I agree. I also think that in order for something to "wash up" on the beach it would mean that it was thrown from a boat or bridge rather than right off of I10. Of course there may be a bridge that goes over this waterway. I'm not familiar with the area, just drove through it a few times but i would think it would be pretty impossible to throw it right at the beach and expect it to float away.
There's a bridge that crosses Bay St. Louis which opens into the Gulf of Mexico. There are a couple of exits for Waveland/Bay St. Louis off of I-10 coming from the west, eg, New Orleans, Lafayette, etc.
Google maps and type in Bay St. Louis.