1!he principle underlying these statements is that the plunging 

 breaker is the "ideal" breaker in the sense that it will occur only under 

 conditions approximating the idealized conditions assumed in theoretical 

 investigations and approximated in the laboratory, namely, infinite crest 

 length and uniform period and height o Any disturbance which causes the 

 wave to peak up more at one point than at adjacent points tends to pro- 

 duce a spilling breaker o 



Bie role of slope seems clearo On a steep bottom the wave must 

 complete its break quickly, leading to plunging and there is the added 

 factor that the distance over which the wave is peaked up approaching 

 the point of breaking is short and the wave is thus exposed during only 

 a short interval to the disturbing influence of superimposed waves, 

 currents, and irregular bottomo On flat slopes, a wave is exposed longer 

 to the possibility of disturbance by the factors causing spillingo 



The role of wave steepness, or H' /Lq, is complexo It appears that 

 smaller values of this ratio lead to more pronounced plunging if the 

 general conditions for plunging are meto For example, if the bottom is 

 smooth, the wave length uniform and the crest long, swells of small 

 Wq/Lq will plunge even on a flat slopeo However, waves of small H'q/Lo 

 which exhibit a greater increase in height before breaking are more 

 easily affected by the factors causing spillingo 



If this conception of the causes of spilling and plunging is 

 correct, it has a bearing on the relationship between depth and wave 

 height at the point of breako The more nearly ideal the conditions, the 

 more the wave peaks up before it breaks and the smaller the ratio $, 

 d^/H|3„ (Water depth at point of break/wave hei^t at point of break) o 

 Spilling is the result of an incomplete transformation before break- 

 ing starts and the ratio d^/H^ is largero The two dimensional latora- 

 tory experiment probably approaches the limiting ratio of depth to height 

 at breaking as closely as ocean waves ever doo It is believed that the 

 ratio di^Mo does not have a single value but may have any value greater 

 than that for the "ideal" breaker of the same H'^/^q moving over the 

 same bottom slope o As the distrubing influences previously enumerated 

 increase in importance, the wave crest breaks earlier in its transforma- 

 tion and the ratio d-j^/H^ increases « 



Determination of breaker heights from aerial photographs may be 

 obtained from either wave velocity or depth but wave velocity itself 

 depends upon depth in shallow water » The value of dj^/^^ to be used 

 must therefore be a matter of judgment based on an appraisal of the 

 effect of all types of irregularities, most of which will be evident 

 in the photographs » Neither d^^/Hu or <i\^Mo (^o ^^ wave height in 

 deep water) has a definite value for all waves and the breaker index 

 values or the relationships between d/h^ and Hq/Lq at the point of 

 breaking is a zone and not a lineo 



The point at which the quantity d-^ is measured must be accurately 

 defined for breakers on steep beaches because an appreciable change 

 in depth occurs in the zone in which the wave breaks o In certain 



