The height, length, and period of wind waves at a site in the open ocean 

 are determined by the fetch (the distance the wind blows over the sea in gen- 

 erating the waves), the windspeed, the duration (the length of time the wind 

 blows), the decay distance (the distance the wave travels after leaving the 

 generating area), and the water depth. Generally, increases in fetch, wind- 

 speed, or duration result in larger wind waves. The water depth, if shallow 

 enough, will also affect the size of waves generated. The wind simultaneously 

 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 waves will reach the 

 beach in nearly the same waveform in which they are generated. Under these 

 conditions, the waves are steep; i.e., the wavelength 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 

 wind areas before reaching the shore. Under these conditions, waves decay — 

 short, steep waves are transformed into relatively long, low waves which reach 

 the shore. Such waves, which have lengths from 30 to more than 500 times the 

 wave height, are called swell. 



Tides are created by the gravitational force of the Moon and, to a lesser 

 extent, the Sun. These forces of attraction, and the fact that the Sun, Moon, 

 and Earth are always in motion relative to each other, cause waters of ocean 

 basins to be set in motion. These tidal motions of water masses are a form of 

 very long period wave motion, resulting in a rise and fall of the water sur- 

 face at a point. There are normally two tides per day, but some localities 

 have only one per day. Tides constantly change the level at which waves 

 attack the beach. 



The range of tides varies tremendously with geographic location. Some 

 areas, such as Eastport, Maine, experience an average tidal range of about 5.5 

 meters (18 feet) while other locations, such as Mobile, Alabama, experience 

 variations of about 0.6 meter. Even more dramatic is the difference between 

 mean tidal ranges at Anchorage (7.9 meters or 26 feet) and Kodiak Island (2.1 

 meters or 7 feet), Alaska. These sites are only 415 kilometers (224 nautical 

 miles) apart. 



Currents and surges sometimes play an important role in the nearshore 

 physical system. When water in one area becomes higher than water in another 

 area, water from the higher elevation flows toward the lower level, creating a 

 current. Significant currents generated by tides occur at inlets to lagoons 

 and bays or at entrances to harbors. Tidal currents in these constricted 

 places flow in when the tide is rising (floodtide) and flow out as the tide 

 falls (ebbtide). Exceptions can occur at times of high river discharge or 

 strong winds. Currents can be caused by differences in water elevation due to 

 (a) wind, (b) waves breaking on a beach, and (c) river discharge. The river 

 discharge to the sea introduces currents into the nearshore zone. 



Wind creates currents as it blows over the water surface, producing a 

 stress on surface water particles and starting the movement of the particles 

 in the direction in which the wind is blowing. Thus, a surface current is 

 created. When the surface current reaches a barrier, such as the coast, water 



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