The President of the Mississippi River Commission ordered the opening of the Morganza Floodway at 3 PM CDT today, May 14 to relieve pressure on the levees at Baton Rouge and New Orleans.
From The Mississippi: A Visual Biography: “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.”
As I noted in yesterday’s posting, before the construction of the levees, much of the flooding on the Lower Mississippi drained south to the Gulf of Mexico along the Atchafalaya River. Hence, if you were going to relieve pressure on the leveed Mississippi, if followed that you would allow floods to once again drain to the Gulf of Mexico through the Atchafalaya Basin. What follows is the reason the Morganza Spillway Structure is where it is.
Morganza Floodway: Forebay
Pointe Coupe Parish, Louisiana
“The first crevasse below Red River Landing was the one on the Batchelor place; the break was a very small one and did little or no damage. The next was the Morganza, discharging directly into the Grand River, and from thence into the Atchafalaya Swamp. It floods portion of Point Coupee, Iberville, West Baton Rouge, and Saint Landry Parishes. This break occurred on the evening of March 14. The first intimation the levee watchers had of the break was the sudden appearance of a stream of water spouting up from one of the borrow pits on the inside and about seventy feet from the base of the levee, when all at once the levee proper gave way. The break occurred at a point where least expected, and at was supposed to be the safest part of the levee. The levee, before the break occurred was a picture of strength; it was perfectly dry and stood from five to six feet above the water surface. The cause assigned for the crevasse was a deep bed of quicksand that existed below the muck ditch, and which connected to the bed of the river.”–John Ewens, United States Assistant Engineer, August 9, 1884[i]
Louisianans made repeated attempts to maintain a levee at Morgan’s Bend, only to see it destroyed by floods. In 1874 the Mississippi blew a mile-wide crevasse in the levee. It was ten years before the state closed the gap and turned it over to the local levee board, only to see it collapse in the flood of 1884, the collapse described by John Ewens. The state repaired it again, only to see it collapse in the flood of 1890.
After the flood of 1928, the Army Corps of Engineers installed a 125-gate weir in the mainline levee just north of Morgan’s Bend at the head of the Morganza floodway. Guide levees funnel water through the floodway to the East Atchafalaya Floodway. The Corps has opened the weir once to siphon water from the Mississippi into the floodway during the Flood of 1973. The engineers did so to protect the Old River Control Structure, which was being pulled apart in the flood. John McPhee described the flood and “Atchafalaya” in the New Yorker in 1987. Read the article; it is still a classic.
The Morganza forebay is seven thousand acres of privately owned land, 4,500 of which are kept free of trees and shrubs to allow for the free flow of water to the structure should it ever be opened. The low Potato Ridge levee separates the forebay from Raccourci Old River, a cutoff that formed in 1848. Anhingas spread their wings to dry as they perch on the limbs of the willows that have taken root at the northern edge of the forebay.
The Corps allows farmers to grow corn and soybeans in the forebay, but persistent flooding that overtops the low Potato Ridge levee makes farming the forebay unproductive. However, it makes for productive crawfishing. Come flood time landowners in the forebay lease out lots to anglers, who drag out “some big, white, easy peeling crawfish.”
More serious is the damage to bottomland hardwoods when water overtops the low levee and floods forebay. It takes engineers up to three to five months to drain the water out through the sluice gates in the Morganza weir. Prolonged flooding during the growing season has destroyed nearly all of the forested land remaining in the forebay and stressed thousands of acres of bottomland forest in the Morganza Floodway.
When the Corps of Engineers drafted the Old River Master Plan in 2005, the engineers recommended developing a Section 1135 project to drain the forebay to the Mississippi rather than to the floodway and restore the bottomland forest lost in the forebay. Doing so would reduce the impact of sluicing water from the forebay onto agricultural lands and bottomland forests in the floodway. It would increase habitat for migratory waterfowl, wading birds, and shorebirds, and bring birders to the forebay. Maintenance of non-forested habitats in the forebay would benefit the least tern. The development of large tracts of bottomland hardwoods would create forested corridors for the sub-population of the Louisiana black bear that lives in the Morganza floodway.[ii]
[i] Department of War, Annual Report of the Secretary of War for the Year 1885, Volume II, Part 4, Washington, D.C.: Government Printing Office, 1885, 2641.
[ii] Bragg, 199; Ruess, Martin, Designing the Bayous: The Control of Water in the Atchafalaya Basin, Alexandria, Virginia: U.S. Army Corps of Engineers, Office of History, 1998, 200-203; U.S. Army Corp of Engineers, New Orleans District, Summary of Old River Control Draft Master Plan, July 2005, http://www.mvn.usace.army.mil/recreation/docs/Old_River_Master_Plan_SUMMARY.pdf; Mississippi Wildlife, Fisheries, and Parks, “Mississippi Waterfowl Questions and Answers: Basshole, February 19, 2006, http://www.mdwfp.com/forums/topic.asp?TOPIC_ID=745; U.S. Army Corps of Engineers, New Orleans District, Draft of the Old River Control Master Plan, July 2005, 109, http://www.mvn.usace.army.mil/recreation/docs/OR_MasterPlan.pdf.