What happens to a submarine ship during a tsunami?

Answer 1

Though modern submarines are affected by surface weather and tidal forces if close enough to the surface (I've experienced 12 - 15 degree rolls at 400' as a hurricane passed over us, and other submariners have reported up to 40-50 degree rolls in Category 4/5 hurricanes, even at test depth), tsunamis are only deadly to submarines as they approach shallower areas of land, which aren't really deep enough for submerged operations.

The only real threat would be a tsunami hitting a coastal area without warning if the boat was underway surfaced in shallow water (e.g., entering or exiting a harbor area), or if it was moored at a pier when one struck. In such a case, it would be subjected to tidal stresses similar to a hurricane storm surge or worse, depending on the strength of the wave. Hurricanes give enough warning that oceangoing vessels can emergency sortie (get underway) quickly to clear port before it hits, as all ships and submarines are designed to handle severe weather at sea. In port however, it's a much different story, as the tidal forces can literally bring a boat out of the water if it isn't prepared properly.

In one such instance I remember, as Tropical Storm Dean headed the East Coast in 1983, we had to emergency sortie to clear Charleston harbor, but one boat, USS Narwhal(SSN-671) was undergoing significant engine repairs and couldn't get underway. The only solution was to submerge the boat at the pier, making her as heavy as possible with variable ballast, hoping that any storm surge wouldn't be strong enough to damage her. Fortunately, the bulk of the storm passed Charleston, and she made it through undamaged. But it could've been a lot worse - Mother Nature has a really bad temper and not respecting the damage she can inflict can be a really bad mistake.