Would think the defense of New Orleans from a naval threat, hinged on defending the river at the forts, which would require a brown water riverine defense to cooperate with the forts. A defense at the city is going to be hampered by the city being below sea level, if they had blew the levees, they might not have been able to take possession of the city......but would have left a land defence with little to defend.While I'm ordinarily heavily Navy-oriented, I do feel that a lot of this (however interesting) is irrelevant. I'd like to reemphasize that the failure of the Confederacy to adequately defend New Orleans with land forces was the most important factor in its loss. If Farragut's fleet had run past the forts and then found New Orleans (or its outer defenses) teeming with a powerful and well-placed defense force, he could have done little other than bombard it or run past it further upriver.
What he accomplished at New Orleans was to bypass and cut off the primary defenses (Forts Jackson and St. Philip) as well as eliminate their naval covering force. Coupled with the (relatively small) Union land force, this effectively placed both forts in a state of siege with no realistic chance of raising it. The fall of the city itself was a strategic opportunity that Farragut perceived and took immediate advantage of, but even then it was a near-run thing-- he couldn't breathe easier until Porter and Butler completed the neutralization of the forts and sent troops upriver to garrison the city.
For their part, the Confederacy's mistakes seem to have been in overestimating the defensive value of the forts (as previously noted). It's important to remember that not everyone in the Confederate command fell into this error-- there was a lot of correspondence from local commanders and civil authorities in New Orleans basically pleading for the retention/return of troops that Louisiana had provided for use elsewhere, so the threat was correctly perceived at the local level. (But I'm sure being able to say "I told you so" was poor compensation for the loss of the South's largest city.)
The problem seems to have been no one realized steam had made forts obsolete by themselves if one was willing to accept losses to simply steam past them.