laboratory experiments, the depth d^ was taken at the point at vrtiich 

 the crest reached its maximum elevation, and this condition occurred 

 before the wave plunged o Consequently, this depth is probably greater 

 than would be observed with less precise methods of measurement in a 

 dynamically similar prototype o 



It is likely that not all of the factors which lead to spilling 

 change the relationship between d^/Htj» For example, short crestedness' 

 leads to spilling but each point of the crest may break at the same 

 depth as would a long crested wave of the same height. 



Observations of breakers made in conjunction vfith studies of 

 landing craft included an estimate of the percentage of breakers which 

 were spilling or plungingo These observations were made for another 

 purpose before the causes of spilling and plunging had become clearo 

 The local wind velocity was measured and it was noticed that the per- 

 centage of spilling breakers increased with wind velocityo This effect 

 was ttie result of superposition of wind waves, usually at an angle, on 

 the main swell. It was also noted that the larger the breaker, the 

 less the tendency towards spilling at any wind velocity. Figure 1 

 shows the observed percentages of spilling and plunging breakers. The 

 breaker heights are shown near points in the range 5 to 10 m.poh. wind 

 velocity to indicate the effect of breaker height. 



In the lee of headlands such as at San Simeon and Halfraoon Bay, 

 California, the filtering effects of refraction results in long 

 crested breakers of small effective steepness. The breakers at these 

 points are almost always of the plunging type, even though small. A 

 relatively slight disturbance, such as the superposition of waves from 

 small boats, changes those waves to the spilling type. At the earliest 

 opportunity, a controlled experiment of this type will be performed 

 using Dukw's to generate waves. 



Observers of landing craft performance agree that plunging breakers 

 are more hazardous for landing craft than are the spilling type. J. D. 

 Isaacs estimates that a plunging breaker presents about the same hazard 

 as a spilling breaker 50 per cent higher. The differential may be even 

 greater for LCVT'So Two important consequences follow, namely: 



1, In the critical range of breaker heights, where casualties 

 increase rapidly with height, weather forecasters should 

 forecast breaker type. 



2. Artificial means of changing plunging to spilling breakers 

 may be feasible. 



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