Took a drive up to the Old River Control Structure and the Morganza Spillway on the 12th of May. I was curious about what is keeping the Corps from opening the spillway until the flow hits 1.5 M CFS. One reason would be that until the river gets to that level, the water is not high enough for the spillway to work. I found that the water is high enough, so that we have to think about the politics.
While the Corps was able to get development easements as well as flowage easements over most of the floodway, it is not clear that these have been enforced. The floodway also endangers Morgan City, which sits at the end of the Atchafalaya river, were the floodway necks down and the water has to flow out into the gulf through a main and minor branch of the river. (For those who are not local, tt is pronounced A CHAF A LIE A – short As.) The Corps trigger point is 1.5 million CFS at Red River Landing, just above the control structure, not the water heights downstream. How that flow corresponds to downstream heights depends on a lot of factors that change through time. The Corps has not publically addressed why it is sticking to the 1.5 MCFS, which was determined when the structures were built. It might have been the right number then, but now it results in very (record) high water downstream – that, or the flow is actually higher and the Corps have not figured it out.
The water level is below the levee tops, but it is well up the side of the levees. The projected crests before Morganza will be opened is pretty close to the tops of the levees. This puts a lot of pressure on the levees and pushes a lot of water under the levees. This water flows through sand boils, which are old stream beds which were covered when the levees were built. The water also flows through the saturated ground. The river is deep in many places and the water spreads out through the soil for a long distance. The longer the water level is high, the more water the clay in the levee absorbs, which weakens the levee. Clay levees do not melt away as did the sand levees on the middle Mississippi which broke during the 1993 flood. But when they develop a crack (crevasse), it erodes quickly and is difficult to plug during high water. In New Orleans, most of the city is below sea level and trapped within flood control levees. A levee break could rapidly put enough water into the city to flood many areas as deep as Katrina, and at least as fast. It would be worse because the water would be flowing through the high ground that was not flooded during Katrina. This might be the French Quarter, or the Ochsner medical center, or other high value areas that escaped Katrina damage.
The advantage to the Corps in putting these areas at unnecessary risk is that it will mute criticism when the spillway is opened and floodway is flooded, displacing people and destroying crops, and perhaps flooding Morgan City. Ironically, it is not a tradeoff – as the Corp’s maps show, if the spillway is not opened, it will be overtopped, as will other levees allowing water into the floodway. If the Corps opened all the relief valves early, protecting the levees downstream in Baton Rouge and New Orleans, it would be accused of not needing to open the spillway – unless the Corps creates a threat downstream, no one in the floodway would accept that there would have been a threat. (I have discussed this in my smallpox paper – politically, you do not want to roll out the smallpox vaccine until there are enough deaths to really scare people. Then no one will second guess you when there are serious vaccine complications. Do it early, stop the outbreak, and all people will see are the complications.)
The down side to the “scare them downstream strategy” (and the smallpox slow reaction strategy) is that if you guess wrong, it is catastrophic. The politicians in Baton Rouge and New Orleans are reassuring the population that the levees are perfect and that there is no risk. This means no evacuations, no thinking about what you would do if there was a break. Lower Mississippi levees are much better than hurricane levees, and much less susceptible to failure. Among other reasons, they are maintained by the Corps, while New Orleans levees were (not) maintained by the local levee boards. But there are probably 200 miles of levees between the Old River Control Structure and New Orleans. It is hard to assure that every mile is perfect. It is also impossible to keep materials and crews to plug a leak along the whole area. It would take a lot of time to get folks in place to repair a break, enough time to do real damage, and perhaps long enough for the levee to break down to the point where it cannot be repaired during high water.
An additional downside is the uncertainty for the people in the spillway. The Corps’ policy of only announcing a decision to open the spillway at the last minute leaves persons in the spillway uncertain whether they will need to evacuate. and when they need to be gone. The Corps predicted the record crests in Baton Rouge and New Orleans weeks ago. If the policy was to prevent levees downstream from being subjected to dangerous loads, the Corps could have announced that the spillway would be opened early in May. This would have given more evacuation time and would also have saved Baton Rouge and New Orleans from being exposed to unnecessary catastrophic risk. It would also have allowed the gradual fill time necessary to protect wildlife without extending the high risk levels downstream.