Waves


   The motion of water waves results from the transfer of energy from the atmosphere to water.  Most water waves are generated by wind moving over the water’s surface.  The size of the resulting waves is dependent upon the wind velocity, wind duration, and the fetch (i.e., the area and distance over which the wind travels).  Deep water waves have a sinuous pattern as illustrated in Figure 1.  The highest point on the wave is called the wave crest, and the low point on the water surface between crests is called the wave trough  The distance from crest to trough is the wave height, and the distance from one crest to another is called the wavelength.   Although not obvious to an observer, a wave extends below the wave trough to a depth that is one-half the wavelength.  This depth is called wave base, below which no water motion attributable to surface waves occurs.
 


    Although deep water waves may visually appear to be moving water over horizontal distances, most of the water is actually moving in a circular pattern (Figure 2).  So, as the wave passes through the water, the water molecules do not move horizontally, but instead sweep out a circular pattern with a diameter equal to the wave height.  The water particles return to approximately the same position from which they started. Thus, most water molecules remain in the same general area, and the same can be said for an object floating on the surface in deep water.  The object traces a circle as waves simply move the object up and down and back and forth.  Therefore, deep water waves cannot move an object like a boat or a surfboard along its surface.  As shown in Figure 2, the diameters of the circles making up the wave decrease with depth until the wave base is reached at one half the wavelength.  Below that wave-induced turbulence is absent.  So if you are a SCUBA diver, when you dive below wave base you no longer feel the surge of the waves.
 



    Waves behave differently once they enter shallow water (water depth < 0.5 WL), the depth at which wave base is intercepted by the seafloor (or lake floor).  Friction slows the water motion near the wave base (Figure 3), but water at the wave crest continues to move at the same speed and the water begins to pile up at the surface.  This continues until the wave height is one-seventh the wave length (or when the water depth is about one to two times the wave height).  At this point the wave can no longer support itself, and the wave breaks.  The piling up and breaking of water within the surf zone generates a horizontal movement of water.  This is what makes it possible for water to move objects such as surfboards forward in the surf zone.
 


    Importantly, the deeper water is off a shoreline, the further into shore waves can move before they begin to "feel bottom" and break.  So erosion along a shoreline well tend to be greater if relatively deep water is found immediately offshore.  In contrast, a wide beach results in waves breaking far offshore and the expenditure of wave energy moving beach sand around rather than eroding a dune, bluff, or cliff.


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LIVING WITH THE GREAT LAKES
BROUGHT TO YOU BY:
GRAND VALLEY STATE UNIVERSITY
DEPARTMENT OF GEOLOGY
ALLENDALE, MICHIGAN 49401