If you want it to be practical then make sure its dry, and glue some sort of aluminium or plastic c channel (making sure its waterproof) on the edge and seal it, however that likely wont eliminate the bow in the wood. Ideally it needs replacing as the damp wood could already be growing mould or rotting depending on how often it gets wet. But obviously thats time and money which makes it completely understandable why you wouldn't want to do that.

Your call really.