(the first large waves with an elevated wave setup) recorded at Gauge A, with 

 a phase lag related to the 6.4-m spacing between Gauges A and I. Other large 

 swash events may also be traced to the occurrence of elevated mean water 

 levels (maximum dynamic wave setup) and large incident waves at Gauge A. 

 These wave records also show the apparent downshifting in wave frequency as 

 187 waves pass Gauge A whereas only 25 waves pass Gauge I in the same 

 length of time. 



In Figure 4-6, the smoothed wave spectra are shown for Gauge I for the 

 total signal, the low-pass signal, and the high-pass signal, respectively. The 

 energy content in this case is concentrated at low frequencies due to the long 

 period of time between successive swash events. It is, in fact, difficult to 

 discern the intended peak wave frequency in the wave spectra. Further evi- 

 dence of the domination of low-frequency components is found in the energy- 

 based significant wave height. The zero-moment wave height for the total 

 wave record is 0.08 m in mis case. If separated into components, however, 

 the low-frequency wave height dominates and is 0.07 m, whereas the remain- 

 ing high-frequency signal has a zero-moment height of 0.04 m. 



Summary of data characteristics 



Figure 4-7 shows a summary of mean water level and selected wave 

 parameters for Run A0509A for all 10 swash gauges. The initial beach pro- 

 file is also shown for reference with elevations defined relative to the still- 

 water-level datum. As shown in Figure 4-7a, mean water levels (mean wave 

 setup averaged over the length of the data run) increase in the shoreward 

 direction from Gauge A to Gauge C (located at the still-water shoreline) 

 where the setup is maximum and equal to 9.9 cm. Shoreward of Gauge C at 

 Gauge I, the mean setup is 0.7 cm, while at Gauge E (next gauge landward 

 from Gauge I) the setup is 0.3 cm. 



Wave transformation across the profile is indicated by both the zero- 

 moment wave height and the maximum wave height. The zero-moment wave 

 height diminishes from 0.45 m at Gauge A to 0.30 m at Gauge C (still-water 

 shoreline). Landward of Gauge C, the zero-moment wave height decays by 

 one order of magnitude to 3.0 cm at Gauge E (next gauge landward from 

 Gauge I) and then decays less dramatically to less than 1 cm at Gauge G 

 (landwardmost gauge). The maximum wave height follows a similar decay 

 across the inner surf zone and swash zone. It may be observed, however, that 

 the maximum wave height does not decrease systematically but actually 

 increases slightly just landward of Gauge C. The reason for this variation is 

 the presence of reflected waves moving seaward from the upper limit of the 

 swash and colliding with incoming waves in this region. 



In Figure 4-7b, the zero-moment wave height is further decomposed into 

 low- and high-frequency components based on the low- and high-pass filtering 

 described earlier. It may be seen that the high-frequency wave component 

 dominates in the surf zone, but, landward of the still-water shoreline, the low- 



Chapter 4 SUPERTANK Swash Measurements 



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