That is the only inlet for many miles in either direction, maybe 20 to the South at Ft Pierce, and 25 or 30 to the North at Cape Canaveral. So the tide really pours out through there when it goes out. I am sure it could have eroded around the jetty if it wasnt well built. The North jetty is like a heavy duty pier now, with a jetty under it. It used to be just a jetty when I was young, and way shorter. The south jetty is not as long and is still just a jetty. They have the sides all beefed up now so it cant erode away. I am sure they used some dredged sand to build the bridge bases, parking areas, campground, who knows. I think they built that bridge that is there now in the mid 60s. I think it used to be a shorter wooden draw bridge. Maybe it was concrete, I dont remember.