large ocean waves as high as 100 feet. (See Figure 1-5.) Wind waves 

 cause most of the damage to the ocean coasts. Another type of wave, 

 the tsunami, is created by earthquakes or other tectonic disturbances 

 on the ocean bottom. Tsunamis have caused spectacular damage at times, 

 but fortunately, major tsunamis do not occur frequently. 



Wind waves are of the type known as oscillatory waves, and are 

 usually defined by their height, length, and period. (See Figure 1-6.) 

 Wave height is the vertical distance from the top of the crest to the 

 bottom of the trough. Wavelength is the horizontal distance between 

 successive crests„ Wave period is the time between successive crests 

 passing a given point. 



As waves propagate in water, only the form and part of the energy 

 of the waves move forward; the water particles remain. 



The height, length, and period of wind waves are determined by the 

 fetch (the distance the wind blows over the sea in generating the waves), 

 the wind speed, the length of time the wind blows, and the deoay distance 

 (the distance the wave travels after leaving the generating area). Gener- 

 ally, the longer the fetch, the stronger the wind; and the longer the 

 time the wind blows, the larger the waves. The water depth, if shallow 

 enough, will also affect the size of wave generated. The wind simul- 

 taneously generates waves of many heights, lengths, and periods as it 

 blows over the sea. 



If winds of a local storm blow toward the shore, the generated 

 waves will reach the beach in nearly the form in which they are gener- 

 ated. Under these conditions, the waves are steep; that is, the wave- 

 length is 10 to 20 times the wave height. Such waves are called seas. 

 If waves are generated by a distant storm, they may travel through 

 hundreds or even thousands of miles of calm areas before reaching the 

 shore. Under these conditions, waves decay - short, steep waves are 

 eliminated, and only relatively long, low waves reach the shore. Such 

 waves have lengths from 30 to more than 500 times the wave height, and 

 are called swell. 



1.33 CURRENTS AND SURGES 



Currents are created in oceans and adjacent bays and lagoons when 

 water in one area becomes higher than water in another area. Water in 

 the higher area flows toward the lower area, creating a current. Some 

 causes of differences in the elevation of the water surface in the oceans 

 are tides, wind, waves breaking on a beach, and streams. Changes in water 

 temperature or salinity cause changes in water density that may also pro- 

 duce currents. 



Wind creates currents because, as it blows over the water surface, 

 it creates a stress on surface water particles, and starts these parti- 

 cles moving in the direction in which the wind is blowing. Thus, a sur- 

 face current is created. When such a current reaches a barrier, such as 

 coastline, water tends to pile up against the land. In this way, wind 



1-7 



