the mississipi has been trying to reroute down the atchafalaya for decades.
river freight is not as economically useful as it once was, at least not to the rest of the country. It's main advantage is transporting bulk, low-value, high volume commodities. However, with these types of cargoes, trains have very good performance, and use only slightly more energy than barges. However, we spend billions through the ACOE on waterway maintenace.
The deepwater Port of New Orleans, which IS important, should be rebuilt at a more sustainable location in the region. I suggest this city be named Newport, but that's just me.
The Vieux Carre of New Orleans, built first, and on the highest ground, is historically and culturally important, and should be protected by new levees and flood control. It is my understanding that most of this area was largely 'spared' by Katrina. A city, based on tourism & entertainment, could be much smaller than the existing New Orleans, and protected by a higher, but smaller, system of levees.
In most cities, roads, streets, and parking take up about 50% of the land area. Eliminate this area in the rebuilt New Orleans, and design a
carfree transit oriented city around 30 or so transit nodes, each serving a fairly dense population of around 10-12,000 (including tourists). A fairly cheap surface tram system running at a line speed of 15 mph (top speed of ~22 mph) could provide maximum trip times in such a city of 40 minutes door-to-door, with most trips being under 20 minutes.