Experimental modelling of tsunamis generated from underwater landslide

Technical Abstract

Tsunami are a fascinating but potentially devastating natural phenomenon that have occurred regularly throughout history along New Zealand’s shorelines, and around the world. With increasing population and the construction of infrastructure in coastal zones, the effect of these large waves has become a major concern. Many natural phenomena are capable of creating tsunamis. Of particular concern is the underwater landslide-induced tsunami, due to the potentially short warning before waves reach the shore.

The underwater landslide research community have designed several standardised experiments to rest the accuracy of computer tsunami models. These standard tests usually model underwater landslides with solid blocks sliding down submerged slopes. The landslide mass and initial submergence, the starting distance of the landslide below the water surface, are varied and the wave heights are measured at a few points with electrical sensors.

Using a similar approach, several laboratory tests are being performed at the University of Canterbury. A new method, shown to be as accurate as traditional electrical sensors, has been developed to measure the water levels at all positions within the wave tank. This technique involves illuminating the water with fluorescent dye and recording its motion with a digital video camera. The ability to measure the entire water surface, instead of as a few specific points, has allowed the tsunami generation process to be looked at in more detail.

In these latest experiments, the mass and initial submergence of the landslide are varied and information about the waves, such as height, shape, speed and run-up at the shore, is measured. The speed and acceleration of the model landslide and the motions of the water below the waves are also recorded. The experiments highlight the complex interaction between the generated waves and the landslide.

By comparing the position and speed of the landslide relative to the waves, it is found that the 1st crest forms over the front half of the landslide and a trough forms over the rear. The point at which these two waves meet is located above the centre of the landslide as it slides down the slope. The changing shape of the water surface also indicates that the wavelengths of individual waves increase as they propagate into deeper water. Waves shorter than the length of the landslide are not generated. Waves further behind in the wave train propagate more slowly than those in front, and new waves are continually generated at the trailing end of the wave train.

The ability to measure water levels across space and time allows wave potential energy time histories to be calculated. It is observed that the maximum wave potential energy occurs later than the maximum landslide kinetic energy. Between 32.9% and 50.4% of the landslide potential energy is converted into landslide kinetic energy. Between 2.8% and 13.8% of landslide kinetic energy is converted into the potential energy of the waves.

The wave trough that initially forms above the rear end of the landslide propagates in both upstream and downstream directions. The upstream-travelling trough causes the large initial draw-down at the shore. A wave crest generated by the landslide as it suddenly slows at the bottom of the slope causes the maximum wave run-up height observed at the shore.

The visualisation of the velocities of the water beneath the surface allows the generation mechanism of the wave field to be examined. The water is forced up over the block as it slides, forming the 1st wave crest. The water over the block travels faster than the surrounding water and causes the water above the landslide to form a wave trough. Once generated the crest propagates freely, whereas the trough remains attached to the slider until it reaches the base of the slope. Several large rotating eddies are left behind in the wake of the sliding block. The fluid particles below the waves move in elliptical orbits that tend to flatten towards the bottom of the water column, with purely horizontal motions at the flume floor.