The following quotation from my upcoming book, The Mississippi, explains the vulnerability of levees:
“As massive as they are, levees are fragile come floodtime. Leave a stick in the levee during construction, when it rots, it creates a cavity, weakening the levee. Let a small animal or crawdad burrow into the levee, it creates a cavity, weakening the levee. Let it rain for days and weeks and months, water will saturate its soil, weakening the levee. Build the levee of light, sandy soil, it is vulnerable to wave wash from wind or barge traffic, weakening the levee. A flooded river roaring downstream might scour its base, weakening the levee.
“The weight of the flood is the greatest danger to the levee. Two, three, four stories of water press against the levee, seek out its vulnerabilities, and saturate it, burrowing underneath it and erupting as sand boils–geysers of river water–on the inside. If the spout is muddy, the river is eroding the core of the levee. The taller the levee, the more massive the crevasse, the greater the damage to the land when it breaks.”
When I opened my post with the above quotation on November 19, 2009 in an article titled Tale of Two Levees, I was referring to the conventional wisdom about levees, all kinds of things make them fragile come flood-time. I should have added that conventional wisdom states, “Never allow trees to grow on levees. The roots break up the soil and undermine the levee.”
In fact the U.S. Army Corps of Engineers is waging nationwide war against all trees that may grow within 15 feet of a levee.
The conventional wisdom for stream banks reads: Allow trees to grow along the banks. Their roots will hold the soil on the bank and reduce or prevent erosion.
If trees on stream banks hold the soil and prevent erosion, why not trees on levees to hold the soil and prevent erosion.
Len Bahr at LaCoastPost has an superb posting on growing live oak trees on levees to hold the soil together and present a breakwater to an incoming storm surge. He calls such a levee a cheniere faux. A cheniere is a natural levee, created when a bayou washes over its bank and anchored by live oak trees, which the French call chenes. If you click on nothing else in the post, look at Bahr’s graphic in his post, “It’s time to consider forested levees.”
The saddest sight along the Louisiana coast are the skeletons of dead live oak trees that once anchored the natural ridges that snake down to the Gulf of Mexico, poisoned by salt water intrusion.
Bahr points out the Louisiana’s single largest expense for hurricane protection are levees, levees to protect New Orleans, levees to protect the Terrebonne and Barataria basins.
The Morganza-to-the-Gulf is a 70-mile monster that follows the ridges, the natural levees, that hold the Terrebonne basin together. But in some cases, it will have to be built across wetlands, which will disrupt the natural flow of water south and north. Fresh water flows south through the wetlands; salt water flows north on the tides. Levees along the Mississippi have long since halted overbank flooding, the major source of freshwater for the wetlands, particularly those in the Terrebonne basin, whose only source of freshwater comes indirectly from the Atchafalaya.
The Corps of Engineers has proposed building floodgates into the Morganza, creating a leaky levee. They would be closed come a hurricane. The Corps is also proposing the construction of floodgates that would close off Bayou Grand Caillou, the Houma Navigation Channel, the Placid Canal, the Bush Canal, and Bayou Pointe aux Chenes, all conveyors of salt water into the wetlands, in the event of a hurricane. Nikki Buskey notes in the Houma Courier that to save money the gates would be manually operated, an all-day operation for just for the gates at the Houma Nav and Grand Caillou.
Finally, Len Bahr discussed the other hurricane protection levee, the Donaldsonville to the Gulf for the Barataria basin in light of Judge Duval’s ruling on MRGO.
One last thing: When Katrina surged across Grand Isle, Louisiana’s only inhabited barrier island, her winds may have stripped the leaves from the trees in the island’s live oak forest, but the trees are credited with saving the island.