LEVEE DESIGNS: EACH SERVES OWN PURPOSE, SAYS CORPS

VICKSBURG, Miss. - Like humans, all levees are not built the same way.

Some are designed and built to protect only rural farmland and small concentrations of population behind those levees. Others, such as the concrete floodwall and earthen levee in downtown Cape Girardeau, or the concrete floodwall in downtown St. Louis, are designed to protect highly populated urban areas.

For economic reasons, some levees also are designed to withstand greater magnitude floods than others. Other levees, such as the Little River Drainage District's Castor River Diversion Channel levee south of Cape Girardeau, protect property. That levee prevents the Mississippi River floodwaters from spreading southwestward from Cape Girardeau to the St. Francis River and into northeastern Arkansas, something which would flood millions of acres of farmland and endanger many cities and towns in its path.

Agricultural and urban are the two types of levees along the Mississippi River and its tributaries. Joe McCormick, chief of the Hydrology and Hydraulics Branch of the Army Corps of Engineers Lower Mississippi Valley Division at Vicksburg, Miss., says each is designed for a specific flood occurrence.

"Urban design levees in the St. Louis District, such as those at St. Louis, are designed and built to protect against the largest flow of record on the Mississippi River, which occurred during the flood of July 1844," McCormick explained.

"The Cape Girardeau floodwall is also an urban design project, but it was based on a different flood, but very similar to the 1844 flood."

During the 1844 event, which is considered the "benchmark flood" for Corps records, army engineers estimate that approximately 1,375,000 cubic feet of water was flowing by Cape Girardeau every second.

"The chance of a reoccurrence of an 1844-magnitude flood at St. Louis or at Cape Girardeau is well in excess of the 100-year average, and in some other locations it could be about a 500-year average," he continued.

The Corps has three other categories of levees: federal, non-federal, and private.

Federal levees are constructed to stringent specifications set by the Corps. They are made with impervious clay that has been compacted and shaped with gentle slopes. The Corps' St. Louis District has built 463 miles of federal levees along the Mississippi River from Cape Girardeau to Alton, Ill., and halfway up the Illinois River.

Non-federal levees are those built by local units of government. Generally, they're lower than federal levees. Some have steeper slopes and are made of more permeable material, and are less tightly compacted than the federal levees. There are 139 miles of non-federal levees in the St. Louis District from just south of Hannibal to Cairo, Ill.

Private levees are those built by individual landowners or businesses using the most convenient material. Compaction and construction standards are up to the builder. There are 55 miles of private levees in the St. Louis District.

Federal levees can be either the urban or agricultural design. The difference is that the agricultural levees are only built to withstand a 50-flood.

McCormick noted most of the levee failures that have occurred during this year's flooding have involved non-federal or private levees, such as the Ste. Genevieve Number Two "Big Field" levee; a private, agricultural levee along the riverbank south of Commerce,in Scott County; and the lower end of the Fayville-Olive Branch-Miller City levee in Alexander County, Ill.

In the St. Louis District, three federal levees have failed so far. But all were agricultural levees designed to withstand a 50-year flood: the Nutwood levee along the Illinois River, and the Kaskaskia Island and Bois Brule (Perry County) levee along the Mississippi River near Chester, Ill.

McCormick pointed out that the three levees that failed actually held well past their design limitation of a 50-year flood since this year's flood is now considered a 100-year model.

So far, none of the federal levees built for a 100-year-design flood have failed in the St. Louis District.

When the 49-foot flood crest passes Cape Girardeau next Thursday, McCormick said, there still will be 5.3 feet of freeboard on the floodwall and earthen levee.

"The top of the downtown floodwall and earthen levee is at a height of 359.1 feet above sea level. For the river to overtop the floodwall at Cape, it would have to go above a level of 54.3 feet on the Cape Girardeau gauge," he continued.

"But at this flood level, (54.3 feet) all other agricultural levees in Illinois and Missouri will have been overtopped by the floodwater."

The 20-mile Diversion Channel levee extends from near Allenville, eastward to the mouth of the Ramsey Creek Diversion, north of Scott City. From there, the levee curves south-southwestward, along the Ramsey Creek Diversion to a point about 3-4 miles west of Scott City. There it ties into a bluff that overlooks the fertile farmland and the industries the levee is designed to protect.

McCormick said the Diversion Channel levee, authorized by the 1928 flood control act, is part of the Mississippi River and Tributaries (MRT) Project system.

"An MRT Project levee is a totally different animal from all other levee systems in the Cape Girardeau area," McCormick explained. "The Diversion Channel levee is designed to withstand an MRT project flood. An MRT project flood is a hypothetical flood that has never occurred as yet at Cairo, Ill. However, it is based on three historical floods that have occurred at Cairo.

The MRT project flood assumes a flow of 2,360,000 cfs of water in the Mississippi River below the mouth of the Ohio River at Cairo. All MRT project levees, including the Diversion Channel levee, are designed and built to withstand this level of flooding."

McCormick said the top of the Diversion Channel levee is 358.5 feet above sea level, which compares with the top of the Cape Girardeau floodwall, which is 359.1 feet above sea level. "You can see both levees offer about the same level of protection, although each was designed under different but similar specifications." he continued.

"If the Diversion Channel levee were overtopped, the water would cover much of Southeast Missouri before reaching the St. Francis River near Fisk. From there, the water would cover a lot of land along the St. Francis River to its mouth on the Lower Mississippi River, just north of Helena, Ark.

Such an event would have catastrophic results in terms of the area involved in the two-state region, he said. "For that reason, there's a higher level of protection built into the Diversion Channel levee than other agricultural levees. It is a critical part of a loop system of levees that extends from Cape Girardeau to Helena."

McCormick said the Diversion Channel levee does not compare to one of the Big Five agricultural levees that extend from the Big Muddy south to Gale, Ill. "A breach in one or more of those levees would certainly be devastating to those in the levee districts, but the impact would be felt over a much smaller area. But a break in the Diversion Channel levee would have an enormous impact."

McCormick said levees that are built or funded by the Corps must have a positive benefit-cost ratio.

He said the Corps is mandated by law to use the benefit-cost equation when considering requests for levee projects or other flood control projects.