How high do you think the water rises(or would lift the bridge)?
Do you need to drive across this when it's flooded?
My initial thoughts are you need certain points that keep the bridge up above water surface. If the wood deck is floating at water surface, you will get a tremendous amount of push from the water flow (and break your tie downs) and it will hang up all the debris floating down creek.
The barrels would work as long as it holds the deck higher than water level when flooded. But keep in mind, when the water recedes the barrels will rest upon the ground. I think they would get crushed when driving on them in dry conditions. You could possibly design some supports that the bridge would set back down on as the water receded and the barrels would then hang from the deck, then it might all work.
