Three kilometers (~2 miles high) tsunamis go around the Earth - what kinds of ruins are left?

Question

I am (still) writing some fiction about the solar system being rearranged, with the Earth losing the Moon. But before she goes, I am having her do a close flyby.

My story is actually happening millennia after the fact. I am trying to come up with some worldbuilding that realistically reflects the destruction caused by such an event, and how much descendants of the survivors of the cataclysm can recover and parse from the past via whatever ruins are left.

I have seen this video from Kurzgesagt where they say that should the Moon come very close to Earth, the tides would pull the oceans into 3km high waves (about 2 miles high, in imperial measure) going around the globe. There would also be global earthquakes and apocalyptic meteor showers, but I think the waves would be the most of the pounding on cities, right?

So the question is what kind of ruins would be left if the oceans were pulled and dragged over the continents in tsunami form. Back of napkin calculation tells me that does for 300 bar for the water column alone pressing on a building, which makes for three Venusians atmospheres, but I don't know what that would do to a city as a whole and what archeology could be done to whatever remains.

Answer 1

Location, location, location

Coast

Coastal cities would be razed to the ground. Bridge foundations tend to be reinforced concrete that's well anchored and stabilized against forces parallel to the river flow, so some of that might survive.

Even then, some underground structures could survive. Probably not basements, but think of subway tunnels. They would get flooded and lots of debris entering, so the entry areas will be a mess, but farther in, friction will slow down the inrush, and you may even find that the water pressing in near the beach will keep entries further up free because of displaced air being blown out at the just right time.
The inside of such an underground structure would have been a deathtrap, and it's somewhat unlikely that entrances would stay accessible, but some freak tunnels might be there for the descendants to wonder about.

Further inland

Distance from the coast will dissipate energy via ground friction.
Terrain height will eat energy that goes into height, though water piling up will regain that energy when rushing down the terrain.

Result

You get absolute devastation near the coastlines. Most cities won't be recognizable as such, but descendants will see all kinds of broken artifacts. Possibly some plastics, some metal fragments. Roads will still stand out because of terrain shaping and because there will be remnants of road surface material, foundations will still be present even if as caved-in depressions filled with concrete rubble. Foundations for bridges and high-volt power lines have no hollow inside so they will be rubble with a more-or-less solid core.

Further away from the coast, the underground stuff will likely be the same but more of above-ground structures will stay discernible. It will be a gradual change until you get to a region where the tsunami never arrived - there are mountains higher than 2000 meters, and horizontal terrain will stop even the highest tsunami. Eventually.

Answer 2

You can look at some historical example to get a sense for what would happen.

On October 9th 1963, a landslide in the Vajont dam caused a tsunami wave 250 meters tall which swept the valley below the dam, bringing death along its course.

This is what was left of a village right below the dam

as you can see it's hard to tell that there was a village there at all. The dam itself was left untouched and still stands there, but I can imagine that a wave 12 times taller would have little mercy also of reinforced concrete.

I think you would be left with scorched ground with, here and there, some leftover from a very lucky or sturdy building.

The ocean would be littered with debris which would bear little similarity to the buildings from which they originated. For example I don't expect a brick wall to hold together while being carried along from the wave, and the crushing during the transportation would drastically affect also the shape of the individual bricks.

Answer 3

Ask and Youtube shall provide! There is this video produced by a scientist who studies tsunamis, which shows a very similar thing to what you have in mind. The premise of the video is slightly different (his hypothetical scenario is that the solid Earth stopped rotating) but the simulation produces waves up to 4 km tall, so in the same ballpark.

The most striking thing is just how fast such tall waves are. They have no problem crossing whole continents in an afternoon. This speed, and not the pressure as such, would be the single biggest destructive factor: the rushing water would effectively apply an enormous amount of torque to any structure sticking out of the ground and snap it off at the base, and then carry it and inevitably smash it into other things and pick these up in turn. It will also have no problem washing out the soil from underneath structures. After the initial wave energy mostly dissipates, water will drain from the interiors of the continents back to the ocean basins and carry the debris with it, where it will be mostly buried in the washed out soil and any other sediment the wave would have picked.

In terms of archeology, not much at all will be left. Most man-made objects will be either smeared haphazardly over the vast deltas deposited by the receding waters, or would have eventually found its way to the bottom of the abyssal plains, and mixed with other debris in both cases. After several millenia, most metal objects would have rusted beyond recognition, and even large blocks of concrete and masonry may have accumulated quite a lot of damage from whatever chemical reactions have been slowly occurring between them and their new environment. Most importantly, even such objects as do survive will be deprived of their original context and therefore difficult to interpret.