How can the water flowing back to sea after a tsunami cause destruction?

The destructive power of a tsunami is not limited to the initial surge of water onto land. While the incoming "run-up" causes significant damage by uprooting trees, destroying buildings, and moving heavy objects like boats inland, the subsequent outflow of water back to the sea is often equally or even more destructive.1

This receding water acts as a powerful, high-velocity current that can sweep away debris, infrastructure, and people that were not carried off by the initial wave. Because tsunamis consist of a series of major oscillations—often occurring over several days—the repeated cycle of rushing water and intense outflow prevents coastal areas from stabilizing, leading to widespread devastation of beaches, peninsulas, and low-lying coastal formations.1

It is important to note that the danger is persistent; the movement of water back and forth continues until the ocean surface reaches equilibrium, posing a severe threat to anyone in the affected area long after the first wave has struck.1