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.”
Last night, November 18, 2009, U.S. District Judge Stanwood Duval, a native of Houma, Louisiana, issued his decision on the culpability of MRGO and the U.S. Army Corps of Engineers in the flooding of St. Bernard Parish and the Lower Ninth Ward of New Orleans. It was.
A reminder: MRGO, the Mississippi River and Gulf Outlet, is the 76-mile long, straight navigation channel the U.S. Army Corps of Engineers dredged in 1965 to bypass the meandering Mississippi and cut a few miles off the trip between the Gulf of Mexico and New Orleans. Read more here and go to the categories section of this blog and scroll down to the MRGO category for other postings on the channel.
The Corps construction MRGO to 650 feet wide. The slumping of the sides of the channel and wave wash from passing ships and other factors eroded to about 2500 feet wide. The Corps built the levees of soft alluvial soil and they slumped. In fact, the Corps had not finished construction of the levees that began after Hurricane Betsy in 1965. When Katrina struck forty years later, the engineers were waiting for the soils to compact so they could put the finishing touches on them.
The judge ruled that the Corps was negligent in the design and maintenance of the channel, that that caused the flooding of St. Bernard and New Orleans, and that the plaintiffs, who could not sue the Federal Government over the collapse of the levees erected between MRGO and their homes, could sue the Corps and the Feds over the design and maintenance of the channel.
This morning the St. Louis Post-Dispatch had an article about the 60-year old levee that runs just north of Lock and Dam 26 at Alton, Illinois. It’s leaking. Water has worked its way under the levee and sand spoils are erupting on its landward site. Engineers attribute the weakness in the levee to the construction of the dam in 1990, which raised the level of the river–a permanent flood– and the hydraulic pressure on the levee. The Corps built the levee and turned it over to the Wood River Levee and Drainage District for maintenance.
The Mississippi in this part of the world has been going up and down like a yo-yo the last two years. Today, it is at flood stage at Lock and Dam 26 and is expected to go higher. The National Oceanic and Atmospheric Administration has a graph of flooding at the dam and a list of which places flood at what levels in the region behind the dam.
This spring when the river was flooded and the gates on Lock and Dam 26 were open, the water level behind the dam was lower than when the gates are close, allowing for moist soil plants, food for waterfowl to germinate on the exposed mudflats.