However, as waves approach the much shallower water near the shore, they start to “feel” the bottom, and they are affected by that interaction (Figure 17.4). Since ocean waves rarely have wavelengths greater than 200 m, and the open ocean is several thousand metres deep, the wave base does not normally interact with the bottom of the ocean. The one-half wavelength depth of disturbance of the water beneath a wave is known as the wave base. Figure 17.3 The orbital motion of a parcel of water (black dot) as a wave moves across the surface. If you look carefully at that animation, and focus on the small white dots in the water, you should be able to see how the amount that they move decreases with depth. Wave motion is illustrated quite clearly on the Wikipedia “Wind wave” site at. This motion is also transmitted to the water underneath, and the water is disturbed by a wave to a depth of approximately one-half of the wavelength. As this happens, a point on the water surface describes a circle with a diameter that is equal to the wave amplitude (Figure 17.3). Very large waves move about five times faster (over 50 km/h), but because their wavelengths are so much longer, they arrive less frequently - about once every 14 seconds.Īs a wave moves across the surface of the water, the water itself mostly just moves up and down and only moves a small amount in the direction of wave motion. Relatively small waves move at up to about 10 km/h and arrive on a shore about once every 3 seconds. How would these ratios change with increasing distance from the wind that produced the waves? Calculate these ratios for the waves shown. The steepness of a wave can be determined from these numbers and is related to the ratio: amplitude/wavelength.ġ. Wind SpeedĮxercise 17.1 Wave Height Versus Length This table shows the typical amplitudes and wavelengths of waves generated under different wind conditions. The duration times listed are the minimum required for the waves to develop fully. Table 17.1 The parameters of wind waves in situations where the wind blows in roughly the same direction for long enough for the waves to develop fully. Waves on our coast that are generated by a storm near Japan will have similar wavelengths but lower amplitudes than those generated by a comparable storm offshore from Vancouver Island. It is important to recognize, however, that amplitudes decrease with distance from the area where the waves were generated. In other words, not only are large waves bigger than small ones, they are also generally more than twice as steep, and therefore many times more impressive. Small waves (amplitudes under a metre) tend to have relatively shallow slopes (amplitude is 3% to 4% of wavelength), while larger waves (amplitudes over 10 m) have much steeper slopes (amplitude is 6% to 7% of wavelength). In the open ocean, with strong winds (92 km/h) that blow for at least 69 hours, the waves will average nearly 15 m high and their wavelengths will be over 200 m. On a large body of water (the ocean or a very large lake) with a fetch of 139 km and winds of 37 km/h, the waves will develop fully in 10 hours the average amplitude will be around 1.5 m and average wavelength around 34 m. In a situation where the fetch is short (say 19 km on a lake) and the wind is only moderate (19 km/h), the waves will develop fully within 2 hours, but they will remain quite small (average amplitude about 27 cm, wavelength 8.5 m). The typical sizes and speeds of waves in situations where they have had long enough to develop fully are summarized in Table 17.1. Figure 17.2 The parameters of water waves The important parameters of a wave are its wavelength (the horizontal distance between two crests or two troughs), its amplitude (the vertical distance between a trough and a crest), and its velocity (the speed at which wave crests move across the water) (Figure 17.2). The stronger the wind, the longer it blows, and the larger the area of water over which it blows (the fetch), the larger the waves are likely to be. Waves form on the ocean and on lakes because energy from the wind is transferred to the water.
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