Are there certain areas along a waterway that are more suitable for levees to be built, or is it a rule of thumb that any community living along a major waterway should protect itself using a levee system?
We shouldn't be constructing levees intended to protect high-value assets close to the low-flow channel without substantive "design-in-depth" protective measures against under-seepage (jpg). This includes slurry cutoff walls, sheetpile cutoffs, under-drains (pdf) and plenty of uplift relief wells (pdf). The closer we construct the levees to the low-flow channel the more pervious the foundation materials beneath such levees. Virtually all non-overtopping-induced levee failures occur due to under-seepage along old channels and sloughs, which have increased permeability. The farther away from the low-flow channel, the foundations tend to become less and less pervious. So, protective levees should really be set back a considerable distance from the low-flow channel if we desire robust and survivable protective systems. This means giving back more of the natural flood plain to the rivers—that is, increasing the "river's share" from 8 to 10 percent of the flood plain to something more like 20 to 25 percent. The levees that are set back from the river channels also tend to comprise more clay, which provides increased soil cohesion, which is critical to the levees' ability to survive overtopping.
What is the purpose of opening up levees—for example, using explosives as the Army Corps of Engineers did on May 2 at Missouri's Birds Point levee?
This was a scheme worked out with physical models designed and monitored by First Lt. Herbert D. Vogel at the Corps Waterways Experiment Station at Vicksburg, Miss., in early 1931. The idea is not new—explosives had been used numerous times since the American Civil War to effect cut-offs. Gen. Grant used these to cut a bypass channel during the Siege of Vicksburg in 1863. And, of course, nine different levee demolitions [were] effected below New Orleans in the Great [Mississippi] 1927 Flood. The Birds Point levee scheme originally envisioned the use of explosives-laden barges, but after the 1993 floods the Corps of Engineers installed embedded pipe charges of a much more reliable and sophisticated design, to minimize unintended damages. When we "open up" a levee using explosives, its only intended to be used once, or at most twice, in 100 years. The whole idea is to save money, in comparison with constructing a multimillion-dollar flow-control structure to divert floodwaters, such as those constructed at Bonnet Carre, Morganza and Old River. Those were very expensive structures and can be expected to be used maybe 10 times per century, much more often than a "fuse plug levee," [which is lower than adjacent levees and designed to be torn down by the rushing water if the pressure becomes too great. As designed, no explosives are needed to remove these levees].
What is a spillway and what impact will opening some in New Orleans have on flood waters there? Where are spillways best placed?
A spillway is simply a "flow-control structure," whereby excess floodwaters can be safely diverted out of a river or around a dam. They come in two varieties. The first is a controllable spillway, which employs movable gates to regulate the volume of flow being discharged. All dams are constructed with spillways, and the vast majority of engineered dams are also equipped with outlet works, essentially conduits which allow the lake levels to be maintained at any desired level, and can draw the reservoir level below the spillway sill level, to provide for flood storage during extra-normal runoff years [like 2011].
The second spillway variety is the uncontrolled weir, or crest spillway, like those used for many dams, where the total volume of water passing through the spillways is uncontrolled once the lake level reaches the sill elevation of the spillway. In my opinion, every large reclamation district along the modern flood plains of major rivers that employs earthen levees—like the Mississippi River—should be equipped with spillways that can be opened before their levees are overtopped and destroyed. These same protective districts should also be equipped with outlet works at their distal downstream ends to safely discharge whatever floodwaters enter the protected basin via its spillways. If we added those two things, we'd prevent an enormous amount of unnecessary flood damage because we'd give the excess water somewhere to be "stored."
How does the present situation compare with the storm surge that New Orleans and surrounding areas faced in the summer of 2005 with Hurricane Katrina?
That's like comparing apples to oranges: not much in common, but both are round—in this case, both are wet. Hurricanes present an entirely different set of problems and load protective structures almost instantaneously, whereas duration, flow cycles (up and down) and the total volume of flood water are the three major variables that require weeks or months of careful observation and emergency mitigation. It's much more difficult to manage a major flood. The worst time to let your guard down is after the flood begins to subside, because that's when most of the failures occur, due to rapid drawdown of pore water pressures built up in the levees during sustained high flows.
Why is the Army Corps of Engineers using high-density polyethylene [HDPE] in Vicksburg? What is the significance of using this on the backside of the Yazoo Backwater Levee?
HDPE sheeting is simply a cheap waterproof membrane that can be employed with sandbags and small bulldozers pushing soil to create temporary "push-ups" on top of existing levees, to increase their effective heights by three to six feet [one or two meters]. They have been increasingly deployed since the 1993 floods with considerable success—not 100 percent success, maybe 55 percent. They have to be removed after the flood waters subside by the local districts—without federal assistance. FEMA usually pays for their initial deployment during the flood fight.
How has the construction of levees along the Mississippi River impacted the river's path?
We reclaimed more of the natural flood plains than we probably should have, because seepage is an invisible phenomenon, which takes time. The more volume of water that seeps though the foundation beneath a dike, the more chance there exists of internal erosion, known colloquially as "hydraulic piping". So, the longer these unseen pipes exist, the more invasive they become, eventually resulting in a levee breach. In many cases those breaches don't occur for five, 10 or even 20 years after the seeps and sand boils are first noted.