Still don't see why Mississippi River silt has anything whatsoever to do with New Orleans possibly flooding. The "fouled up" silt deposition is why the coastal swamps shoreline are creeping north, not why New Orleans is below sea level. Of course, fixing that silt distribution would be very easy---just cut two "east-west" channels well up the isthmus---one from the Missippi towards the east, and one from the Mississippi towards the west.
"Secondly, levees could breach in multiple places. Lake Pontchartrain AND the river...but that's not the point. Why is there a levee for Lake P.? Perhaps because of its elevation relative to the the developed areas? Why such a difference? Again, think of long-term policies, not just today."
Breaching of the Mississippi River levees by a hurricane is so unlikely as to approach zero, as the levees are higher compared to Ponchartrain. The reason New Orleans is below sea level is because the LAND IS SINKING, due to the huge weight of all the buildings (built on Jello), and the pumping out of groundwater. Again--nothing to do with river silt.
"(And I note that the river elevation is "high," and the lake elevation is the "normal.")"
I assume you mean that for your graphic, which probably represents the Mississippi level at "flood stage", which only occurs in early spring. The river level during hurricane season is obviously much lower.
There you go. We're on the same page now.
The reason New Orleans is below sea level is because the LAND IS SINKING, due to the huge weight of all the buildings (built on Jello), and the pumping out of groundwater. Again--nothing to do with river silt.
You got two of the three parts right. Subsidence is definitely important, but it's not a differential subsidence based on the river water holding up the river or something--water is heavy, too. It is also the silt that builds the elevation of the river up. (Plus, note that the the weight of the thickness of sediments themselves bends the lithosphere regionally, not just weight of buildings.)
Without the river containing silt, it could cut deeper into the channel and carry that load out into the Gulf...making the river lower. However, with the silt load deposited, the elevation of the river increases as the city subsides.