Gumbo and jazz, beignets and accordions, tipsy revelers festooned with plastic beads. The city of New Orleans evokes zesty images. Add to your mental slideshow two more images: that of a major economic crossroads it’s part of the largest port complex in the country, handling more cargo than the next two largest ports, Houston and New York, combined and that of a thriving coastal fishery that supports a $3.5 billion a year commercial and recreational fishing industry. Culturally and economically, it’s one of the great cities of the world.
Yet a perfect storm is brewing on the horizon.
It took thousands of years for the Mississippi River, a continent-crossing artery bearing a broth of sand, mud and water, to pile up the massive bird’s-foot-shaped Mississippi River Delta. In the past century, the delta has shrunk by about a quarter, an area roughly the size of Delaware. And a bit more land slips away each day.
The delta is literally drowning as the sea rises in response to a warming planet and the land itself sinks from the compaction of the thick sediment pile and the pumping out of water, oil and gas from beneath it. If the Mississippi River were a natural system, it might be able to keep the delta’s head above water by depositing fresh sediment. Unfortunately, the river has been massively re-engineered to prevent flooding or shifting course. Dams in the upper reaches have cut the sediment load in half, and levees have turned the river into what one recent scientific review paper calls “a superefficient pipeline channel” shuttling what sediment remains straight into the Gulf of Mexico.
With the rapid disappearance of its protective buffer of wetlands, New Orleans is becoming more vulnerable to storms every year. But recent research at The University of Texas at Austin has revealed some important clues about how to shore up these vanishing wetlands and has generated new optimism about saving the delta.
“With the insights we gain, it might be possible to dramatically reshape the U.S. Gulf Coast,” says David Mohrig, a professor at the university’s Jackson School of Geosciences,. “For example, we might learn how to create hundreds of square kilometers of productive new wetlands and help protect New Orleans from future storms.”
Wax Lake Delta
Mohrig is learning the art of building land in the shrinking Mississippi River Delta at the foot of the masternature.
While most of the Louisiana coast is receding, there are a couple of bright spots. One of those is Wax Lake Delta, a broad lobe of fresh wetlands that began poking up out of the water in the 1970s near Morgan City. It’s growing because the Army Corps of Engineers frequently opens a spillway about 200 miles upstream from New Orleans and diverts about 30 percent of the Mississippi’s flow into the Atchafalaya River. Wax Lake Delta receives about half of that diverted flow. All of that water carries sediment with land-building potential.
The phenomenal success of Wax Lake Delta, a happy byproduct of flood prevention, inspires scientists and engineers to dream big: Perhaps one day river diversions below New Orleans expressly designed for the purpose could shore up the dissolving delta and strengthen the city’s natural defenses against storms such as 2005’s devastating Hurricane Katrina.
This fall Mohrig and his colleagues will begin setting up a network of automated sensors across Wax Lake Delta to continuously monitor water conditions such as flow, sediment concentration, temperature, and salinity. The data they collect will be fed to scientists across the country in real time.
It’s part of a $5 million National Science Foundation-funded collaboration between The University of Texas at Austin and seven other universities directed by Mohrig. His two UT Austin co-principal investigators are Wonsuck Kim, assistant professor in the Jackson School, and Paola Passalacqua, assistant professor in the Cockrell School of Engineering.
Using drone aircraft outfitted with cameras and other devices carrying laser and acoustic altimeters, the researchers will periodically map the topography of the delta and take high-resolution snapshots of the plant communities that form an integral part of the wetlands. Biologists will conduct on-the-ground surveys of the types and densities of plants and their relationships to the physical land surface.
The ultimate goal of the collaboration is to build a comprehensive set of computer models that can reliably predict the physical and ecological evolution of river deltas. These models could help evaluate different proposed delta restoration projects.
Mr. Sandman
Most experts agree that if you want to build new land in the delta, you need sandand a lot of it. Sand makes up half of actively growing deltas. The rest is fine-grained mud and silt. It’s thought that rough, course-grained sand forms a stable template for the mud to stick to. Without that template, the mud slips into the deeper reaches of the Gulf where it becomes useless for land building.
