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.
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11 Comments
Add CommentSo what do the global warming deniers have to say now that the effects are being seen on the Mississippi River?
Reply | Report Abuse | Link to thisWithout denying or confirming global warming, waiting until the rivers crest has been proven to be not the way to go. If diversion had taken place when the rivers approached the spring melt and rain (which everyone knew was coming)might have been a better choice to open flood gates for gradual relief than waiting for the torrent. Once again I see the water flooding the plain could be used in drought areas that are suffering at the same time as the plains are flooding. Present technology can offer diversion techniques without flooding additional land while putting many many Americans to work building the diversion system. No doubt improvements to present technology could be accomplished but that would only put more people to work. Build conduits along the interstate system now to divert water from future floods and store it in resevoirs for the future global water shortages that have been forecast or route it to drought areas immediately as it is diverted. The Everglades are almost dry and other parts of the nation need water to save crops. Too late this time but not to avoid the next crisis.
Reply | Report Abuse | Link to thisI'm not a dedicated global warming denier; although, I question some of the so-called proof. The Mississippi has flooded for hundreds of years during our occupation of North America. Recent floods have been exacerbated by poor engineering decisions made since 1927. As more people inhabit the area, the floods will become more and more disasterous. But to say the the current flood is caused by GW is a real stretch.
Reply | Report Abuse | Link to thisAlso, by your reasoning, the coldest April on record in the Northwest poinst to global cooling and an imminent Ice Age.
Reply | Report Abuse | Link to thisThey don't wait for the crest. The ACOE uses a trigger flow rate, then opens the gates. This is well before the crest occurs.
Reply | Report Abuse | Link to thisIt will be interesting to see if the flooded farm land produces better after the nutrients and sediments are deposited...maybe flood gates should be built that can flood certain areas in one year and other areas in another year...replenishing used/extracted nutrients...
Reply | Report Abuse | Link to thisJust a couple of observations stewie;
Reply | Report Abuse | Link to thisa) Tectonic activity makes your idea of subteranian cities really silly.
b) Rumor has it that you are mentally ill.
It is nice to see that someone has the ability to think rationally. Flood plains that flood regularly are some of the most fertile in the world. Flood plains that are prevented from flooding have significant drops in productivity over time. Too bad the powers that be don't have that much mental capacity. This A.C.E. guy makes more sense than most but still isn't quite there yet.
Reply | Report Abuse | Link to thisThere's not much can be done about the rivers' flooding. But there's alot to be done to save property:
Reply | Report Abuse | Link to thisThe gov't should require [through insurance companies, FEMA, or other] that all new building in floodplanes shall be mounted on steel barges. Instead of basements, homes shall be built atop a steel or re-enforced concrete buoyant platform. They shall have sloped sides like the bow of typical river barges. They shall be anchored, in four directions, with buried mushroom anchors and heavy chains painted with rubberized epoxy paint. The "barges" may be inset into the ground. When the flood comes the building will simply float up and not drift away. Perhaps the four corners of each building should have retractable/extendable legs, like a PU camper shell, so that after the flood the house can be held aloft and prevented from resettling into its hole until such time and the hole is re-excavated to it's original shape, and the house can be lowered back down.
The added cost can be somewhat amortized by reduced insurance premiums, perhaps in a plan administered by the Federal Government and financed by 0.5% bonds sold to rich Republicans and rendered invisible to the poor blokes who can't afford to live on hillsides.
Perhaps there should be pre-planned designated areas to be flooded, like as you say, during low stages ahead of the crest. This should be done, to varying degrees, most years, during spring floods. Rotate designated areas succeeding years.
Reply | Report Abuse | Link to thisAlso flood large areas and retain the water, not let it drain back into the river after flood stage. These areas could be an agricultural reserve. Also, if large enough in areal extent would have a significant effect on the weather; retaining a wetter atmosphere in the center of the continent later in the year, thus encouraging more reforestation and enabling crops higher in carbon content downwind of the "flood plains".
So, the farther west [ie: upwind] water can be captured the better.
Not an imminent ice age, simply a reordering of weather patterns due to a steepening temperature gradient with altitude [warmer near the ground, and cooler near the stratosphere] due to the increasing "foggy" appearance of the atmosphere as would be seen in the infrared if we could see in the infrared.
Reply | Report Abuse | Link to thisMost areas of the globe are warming, fewer areas are cooling. Various computer climate models showed this would happen even back in the 80's. Remember, higher temperature at the ground -> greater evaporation -> more cloud cover downwind -> cooler daytime ground temperature there -> change in surface wind patterns, etc. This is why we pay smart people to figger it out!