What determines how devastating a tidal wave can be?
Are you talking about tsunamis or tidal waves? If tidal waves, which kind? For tsunamis it would be a combination of its wavelength, distance from crest to crest, magnitude, distance from crest to trough, speed, and configuration of the coast it hits. There is no scale such as the Richter for earthquakes. A ship at sea can pass over a tsunami without even noticing. What causes the damage is when the tsunami approaches the coast and the depth of the water decreases, it produces wave shoaling which compresses the wavelength (pushes the crests closer together) which in turn increases the magnitude (the height of the wave). The higher the wave the more potential damage.
