Overall, has the system of levees along the Mississippi made communities along the river safer from flooding or have they exacerbated the problem?
This depends on where you look. Certainly, in a grandiose view of the entire MR&T system, it is much safer than it was in 1927 in terms of the flood threat posed to those areas that were then under cultivation, like the cotton fields surrounding Greenville, Mississippi. But, high-value areas like Memphis and places where engineering infrastructure crosses the rivers—bridges for example—we have constrained the channels with considerable severity, for which we later pay an exacting price. One of the biggest challenges in flood routing is shifting flow regimes. The Mississippi River has been narrowed down to just a 3,000-foot [900-meter] width where at passes downtown Saint Louis, when the natural low-flow channel, prior to Corps of Engineers involvement, was about 5,000 feet [1,500 meters] wide. When you squeeze the channels adjacent to densely populated urban centers, the result is usually back-eddy under-scour problems in the flow transitions upstream and downstream of such constructions. And, you heighten the flood depths in the very areas you are seeking most to protect. You have to "store" the excess floodwater somewhere. The big debate is always about where the somewhere else is going to be.
Are earthen levees still being built, or have they largely been replaced by newer technology? What are some of the alternatives—for example soil reinforcement?
Earthen levees are still being built because they are cheap, not because they are good. The average levee in the United States is 10,000 times more likely to fail than the average dam. That's not a matter of expert opinion. It's simply the actuarial figure. We can make levees more survivable by engineering them—hardly any levees are "engineered," per se—which means examining their respective foundation conditions and designing the dikes accordingly, instead of employing a one-size-fits-all mentality, which had been the norm until Katrina. There are all sorts of engineered measures that could be undertaken to make levees more resilient and survivable, if the will exists to do so.
That said, it is very tough to construct robust levees in the Mississippi–Atchafalaya delta because of grievous and pervasive ground settlement and low soil strength. When you stack up three feet [one meter] of earth to begin a levee, it settles about two and a half feet, due to exceedingly low bearing capacity. In those kinds of conditions, we need to develop methods and techniques that are more cost-effective than simply piling up earth like they did during the Civil War. This includes using clay as a binder. Clay wasn't used in New Orleans's levees prior to Katrina, so they didn't have any soil cohesion within the levees [making them more susceptible to erosion once they were overtopped].
What progress has been made in building levees more as energy-dissipation systems as opposed to simply being used as barriers?
I have been one of the advocates of energy dissipation systems because that is the type of system that has been developed by coastal engineers to design resilient breakwater systems. It could easily be applied to protective structures for hurricanes and tsunamis, which are much easier engineering problems than 100-year floods because of their very short duration. You only have to survive six to eight hours of overtopping, even in the largest hurricane.
What are "green" levees?
This is a term I've coined to describe a new type of protective barrier system, which would employ woody vegetation as a critical defensive component, along with a series of low-weight reinforced soil berms, which would be crafted to enhance recreation opportunities—hiking, biking and boating paths, for example—and be capable of surviving overtopping by the largest hurricanes.
View a slide show of the Army Corps of Engineers opening up the Bonnet Carre Spillway and blowing up a portion of the Birds Point Floodway.
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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!