150 WAVE IMPACT ON 



In certain of the experiments an air valve, J inch diameter, 

 was mounted at the upper side of the horizontal pipe at its 

 inner extremity, and was left open during impact. Where 

 the diagrams showed the phenomenon to be due to air com- 

 pression, the effect of this in reducing the maximum pressures 

 was very marked. Where water-hammer occurred the effect 

 was very erratic, the maximum pressures being in some cases 

 as high as, and in other cases much lower than, when the valve 

 was closed. 



The question as to how far the results may be considered 

 to apply in the case of wave impact on a sea-wall, is of some 

 interest. Probably the average sea-wall will be comparable, 

 as regards rigidity, with the experimental pipe line, and, 

 except as regards porosity, pressures of the same order of 

 magnitude may be anticipated with the same velocity of wave 

 impact. Any such porosity will, however, considerably 

 reduce the maximum pressures obtained, whether due to 

 water-hammer or to air compression, while the presence of 

 any cavity forming an air chamber at the inner end of any open 

 joint will effectively prevent hammer action. Still, since the 

 magnitude of the hammer pressures are directly proportional 

 to the velocity of impact of water surface on water surface, 

 and since this is probably approximately proportional to the 

 velocity of wave impact in all cases, it is evident that with 

 velocities in the neighbourhood of 80 feet per sec. (6*3 times 

 those, obtaining in these experiments), internal pressures of 

 the order of 40 tons per square foot may, under favourable 

 circumstances, be developed. Fortunately each application 

 of such a pressure only lasts for an almost infinitely small 

 interval of time, and the shorter the effective length of the 

 joint, and the less in consequence the modifying effect of the 

 entrapped air and the greater the pressures attained, the less 

 is the time over which the pressure is exerted. Still, even so, 

 its effect, in gradually breaking down the adhesion of block 

 to block, is likely to be extremely serious. 



