I couldn't remember the name of the place or where I read it for a while, but this (important) article about Algiers Point is another example of what happened after Katrina. Algiers Point is high ground, just across the river from the Lower Ninth Ward, although not directly accessible from there by any bridges.

You had roving bands of armed white residents basically executing any non-whites who ended up in their corner of New Orleans--and they brag about it to the reporter. The veneer of civilization is much thinner in some places than others, it seems.

So, Gretna was not necessarily typical, but it wasn't a lone outlier either when it comes to extreme outsider-vs-insider attitudes.