Locks for boats. With a double lock a boat can be lowered so as to pass under a relatively low bridge. Boat enters, drops say 60 feet, passes under roads and then raised to the water level inside.
There is also the 'Drop Lock', where there is only one lock, emptied for a vessel to pass under the obstruction, filled when it is the other side. There is less engineering but a safety risk related to filling the lock with a vessel under the obstruction.
I'm not sure how practical or economically viable dropping the waterway would be. Experience with canal restoration is that the amount of engineering required and other difficulties involved are often underestimated by engineers more used to road and rail.
Oh, and limiting ships to Panamax size would halve the size of shipping that can use Avonmouth.
There is a _lot_ of water involved in dropping a Panamax ship 60ft. Moving that amount by allowing it to run downhill, as on the Panama Canal, is relatively easy and fast. Moving it with pumps is going to involved a lot of engineering and power and may take some time, especially if two lockings are required. To give you some scale, it takes two 22ft diameter culverts to fill and drain a Panama Canal lock - that's about the size of a single track rail tunnel.
And that's after you have constructed the chamber - which, when you allow for water draft will be 100ft deep to drop a Panamax ship 60ft. There are good engineering reasons why the Panama Canal uses two locks to lift ships 54ft. The walls are 55ft thick at the base to handle the water pressure. A lock with a lift of 60ft (over twice the lift of a Panama lock) would need to be even thicker. Avoiding reducing the size of ships using Avonmouth would require locks capable of handling something around New-Panamax size (There is work going on to construct new, larger, locks on the Panama Canal). This would require the locks to be 120ft deep.
Looking around the web there are several plans for road and rail. Some cross the lock on a bridge. All require the road (and I assume the railway) to be elevated above the level of the Barrage to avoid the effects of storm waves.
The most likely solution for rail, and maybe road, if the crossing is 'flat', is _two_ movable bridges, one at each end of the lock. By definition only one set of gates is going to be open at a time. So traffic is diverted over the other, still open to rail traffic, bridge. I'm not sure how well it would work for road traffic though.
I would expect to see the moveable bridge(s) at least one end of the lock feature some modest amount of elevation, to allow vessels with a smaller air draft to transit the lock without requiring a bridge closure. If the elevated bridge was at only one end this would be the 'normally in use' bridge for rail and road traffic.
A mix of double movable bridges for rail traffic and an elevated bridge for road traffic, where gradients matter less, is obviously also an option.
