The barge will be in a channel that connects to the main channel of the Achfalalaya. As the water in the Atchfalaya rises it would back up(back water flooding) into the connecting channel if the barge wouldn't be there to block it. A crude valve. As long as the top of the barge is above water they can refloat it by pumping the water out.
If it rains hard enough the area upstream of the barge may flood. If the barge is not put in the channel the area upstream WILL FLOOD.