APPENDIX C: WAVE SETUP AND SEICHE EFFECTS 



1. During analysis of the overtopping test results, it was determined 

 that wave setup and seiche were present in the wave tank. Wave setup is the 

 superelevation of the water surface above normal still-water levels (swl's) 

 and is related to wave breaking. Wave setup is caused by the radiation stress 

 (wave-induced transport or momentum) of the waves progressing toward the shore 

 (Seelig and Ahrens 1980). A seiche is a long-period standing wave which oc- 

 curs in an enclosed body of water such as a wave tank. Seiches are commonly 

 found in the prototype in lakes and embajnnents; consequently, they are un- 

 likely to be found along the open coast of Virginia Beach. 



Wave Setup 



2. Wave setup which occurred in the wave tank increased as the percent 

 gain of the design wave height at the wave board (DWHAWB) increased. Setup 

 became significant only at the higher gain settings. Wave setups as high as 

 0.3 and 0.7 ft (prototype) were reached in the Phase I and Phase II studies, 

 respectively (Figures CI and C2) . As would be expected, the setup was greater 

 for the lower swl conditions of +8.0 and +7.0 ft National Geodetic Vertical 

 Datum (NGVD) . The effect that setup had in the model was to effectively in- 

 crease the local swl at the structure. This increase was accounted for in the 

 relative freeboard parameter F' by simply adding the measured setup to d , 

 the water depth at the structure toe. By increasing d , the average free- 

 board F and significant wavelength L were also adjusted. Accounting for 

 setup in the data analysis in this manner implies that wave setup which oc- 

 curred in the wave tank was the best estimate of wave setup that would occur 

 at Virginia Beach. 



Wave Tank Seiche 



3. Initial data analysis showed that a seiche was occurring in the wave 

 tank, and it was determined that much of the data scatter seen in Figures 8 

 and 9 (main text) was due to this seiche. Figures C3 and C4 show the calcu- 

 lated seiche wave amplitude plotted versus the percent gain DWHAWB. The fig- 

 ures clearly show how the seiche wave amplitude increased as the percent gain 



CI 



