The plan is not to bring supertankers into the Fore River, but rather enlarge the channel at the bridge so as to facilitate safe passage of so-called "PanaMax" ships at the bridge and so meet current federal specs NECESSARY to secure federal funding.
A PanaMax ship is one that will be able to transit the Panama Canal once its upgrade a somewhat larger ship size handling capacity is completed during this decade.
Absent meeting the current federal design and performance specs or securing an all but impossible to secure congressional waiver, a new bridge at the Fore River will not be build and the state would then have to figure out a way to make do with the current temporary bridge.
A bascule design cannot be built for the same money and provide a 250 foot wide clearance.
A bascule design would also entail a much lower clearance when down and would thus increase the number of times it would have to be opened as compared with a vertical lift bridge. The lower clearance of a bascule design also results in a potential point of vulnerability at its hinging points should a large ship drift up against one of the two bridge foundations. diagonally
A verticle lift bridge will also open quickly (2 and a half minutes) and close as quickly. It will also be much easier as well as much less expensive to maintain over its life expectancy, especially as it will require far fewer custom as well as much less difficult to fabricate complex parts in the event of a structural failure.
Finally, the Fore and Town River Channels are COMMERCIAL harbors, not Marblehead. As such, to demand a less capable and more expensive bridge for reasons of esthetics is, well, neighborhood level nonsense at its most selfish.
The bridge will cost $326+ million dollars up from the original $242 million and it is only at the 25% design stage. Does this sound like the "Big Dig"? They also want to run oversized supertankers down the undersized channel. Does this sound like another potential Exxon Valdez? They have a big bascule in Casco Bay. Google it and see what you could have.