This required condition can be attained by installation of an airtight 

 cover on the test flume so that the air pressure above the water surface 

 can be adjusted in accordance with the linear scale. In addition, for 

 the fifth 7r term. 



from which 





M 

 (Q^), 



^^^r 



2/3 



(6- 19a) 



(6- 19b) 



Other model-to-prototype 

 equation (6-9), 



and 



ratios derived from the above relations are 



where 



= time 



= velocity 



PwS 





1/2 



(Pat), 



+ (7Z). 



(Pat)p^(n 



(6-20) 



p = pressure at any depth z below the water surface 



g. Hydraulic Breakwaters . Hydraulic breakwaters reduce wave heights 

 in the same manner as pneumatic breakwaters, except that the horizontal 

 surface currents are generated by water jets formed by forcing water 

 through a series of nozzles in a pipe situated at or near the mean still- 

 water level, and alined so that the currents generated are opposite in 

 direction to the incident wave velocities. Thus, the important forces 

 that must be modeled are inertia, gravity, and viscosity; the variables 

 are presumed to be Hj^, H^, X, d, g, Cl^, p^, and u, where Q,^ is the 

 quantity of water emerging from the orifices per second per foot of mani- 

 fold (cubic feet per second per foot). From dimensional analysis. 



and 



f(Hi,H^,X,d,g,Q^,p^,p)= 



X ' X' o2/3 ' 



(6-21a) 



(6-21b) 



332 



