Jim Thomson is principal oceanographer at the Applied Physics Lab at the University of Washington. He studies ocean surface waves and coastal processes and wrote about his expedition to the North Pacific in the fall.
Jan. 9, 2013
For thousands of years, the tides have fascinated and vexed civilizations around the world. In classical Greece, Seleucus of Seleucia was the first to relate the pull of the moon with the rise and fall of the ocean. In the 19th century, William Whewell mobilized the global British empire to gather data for the first ?cotidal? charts, only to find that local phenomena often trumped global tidal predictions where and when the British Navy needed them most.
My own small fascination with the tides began as a boy learning to sail on the coast of Maine. Our local harbor, Biddeford Pool, had a huge expanse of mud flats at low tide that became a square mile of good sailing water at high tide. All that water came and went thru a small opening that we called, simply, the ?cut? (or, as the old timers referred to it, the ?gut?). The water would rip through the channel at speeds that alternately terrified and excited me, depending on whether I was in a boat or on the nearby pier.
When my parents moved recently, I cleaned out the closet of my childhood bedroom and found a hand-written logbook of Biddeford Pool tides that I had made for a school project.
Now, 30 years later, I am studying the tides again. As part of a large, collaborative team within the Northwest National Marine Renewable Energy Center, I?ve been measuring the tides in Admiralty Inlet near Seattle for the pilot installation of ?hydrokinetic? turbines to generate electricity from tidal currents. These turbines are analogous to wind turbines, in which rotating blades convert the kinetic power of the moving air.
Admiralty Inlet is the large-scale manifestation of that little harbor in Maine: a lot of water must pass through a small opening to produce the rise and fall of the tides inside Puget Sound. Thus, the water must flow fast. Our contribution to the pilot project in Admiralty Inlet is to measure how fast, so that turbines can be designed that are appropriate for the site.
This month, we?re taking the same tools to a similar location in Chile: the Canal de Chacao, or Channel of Chacao. This channel separates the Island of Chiloe from the mainland in south Chile.
The prospect of tidal power generation in southern Chile is still at an early stage, but it has the potential to provide renewable energy to a remote region that is otherwise dependent on fossil fuels and hydroelectric dams. Just like in Admiralty Inlet, a lot of data is required to assess the site, both as a potential energy resource and as a marine ecosystem with specific sensitivities.
Our collaborators in Chile have already measured the tidal currents in Chacao for several months ? long enough to have a good description (and prediction) of the rush of seawater through the 5-kilometer-wide wide channel. The fundamental forces for the tides (from the moon and the sun) is harmonic, thus the tides are largely predictable.
These flows, however, generate smaller features that are not as predictable: turbulence. It is the turbulence that we aim to measure. Turbulence can cause vibrations, fatigue and eventually failure of turbines. Turbulence is also part of what makes coastal regions so full of marine life. Our objective is to measure the magnitude and scales of turbulence, both to aid in the design of turbines for the site and to understand the fundamental dynamics of the channel.
The equipment we will use is standard enough; the logistics of working in Chile are not. We shipped the equipment well before Thanksgiving, yet there?s still a concern it won?t make it for the expedition, which is scheduled to begin on Jan. 18 (and last about two weeks). We have only a vague idea how long it will take the shipment to clear customs.
A team of four of us will arrive in advance, and we?ll have just a few days to assemble a mooring and deploy it. A lot of things have to go right to pull this off, not least of which is explaining to the captain of the local research vessel the intricate sequencing of the mooring deployment.
My Spanish is limited, to put it kindly, but there are a few ringers on the team with Spanish to spare. I am hoping that will leave me free to focus on the turbulence and the physics; those should be the same in any language.
Source: http://scientistatwork.blogs.nytimes.com/2013/01/14/in-chile-turbulence-ahead/?partner=rss&emc=rss
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