low end of subsidence rates that have been measured across the coastal plain. In 1988 Shea Penland, who later became the director of the Coastal Research Laboratory at U.N.O., used tidal gauges in the interior marsh to estimate rates of relative sea level rise as high as 23 mm/yr (or a local subsidence rate of 20 mm/yr). Timothy Dixon used data derived from satellite imagery to estimate subsidence rates in the greater New Orleans area, and found rates as high as 29 mm/yr in the Central Wetlands Unit – Golden Triangle area. Both of these studies show that the distribution of subsidence rates across the coast is a pattern of “hot spots” of high subsidence within a broader pattern of a slower rate of sinking. Rates of subsidence in these hot spots over 20 mm/yr is equivalent to about 8 inches per decade.

    Several studies by authors including Harry Roberts of the Coastal Studies Institute at L.S.U., Robert Morton of the U.S.G.S., Roy Dokka of the L.S.U. Geology Dept., and Woody Gagliano of Coastal Environments, Inc. have shown that these hot spots of subsidence are very often being caused by the vertical movement of geologic faults. Gagliano showed that subsidence on the Empire Fault just west of the town had submerged the marsh to a depth of 3.5 feet below sea level in about 27 years – making it one of the most obvious subsidence hot spots on the coast. This hot spot is the exact location of the proposed site of Lower Barataria Diversion. With rates of subsidence this high it doesn’t matter how much sediment a diversion can move – it will be below the surface in a couple of decades. A 2005 study by Robert Morton showed that there are similar rates of subsidence at Ironton near the proposed location of the proposed Mid-Barataria Diversion. There are even preliminary indications that concentrating sediment in one place in a restoration effort, such as sand piled on a barrier island or deposition at the mouth of a diversion project, may even accelerate the rate of subsidence.

    Much is said about trying to draw an analogy between proposed sediment diversions and the Wax Lake Diversion of the Atchafalaya River. The primary take-away from such analogies is that the Wax Lake Delta is in a very stable, low subsidence area where vertical accretion of land is possible. The locations of the Mid-Barataria and Lower Barataria Diversions should be reconsidered in light the fact that both of them are designed to divert river flow directly into hot spots of subsidence where any sediment does accumulate has a very low chance of building a vertically accreting wetland.