It would seem a worst case scenario for New Orleans would be a direct hit by a major hurricane, with the levees remaining intact. As it is, with the Pontchartrain levee failing the above-sea-level high ground was not flooded. However, if the levee had remained intact, the water could have risen above sea level.
The question: Could a direct hit by a major hurricane (with levees holding up) generate enough rain to fill the New Orleans “bowl” above sea level.
It would depend on tha actual contours of the “bowl” shape of the land, but a true spherical bowl would require anywhere from 0.7" to 0.8" of rain to accumulate each inch of rain, depending on how much of the bowl you’re trying to fill. Given that New Orleans is - what? - at least 6 feet below sea level, you’d have to have a storm dump at least 4-1/2 feet of rain to get it up to sea level (assuming the pumps and drains are defeated). So, at least 54" of rain in one storm. That’s a lot of rain.
The storm surge could do it, but I doubt it could be done strictly with rain water.
Yes. The worst-case scenarios I read about a few years ago described a situation where the storm surge tops the levees. This would send a wall of water through the city rapidly flooding it, as opposed to the (relatively) slow rise of water that followed the levee breach two weeks ago.
After this hypothetical perfect storm passed the city would be left with water levels above sea level trapped behind the levees. Quite possibly the Corp of Engineers would have had to demolish one of the levees on purpose to begin draining the city.
If Katrina hadn’t taken a turn to the east in it’s final hours, this could well have happened … .
the pumping stations in new orleans are the best in the world. i’m rusty on the specifics but the pumps are able to pump an inch of rain per hour out of the city for the first approx 5-hours then it falls to 1/2 inch per hour. it would have to dump some continuous hard rain to flood that high without a levee break.
the true worst case would be for a storm to enter the pontchatrain estuary and sit over the lake. that would basically displace the lake into the city.
The OP asks about rain, not storm surge…