Chapter 2- The Diving Environment The Diving Environment 37 When a wave front, or group of waves, approaches a shoreline at an angle, successive portions of the wave front are slowed as the waves encounter resistance with the bottom. Since different segments of the waves are moving in water of varying depths, the crests and the direction of the waves refract (bend) until the wave front nearly parallels the contours of the bottom. Thus, waves become parallel to a straight shoreline, concentrated on points of land, and dispersed in coves or bays. Entries and exits on rugged points of land are usually unwise when wave action is significant. SURF As the lower portion of a wave is slowed in shallow water, the top portion moves faster than the bottom and an unstable condition results. When the depth is about twice the wave height, the crest begins to heighten and peak, the wave velocity decreases, and the wavelength decreases. Finally, at a depth of approximately 1.3 times the wave height, when the steepest surface of the wave inclines forward more than 60 degrees, the wave becomes unstable and the top portion plunges forward. At this point, water within the wave actually moves with the wave. The broken wave, known as surf, forms a “white water” area in which the waves give up their energy and where systematic water motion gives way to violent turbulence. This area is known as the surf zone (figure 2-6). The white water results from air bubbles entrapped in the water. Aerated water is less dense and, therefore, provides less buoyancy. The breaking wave continues landward until finally the momentum carries it into an uprush or swash on the face of the beach. At the uppermost limit, the wave’s energy is expended. The water transported landward during the uprush must now return seaward as a backrush, or counter current flowing back to the water. This offshore movement of water is usually not evident beyond a depth of 1 m (3 ft), and is not to be considered an undertow. An undertow, supposedly a current that flows seaward and pulls swimmers under, is a myth; “undertow” does not exist. The backrush on a steep beach can be quite strong, however, and may require you to crawl out of the water to avoid having your feet swept from beneath you. If the water deepens again after a wave has broken, such as where sand bars or reefs are adjacent to shore, the wave may reform into systematic orbital motion. The new wave will be smaller than the original one. It will proceed into water again approximately 1.3 1 10 11 12 13 2 3 4 5 2 14 19 6 15 18 7 8 9 16 17 FIGURE 2-6. SURF ZONE AND SURF. 1. SURF ZONE 2. TRANSLATORY WAVES 3. INNER LINE OF BREAKERS 4. PEAKED-UP WAVE 5. REFORMED OSCILLATORY WAVE 6. OUTERLINE OF BREAKERS 7. STILL-WATER LEVEL 8. WAVES FLATTEN AGAIN 9. WAVES BREAK-UP, BUT DO NOT BREAK ON THIS BAR AT HIGH TIDE 10. LIMIT OF UPRUSH 11. UPRUSH 12. BACKRUSH 13. BEACH FACE 14. INNER BAR 15. OUTER BAR (INNER BAR AT LOW TIDE) 16. DEEP BAR 17. MEAN LOWER LOW WATER 18. BREAKER DEPTH 19. PLUNGE POINT.
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