The real challenge is getting enough sand where it’s needed. Sand makes up less than a quarter of the sediment flowing down the Mississippi. Much of it comes in pulses during big flood events such as last year’s torrent on the Mississippi. In many stretches of the river, sand tends to hug the bottom of the riverbed. Yet because deep diversions are costly to build, all of them built to date decant water from just the top few meters.
Mohrig says effective diversions need to coincide with big flood events, be located in areas with naturally high sand concentrations, and be engineered to extract as much sand as possible. But it’s a fine balancing act.
“Basically the idea is to maximize sediment and minimize water going through the diversion,” says Mead Allison, a senior research scientist in the Jackson School’s Institute for Geophysics.
Formerly a professor at Tulane University who moved to Austin after Hurricane Katrina, Allison has studied how sediments move through the river system for more than a decade.
He says too much freshwater or nitrates from fertilizer runoff may damage marsh ecosystems and fisheries. He also says removing large volumes of water can create shoals, or piles of sediment, in the main river channel downriver of the diversion that may affect navigation. His observations will help calibrate and validate computer models that he and his colleagues are using to examine how much water can be withdrawn before significant shoaling occurs.
Sprouts
Besides sand, another key ingredient for growing deltas is plants.
They play a critical role in capturing fine sediments, producing organic soil, speeding delta growth and making deltas more resilient in the face of storms and floods. But no one really understands which combinations of plants and which densities are optimal for land building. The work at Wax Lake Delta will try to answer these questions by correlating changes in plant communities with changes in the physical shape of the delta itself over several years.
Scientists are also tackling the problem in the lab.
Anastasia Piliouras, a PhD student working with Wonsuck Kim, is spending her summer growing deltas from scratch in a glass-walled tank that’s big enough to park a small car. Everything is downsized and sped up to allow researchers to get their hands around natural processes that would otherwise be too big and slow. Water flows gently into a channel at one end of the tank at a steady rate for weeks. Once a week, for one hour, the water flow is cranked up and sand is added to mimic a big flood, piling up a lobe of sand. Then alfalfa seeds are cast on top of the fresh delta surface. The little sprouts that form represent much larger plants on the real delta.
One hypothesis she’s testing is that the shape and general stability of the delta depends on two factors: how quickly the ecosystem (alfalfa) grows and how quickly the delta itself (sand pile) grows. Specifically, if the ecosystem grows quickly relative to the delta, then she predicts the delta will form stable channels and be shaped like the branches of a tree. On the other hand, if the delta grows quickly relative to the ecosystem, then she predicts the delta will be fan shaped, with channels that migrate sideways over time. Piliouras says the tree-shaped delta would probably be more resilient against waves and storms.
So far about a third of the way through their latest 12-week run Piliouras’ delta is looking less like a tree and more like half of an extra-large one-topping pizza with alfalfa. Still, she says it could all look very different by the end of the experiment.
Eventually, she plans to take her results out into the field to see whether they match up with how Wax Lake Delta evolves.
Losing Battle?
Just a few years ago, some critics asserted that dams in the upper reaches of the Mississippi River had reduced sediment flow so much that there wasn’t enough raw material left to rebuild the delta. They also argued that sea level rise and land subsidence would take away land faster than anyone could hope to replace it.
Then in 2009, Kim and Mohrig published results of a computer simulation showing that if a proposed diversion were built 93 miles downstream from New Orleans, it would offset nearly half the area of land expected to disappear from the delta during the next century. That study showed that well-planned diversions of water and sediment could help turn the tide on land loss.
Then, last year the Mississippi River swelled to levels not seen since 1927. The U.S. Army Corps of Engineers opened a spillway near New Orleans to prevent flooding. Jeffrey Nittrouer, a University of Texas alumnus and post-doctoral researcher at the University of Illinois, discovered that even though only about 10 to 20 percent of the river’s flow was diverted, about 31 to 46 percent of the entire sand load carried by the river during those six weeks was carried through the spillway. It turns out the twisty shape of the river at that location stirred up shallow sand from a sandbar next to the spillway.
“A tremendous amount of sediment made its way out from this diversion, indicating that if the sites of diversions are properly located,” says Mohrig, Nittrouer’s former Ph.D. adviser, “there’s the real opportunity to produce much more land than any of us were predicting.”.
