2 Field Wave Measurements 



Planning 



Wave data were required at Kahului Harbor to document present conditions 

 and provide data to validate numerical and physical models of harbor response to 

 incident wind waves and long waves, also called seiche, or infragravity waves 

 (typically, waves with frequencies lower than 0.03 Hz or wave periods longer 

 than 33 sec). Tidal response was not included. The numerical model calculates 

 the amplitude of the response at each grid point to an incident wave of a 

 particular height, frequency, and approach angle. For each frequency and 

 direction, validation involves driving the model with measured data of known 

 energy and comparing the model's output to the measured energy at one or more 

 sites within the harbor. 



Planning the measurement program requires specifying the location, duration, 

 and type of data collected. Ideally, incident measurements coincide with the 

 outer boundary of the model, and there are sufficient interior measurements to 

 define spatial variability within the harbor. Finally, the types of data (wave 

 energy, wave direction, currents, etc.) and the range of frequencies measured 

 should equal or exceed the requirements of the model. Fiscal, logistic, and 

 schedule limits always constrain the ideal measurement plan. 



Due to the random nature of waves, it is always difficult to schedule the 

 duration of a wave study in advance based solely on engineering considerations. 

 It is desirable to continue measurements long enough to obtain a broad range of 

 incident conditions - up to or exceeding design conditions - but study schedules 

 and budgets usually override this issue. The plan for Kahului was an initial 

 deployment of one year. A decision to continue measurements would be based 

 on the amount and type of measurements obtained by the end of that year. 



Instrument Type and Site Selection 



Incident waves in deep water are used to define the wave field before it is 

 affected by local shallow water. For ocean swell with a period of 25 sec, deep 

 water is considered greater than about 500 m. Surface-following buoys are 

 typically used to measure waves in deep water, but the accelerometer-based 



Chapter 2 Field Wave Measurements 



