EXPERIMENTAL DETERMINATION OF WAVE PRESSURE ATTENUATION 



by 



George W. Simmons 

 Research Division, Beach E r osion Board 



Several attempts have been made previously to determine the relation- 

 ship between measured and theoretical underwater pressure fluctuations due 

 to surface waves. In most of these cases, controlled waves were not avail- 

 able and the tests were dependent on natural conditions, where the analysis 

 of actual surface wave action is difficult. Observations under controlled 

 conditions permit more accurate determination of this relation. 



Theoretically, the relationship between bottom pressure fluctuations P 

 at depth Z during passage of a wave and wave height H is given by: 



d-Z 

 P L 



H 



Cosh(2ff ~) 



Cosh(2n f ) 



where d is the depth of water from still water level to the bottom and L 

 is the wave length at the particular location. 



Based on field data, Seiwell* found that a correction factor of 1.35 

 should be applied in deriving wave heights from bottom pressure fluctua- 

 tions by the theoretical formula. Later laboratory investigations by the 

 University of California Fluid Mechanics Laboratory group 2 indicated the 

 factor to be about 1.1. 



Additional data on the relationship have been obtained by the use of 

 two Stratham Laboratories (Strain gage type) pressure gages in the Beach 

 Erosion Board's prototype tank where wave characteristics were governed. 

 This tank measures 635 feet long; however, its effective length for these 

 tests was only 490 feet because of a rubble-mound breakwater located 145 

 feet from the end opposite the wave generator. The pressure data were 

 obtained in conjunction with a study on the stability of rubble mounds. 



Measurements were made at two locations along the tank; 210 feet and 

 300 feet seaward of the rubble structure. Surface fluctuations at both 

 locations were measured by step-resistance type wave gages. In addition, 

 at Station 210, visual readings were made for each individual wave by 

 observing the highest and lowest water line seen on a fixed water stage 

 gage. Bottom pressure fluctuations at the same locations were obtained 

 by the two pressure gages. These were mounted to heavy 3-inch iron plates 

 to keep the gages stable on the tank floor. All gages were calibrated 

 statically by raising and lowering the gage. Figure 1 shows the tank 

 set-up and the combination of gages and station locations to give three 

 individual cases of bottom-to-surface pressure fluctuation comparisons. 



1, 2, see references on next page. 



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