WaterMarks Interview with Mead Allison

The process of taking mud and sand dredged from the Mississippi River or offshore sites and moving it by pipeline or barge to project sites is known as sediment transfer. Dr. Mead Allison, professor of earth and environmental sciences at Tulane University, discusses the significance of this process to the protection and restoration of Louisiana’s coastal wetlands.

WaterMarks: In the last couple of years, the idea of transferring sediment to coastal wetlands has been the focus of conferences, papers and coffee room discussions. Why the interest?

Allison: Sediment transfer does something that no other restoration tool can — it builds land quickly. A project using sediment transfer can pour thousands of cubic yards of material into an area in a matter of weeks. The result is the replacement of open water with solid ground, and that can’t be accomplished in any other way.

WaterMarks: How is it different from freshwater diversions? When water from the Mississippi flows into the marsh, doesn’t it carry sediment and build land?

Allison: Diversions bring nutrients and fresh water to the marsh, but only relatively small amounts of sediment. In the short term, even “sediment” diversions that cut deep into the water column to capture more suspended material don’t deliver nearly the volume of material available in sediment transfers. Freshwater diversions are excellent options for protecting an existing marsh, but as a reconstructive tool, they aren’t the first choice.

WaterMarks: Thousands of cubic yards of sediment is a lot of material to place in a matter of weeks. Can engineers control where it all ends up?

Allison: That’s another advantage of sediment transfer — it offers pinpoint control. An engineer can send a stream of sediment across and around healthy marsh and place it into an area that has turned into open water or a subsidence pond. And since the pipeline slurry contains a relatively small amount of water, that part of a marsh can be rebuilt at minimal impact to surrounding habitat. For example, the slurry wouldn’t change salinity levels or water column turbidities beyond the immediate area, and therefore wouldn’t affect the value of oyster leases.

WaterMarks: It’s going to take a lot of sediment to raise the floor of subsidence ponds in coastal Louisiana. Where’s it all going to come from?

Allison: Two main sources have been identified, the Mississippi-Atchafalaya River and offshore sites such as Ship Shoal. But the Mississippi is a cost-effective option — as you might imagine, it’s expensive to build and maintain a 20-mile pipeline on the open sea, or barge sand to land. So the Mississippi’s the first choice for areas reasonably close to the channel, but even so, additional decisions have to be made since several different types of sediment are found in the river. For example, there’s a layer that’s actively moving along the river bottom called the bedload sand sheet, but there is also relict fluvio-deltaic sediment below the sand sheet that the river has incised. Both could be used in sediment transfer projects.

WaterMarks: So what’s significant about the distinction between the two?

Allison: Among other things, the relict layer is nonrenewable and its removal will deepen the river channel, while the active sand sheet is continually replenished from sand supplied from upstream. The relict source is also more diverse in its composition and degree of consolidation.

WaterMarks: Does this mean anything to restoration planners?

Allison: It means a great deal. Sediment transfer has exceptional potential as a restoration tool, but that potential is limited to the amount of sediment that’s available. The relict source, while containing substantial amounts of sediment, is finite. When it’s gone, it’s gone. The sand sheet is renewable, but it’s also finite. There’s only so much that moves down the Mississippi in a year.

WaterMarks: And is it possible that at some point the demand for sediment will exceed the supply?

Allison: If we bring our restoration projects up to the level they need to be, it will certainly happen. Today we are casually watching this resource slide into the gulf. In the future, we’ll see it as white gold and we’ll be doing everything we can to capture every last grain.

WaterMarks: What is the downside of extracting large quantities of sediment from the river? Aren’t there significant contaminants trapped in the sediment that will be dumped into the marshes?

Allison: There’s a common misconception that the river sediments are badly contaminated. A recent study by the U.S. Geological Survey shows that the total suite of contaminants in fine grained sediments from the river channel is surprisingly low. Sands have an even lower potential for contamination. We also know that these same fine grained sediments discharge out of the mouth of the Mississippi and are drawn up into the marshes with tides and storm surges. And there’s no evidence that the marshes are suffering any negative effects.

WaterMarks: There’s also a concern that removing sediment will unbalance the vegetative and aquatic life in the river as well as where it’s deposited.

Allison: The bed of the Mississippi is a harsh environment. It’s constantly moving and shifting and doesn’t support a complex ecosystem. Removing sediment is unlikely to have a significant consequence. At the project site, however, the effect of large-scale deposition of sediment may initially be negative, especially on organisms such as the bay bottom benthic community. But that won’t be long term. These organisms will reestablish themselves, or others will take their place in the newly established marshes, and the ecosystem will come back into balance. Mobile aquatic life such as fish should be relatively unaffected.

WaterMarks: And levees won’t be threatened?

Allison: I’m not an engineer, but I know the levees are underpinned by relict layers,

which are highly consolidated. It’s likely that removal of sediment from the thin, surficial sand sheet won’t have an effect on levees. Mining the relict source will have to be done judiciously, however, to avoid undermining the sediments that provide a foundation for the levees. Fortunately, there’s a lot of geotechnical expertise around that will make that possible.

WaterMarks: You’ve addressed the common concerns about the possible downside of sediment transfer as a restoration tool. Are there other misunderstandings among lay or even technical people that concern you?

Allison: I do have a concern that even professionals forget that the amount of sediment carried by the Mississippi is finite. There will come a time when there isn’t enough to meet the needs of all the projects proposed, at least not on a cost effective basis. In the present, however, it’s frustrating that we have barely begun to tap this restoration resource. I’m also concerned that our citizens think that restoration projects last forever — they don’t realize that they have a limited lifespan. Whatever we do in restoration won’t be a permanent fix. If we successfully restore a barrier island or marsh, it’s possible that a single major hurricane will slice it up again. But if that happens, it will have done its work by protecting thousands of people, industries and infrastructure. As taxpayers, we shouldn’t see that as failure, we should see it as success.