I still don't understand why anyone building a new canal through Central America would not be trying to maximise the effectiveness of their work by building a sea level canal wide either enough for continuous crossing in both directions or dig two canals and make each one continuous flow in a single direction.
Yes it's a bigger challenge, but these projects are some of the few remaining opportunities in modern economics to say "we expect payback time of two decades" and not get laughed at. No locks no lock maintenance, continuous flow, more ship, more money.
It's also possible to use novel techniques from the last hundred years of progress to "dredge forward" using water itself as an an active tool to wash away all but any hard rock terrain that needs clearing. The rain and loose soils that hurt the first attempt at the Panama Canal could be turned into a positive factor with today's technology.
a) You can not escape the need for locks because there is a 8" height difference of the Pacific Ocean and the Caribbean. Additionally you have tidal variation.
b) The Panama Canal locks are already in pairs to allow simultaneous bi-directional traffic. They are currently building a third set of larger locks that will increase maximum dimensions of vessels transiting the Panama Canal.
They are nearly finished with the third locks (operation expected in 5 months). They are now contemplating a fourth set of locks, which would provide competition with a pending Nicaragua supercanal at a small fraction of the price, and provide room for the sort of ships that are launching now.
Cargo and cruise ships ships got very big, very fast this century amidst the sustained high oil prices. 20 years ago the record holder was around 5k TEU, the current Maersk Triple E, produced in quantity, runs 18k TEU.
An 8" height difference seems fairly trivial in the broad scheme of things. This would be a very long canal. Far lesser ships navigate rivers against much steeper drops.
Just because there's a height difference with locks, doesn't mean some kind of singularity would develop without them. Either the height difference would disappear, or continuous currents through the canal would maintain it.
In the Panama canal, they benefit from keeping the inland lakes above sea level so that they're deep enough. If they were connected to the sea, they'd lower significantly and even more excavation would be needed to make a path for ships.
There would also be fascinating effects of strong currents through it, sea level changes, fresh water lakes becoming salt water (sorry local people!), and fish migrating.
The average difference is 8" but with tides it can be up to 12' and it changes constantly. It'd be really tough for the ships to handle in the confined space especially when eddies form along the edges and with currents changing over a period of hours. A lot of these ships only travel at 10-15 knots and they aren't very maneuverable.
I was recently on a sailboat going through Hell Gate on the East River in NYC. It has about a 6' tidal range. We can motor at 6 knots. When the tide was at peak flood into Long Island Sound we were doing about 1/2 knot over the ground. You can time an East River transit to work around the tides, the Panama Canal is too long for that to work.
(I don't recommend transiting Hell Gate under those conditions, the UN closed the river longer than they said they would and we only managed because there wasn't any wind. We should have anchored and waited a few hours, we would have gotten through almost as quickly.)
That tech aside, Chinese ventures have had their eyes on the idea of a canal there for a long time. Six months ago they apparently put out a premature press release (as Chinese firms tend to do with construction megaprojects for some reason) in this direction, which was disavowed.
Yes it's a bigger challenge, but these projects are some of the few remaining opportunities in modern economics to say "we expect payback time of two decades" and not get laughed at. No locks no lock maintenance, continuous flow, more ship, more money.
It's also possible to use novel techniques from the last hundred years of progress to "dredge forward" using water itself as an an active tool to wash away all but any hard rock terrain that needs clearing. The rain and loose soils that hurt the first attempt at the Panama Canal could be turned into a positive factor with today's technology.