Refraction is the bending of wave crests due to the slowing down of that 

 part of the wave crest which is in shallower water (see Ch. 2). As a result, 

 refraction tends to decrease the angle between the wave crest and the bottom 

 contour. Thus, for most coasts, refraction reduces the breaker angle and 

 spreads the wave energy over a longer crest length. 



Shoaling is the change in wave height due to conservation of energy flux 

 (see Ch. 2). As a wave moves into shallow water, the wave height first 

 decreases slightly and then increases continuously to the breaker position, 

 assuming friction, refraction, and other effects are negligible. 



Bottom friction is important in reducing wave height where waves must 

 travel long distances in shallow water (Bretschneider , 1954). 



Nonlinear deformation causes wave crests to become narrow and high and 

 wave troughs to become broad and elevated. Severe nonlinear deformation can 

 also affect the apparent wave period by causing the incoming wave crest to 

 split into two or more crests. This effect is common in laboratory exper- 

 iments (Galvin, 1972a). It is also expected to be common in the field, 

 although only limited field study has been done (Byrne, 1969). 



Offshore islands, shoals, and other variations in hydrography also shelter 

 parts of the shore. In general, bottom hydrography has the greatest influence 

 on waves traveling long distances in shallow water. Because of the effects of 

 bottom hydrography, nearshore waves generally have different characteristics 

 than they had in deep water offshore. 



Such differences are often visible on aerial photos. Photos may show two 

 or more distinct wave trains in the nearshore area, with the wave train most 

 apparent offshore and decreasing in importance as the surf zone is approached 

 (e.g., Harris, 1972a, b). The difference appears to be caused by the effects 

 of refraction and shoaling on waves of different periods. Longer period 

 waves, which may be only slightly visible offshore, may become the most 

 prominent waves at breaking, because shoaling increases their height relative 

 to the shorter period waves. Thus, the wave period measured from the dominant 

 wave offshore may be different from the wave period measured from the dominant 

 wave entering the surf zone when two wave trains of unequal period reach the 

 shore at the same time. 



c. Winds and Storms. The orientation of a shoreline to the seasonal 

 distribution of winds and to storm tracks is a major factor in determining the 

 wave energy available for littoral transport and the resulting effect of 

 storms. For example, strong winter winds in the northeastern United States 

 usually are from the northwest and, because they blow from land to sea, they 

 do not produce large waves at the shore. 



A storm near the coastline will influence wave climate owing to storm 

 surge and high seas; a storm offshore will influence coastal wave climate only 

 by swell. The relation between the meteorological severity of a storm and the 

 resulting beach change is complicated (see Sec. 111,5). Although the 

 character, tracks, and effects of storms vary along the different coasts of 

 the United States (Pacific, Atlantic, gulf, and Great Lakes), they can be 

 classified for a particular region. 



4-30 



