along the crest of a wave moving at an angle to underwater contours because 

 that part of the wave in deeper water is moving faster than the part in 

 shallower water. This variation causes the wave crest to bend toward align- 

 ment with the contours, (See Figure 2-17.) This bending effect, called 

 refraction, depends on the relation of water depth to wavelength. It is 

 analogous to refraction for other types of waves such as, light and sound. 



In practice, refraction is important for several reasons such as: 



(1) Refraction, coupled with shoaling, determines the wave height 

 in any particular water depth for a given set of incident deepwater wave 

 conditions, that is wave height, period, and direction of propagation in 

 deep water. Refraction therefore has significant influence on the wave 

 height and distribution of wave energy along a coast. 



(2) The change of wave direction of different parts of the wave 

 results in convergence or divergence of wave energy, and materially affects 

 the forces exerted by waves on structures . 



(3) Refraction contributes to the alteration of bottom topography 

 by its effects on the erosion and deposition of beach sediments. Munk and 

 Traylor (1947) confirmed earlier work by many indicating the possible inter- 

 relationships between refraction, wave energy distribution along a shore, 

 and the erosion and deposition of beach materials. 



(4) A general description of the nearshore bathymetry of an area 

 can sometimes be obtained by analyzing aerial photography of wave refraction 

 patterns. While the techniques for performing such analyses are not well 

 developed, an experienced observer can obtain a general picture of simple 

 bottom topography. 



In addition to refraction caused by variations in bathymetry, waves 

 may be refracted by currents, or any other phenomenon which causes one part 

 of a wave to travel slower or faster than another part. At a coastal inlet, 

 refraction may be caused by a gradient in the current. Refraction by a 

 current occurs when waves intersect the current at an angle. The extent 

 to which the current will refract incident waves depends on the initial 

 angle between the wave crests and the direction of current flow, the 

 characteristics of the incident waves, and the strength of the current. 

 In at least two situations, wave refraction by currents may be of practical 

 importance. At tidal entrances, ebb currents run counter to incident waves 

 and consequently increase wave height and steepness. Also, major ocean 

 currents such as the Gulf Stream may have some effect on the height, length 

 and direction of approach of waves reaching the coasts. Quantitative 

 evaluation of the effects of refraction by currents is difficult. Addi- 

 tional research is needed in this area. No detailed discussion of this 

 problem will be presented here, but an introduction is presented by 

 Johnson (1947). 



The decrease in wave celerity with decreasing water depth can be con- 

 sidered an analog to the decrease in the speed of light with an increase in 



2-63 



