Thursday, December 15, 2011
Waves are introduced early in oceanography and coastal geomorphology books, since they are a fundamental way by which energy moves through water and by which water moves sediment. One of the most important aspects of waves is their behavior as they reach shore - and terms like reflection, refraction, and diffraction are important concepts, but often misapplied words. There were some nice examples in the little tiny waves at Ala Spit in Monday's low early morning light.
Reflection is the simplest notion, in part because we can all relate to other examples in day to day life - like billiard balls, canyon echoes, and shiny countertops. Here, waves are striking a vertical wall at about 45 degrees. The reflected waves leave the wall at the same angle, creating a mesh of intersecting waves. Where the wave peaks cross, their heights add, doubling the height of the individual waves. This photo also shows how the small waves steepen and grow in height as they arrive in shallow water. One of the most dramatic examples of wave reflection here in the Sound is the havoc on the south side of the Hood Canal Bridge during a strong south wind (Shine, January 2007).
This second picture illustrates both diffraction and refraction (though tough to separate here), as waves wrap around the small bar (and diffract from its ends). Diffraction seems to be the hardest concept to pin down and takes me back to college physics labs, the diffraction of microwaves through narrow slits, and the relevance to the wave-particle character of electromagnetic energy. Refraction is the bending of waves that occurs as waves slow down in shallower water and is an incredibly important concept to coastal geomorphologists and to surfers (these avocations frequently overlap). Reflection also reappears in this photo, since there are a series of waves reaching the far side of the bar that have reflected off of the log in the distance.