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 decreasing in importance as the surf 

 zone is approached, (e.g., Harris, 1972.) 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 off- 

 shore, 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 less than 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. 



4.323 Winds and Storms . The relation of a shoreline locality to the 

 seasonal distribution of winds and to storm tracks is a major factor 

 in determining the wave energy available for littoral transport. For 

 example, strong winter winds in the northeastern United States usually 

 are from the northwest, but because they blow from land to sea, they do 

 not produce large waves at the shore. These northwest winds often imme- 

 diately follow a northeaster - a low pressure system with strong north- 

 east winds that generate high waves offshore. 



A storm near the coastline will influence wave climate with storm 

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

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

 and the resulting beach change is complicated. (See Section 4.35.) 

 Storms are not uniformly distributed in time or space: storms vary 

 seasonally and from year to year; storms originate more frequently in 

 some areas than in others; and storms follow characteristic tracks deter- 

 mined by prevailing global circulation and weather patterns. 



An investigation of 170 damaging storms affecting the east coast of 

 the U.S. from 1921-1962 (Mather, et al., 1964), classified the storms into 

 eight types based on origin, structure, and path of movement. Of these 

 eight types, although 33 percent were hurricanes, two types, comprising 

 only 19 percent of the total, characterized by weather fronts east and 

 south of the U.S. coasts produced more damage per storm because of long 

 fetches. (Damage is defined by Mather, et al., as "at best some water 

 damage," and includes "wave damage, coastal flooding, and tidal inunda- 

 tion," but specificially excludes wind damage.) 



The probability that a given section of coast will experience storm 

 waves depends on its ocean exposure, its location in relation to storm 

 tracks, and the shelf bathymetry. 



4.33 NEARSHORE WAVE CLIMATE 



4.331 Mean Value Data on U.S. Littoral Wave Climates . Wave height and 

 period data for some localities of the U.S. are becoming increasingly 

 available (e.g., Thompson and Harris, 1972), but most localities still 

 lack such data. However, wave direction is difficult to measure, and 

 consequently direction data are rarely available. 



4-29 



