The mighty Mississippi tamed?
Tuesday’s flood crest at Memphis exceeded the 1927 high-water mark and fell just short of the all-time 1937 record. Yet as headlined by the Associated Press: Despite flooding, Memphis remains open for business.
In the eight decades since passage of the Flood Control Act of 1928 the Mississippi has become less a river and much more an engineered system of maintained channels, levees, and floodways. In 1927 there were at least a thousand deaths and flooding left more than 600,000 homeless. This week the threat was largely contained (broadly understood, see below).
The engineered system worked largely as planned. Many aspects of planning and implementation are undeniably impressive. I am especially impressed that floodways and the Bonnet Carre Spillway were built into the original and subsequent legislation and appropriations. Moreover water flow easements and other legal requirements for the floodways have been sufficiently maintained to survive a Supreme Court challenge.
The seldom-used floodways and spillway are a great example of trans-generational resilient design. Resilience is an outcome of anticipating turbulence by broadening and deepening the basin in which turbulence can occur while the system maintains its essential character. The US Army Corps of Engineers estimated that opening the Bird’s Point Floodway in Southeast Missouri may have redirected as much as 550,000 cubic feet per second of water volume. (Read more about the floodways at the USACE Mississippi River and Tributaries Project website.)
A couple of cautionary notes: The typical flow rate for the Mississippi is about 450,000 cubic feet per second. This week at Memphis the river was running about 1.4 million cfs. In 1927 the flow rate reached 2,345,000 cfs. How would the levees at Memphis have dealt with an extra million cubic feet per second? When the flood peaks next week at Vicksburg it may over top some levees by up to 12 inches. In non-protected or low-priority areas along the river and in the delta the high water will consume everything in its path. This is part of the system’s design.
The US Army Corps of Engineers organizes its design, development, and management around a “hypo-flood” that anticipates 2,710,000 cfs at the confluence of the Mississippi and Yazoo Rivers near Vicksburg. Fortunately, the mid-South precipitation forecast is mostly scant until the middle of next week. The upper Mississippi is past its surge. This suggests continuity of the current system.
Many engineered systems (and ecological systems) are characterized by a certain precariousness. The system is adaptive and resilient until – suddenly — it is not. When the adaptive capacity of a system is exceeded, a cascade of failures can result in catastrophic collapse. Tuesday in Memphis we stayed on this side of that terrible threshold. Barring some surprise (this would be a bad moment for the New Madrid fault to rock-and-roll) the lower Mississippi “system” should continue on its present edge for the next week and more.
We pay a price — and not just financial — for engineering the Mississippi. There are several negative, often unintentional consequences worth our attention (in another blog). But at Cairo and Memphis the system performed as planned.
Fundamental to our comparative success this Spring has been an acceptance of our limitations. The floodways reflect prudent preparedness for the extraordinary, the infrequent, and the potentially catastrophic. How might we systematically apply this principle to policy, strategy, planning, and management for other areas of homeland security?
