Q.Will St Augustine Grass Grow In A Sandy Environment? (South Louisiana)
We just filled in our yard using 18 yard of river sand and will be planting in this.

Certified GKH Gardening Expert
Here's a a good article from a Texas extension service that should be helpful.
https://aggie-horticulture.tamu.edu/plantanswers/turf/publications/staug.html
I'm assuming your sand is mixed with soil; I'm not sure this grass will grow in sand alone. You may want to contact your own local extension service and have a discussion about this. Here's a link for that: https://www.gardeningknowhow.com/extension-search
https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-9-10-11/zone-9-lawn-grass.htm