It is admitted axiomatically by niany that the pits observed at the 

 foot of vertical walls and dikes against Tihich the wave breaks are pro- 

 duced by the fall of spray and not by the direct action of the wave in 

 breaking against the surface o Profcably by extension of this theory and 

 by judging that the damage to rock-fill dikes always is caused by the 

 displacement of the stones on the exterior slope and not by the passage 

 of waves over the top of the dike and toward the interior of the port, 

 Messrs o Castro and Briones assur/ie that the disintegration of the dike is 

 caused by the fall of the mass of water raised by the breaking wave and 

 not ty the direct action of the same in breaking over the exterior slope, 



It is my opinion, in spite of test results to the contrary, that 

 the pitting at the foot of walls and dikes is not necessarily due to 

 wave sprayo 



A wave breaks because the water depth is shallow and approximately 

 equal to the height of the wave (see Figure l)o The energy of the wave 

 acts downward chieflyo The spray above^ which is all we can observe, 

 represents only a small part of the total energyo The submerged por- 

 tion of the breaking wave possesses a great amount of energy and causes 

 violent currents on the shallow bottom, occasioning the pitting at the 

 foot of the dike or wall„ 



Even if it is admitted that the energy of the wave is divided 

 equally between spray and submerged breaking, that of the latter wouM 

 act downward undiminished while a large part of the energy of the 

 upper spray would be lost in its ascent and descent through liie aire 

 For this reason it can hardly cause effects comparable to those of the 

 submerged portion of tte breaking waveo 



It is easy to understand the low intensity of both portions of the 

 breaking wave, particularly the upper, on the rough and permeable sur- 

 faces of rock-fillo The cavities in the rock-fill permit the passage 

 of water through them thus avoiding the formation of much spray, and 

 "Uiis, together with the surface unevenness causes the upper spray to be 

 of little importance o Only the inner submerged part descends along the 

 slopeo The remainder of the water filters and descends, almost verti- 

 cally, through the mass of the rock-fillo See Figure 2o With these 

 details in mind it does not appear possible that currents of descending 

 water capable of washing out stones in their descent can be produced 

 upon the slopes of rock- fill dikes o 



A further proof of the slight importance of the upper spray in 

 this type of dike lies in the fact that it is sufficient to raise the 

 top elevation but little above the crest of the wave to avoid over- 

 topping o 



What can be the cause of the downward displacement of stones of 

 sufficient weight if not the descent of the water along the slope? 

 Let us try to reason by logic, expressed as follow.'-s: 



If a current, of velocity V, acts on a plane normal to its course, 

 the pressure of the impact, directed normal to the plane, in terms of 



