My story opens up with a disaster at sea in the middle of the night on August 14th. A few hours later, a small tidal wave hits the gulf coast. These two events set the entire story into motion. One of my characters is collecting sedimentary samples on a barrier island the day before the disaster at sea and spends the night on the island thus becoming trapped in the tidal wave. My problem is that I have written an entire scene describing her actions in collecting soil samples on August 13th. The scene is important because it develops her character, describes her place in the plot, and helps contribute to the overall plot of the book. I don't want to get rid of it and I don't want to turn it into backstory because I feel like there is too much there. I don't want to start the story with this scene because it feels wrong. My only thought was to possibly start the scene with something like this: Eight hours earlier, Alley stood on the back of her truck driving the core sampler into the sand... Does that work? It seems sort of stilted to me but I can't think of any other ideas.
It's hard to say what you should do here, but it's good to keep in mind that you don't have to make this decision right now. Why not just make it the first scene, then come back to it later (maybe the second draft) when you've made the decision about where it should go? Several times I've written stories in which I've changed the beginning drastically between the first and second drafts. I've learned that agonizing over this kind of decision before I really get the story going can stop me cold at the exact time I need to get on my horse and go.
You could title the chapter, "Eight Hours Earlier". I see no problem. I believe it is commonly done. And what Minstrel said.
