I had a 16' bridge of mine float away (made it out of 6x6's) when the 2" deep creek flooded. Freak of nature... anyway, in order to hopefully help this from never happening again, I fastened thick rebar to the side of it. I got some metal brackets to hold the rebar in place. Then I drove the rebar 4' into the ground. I don't know if it will work, but I sure hope so.
