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The force of a tsunami is enough to put a gash in a tree about pi (3.14) feet deep. Tsunamis are pretty powerful, so the force of the water alone is enough to cut into a tree several feet deep.
Some tsunamis cut so deep that they flat out chop the tree right off its stump. In areas like this, you have to be careful not to bang a shin (しん) on them. These stumps are right about shin height, so you want to be cautious when walking around them as to not injure yourself.