Suppose further that instead of a supernatural phenomenon, this was an irrigation method for creating a barrier that had the purpose of defending from marauding armies for the 6 days when the men were in the fields, but inviting an attack on Shabbos when the men were assembled at home. Then what ? What good would that do you?
The problem with this hypothesis is that the Sambatyon is held up as proof of the concept of Shabbos. Thus, by definition, the fact that it prevented crossing for six days of the week must be a natural or supernatural occurrence. For it to be a man-made occurrence (as the way you describe it) would defeat the entire point.
The Wolf