Feature article inter sci waves


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Feature article inter sci waves

  1. 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 1
  2. 2. InterSCI FINAL DRAFT - May 28, 2009 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 2
  3. 3. InterSCI FINAL DRAFT - May 28, 2009 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 3
  4. 4. InterSCI FINAL DRAFT - May 28, 2009 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>. 4