A New Model Better Captures the Forces Behind A Tsunami

A tsunami, as we know it, is often caused by earthquakes that occur under the ocean. However, there are other factors and events that could cause this destructive tidal wave.

The Need to Understand Tsunamis

The word "tsunami" has been widely associated with highly destructive waves, reaching several meters high and travels at speeds of up to 250 miles per hour. Among the most devastating human experiences involving a tsunami include the 2004 Indian Ocean tsunami, whose epicenter was in Sumatra, Indonesia, and the 2011 tsunami in Tohoku, Japan.

Now, a new study appearing in the latest Journal of Fluid Mechanics presents a new model that offers new insights into the behavior of a tsunami as well as the factors that influence its motion. The report, titled Nonlinear regimes of tsunami waves generated by a granular collapse, was led by mechanical engineer Alban Sauret from the University of California Santa Barbara. He is joined by colleagues Wladimir Sarlin, Cyprien Morize, and Philippe Gondret from the Fluids, Automation, and Thermal Systems (FAST) Laboratory at the University of Paris-Saclay as well as the French National Centre for Scientific Research (CNRS).

A 2018 tsunami that killed more than 400 people in Indonesia was caused by the partial underwater collapse of the Anak Krakatau volcano. Events like this have spurred scientists to understand how far can a tsunami be predicted and modeled. Most existing tsunami models are generated from landslides, coming from the concept that the size and power of a tidal wave depend on the thickness, or depth, of the landslide and the speed of the "front" that meets the moving water.

Factors that Affect Tsunamis

The latest research, which is a part of a series of ongoing studies by the same team on environmental flows, takes a look at a tsunami generated by a landslide. An earlier study, released last January also on the Journal of Fluid Mechanics, demonstrated that the collapse velocity — or the rate at which the landslide travels as it enters the water — affects the amplitude, or the height of the tidal wave.

In their recent effort, researchers measured the volume of the granular material before releasing it, simulating a natural collapse of a cliff toward a narrow channel of water. They then discovered that while the density and diameter of the grains constituting the landslide only had little effect on the wave amplitude, the total volume and the depth of the liquid had more notable effects on the tsunami.

"As the grains enter the water, they act as a piston, the horizontal force of which governs the formation of the wave, including its amplitude relative to the depth of the water," Sauret explains in a news article from UC Santa Barbara. He adds that their experiments show that the geometry of the initial column, which is the material that first flows into the water before it collapses, as well as the water that receives this column, determines the amplitude of the wave.

Current prediction attempts for tsunamis are based on simplified models that consider the field complexity instead of the physics of the land entering the water. With the new study, researchers compare their data from real-life case studies to see how these correlate and if other geophysical factors are in play.