wall, Bagnold (1939) theorized that the short duration shock pressures result 

 from the rapid compression of an air pocket trapped between the face of a 

 breaking wave and the wall. In the past, this phenomenon has been studied by 

 several investigators (Minikin 1946; Carr 1954; Kamel 1968a, 1968b; Garcia 

 1968; Kirkgoz 1982). However, there is still debate concerning the relative 

 importance of these shock pressures to the actual design of a seawall. A com- 

 mon opinion among many designers is that pressures of such short duration 

 should not be used for establishing design loadings; thus, it is their opinion 

 that the lesser surge pressures of longer duration are more suitable indica- 

 tors of critical dynamic loadings. 



Testing Procedure 



33. For the purpose of this study, shock and surge pressures were mea- 

 sured in response to waves characteristic of the same two storms used in the 

 overtopping study. These storms were simulated at swl's of +7.0, +8.0, and 

 +9.5 ft NGVD. Signal generations and resulting zero-moment wave heights were 

 accomplished with gains set at 50 and 100 percent. Test conditions to which 

 the wall was subjected are summarized in Table Dl (Appendix D) . Because of 

 limited data storage capacity of the computer facilities used for data acqui- 

 sition, the duration of each test was dictated by the particular sampling rate 

 at which pressures were measured. As stated above, durations of shock pres- 

 sures are characteristically quite short (in the range of prototype milli- 

 seconds); therefore, to acquire a definitive record of these portions of the 

 pressure response, a high sampling rate was imperative. Tests were initiated 

 using a 2,000-Hz sampling rate which, due to data storage capabilities, lim- 

 ited the actual data acquisition interval to approximately 30 sec. Therefore, 

 with a 2,000-Hz sampling rate, pressure data in response to roughly seven to 

 nine waves in sequence could be obtained. Analyses of these first runs indi- 

 cated that the 2,000-Hz sampling rate resulted in good resolution of most max- 

 imum pressures; however, since the duration of individual tests was so limited 

 (30 sec) , a series of tests using various slower sampling rates was under- 

 taken. These tests indicated that an acceptable resolution of most shock 

 pressures could be achieved at a 1,000-Hz sampling rate, thereby increasing 

 the allowable length of each test to 60 sec. Table Dl shows that 16 tests 

 were executed with an 80-Hz sampling rate. These tests were conducted to 



29 



