A LEVEE IS NOTHING MORE than earth mounded into a hill to contain water. Babylonians leveed the Euphrates. Rome leveed the Tiber and Po. By 1700 the Danube, the Rhone, the Rhine, the Volga, and other European rivers had levees, while Holland made the most extensive use of them (a levee and a dike are the same thing).SOURCE: Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America, by John M. Barry (Touchstone, 1998), pp. 40-41 (reviewed here)
The Mississippi creates natural levees. When the river overflows, it deposits the heaviest sediment first, thus building up the land closest to the river. Generally, these natural levees extend for half a mile to a mile from the riverbank. "Bottomlands" farther away are lower and often marsh and swamp. New Orleans was founded on a natural levee, and its French Quarter is the highest ground in the region. By 1726, artificial levees with a height ranging from four to six feet also protected the city.
But levee building never stopped; levees were extended above and below New Orleans, then to the opposite bank. Those levees increased the pressure on old ones. The reason is simple: when the river was leveed on only one bank, in flood it simply overflowed the opposite bank. But with both banks leveed, the river could not spread out. Therefore, it rose up. Thus the levees, by holding the water in, forced the river higher. In turn, men tried to contain the flood height by building levees still higher. By 1812, levees in Louisiana began just below New Orleans and extended 155 miles north on the east bank of the river and 180 miles on the west bank. By 1858, levees on the two sides of the river totaled well over 1,000 miles.
In some stretches the levee rose to a height of 38 feet. These heights changed the equations of force along the river. Without levees, even a great flood—a great "high water"—meant only a gradual and gentle rising and spreading of water. But if a levee towering as high as a four-story building gave way, the river could explode upon the land with the power and suddenness of a dam bursting.
From the first, some critics argued that building the levees higher simply increased the dangers should a crevasse, or levee break, occur, and insisted that a means to lower flood heights be used in conjunction with levees. There were three main ways to lower the flood level. One was to build reservoirs on tributaries to withhold water from the Mississippi during floods. A second was to cut a line through the sharp S curves of the river; these cutoffs would move the water in a shorter and straighter line, increase its slope, and hence its speed (a book arguing for cutoffs would later be titled Speeding Floods to the Sea). A third way was to let water escape from the river through outlets. All three proposals had detractors, but outlets had the most—because it also had the most advocates.
As early as 1816, proposals were made to create artificial outlets, also called spillways or waste weirs, on the east bank of the Mississippi near New Orleans. One proposal called for a spillway above the city to drain Mississippi floodwater into Lake Pontchartrain, while another called for one below the city to drain into Lake Borgne. Both "lakes" are really more akin to saltwater bays and empty into the sea, and at the proposed sites the river flowed within five miles of them.
Simple logic drove the argument for outlets. Removing water from the river would lower flood levels, proponents of the scheme insisted, just as removing the plug in a bathtub lowered the water level there.
Critics of outlets who instead insisted upon levees, and levees only—it soon became known as the "levees-only" position—generally subscribed to an engineering theory developed from observations of the Po made by the seventeenth-century Italian engineer Guglielmini. Guglielmini argued that alluvial rivers, like the Mississippi, always carried the maximum amount of sediment possible, and that the faster the current, the more sediment the river had to carry. His hypothesis further argued that increasing the volume of water in the river also increased the velocity of the current, thus compelling the river to pick up more sediment. The main source for this sediment had to be the riverbed, so confining the river and increasing the current forced a scouring and deepening of the bottom. In effect, adherents of this theory argued, levees would transform the river into a machine that dredges its own bottom, thus allowing it to carry more water without overflowing.
Levees-only advocates argued that outlets, by allowing water to escape from the river, were counterproductive since they removed volume from the river, lowered the slope, and caused the current velocity to slow. This not only prevented the current from scouring out the bottom, but actually caused the deposit of sediment—thus raising the bottom and in turn the flood height. According to the levees-only theory, using outlets was like taking water out of a bathtub, then dumping so much gravel into it that the tub ended up holding less water. The levees-only hypothesis argued that outlets, rather than lowering the flood height, would actually raise it.
04 September 2006
The Levees vs. Cutoffs vs. Outlets Debate
Posted by Joel at 9/04/2006 08:07:00 PM