A tsunami that hit New Guinea in 1998 had waves of possibly up to 30 feet, and at least 3,000 people were killed. Any time the waves get over ten feet, it can get bad. Knee-deep water can be enough to sweep you away, especially when it is filled with debris and is moving back to the ocean.
They were saying earlier it was just 10 feet, which while bad....is pretty tame compared to 2004.
But, 23 feet is not good at all....easily sweep away houses, everything.