How does a tsunami get bigger as it reaches shallow coastal water?

In deep ocean water, a tsunami may be only tens of centimetres high, even though it can travel very fast. Typical speeds in the open ocean can reach about 700 to 800 kilometres per hour, similar to a jet aircraft. As the wave reaches shallow water near shore, friction with the seafloor slows it down, but the wave’s energy is conserved, so the water piles up into a much higher wave. That is why a tsunami often looks more like a rapid rise in sea level than a curling surf wave. The danger usually comes from multiple surges, not just one wave.