1. InterSCI FINAL DRAFT - May 28, 2009
The Final Countdown For California’s Coastlines
By Kevin Davison
Despite several movie plots to the contrary, California will not collapse into the ocean during one
violent and catastrophic earthquake. The state is more likely to lose its delicate edge to the ocean
from more gradual, and no less destructive, eroding actions of waves, wind, and currents. Recent
analysis of the potential impacts from sea-level rise prepared by the Pacific Institute for three
government agencies in Mar. 2009 determines how real this may become to California’s
coastlines, between now, and the year 2100.
According to the Pacific Institute, floods resulting from sea levels rising 1.0 to 1.4 meters
before the year 2100 could cost $100 billion in mostly residential property damage. Two-thirds
of this threatened property is concentrated on San Francisco Bay. Flooding may potentially affect
almost half a million people who live in these areas. The report also estimates that 3,500 miles of
roads, 30 power plants, 29 wastewater treatment plants, and San Francisco and Oakland
International airports are all at risk of being flooded. Many other counties along the coast are
principally threatened by erosion. The dominant forces driving coastal erosion involve ocean
waves crashing and transferring their energy and momentum onto the faces of cliffs, rock
formations, and sand dunes. Signs of accelerated erosion became particularly apparent during the
1997 El Nino, when homes situated upon soft, flaky cliffs of sedimentary rock in places like
Pacifica, Santa Barbara, and Malibu began to slide into the ocean.
The allure of less threatening ocean waves gently breaking on distant tropical shores
(with a Corona in the foreground) inspires many to travel. Students who attend SFSU can walk
less than a mile to see the rhythmic motions of long-period swell waves arriving from distant
storms. Their transformation into turbulent, breaking white waters in the surf is a fascinating
spectacle that is well known to any beachcomber. Ocean waves rumbling on our coastlines seem
far removed from a cataclysmic, seismic event that few still speculate would cause California to
slide into the Pacific Ocean. In reality, waves are equally as destructive, only taking a little
longer to achieve the same results.
Enhanced erosion of fragile coastlines associated with rising sea levels will affect more
than 30 million Californians living in coastal counties. Despite the potential impact in the life of
millions of Californians, many aspects of coastal wave propagation and their interaction with the
coastal zone are poorly understood. California depends on dedicated Oceanographers to learn
more about how and why the fundamental eroding actions of waves, wind, and currents directly
affect California’s populated coastlines.
Tim Janssen, Assistant Professor of Oceanography in the Department of Geosciences at
San Francisco State University, concentrates with other scientists on developing accurate wave
models to help predict the statistical properties of ocean waves on the continental shelf and in the
nearshore. Originally from the Netherlands—a country that rests partially below sea level, and
continually under threat of being flooded by the sea—the effect of waves on coastal
environments is a personal matter to him. Janssen works to understand coastal wave propagation,
and the interaction with the coastal environment, to enable predictions of changes in the wave
climate and its impact on the coast.
Anyone visiting an exposed beach notices how irregular ocean waves are. No two waves
are exactly the same. Pixar studios may very realistically animate ocean waves on the movie
screen, yet this randomness makes detailed predictions on oceanic scales impossible. “Even if we
do know the details, we can’t compute it for the whole ocean because we could spend 10,000
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years computing that for the next day!” Oceanographers attempt to estimate the evolution of
statistics (not the details) of the sea state, while accounting for the effects of wind blowing over
water and dissipation of energy in breaking waves.
Deep-ocean wave models are remarkably accurate, despite our incomplete understanding
of these complicated, natural processes. This is not so with the current wave models for our
coastlines. Shallower water and irregular seafloor interactions cause predictions to be far less
accurate, where it matters the most.
The Pacific Institute’s report calls for increased awareness to the threat of sea level rise
on California’s coastline. It recommends that coastal protection be improved with “three
principal forms of vertical shoreline walls used to protect upland areas from storm surges and
high tides: seawalls, bulkheads, and revetments.” The Dutch have been fighting the sea for as
long as they remember. Perhaps California could learn from their insights and mistakes.
Many Californians are very familiar with the various types of waves, including wind
waves, tsunamis, and “freak waves.” Asking Janssen about the fundamental differences between
these wave phenomena is like speaking with a dedicated San Francisco Giants fan about
baseball. Waves intrigue this scientist, and his energy will inspire anyone to learn more. One of
his fascinations is with freak waves, which are extremely large waves that unexpectedly rise up
from nowhere and surprise cargo ships crossing the Pacific Ocean. Of course, these big waves
are not that surprising when considering the ocean as a random process. They should occur, just
not that often. The problem is that they seem to occur far more often than standard models
predict.
