This is a bit of a trick question because the book Bayou Farewell actually predates Hurricane Katrina by a year. (The book was first published in 2004 and Hurricane Katrina hit New Orleans in 2005.) Therefore, in the original publication, Tidwell doesn't really "accept" or "resign" anything, but one could say that he "predicts" the outcome and aftermath of Hurricane Katrina by the end of the book.
Why is the coastline of Louisiana eroding even before Katrina? Tidwell places a lot of blame on the channels dug by companies in the 1920s and 1930s to create small routes to support their businesses. These small channels have become huge waterways now with no way to stop their growth by erosion.
The disappearance of the Louisiana coastline is not an act of God. . . . There’s nothing natural about it, in fact. Human beings have made it so.
Tidwell further says that a large hurricane would be cataclysmic. Little did he know that Katrina would arrive a year later. Tidwell states that both people and homes would be washed away. The seafood industry of the region would be destroyed. The marshlands, eroded away by lack of care by humans, would no longer serve as a buffer between Mother Nature and humanity. Bayou life would be decimated.
If the reader doesn't accept this prediction as the "acceptance" or "resignation" you speak of in your question, then Tidwell's solution he presents by the end of the book certainly should. Tidwell suggests a huge channel to serve as a diversion for the great Mississippi river. This would re-flood the marshes, building back up the bayous and the barrier islands. Tidwell admits that it's both costly and necessary.
In conclusion, it is important to note that by the end of the book (and after many engineers and environmentalists have given their testimony), Tidwell predicts what must happen after Hurricane Katrina in order to save a very unique population of people living on the Louisiana coastline in the bayou country.