re: Neighborhood developments in South Louisiana

Part of having a subdivided plan approved by the local gov'ts Zoning Commission is to have a drainage study conducted and then provide the drainage plan with your subdivision plat. In that plan, the city is responsible to make sure you have sufficiently addressed any current flooding issues as well as any new issues your runoff would cause.
The last one I did, I had to change culverts under a road to increase the runoff rates sufficient to offset the developed property. It had needed to be done for 20 years but Lafayette waited until somebody else would pay for it to do the job.
Baton Rouge more than likely does the same thing, that's why you don't see all those new neighborhoods flooding ... it's because they are changing the retention capacity of the property itself with ponds/open ditches or improving the downstream flow rates of the surrounding drainage system.