THE CAUSES OF PLUNGING AND SPILLING BREAKERS 



The following notes first appeared in limited 

 issue as Technical Report HE-ll6-192s Fluid 

 Mechanics Laboratory, University of California < 

 They are reproduced here to bring the concepts 

 therein to the attention of research workers 

 and other persons having an interest in ocean 

 waves o The notes were prepared by Dean Mo Po 

 O'Brien, University of California in January 

 19-460 



Plunging breakers are more hazardous for landing craft than spill- 

 ing breakers o As yet there has been no method developed to forecast 

 the type of breakero "Rie following notes discuss this problem quali- 

 tativelyo It is hoped that quantitative data will followo 



Plunging breakers are characterized by uniformity of breaker 

 height and by long-crestedness but there are other factors involved 

 in this phenomenon o Field observations and examinations of photo- 

 graphs have led to the following general conception of the conditions 

 causing plunging or spilling breakers; 



Plunging 



ao Long crests and uniform wave heighto 



bo 3niall curvature of crests o (small angle of breaker with 



shore line ) 

 Co Steep bottom slope o 

 do Regularity of bottomo 

 eo Absence of other swells or wind waves and absence of 



irregular current o 

 f o Size; large waves tending to plunge » 



Spilling 



ao Short crest length or variations in heighto 



bo Large curvature of crests at the break o 



(Abrupt change of crest angle by refraction o) 

 Co Flat bottom slope o 

 do Irregularity of bottom, 

 60 Superimposed waves, such as wind waves making an angle 



with the main swell in the breaker zone o 

 f o Irregular current, particularly "tide rip" and tidal 



current o 

 go Obstacles such as boats, piers, etCo 

 ho Variation in strength of proceeding backrush ctirrent due 



to variations in height and period o 



