sensors have a low frequency cutoff near 0.05 Hz (20 sec), or about at the 

 infragravity band, so only wind waves are measured. In January 1993, a 3-m 

 discus buoy, station number 51026, was installed at latitude 21.37° N, longitude 

 156.96° W - about 20 n. m. north of Molokai in 7,618 ft (2,322 m) of water. 

 While not direcdy offshore of the study site, the difference in the deep-ocean 

 conditions over distances less than 100 n. m. was considered small. The wave 

 climate portion of this study (Chapter 3) helped confirm this judgement. The 

 buoy, which measures directional wave energy and meteorological data, is 

 operated by the National Weather Service (NWS) National Data Buoy Center 

 (NDBC). The station was installed prior to, and maintained after, the Kahului 

 Harbor study by the Corps of Engineers' Field Wave Gaging Program. During 

 the scheduled Kahului field data collection period, the station was funded by 

 HDOT. 



The range of frequencies of interest for wave energy inside the harbor extends 

 from approximately 0.001 Hz to 0.2 Hz. Experience has shown bottom-mounted 

 pressure sensors provide the desired frequency response, flexibility of placement, 

 reliability, and survivability in the coastal environment. Due to the attenuation 

 of wave-induced pressure fluctuations with depth, measurement of the higher 

 frequency wind waves limits the allowable water depth of the bottom-mounted 

 sensors to around 10 m. (This constraint indirectly affected the offshore extent 

 of the numerical model grid boundary.) Directional information was needed for 

 incident energy at the model boundary. Three or more pressure sensors in an 

 array provide a two-dimensional (energy and direction) spectrum. Only non- 

 directional wave energy, provided by a single pressure sensor at each site, is 

 required inside the model domain. Design, installation, and operation of the 

 shallow water gaging system was provided by the Coastal Data Information 

 Program (CDIP), a joint effort of the Corps and the California Department of 

 Boating and Waterways. The CDIP is a network of wave gages operated by the 

 Scripps Institution of Oceanography (SIO). Gages in the network are linked by 

 radio and/or telephone to a central computing facility in La Jolla, CA, where data 

 are collected, analyzed, qualified, and stored. 



Given the size and complexity of the harbor, a minimum of three interior 

 sites, in addition to the incident, or boundary site, were planned. Usually, these 

 sites are selected based on engineering judgement and logistics (Basco and 

 McGehee 1990). For this study, the numerical model itself was used to optimize 

 the measurement sites (Okihiro et al. 1994). Four interior sites were used in this 

 study. Gage locations are summarized in Figure 10 and Table 1. 



Data Acquisition 



The NDBC buoy measures directional energy with a pitch-roll-heave sensor 

 and magnetometer. The superstructure supports dual anemometers and baro- 

 meters for wind velocity and atmospheric pressure. Thermistors measure near- 

 surface sea and air temperature. Signals from the sensors are time averaged or 

 spectrally analyzed with on-board computers. Reduced parameters are 



10 



Chapter 2 Field Wave Measurements 