In the standard linear models, the ocean wave field is considered as large as the sum of
independent waves, all propagating at different speeds and directions. In more sophisticated non-
linear models, these individual wave components interact, or “communicate” with each other.
“Because of non-linearity... they are no longer independent. They basically fuel each other, and
adapt to each other. If they do that in a coherent way, sometimes you’ll get a much bigger wave
that you would not otherwise expect. Non-linearity basically couples the different waves rather
than leaving them independent.” It’s as if at one point, certain waves “talk” and develop
“coherent structures in the wave field” (building upon each other) that fuel each other to create a
freak wave. From a physical point of view, nonlinear interactions in ocean waves are rather
weak, and it is remarkable that they can unleash such destructive power!
Tsunamis are a familiar phenomenon to most of us since the tragic events that occurred in
2004, when a tsunami wave killed more than 225,000 people in eleven countries surrounding the
Indian Ocean. This was caused by the “Great Sumatra-Andaman earthquake,” which carried the
energy equivalent of 1,502 times that of the Hiroshima atomic bomb. Tremendous energy
propagated throughout the ocean as one, vast wave. Fundamentally, tsunamis are very similar to
common wind-generated ocean waves, except that they are generated by earthquakes or
landslides and are much longer.
The incredible power of a tsunami wave is obvious on the shore, but not nearly so
spectacular in the deep, open ocean. They are surprisingly hard to detect. For a typical tsunami in
deep water, the surface level changes only about one foot over approximately 100km (the
wavelength) in about 10 minutes (the period). Special equipment is necessary to detect such
gradual variations. To develop a worldwide tsunami warning system, NOAA has deployed
DART (Deep-ocean Assessment and Reporting of Tsunamis), which involves an array of very
sensitive bottom-mounted pressure sensors that can detect small pressure variations in water due
the presence of a tsunami. The DART network is “part of a cooperative effort to save lives and
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protect property through hazard assessment, warning guidance, mitigation, research capabilities,
and international coordination,” and measure tiny differences between routine tidal movements
and tsunamis.
Like wind waves, tsunamis are directly affected by seafloor topography in coastal waters.
These underwater interactions complicate predictions in critical, coastal areas. Nearshore shoals
and reefs can act like a magnifying lens, focusing and intensifying wave energy into a
concentrated area. The consequences of such refractive focusing range from a spectacular surf
spot (Mavericks, in Half Moon Bay), to an erosion hot spot (south Ocean Beach, in San
Francisco), or the catastrophic amplification of a tsunami wave.
Tim’s research over the next five years is dedicated to developing the next generation of
wave models, which will account for much of the ‘missing physics’ in today’s models. However,
predicting waves and persistently fighting them like the Dutch are only part of the solution.
Oceanographers will continue studying how the actions of waves, wind, and currents affect our
coastlines. It’s ultimately the state’s decision whether or not to save California’s coastlines from
excessive erosion and flooding, because a sustainable strategy for the management of
California’s coastlines requires dedicated resources, funding, and political support.
Signature for authorization to publish ____________________________________
Tim Janssen
Pull Quotes
“Even if we do know the details, we can’t compute it for the whole ocean because we could
spend 10,000 years computing that for the next day!”
“Because of the non-linearity of those waves, they are no longer independent. They basically fuel
each other, and adapt to each other. If they do that in a coherent way, sometimes you’ll get a
much bigger wave that you would not otherwise expect. Non-linearity basically couples the
different waves rather than leaving them independent.”
Student Biography
Kevin Davison is a graduate (by Summer ‘10) of the Technical and Professional Writing
program at San Francisco State University. When not fixated on science as a personal hobby,
Kevin is busy blogging about web technology, writing technical documentation, and producing
business and eCommerce websites at Quevin, LLC (www.Quevin.com).
Kevin Davison
3993 24th Street, APT E
San Francisco, CA 94114
Phone: 415.279.7764
Email: kdavison@sfsu.edu or Kevin@Quevin.com
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References
California Climate Change Center. The Impacts of Sea-Level Rise on the California Coast.
Analysis. Oakland: Pacific Institute, 2009.
Janssen, Tim. Assistant Professor of Oceanography, SFSU Kevin Davison. 2 April 2009.
Parker, Virgil. "Biology 317 Ecology of California." 2009. SFSU. 20 April 2009
<http://userwww.sfsu.edu/~parker>.
Schaub, Jeffrey. Rising Sea Could Cause Big Bay Area Property Loss. 11 Mar 2009. 10 April
2009 <http://cbs5.com/environment/global.warming.seas.2.956798.html>.
Wikipedia. Nonlinear System. 28 March 2009. 4 April 2002 <http://en.wikipedia.org/wiki/Non-
linear>.
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