I'm running a D&D campaign and, well, the players had blown up a cave filled with explosives under a 40-metr cliff. I told them the cliff cracked and fell into the ocean. 40 meters tall, 30 meters into the land, around 60 meters along the coastline from start to finish of a crack. How would it affect the marina in a city nearby? How tall would a wave be? The cliff, I'm assuming would just slide into the water quite fast.

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    $\begingroup$ Argh. I cannot find the citation!!! I once read about a particular cliff (somewhere in the Caribbean) that has a near perfect parabola shape that may cleave off the side of its island, entering the water on a knife edge and widening. Computer models estimated a single wave a couple hundred feet tall resulting, heading toward a Mexico/USA. But I cannot find citation. Maybe your Google karma is better than mine. I do recall that shape mattered as much as mass in the wave created. $\endgroup$
    – SRM
    Feb 8 '20 at 14:15
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    $\begingroup$ Found it! Didn’t start in the Carribean, but it ends there... starts at Isle of Man: theguardian.com/uk/2004/aug/10/science.spain someone else can turn this into a proper answer. $\endgroup$
    – SRM
    Feb 8 '20 at 14:21
  • $\begingroup$ Looks like big, but not massive. $\endgroup$ Feb 8 '20 at 15:42
  • $\begingroup$ this depends a lot on the sea floor in the area, how deep is the water it is falling into? where do you want the height measured right next to the impact, half a mile away? $\endgroup$
    – John
    Feb 8 '20 at 15:57
  • $\begingroup$ @SRM the article says it starts in the Canary Isles but the chunk of rock is the size of the Isle of Man. Good find though. $\endgroup$ Feb 8 '20 at 19:04


A tsunami with a record run-up height of 1720 feet occurred in Lituya Bay, Alaska. On the night of July 9, 1958, an earthquake along the Fairweather Fault in the Alaska Panhandle loosened about 40 million cubic yards (30.6 million cubic meters) of rock high above the northeastern shore of Lituya Bay.



Your cliff face (dimensions and size) is like icebergs calving off the glaciers around the world. Cruise ships carry passengers up to the face for pictures and occasionally they are gifted with a new iceberg breaking off the face. It makes a small wave. Your wave might be a bit bigger but not catastrophic.

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    $\begingroup$ A wave passing beneath a ship at sea and a wave impacting a marina are two different things... $\endgroup$
    – Willk
    Feb 8 '20 at 14:56
  • $\begingroup$ About like this... youtube.com/watch?v=F6EZOw5wlEI $\endgroup$
    – user72081
    Feb 8 '20 at 15:08
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    $\begingroup$ A landslide would also be at least thrice as dense as an iceberg is, making for a lot more kinetic energy once it gets moving. $\endgroup$
    – Hyfnae
    Feb 8 '20 at 17:26
  • $\begingroup$ A lot of Antarctic ice calving is done from glaciers currently fully on land. I can assure you that these make very large and dangerous waves. If the iceberg is calving and is already mostly submerged, as I suspect to be the case for cruise chips being 'at the face', then there would be a lot less water perturbations. $\endgroup$ Feb 10 '20 at 19:40

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