Hi
When building a bridge, you spend all the trouble holding a road deck in the air. ---- Guess what is expensive with most bridges, are to support the actural road bridge deck.
With a floating bridge you don't worry about this, as this cheap water replace a lot of steel, ---- beside if you think low or high water level will be a problem, then the system work like this ; when high water ballast water are taken into tanks within the bargebridge hull, and this way the bridge are level with where it's connectet to land. With low water, you pump out water from the ballast tanks to highen the road deck, ------ offcaurse this can be done with quite primitive means, but a satelite navigator will also be quite handy for maneging the pumps.
Btw, in Copenhagen where we need a tunnel, a number of these barges in a row, will solve any problem about trafic across the harbour, and there will be several way's to place these ;))