Instrumentation and Calibration 



Wave data were collected with electrical capacitance wave gauges which were 

 calibrated daily with a computer-controlled procedure incorporating a least-square 

 fit of measurements at 1 1 steps. This averaging technicae, using 21 voltage 

 samples per gauge, minimizes the effects of slack in the gear drives and hysteresis 

 in the sensors. Typical calibration errors are less than 1 percent of fiill scale for 

 the capacitance wave gauges. Wave signal generation and data acquisition were 

 controlled using a DEC Micro Vax I computer. Wave data were analyzed with a 

 DEC VAX 3600. 



Water velocity data were collected with Sontek 2D Acoustic Doppler 

 Velocimeters with a side-looking probe which is oriented to collect in- 

 formation on the two velocity components in tiie horizontal plane. Samples were 

 collected at 10 Hz, though the instrument makes 250 pings per second and 

 averages for each output sample. Accuracy is ±0.5 percent of the measured 

 velocity, with resolution of 0. 1 mm/s and threshold of 0. 1 cm/s. The probe 

 samples a 0.25-cm ^ volume located 5 cm from the sensor heads. 



The sensors were placed as seen in Figure 3. An offshore array of wave 

 gauges in firont of die wave generator measured the waves outside the region of 

 currents. In the inlet region a gauge rack was designed to hold both ttie wave and 

 current meters in a colinear manner, with a 0.61-m (2.0-ft) separation between 

 alternating sensors. The rack was then moved to other locations for test reruns of 

 the same wave and current condition to complete a data set. 



Experiment Series 



Wave conditions for the study were zero-moment wave height H^=3J and 

 5.5 cm, peak spectral period 2^=0.7 and 1.4 s, and incident wave direction 

 perpendicular to the jetties and at 20 deg relative to the jetties. All waves were 

 generated with a TMA spectral form using a gamma value of 3.30. The current 

 velocities were 0, 12, and 24 cm/s as determined fom a reference gauge located 

 in the center of the channel seaward of the inlet gorge but between the jetties. 

 Magnitude of the current decreased approximately 20 percent seaward of the 

 jetties (between current meters 3 and 1). Twelve runs were analyzed for the work 

 presented here (two periods, two heights, three current speeds, and normal wave 

 incidence). Wave parameters considered in this study are summarized in Table 2 

 (lab and prototype scale), where U is the average current magnitude, C is the 

 wave celerity, and k is wave number. Table 2 gives the target incident wave 

 conditions and maximum current in the inlet throat. The wave and current 

 parameters cover a wide range of values, which makes the data useful to evaluate 

 the wave dissipation formulations for current-induced wave breaking. Each 

 experiment run was repeated three times, first with the wave gauge and current 

 meter array centered between the jetties and then offset to the left and to the right 

 of the center line by 1 m. Waves and currents at the three positions across the 

 channel were similar and were averaged. 



Chapter 2 Experiment Arrangement 



