Force of Tsunami Impact and Backwash

When tsunamis crash into coastal areas they are typically moving at about 22 miles per hour (35 km/ hr). The speed as the wave moves inland changes dramatically, decreasing to a few miles per hour (several km/hr) over short distances, depending on the slope of the beach or shore environment and how much resistance the wave encounters from obstacles on shore. The force associated with a debris-laden wall of water 50-70 miles (80-120 km) wide moving inland at that speed is tremendous. As tsunamis impact the shoreline and move inland they rapidly pick up debris and move this with the wave front, and these objects smash into whatever is in the path of the water, destroying almost anything in the way. The force of the tsunami can be appreciated by considering the impact of a series of rocks thrown from a moving car, or a train hitting a building at 22 miles per hour (35 km/hr). After the first impact the force of the wave does not stop but keeps on pounding into the coast until the crest passes; then the water continues to move inland and remain high for 30, 40, 50 minutes or more before retreating back to sea. The force of the tsunami backwash can be just as strong, and in some cases stronger than the initial impact. some waves take five minutes or more to move inland, and less than two minutes to wash back out to sea, so the outgoing velocity may be greater than the initial surge. The outgoing waves often take the loose debris from the destruction of the incoming wave with them, placing projectiles in the water for the next crest to launch when it moves inland.

Many tsunamis have been observed to form a thin wedge of turbulent water that shoots out in front of the wave crest with tremendous speed and force. These turbulent wedges of foamy debris-laden water do a lot of damage to buildings and vegetation just before the wave crest hits, and may be associated with much higher velocities of projectiles than found in the main wave. These wedges may form by the weight of the wave compressing air trapped in front of the wave and shooting this air-water-debris mixture out as the wave moves inland.

In many cases the area of land that is flooded by a tsunami is roughly equal to the area found beneath the wave crest when it is close to shore. Larger tsunamis flood larger areas. The amount of flooding is greatest for flat open areas such as mudflats, pastures, etc., where the wave can move uninterrupted inland. The amount of inland penetration decreases for areas that have forests, buildings, or other obstacles that slow the wave down. A moderate tsunami, about 30 feet (10 m) high, might penetrate a little less than a mile (1.6 km) inland in flat but developed coastal areas, half a mile (less than a kilometer) in a developed downtown city environment, and perhaps four miles (6 km) on an undeveloped open coast. Dense coastal forests are able to significantly decrease the amount of inland penetration, taking much of the energy of the wave away as it must snap and move the trees to move further inland. Large tsunamis, greater than 160 feet (50 m) tall, can move inland 5-7 miles (9-12 km), while great tsunamis can theoretically reach tens of miles (km) inland.

Continue reading here: Reducing The Threat From Tsunamis

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