Environment
 

As river slowly gives up its secrets, planning diversions gets more complex

For decades the solution to the state’s coastal land loss seemed simple: Just punch a few holes in Mississippi River levees and let the mud-rich water spill out over marshes to build new land.

After all, that’s just what the river did for millennia before those mud walls went up after the epic 1927 flood.

But as the first in-depth study of the lower river in 50 years pushes past its mid-way point, the scientists involved have a few words of advice: It isn’t that easy — not even close.

There may be plenty of land-building material in the lower river, but getting it into the basins in a way that can build land efficiently is a major scientific and engineering challenge.

“It’s not as simple as cutting a hole in the levees and you build land,” said University of New Orleans researcher Alex McCorquodale, one of dozens of scientists working on the  Mississippi River Hydrodynamic and Delta Management Study.

“That’s because everything you do at one point of the river — anything you take out or put in — will have an effect on the rest of the river, as well as what effects you can have on the wetlands outside the river.

“We’re discovering just how large a puzzle this is. We’re collecting some of the pieces, but we have a long way to go.”

Research papers presented at last week’s State of the Coast conference underlined how much work lies ahead. But results pouring in from measuring devices as well as complicated computer models are beginning to fill in a still sketchy picture of what the river can contribute to the Master Plan’s fight against coastal land loss.

Some of the questions being answered:

  • How much water and sediment actually flow past New Orleans for land building on the lower river?
  • How much water can be taken out of the river by diversions and still leave enough for shipping other uses?
  • What is the ratio of coarse sands to the powdery light sediments in the river water?  The sand is needed to form the framework of new land; the fine muds settle to fill in the frame and nurture plant growth.
  • Which water levels on the river move enough of the heavy sands required for land building?
  • Which locations on the river are best for capturing the volume of water needed to move those sands and mud?
  • Which locations in the marsh are best for building land? Open bays need more sand, broken marshes can more readily capture the fine sediments.
  • Where are the deep sand bars in the river and how do they travel inside the river bed after different flows?
  • How much water can flow from the river into the basins without increasing the danger of flooding in surrounding communities?

Some of the findings were expected, others are surprises.

McCorquodale’s group has been looking at the flow of water and sediment in the lower river, and what happens to those flows when a diversion is opened.

“We modeled one large diversion (250,000 cubic feet per second) on the upper section (of the lower river) and we found it dramatically reduced the amount of water and sediment available on the lower river,” he said.

That finding had greater implications when tied to another of the team’s discoveries: 50 percent of the river flow that passes Belle Chasse left the system before the river reached Head of Passes south of Venice. Most of that was exiting a series of natural openings on the east bank.

“You need a certain amount of energy available to move not just sediment and sand in the river, but to move it through a diversion and into the wetlands,” McCourquodale said. “So we have to look at how much we can take out higher in the system and still have enough for purposes in the lower reaches.

“And, of course, we have to have enough water left in the river for shipping.”

Even with weaker flows than expected, the lower river still has enough building material to supply the projects in the 2012 Master Plan, McCourquodale said.

“The pieces of the puzzle we’re getting make us believe we can still accomplish those goals,” he said, “and I’m hoping in a couple years when we complete this study we’ll be much more confident.

“Of course, then we’ll have to look at the Delta Management part of the study.”

That portion of the research will assess just what can be done with the river’s material, and what those impacts will be on the current ecosystem.

Correction: This story originally misspelled Alex McCorquodale’s name. (March 27, 2014)

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  • Clay

    The “we just gotta dynamite some levees and let nature take back over”-idea is just plain wrong.

    Diversions need careful design if they are to work as desired.

  • KC King

    This sounds like yet another instance of ignoring the river and its flooding dynamics as a total, integrated system. When will the amateurs driving this train ever learn.

    This confirms that entire master plan needs to be rearchitected into systems, subsystems and sensible system process components.

  • Ezra Boyd

    In my view, ‘Planning gets more complex’ is an accurate & helpful synopsis on our situation. Unfortunately, there are narratives out there that confuse ‘complex planning’ with ‘impossible so resettlement is the only option’ and others that confuse it with ‘diversions will destroy our fisheries’. Neither of which are particularly accurate, and I don’t see them as particularly helping. Complex – Yes. Requiring detailed, science based planning – YES! YES! YES!

  • Chris McLindon

    Thanks Bob, another well written and balanced article. IMHO the revelations of this study all underscore the reality that the avulsion of the Mississippi River to a new course has already occurred. Forcing flow down what would otherwise be its abandoned channel does not constitute a river. The sooner we stop pushing back against nature and start working with it the better off we will all be.

  • nickelndime

    Humans ALWAYS have a Master Plan. Perhaps that may be our greatest flaw – in the end. OLD JOKE: Do you want to make Gawd (the one in Nawlins) laugh? TELL Gawd what you’ve got planned.