The biggest threat to the massive levee system protecting Memphis from the flooding Mississippi River: sand boils. Sand boils are bubbling water on the dry side of a levee, a danger sign that the mighty river is finding its way through the earthworks.
"Throughout the Mississippi, the delta area, typically you have fine-grain materials like clays and silts, under that you have sands and aquifer, where we get our drinking water," said Memphis Area Commander Shane Callahan of the U.S. Army Corps of Engineers. "When you have a high water event, the water is actually pushed down through that aquifer and there's a pressure for it to emerge on the land side, immediately landward of the levee."
Teams from the Corps of Engineers, approximately 150 engineers in all, have been walking the levees and floodwalls, searching for evidence of breaching.
They are looking for four distinct threats to a levee and flood wall system like the one keeping the Mississippi out of the streets of Memphis.
1. Overtopping: Water laps over the top of the levee.
The Army says that the threat of overtopping has passed and, in fact, the river could safely rise another six feet before going over the flood wall.
There are no signs of erosion on the levees.
3. Strength of Flood Walls
The flood walls protecting downtown appear to be holding.
4. Sand Boils
Sand boils offer just enough risk to keep engineers worried for another couple of days.
"My confidence level is high," said Callahan. "At this point we are monitoring, we would envision that we would have some signals that we would see before we would see any catastrophic-type failure."
The engineers say that if the levees and flood walls are able to sustain another two days they'll likely be through the worst in Memphis. However, for cities further downriver, the test is just beginning.
The river is cresting today in Memphis and is expected to be at full height in Vicksburg, Miss. by Wednesday morning, cresting in Baton Rouge, La., on Thursday.
New Orleans has already opened floodgates northwest of the city, saturating farmland but protecting New Orleans itself from the high water that is expected Friday.
Without the emergency measures that were put into place following the great flood of 1937, John Barry, author of a book on the flooding Mississippi, says there would have been immense devastation.
"If we didn't have the elements that were put in place ... there today, this would be a massive disaster that would dwarf hurricane Katrina," said Barry. "It's that simple … you would have several million people flooded. The water would keep pouring out of the river for weeks. It would be months before all of the water left the land, it would be easily the worst disaster in U.S. history had those things not been put in place."