132 WAVE IMPACT ON 



the level of f tide, obtained maximum values of 3 and 3 tons 

 per square foot at Dunbar and on the Banffshire coast. These 

 values correspond to heads of 122 and 105 feet and to velocities 

 of 89 and 82-3 feet per second respectively. This mass of 

 water being diverted by the face of the breakwater wip then 

 be projected upwards to a height approximately the same as 

 that corresponding to this pressure. As the result of observa- 

 tion it is known that on the breaking of a wave, during a 

 storm, masses of water are, on occasion, hurled to heights in 

 the neighbourhood of 150 feet, and even greater heights have 

 been occasionally recorded. 1 



As, however, the breakwater causes an upward deflection 

 of the air currents in such an onshore wind as commonly 

 accompanies the production of large waves, this will probably 

 account to some extent, for the extreme height to which the 

 spray is often thrown, and there would appear to be some 

 doubt as to whether, apart from this action, the height exceeds 

 some 100 to 120 feet, except in very abnormal cases. 

 Throughout the present paper calculations have been made 

 on the assumption that this face pressure may attain a value 

 of 6400 Ibs. (2 - 86 tons) per sq. foot, corresponding to a head 

 of 100 feet and to a velocity of 80 feet per second. 



If water accumulates in the interior of a breakwater and 

 is in communication with the sea-face through one or more 

 crevices or open joints which are themselves full of water, 

 such a face pressure may be transmitted hydrostatically to 

 every portion of the interior. Moreover, as all portions of 

 the face may not be exposed to direct wave impact at the 

 same instant, this may give rise to an internal pressure tending 

 to cause outward displacement of the blocks, which may thus 

 attain an effective value of 6400 Ibs. per square foot of projected 

 area of the block. 



Effect of air compression in the joints Let the joint 

 Fig. 1 a, have a length I feet, perpendicular to the sea-face, 



1 Harbour Construction, Shield, pp. 80 and 20S. 



