4.32 FACTORS DETERMINING LITTORAL WAVE CLIMATE 



The wave climate at a shoreline depends on the offshore wave climate, 

 caused by prevailing winds and storms, and on the bottom topography that 

 modifies the waves as they travel shoreward. 



4.321 Offshore Wave Climate . Wave climate is the distribution of wave 

 conditions averaged over the years. A wave condition is the particular 

 combination of wave heights, wave periods, and wave directions at a given 

 time. A specific wave condition offshore is the result of local winds 

 blowing at the time of the observation and the recent history of winds 



in the more distant parts of the same water body. For local winds, wave 

 conditions offshore depend on the wind velocity, duration, and fetch. 

 For waves reaching an observation point from distant parts of the sea, 

 a decay factor is added which preferentially filters out the higher, 

 shorter period waves with increasing distances. (Chapter 3 discusses 

 wave generation and decay.) 



4.322 Effect of Bottom Topography . As waves travel from deep water, they 

 change height and direction because of refraction, shoaling, bottom fric- 

 tion, and percolation. Laboratory experiments indicate that height and 

 apparent period are also changed by nonlinear deformation of the waves in 

 shallow water. 



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 Section 

 2.32.) As a result, refraction tends to decrease the angle between the 

 wave crest and the bottom contour. Thus, for most coasts, refraction re- 

 duces 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 Section 2.32). As a wave moves into shallow water the wave 

 height first decreases slightly, and then increases continuously to the 

 breaker position, assuming friction and refraction effects are negligible. 



Bottom friction is important in reducing wave height where waves 

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



There has been only limited field study of nonlinear deformation in 

 shallow water waves (Byrne, 1969), but because such deformation is common 

 in laboratory experiments (Calvin, 1972), it is expected that such phe- 

 nomena are also common in the field. An effect of nonlinear deformation 

 is to split the incoming wave crest into two or more crests affecting 

 both the resulting wave height and the apparent period. 



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. 



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