Gate off the water for the canal separate from the sea, so that it doesn't fluctuate. Usually this involves building a set of locks so that ships can traverse between the different levels - basically, a pair of floodgates with mechanisms to drain water into and out of the pound between them. Push ship in, lock the near gate, drain or fill the pound, unlock other end.

No, that's not how it's going to work. This proposed tunnel has no locks, the tunnel is open to the ocean on both ends.

You sure? 100 ships a day means 14 minutes per ship, and a lock cycle takes 10-20 minutes, so that's awfully suggestive to me. OTOH, I'm not sure of the relative difficulty and expense of adding height to a tunnel vs creating and operating a lock - I'd assume at some length it makes more sense to use a lock, since tunnel cost scales with length and lock cost doesn't.

I don't know why the limit is 100 per day, I'm just going by the article & the pictures in it which very clearly show a tunnel open to the ocean with no lock system.

The photos also show a mountain going straight into the ocean, which means raising the ship with a lock wouldn't save you any money, since you'd need an unbroken sequence of locks going all the way to the top of the mountain.

It'd save money by reducing the height of the tunnel you need to bore.

So it's a dock that is open at both ends and has a stone roof?

Docks are shoreside structures used for mooring vessels. It's basically an artificial lake inside an artificial cave.

How much the tide varies in any given area is well-known, you just take that into account when deciding how high to make the ceiling.

Presumably the tide isn't ever sufficiently different on either side of the tunnel to cause a large current through the tunnel.

It has a significant east-west displacement so you'd think there would be some tidal flows.

But a storm surge would be unpredictable, right?

How do the physics of "crushing" a boat by raising water level in a confined space work?

    > But a storm surge would be unpredictable, right?

No we know what sort of storm surges are 10, 100, 1000 year events etc. At that point it's just a question of what you design the tunnel to withstand.

    > How do the physics of "crushing" a boat by
    > raising water level in a confined space work?

This is an elementary application of Archimedes' principle. The weight of the ship is equivalent to the weight of the water it displaces[1].

If the ship is stuck against the ceiling of a confined space the pressure it's excreting on the ceiling is going to be the equivalent to the displacement of that "extra" water.

1. Leaving aside the full calculation which takes the difference in the weight of air & water into account for the purposes of this explanation.