It is said that Baton Rouge is protected to 48-51 feet and New Orleans is more in the 17-20 foot range. Can you explain the difference in 'protection' from the river? How do the operations of Morganza Spillway compare to that of Bonnet Carre?
You make a good point ... what does "protected" really mean? I loosely use it to reference the height of the levees relative to sea-level ... since the river levels are also reported with reference to sea-level. So the tops of the levees in BR are around 48-51 feet, according to a recent LSU-scientist's survey. He used GPS technology, and claims accuracy within well under one inch!!
Of course, as you head "down river," the river and the levees are "falling off" towards sea-level. So yes, the levees are "higher" in B.R. than in N.O., but so is the river. I don't know this for a fact, but my guess is that the relative heights (river-to-levee) are similar for both locations.
As for the Morganza Spillway vs. the Bonnet Carre Spillway: outside of my real expertise ... but I think that the general operation is the same, although memory tells me that Morganza is not composed of "needles" (vertical logs) like Bonnet Carre, but rather "gates" (vertical panels) that are lifted. Morganza was built much later ... 1954, I believe.
A Google search (from a Times-Pic) article indicates Bonnet Carre has been opened 9 times (2008 is #9) since 1937. Morganza has been opened only ONCE ... in 1973. And that was NOT to reduce flood threats, but to ease the pressure on the Old River Control Structure (ORCS, located about 60-70 miles north of BR).
The high water in 1973 caused a partial failure of the ORCS, so the Corps of Engineers opened Morganza to ease the pressure on the structure. The ORCS lost a "wing wall" and was in jeopardy of collapsing (I believe that the wing wall has never been replaced). Had it done so, the vast majority of the Mississippi River flow would have changed course, with most of the water heading down the Atchafalaya River (which is EXACTLY what Mother Nature REALLY wants to do). That would mean "goodbye, Morgan City!" ... and would have effectively closed the ports of Baton Rouge and New Orleans because of the huge drop in water levels along the current Mississippi river channel!
The ORCS acts like an adjustable "sieve" to send a portion of the Mississippi River flow down the Atchafalaya. Actually, the thing is a bit of an engineering marvel - a dam would have been easy to build, but an adjustable sieve?
The object, as mandated by the U.S. Congress: add up the total daily flow of the Mississippi River and the Red River, then send 30% of that total down the Atchafalaya ... and retain 70% of the two-river total and send down the Mississippi. Of course, what this really means is determine what percentage of the Mississippi flow must be shunted into the Atchafalaya, since the Mississippi ALWAYS overwhelms the Red in terms of volume of water.
The ORCS is then adjusted regularly to meet this 30:70 mandate.
Hope this is readable ... and understandable!