The Louisiana Coast: Last Call
12:26 pm
Mon June 3, 2013

The Louisiana Coast: Last Call — Measuring The River

The Bonnet Carré Spillway when it was opened in 2008. Scientists now say much of the sediment and water the Mississippi River carries into Louisiana never makes it to the Gulf of Mexico.
The Bonnet Carré Spillway when it was opened in 2008. Scientists now say much of the sediment and water the Mississippi River carries into Louisiana never makes it to the Gulf of Mexico.
Credit Jason Saul / WWNO

If there is one underlying justification for Louisiana’s $50 billion Master Plan for coastal restoration, it’s this: We actually have a chance to prevent Southeast Louisiana from drowning in the Gulf, because the Mississippi River carries the sediment necessary to keep pace with sea level rise.

However, it turns out that assumption was a “best guess” at the time. In fact, the definitive evidence for the state’s claim that the river carries hope for the coast is just now being collected by a small army of state and federal researchers embarked on an historic effort — the first comprehensive study of the lower river.

The Mississippi River Hydro-Dynamic and Delta Management Study, as it’s called, is a massive $25 million, five year project begun in 2011 that seeks to provide detailed information about the building materials today’s river has to offer restoration projects. In particular, where and how those projects should be managed; how effective they can be in building and protecting land in the adjoining basins; and how they will impact the U.S. Army Corps of Engineers’ traditional river responsibilities — navigation and flood control.

“I think it’s taken us a long time to get to the point of realizing that we really need to understand the river as a dynamic system, says Denise Reed, the chief scientist for The Water Institute of the Gulf and one of the nation’s foremost experts on the river. “We’ve always known there were floods and droughts and things like that, but I think what we haven’t realized previously is how much variability there is along the channel — and there have been a number of studies of different aspects of the Mississippi River over time.”

Reed says most studies take detailed measurements at one point or another point and try to infer what happens in between.

“I think what’s happened now is we’ve done a few site-specific studies of some areas, and we realize that they change, really quite a lot, from one side of the river to the other, or up and down within the space of just a few kilometers,” she says. “And so, that recognition, along with the fact that we now — in the early 21st century — have much better instrumentation and techniques for measuring things like water flow and sediment concentration over large areas very, very quickly, that we can actually now do this. I think 20, 30 years ago we basically didn’t have the same equipment that we have now.”

Planned since 2004, the hydro study has lately taken on new urgency after research in 2011 raised questions about some long held assumptions about the river. The report found that much of the sediment and water the river carries into Louisiana never makes it to the Gulf of Mexico, disproving the assumption that most of the sediment the river gathers from its vast drainage system eventually sluices out the river’s mouth and into the Gulf of Mexico.

In fact, the study found only 19 percent of the total suspended sediments that pass the mouth of the Atchafalaya River north of Baton Rouge exits the mouth of the Mississippi River. In fact, a significant portion of the sediment downstream of the Atchafalaya is trapped on land between the riverbank and levees during times of high water, and even more drops to the bottom of the channel before it reaches New Orleans.

That last finding was attributed to one of the study’s most surprising and significant discoveries: less than half the water the Mississippi carries past the Atchafalaya exits the mouth of the river, and since lower water volume equates to less power for moving sediment, the finding casts new light on the potential of manmade and natural spigots drawing water from the main channel.

The scientists who made the report were not saying there still isn’t enough sediment and mud in the river to build new land, only that there were different forces at work south of New Orleans, and moving sediment in different ways than there had been north of the city. The Hydro-Dynamic study is designed to provide the engineers and scientists molding the state’s Master Plan with the raw numbers to determine the efficacy of restoration projects that depend on the river’s volume and sediment capacity.

But, as Denise Reed points out, everything is based on assumptions, since there can be no surety of just how big a river we’ll have from year to year.

“The hydro study is going to give us new information, and is developing some really useful tools that will allow us to say — what if we do this, and what if we do that,” Reed says. “But we’re still going to making a lot of assumptions if we’re trying to think about the future. The hydro study is not going to be able to tell us what’s going to happen in 20 years — nothing is going to be able to do that in a very, very specific way.”

Reed says what the study will allow scientists to do is make assumptions about the river in a more informed way.

“There is water exiting, perhaps at much larger magnitudes than we previously thought, out of what look like relatively small gaps in the banks, from the maps anyway — and so there is some material, and some water, leaving the river channel before it gets down to heads of passes,” she says. “I think we now have a better handle on how much that is, but only on the basis of some very specific surveys. So, we might know how much sediment goes out of those during the 2010 water year, or the 2011 water year, or the 2012 water year.”

Reed says the challenge with planning for the coast is scientists have to think about what’s going to happen in 2035, or 2040, or 2045 — so these measurements are really helpful, since they give an idea of how variable the river can be.

“They make us realize that when we’re going into the details of planning some of these diversions, we have to get some more site-specific information,” she says. “I don’t think that’s a surprise, though. I think that when you’re doing a large-scale broad planning effort like the 2012 Master Plan, I think you always recognize that you have to make some pretty coarse assumptions about these complexities in the system and you have to think a long way ahead. We could have made the assumption, perhaps, that every year for the next 50 years would have been like the year 2010. I’m not sure that that would have been a very reasonable assumption, either.”

That’s why this hydro-dynamic study of the lower river is so important. As Reed said, the river is a living organism that changes yearly.

According to the scientists working on the study for the Army Corps of Engineers and the state, the hydro study will determine what resources are available for restoration — while the delta management study will take that information and determine the quantity and quality of sustainable wetlands that can be built with large-scale diversions.

So this study will provide answers — but ones that can be adapted in the years ahead.

Support for The Louisiana Coast: Last Call comes from the Greater New Orleans Foundation, an organization that addresses the challenges facing people who live and work in the coastal communities of Southeast Louisiana.

The Louisiana Coast: Last Call is written and reported by Bob Marshall of The Lens, and produced by Fred Kasten.