Page 538 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

502 part III The earth–atmosphere interface
undulations known as waves of transition (Figure 16.9a). Individual water particles move forward only slightly, in a vertical pattern of circles. The diameter of the paths traced by the orbiting water particles decreases with depth.
As a deep-ocean wave approaches the shoreline and enters shallower water (10–20 m), the orbiting water par- ticles are vertically restricted, causing elliptical, flat- tened orbits of water particles to form near the bottom. This change from circular to elliptical orbits slows the entire wave, although more waves keep arriving. The result is closer-spaced waves, growing in height and steepness, with sharper wave crests. As the crest of each wave rises, a point is reached when its height exceeds its vertical stability, and the wave falls into a charac- teristic breaker, crashing onto the beach (Figure 16.9b).
In a breaker, the orbital motion of transition gives way to elliptical waves of translation, in which both energy and water move toward shore. The slope of the shore determines wave type. Plunging breakers indicate a steep bottom profile, whereas spilling breakers indicate a
gentle, shallow bottom profile. In some areas, unexpected high waves can arise suddenly, creating unexpected dan- gers along shorelines.
Another potential danger is the brief, short tor- rent called a rip current, created when the backwash of water produced by breakers flows to the ocean from the beach in a concentrated column, usually at a right angle to the line of breakers (Figure 16.9c). A person caught in one of these can be swept offshore, but usually only a short distance.
As various wave trains move along in the open sea, they interact by interference. When these interfering waves are in alignment, or in phase, so that the crests and troughs from one wave train are in phase with those of another, the height of the waves becomes amplified, some- times dramatically. The resulting waves, called “killer,” “sleeper,” “rogue,” or “sneaker” waves, can sweep in un- announced and overtake unsuspecting victims. Signs along portions of coastlines in British Columbia and the Pacific coastal states warn beachgoers to watch for such waves. Coastal areas near popular tourist attractions such
▼Figure 16.9 Wave formation and breakers. [(b, c) Bobbé Christopherson.]
Wavelength
Crest Crest
Height
Trough
Shallow depth
shortens wavelength
(depth less than one-half wavelength)
Waves of transition
Surf
Waves of translation Breaker
Beach
Animation
Wave Motion/ Wave Refraction
Path of water particles
Deep water
Little motion below half wavelength
(a) The orbiting tracks of water particles change from circular motions and swells in deep water (waves of transition) to more elliptical orbits near the bottom in shallow water (waves of translation).
(b) Breakers along the coast of Baja California, Mexico.
(c) A dangerous rip current interrupts approaching breakers. Note the churned-up water where the rip current enters the surf.
Movement of wave energy