This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Monday, January 24, 2011

Half Moon Bay surfer in critical condition after large wave strikes

January 23, 2011 big wave at Half Moon Bay
Photo by Nikki Brooks


A California surfer was caught in this big wave at Mavericks surfing area near Half Moon Bay, California.  He is in critical condition at Stanford Medical Center. Half Moon Bay is famous for its big, and dangerous, surf. On February 13, 2010, spectators crowded onto a narrow beach were hit by a wave estimated to be 50' in height. This wave was apparently caused by a combination of high tides and high surf. When the waves exceed 20' height, there is a contest held on short notice, with $150,000 prize money for competitors.  Just the day before the accident this weekend, the waves had been declared close, but not at, the 20' height. There is a web site dedicated to updating surfers on conditions here.
map of the sea floor near Mavericks (NOAA)
from this WWW site where you can find more discussion:
What causes such big waves in Half Moon Bay? A combination of three factors: the waves generated by winter storms in the Pacific Ocean, the geology of the bay itself, and the complex topography of the underwater ocean bottom.  The geology of the area was described in 1965, by Warren Yasso who published an article in the Journal of Geology titled "Plan Geometry of Headland-Bay Beaches" (J. Geol. , v. 73 (5), pp. 702-714). A "headland bay" lies in the lee of a headland (obstruction) that causes waves to refract (bend) around it. The headland at Half Moon Bay is Pillar Point.  The beach formed in these settings has a characteristic concave shape that is caused by refraction, diffraction, and reflection of waves into the shadow zone of the headland.  To a good approximation, the concave shape of these beaches is given by a logarithmic spiral.  Half Moon Bay beach joins the beaches at Sandy Hook, NJ, and Drakes Beach and Limantour Spit along the Drakes Bay area, CA, in having this shape. The sea floor under this region is contorted because of the active tectonics and faulting that have occurred in this part of California. On the map above, dark blue is deep; red is shallow.  The center of the box is "Sail Rock", and the tan area to the northeast of it is Pillar point. The waves break in the area in the black box. The dominant wave direction through most of the year is from the northwest, and these waves do not generate large surf. However, sometimes during the winter, the strong storms in the north Pacific generate waves that come in from the west.  When they hit the shallow water at Sail Rock, they break over the bedrock reef just to the east of Sail Rock.  See here for more discussion, and an animation of the underwater topography.

To see a very different, and much gentler, surf posting on this blog, click here!

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