The Science of tsunamis

Date:
June 21, 2021
Source:
University of California - Santa Barbara
Summary:
The word 'tsunami' brings immediately to mind the havoc that can be
wrought by these uniquely powerful waves. The tsunamis we hear about
most often are caused by undersea earthquakes, and the waves they
generate can travel at speeds of up to 250 miles per hour and reach
tens of meters high when they make landfall and break. They can
cause massive flooding and rapid widespread devastation in coastal
areas, as happened in Southeast Asia in 2004 and in Japan in 2011.



FULL STORY ==========================================================================
The word "tsunami" brings immediately to mind the havoc that can be
wrought by these uniquely powerful waves. The tsunamis we hear about
most often are caused by undersea earthquakes, and the waves they
generate can travel at speeds of up to 250 miles per hour and reach
tens of meters high when they make landfall and break. They can cause
massive flooding and rapid widespread devastation in coastal areas,
as happened in Southeast Asia in 2004 and in Japan in 2011.


==========================================================================
But significant tsunamis can be caused by other events as well. The
partial collapse of the volcano Anak Krakatau in Indonesia in 2018
caused a tsunami that killed more than 400 people. Large landslides,
which send immense amounts of debris into the sea, also can cause
tsunamis. Scientists naturally would like to know how and to what extent
they might be able to predict the features of tsunamis under various circumstances.

Most models of tsunamis generated by landslides are based on the idea
that the size and power of a tsunami is determined by the thickness,
or depth, of the landslide and the speed of the "front" as it meets the
water. In a paper titled "Nonlinear regimes of tsunami waves generated by
a granular collapse," published online in the Journal of Fluid Mechanics,
UC Santa Barbara mechanical engineer Alban Sauret and his colleagues,
Wladimir Sarlin, Cyprien Morize and Philippe Gondret at the Fluids,
Automation and Thermal Systems (FAST) Laboratory at the University of Paris-Saclay and the French National Centre for Scientific Research
(CNRS), shed more light on the subject. (The article also will appear
in the journal's July 25 print edition.) This is the latest in a
series of papers the team has published on environmental flows, and on
tsunami waves generated by landslides in particular. Earlier this year,
they showed that the velocity of a collapse - - i.e., the rate at which
the landslide is traveling when it enters the water - - controls the
amplitude, or vertical size, of the wave.

In their most recent experiments, the researchers carefully measured the
volume of the granular material, which they then released, causing it
to collapse as a cliff would, into a long, narrow channel filled with
water. They found that while the density and diameter of the grains
within a landslide had little effect on the amplitude of the wave, the
total volume of the grains and the depth of the liquid played much more
crucial roles.

"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," said Sauret. (A remaining challenge
is to understand what governs the speed of the piston.) "The experiments
also showed that if we know the geometry of the initial column [the
material that flows into the water] before it collapses and the depth of
the water where it lands, we can predict the amplitude of the wave."
The team can now add this element to the evolving model they have
developed to couple the dynamics of the landslide and the generation of
the tsunami. A particular challenge is to describe the transition from
an initial dry landslide, when the particles are separated by air, to
an underwater granular flow, when the water has an important impact on
particle motion. As that occurs, the forces acting on the grains change drastically, affecting the velocity at which the front of grains that
make up the landslide enters the water.

Currently, there is a large gap in the predictions of tsunamis based
on simplified models that consider the field complexity (i.e., the
geophysics) but do not capture the physics of the landslide as it enters
the water. The researchers are now comparing the data from their model
with data collected from real-life case studies to see if they correlate
well and if any field elements might influence the results.

========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Barbara. Original written by James
Badham. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Wladimir Sarlin, Cyprien Morize, Alban Sauret, Philippe Gondret.

Nonlinear regimes of tsunami waves generated by a granular collapse.

Journal of Fluid Mechanics, 2021; 919 DOI: 10.1017/jfm.2021.400 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/06/210621160524.htm

--- up 6 weeks, 3 days, 22 hours, 45 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)