84 



Sample Data 



Gauges located seaward of still-water shoreline 



Typical results from a location seaward of the still-water shoreline are 

 shown in Figures 4-3 and 4-4. The sample time series in Figure 4-3 is from 

 Gauge A during Run A0509A. The incident wave conditions for this run 

 were based on a narrow spectrum (7 = 20) with a peak period of 3 sec (peak 

 frequency of 0.33 Hz) and a zero-moment wave height of 0.8 m near the 

 wavemaker. Gauge A was located inside the surf zone approximately 3.7 m 

 seaward of the still-water shoreline where the water depth was initially 

 0.37 m. 



In Figure 4-3, the initial zero offset has been removed from the time series 

 and every fourth data point has been plotted to show a smoothed version of 

 the original wave record. In addition, the low-pass signal showing time- 

 varying wave setup and setdown is plotted as a dashed curve. As shown in 

 Figure 4-3, waves arrived at Gauge A about 40 sec after the start of data 

 recording. Data recording was initiated at the same time that wavemaker 

 commands were initiated by the host computer. The 40-sec delay in waves 

 reaching the wave gauge is then due to two factors: (a) a time delay between 

 the initiation of these wavemaker commands and the start of the wavemaker 

 motion, and (b) the propagation time of waves travelling from the wavemaker 

 to the wave gauge. 



The wave record in Figure 4-3 is fairly typical of gauges that were located 

 inside the surf zone but seaward of the shoreline where the sand bed was 

 never exposed during the wave run. Almost all waves are breaking or broken 

 as they pass this gauge and individual waves exhibit the characteristic "saw- 

 tooth" shape of shallow-water waves in the inner surf zone. The incident 

 waves were highly grouped in this example and some evidence of the wave 

 group structure is shown in Figure 4-3, both from the individual (high- 

 frequency) waves and from the low-pass signal. The low-frequency signal 

 may be interpreted as a record of the dynamic wave setup and setdown that, 

 relative to the high-frequency waves, represents a slowly varying mean water 

 level. By averaging the low-frequency signal, the mean wave set-up averaged 

 over the entire data run was determined to be 4.5 cm at Gauge A. 



In Figure 4-4, the smoothed (block-averaged) wave spectra are shown for 

 the total water surface signal, the low-pass signal, and the high-pass signal, 

 respectively. Dominant wave energy is concentrated around the intended peak 

 frequency of the incident spectrum near 0.33 Hz, but significant low- 

 frequency energy is also found. Higher harmonics are evident at about two 

 and three times the peak frequency. In this case, the zero-moment height 

 from the high-frequency signal dominates and is about 0.39 m, whereas the 

 low-frequency zero-moment height is about 0.22 m. 



Chapter 4 SUPERTANK Swash Measurements 



