offshore islands might not shield coastlines from tsunami ... · “this causes the wave to swing...

Offshore Islands Might Not Shield Coastlines fromTsunami Waves

Rather than offering protection, islands sometimes cause increased wave

run-up on shorelines, experiments in a wave laboratory suggest.

Although prevailing wisdom holds that offshore islands protect coastlines from tsunami waves,

new experimental research suggests otherwise. Credit: fotoVoyager/E+/Getty Images

By Katherine Kornei! 12 December 2017

An offshore island might appear to block incoming tsunami waves, but safety is far from

guaranteed in an island’s shadow, new research indicates. Offshore landmasses in fact cause

higher levels of wave run-up on mainland coastlines for some types of tsunami waves, according

Offshore Islands Might Not Shield Coastlines from Tsunami Waves - Eos https://eos.org/articles/offshore-islands-might-not-shield-coastlines-fr...

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to scale model tests.

These new results have emerged from the first experimental investigation of tsunami inundation

on beaches behind islands, conducted by civil engineer James Kaihatu (https://ceprofs.civil.tamu.edu

/jkaihatu/) of Texas A&M University in College Station and his colleagues.

In a sprawling, indoor laboratory in Oregon known as the Directional Wave Basin

(http://wave.oregonstate.edu/directional-wave-basin), the researchers tested artificial tsunami waves

passing over and around fabricated islands and washing up on a gently sloping steel ramp that

serves as the facility’s beach.

“The Directional Wave Basin is the best place to do this work,” said Kaihatu. “There’s a lot of

power in the wave maker, so you can generate really nice tsunami-type waves.”

Turning a CornerWhen they did make waves, the team found that within the shadow of the island, the beach run-

up, or vertical gain in water height above the mean level, surpassed the run-up in areas outside

the shadow zone by roughly 15%–25%.

“The two parts of the wave meet on the lee [side] of the island…roughly doubling the wave height

there.”

This increase results from refraction, Kaihatu explained. As a wave approaches an island, the part

closest to the island slows down because the speed of a wave is largely determined by water depth,

with shallower depths resulting in slower waves (http://itic.ioc-unesco.org/index.php?option=com_content&

view=article&id=1164&Itemid=2031). However, the other parts of the wave farther from the island

continue to move swiftly. “This causes the wave to swing around the island,” said Kaihatu. “The

two parts of the wave meet on the lee [side] of the island…roughly doubling the wave height

there.” But wave energy is partially dissipated before the water reaches the shoreline, which is

why the increase in run-up is less than a factor of 2, Kaihatu added.

The experimenters’ observations confirm computer simulations and can help inform disaster

preparedness in tsunami-prone regions. “From an education and hazards mitigation perspective,

it’s important to realize that it can be more dangerous to be behind an island,” said Hermann

Fritz, a tsunami scientist at the Georgia Institute of Technology in Atlanta who is not involved in

the research.

The findings are important, according to Kaihatu, because many areas in which tsunamis are a


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hazard—from the western coast (https://eos.org/research-spotlights/revising-an-innovative-way-to-study-

cascadia-megaquakes) of the United States to Indonesia (https://eos.org/articles/indonesian-cave-reveals-

nearly-5000-years-of-tsunamis)—are home to offshore islands. What’s more, coastal communities often

have the mind-set that offshore islands afford protection against incoming tsunamis. “Locals pass

down [this folklore] from generation to generation,” said team member John Goertz

(https://ceprofs.civil.tamu.edu/jkaihatu/research/students.html), also a Texas A&M civil engineer.

Goertz presented (https://agu.confex.com/agu/fm17/meetingapp.cgi/Paper/275573) the new study’s findings

yesterday at the American Geophysical Union’s 2017 Fall Meeting in New Orleans, La.

Fabricated Islands and WavesTo conduct the tests, which began in July and August 2016, Kaihatu and his team crafted four

conical sheet metal “islands” about 3–4 meters in diameter at their bases. The researchers

lowered the tops of two of the islands to roughly water level, to resemble shallow reefs, and

brought all of the structures to the Directional Wave Basin.

“You’re generating a wave that breaks pretty far offshore and hits the island as a big mass of

foam.”

The wave maker at the enormous facility sweeps waves into a roughly 50 × 30 meter basin,

located at the O.H. Hinsdale Wave Research Laboratory (http://wave.oregonstate.edu) at Oregon State

University in Corvallis. The team used it to simulate two types of waves: solitary waves and “full-

stroke” waves.

Being an isolated hump of water, a solitary wave does not change shape unless the water depth

changes, whereas the shape of a full-stroke wave—whose name refers to the motion of the wave

maker—can vary even when the seafloor is flat, said Kaihatu. In the latter case, “you’re generating

a wave that breaks pretty far offshore and hits the island as a big mass of foam.” Both types of

waves occur in nature, but most tsunami waves closely resemble full-stroke waves when they

make landfall.

Distance EffectsInitially, the researchers placed one island at a time in the basin at varying distances from the

beach and adjusted the water depth to either 30 or 50 centimeters. They then triggered a solitary

wave or a full-stroke wave and measured water level, water velocity, and beach run-up using two

video cameras and sensors mounted near and in the water.


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They found that inundation generally increased as an island was moved closer to the beach.

However, this trend reversed in cases in which the tsunami wave broke before reaching the island,

with more inundation recorded for islands farther offshore. Whether an island was fully conical or

more reeflike had little impact on how much inundation occurred, the researchers noted.

Multiple Islands“We can figure out which areas are going to have significant inundation [when a tsunami

strikes].”

This past summer, the team returned to Corvallis to test how inundation levels changed when

tsunami waves propagated over two islands arranged either in line or side by side. In the case of

islands side by side, preliminary analysis suggests that the coastline behind the gap between the

islands experiences less inundation than the areas shadowed by the islands, Kaihatu said.

The new findings about how island characteristics, location, and wave conditions affect

inundation (https://eos.org/project-updates/a-test-bed-for-coastal-and-ocean-modeling) might help city

planners and officials involved in emergency management, ventured Kaihatu. “We can figure out

which areas are going to have significant inundation [when a tsunami strikes],” he said.

—Katherine Kornei (email: [email protected] (mailto:[email protected]); @katherinekornei

(http://twitter.com/katherinekornei)), Freelance Science Journalist

Citation: Kornei, K. (2017), Offshore islands might not shield coastlines from tsunami waves, Eos, 98, https://doi.org

/10.1029/2017EO088781. Published on 12 December 2017.

© 2017. The authors. CC BY-NC-ND 3.0


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