The sheltering effect of a breakwater of finite length and vertical 

 impermeable walls depends upon diffraction of the incident waves around 

 the ends of the breakwater „ The phenomenon is analagous to the diffraction 

 of light., and a theory of breakwater diffraction has been adapted from the 

 theory of physical optics. To check the applicability of this theory to 

 problems of breakwater design a series of model studies was made in the 

 wave tank at University of California,, Berkeley, California,, The tests 

 consisted primarily of generating waves which moved toward a model break- 

 water that could be given various orientations „ Heights of the incident 

 waves as well as wave heights at various points in the lee of the break- 

 water were measured by recording instruments The conclusions from these 

 tests were that the general form of the wave-diffraction theory was 

 verified o Good agreement was obtained for the region sheltered by the 

 breakwater for all angles of incidence investigated (that is, from to 

 135°) o Outside of the geometric shadow the experimental wave heights were 

 considerably less than the theoretical heights „ In general the theoretical 

 solution may be applied to the location and design of breakwaters with 

 conservative results „ In other words, the predicted wave heights in 

 disturbed regions in the lee of the breakwater will be somewhat larger 

 than the height of waves that may be expected in nature , No effect on 

 diffraction resulted from rounding the tip of the breakwater., It should 

 be noted that the diffraction theory applies only to waves in deep water 3 

 that is, refraction effects are not included in the treatment,. 



The use of the wave diffraction theory in breakwater design problems 

 is made convenient when summarized in a diagram which shows curves of 

 equal values of diffraction coefficients on a coordinate system in which 

 the origin is at the breakwater tip 3 the diffraction coefficient in this 

 instance being defined as the ratio of the diffracted wave height to the 

 incident wave heighto Figure 7 shows such a generalized diagram where 

 coordinates at a particular point are expressed as a multiple of the 

 wave lengtho It is of interest to note on this diagram that along the 

 geonetric shadow the wave heights are one-half of the height of the in- 

 cident waves, and that waves slightly greater in height than the incident 

 waves are possible beyond the breakwater „ 



The use of a diffraction diagram in arriving at the most desirable 

 alignment of a breakwater first necessitates the selection of the design 

 wave from a consideration of the factors discussed above; that is, the 

 height, period and direction of the incident wave from which protection 

 is to be provided o A large scale map of the general area at the proposed 

 breakwater is required and the direction (or directions) of the critical 

 waves is indicated „ For the given wave period the wave length is 

 computed and a diagram showing curves of equal wave diffraction coeffic- 

 ients similar to Figure 7, is plotted on transparent paper with the 

 same length scale as the map of the general area„ This transparent 

 paper with the same length scale as the map of the general area„ This 

 transparent overlay then may be moved around on the map (keeping the geo- 

 metric shadow parallel to the direction of wave travel) until the desired 

 degree of protection for a selected reach of the shore line has been ob- 

 tainedo The location of the breakwater tip is thus determined,, For 



