Measuring directional wave spectra using the 3D-ACM WAVE on fixed and taut-wire moorings

Field data were analyzed from a simultaneous deployment of two 3D-ACM WAVE instruments; one on a fixed seabed frame in the nearshore zone, and the oth er further offshore on a taut-wire mooring. An inter-comparison of measurem ents of vertical and horizontal wave-orbital currents with pressures was us ed to evaluate the velocity sensor response under field conditions. Results using the fixed frame have validated the measured horizontal wave-orbital velocities, but found the vertical velocities to be less coherent with the pressure time-series. The influence of the instrument mooring system on the velocity measurements was investigated. The oscillation of the taut-wire m ooring was found to influence the magnitude of the measured horizontal wave -orbital velocities and induce a phase lag between velocity and sea-surface elevation. Examination of other data from similar taut-wire moorings indic ates a systematic relationship between the length of the mooring cable and the measured phase lag, consistent with the behavior of the mooring system considered as a forced, linearly damped oscillator. A comparison was made b etween the spectra of wave direction derived from both velocity and pressur e data with that derived solely from velocity data. The results show a high coherence for the fixed mooring, but significant directional variability i n the higher frequencies (>0.13 Hz) on the taut-wire mooring we employed, w hich we attribute to the mooring oscillation. The analysis further indicate s that on taut-wire moorings, the spectra of wave direction should be resol ved solely from velocity data. Using these findings, directional wave spect ra were produced for the nearshore and offshore sites from 233 coincident e vents over a two-month period, and these data are presented in a time-avera ged spectral format.