NAUI Master Scuba Diver 36 The Diving Environment When the wind velocity decreases or the waves leave the fetch area; their height decreases, their crests round, their wavelength shortens, and they are called swells. A swell is a wave that is fairly regular in height, period, and direction. In this form, waves can travel thousands of miles while retaining much of their energy. Extremely large, destructive waves can be generated by earthquakes beneath the seas or near the shore. These rare occurrences are popularly called tidal waves or “Tsunamis.” In reality, they have nothing to do with the tides, and are now properly called seismic waves. These great waves travel at tremendous speed and have extremely long wavelengths, carrying a huge volume of water. For this reason, diving, swimming, and even beachcombing must be avoided following major geological disturbances even if the seismic event occurs many miles away. Seismic wave predictions are announced on the marine radio and sometimes on television. Waves in Shallow Water As a train of waves moves into shallower water, the bottom begins to interfere with the orbital motion of the water moving within the waves. The orbits flatten into ellipses, and the net movement of the water becomes the back-and-forth surge motion. Surge is greatest when the water is shallow, the waves are large, and the wavelength is long. When you are diving, surge will sweep you back and forth as the waves pass overhead. Diving in strong surge can be hazardous and should be avoided. It is also better to drift back and forth with moderate surge than it is to waste energy fighting it. In light surge it is possible to secure yourself by holding onto a structure or sticking your knife into the sand while the surge passes. Generally, the movement of water toward shore is greater than the movement of the water away from shore, so it is possible to use this tendency to assist you in shallow water when returning to shore. Simply hang on or dig in while the motion is offshore and move with the motion when it is toward shore. As wave forms approach shore and move across shallow bottoms, they are reflected, diffracted (bent), and refracted. When a wave encounters a vertical wall, such as a seawall or a steep cliff rising from deep water, the Island Undisturbed Water Water Fronts FIGURE 2-4. WAVE DIFFRACTION wave is reflected back upon itself with little loss of energy (figure 2-5). The surface chop resulting from such reflected waves can produce uncomfortable situations for divers. Divers should avoid being at the surface in areas of reflected waves. When waves encounter an obstruction, their motion is diffracted around it (figure 2-4). As the waves pass the obstruction, some of their energy is propagated sideways due to friction with the obstruction. The waves bend around and pass into areas that might otherwise appear to be sheltered. Orthogonals Wave direction offshore Divergence of orthogonals produces low waves in this area Convergence of orthogonals produces high waves in this area Wave Fronts FIGURE 2-5. WAVE REFRACTION
20047TC
To see the actual publication please follow the link above