Improved Estimates of Swell from Moored Buoys

The National Data Buoy Center (NDBC) began posting buoy estimates of swell height and period in 1997 because of numerous requests from mariners. Until then, only significant wave height, dominant period, and spectral wave data were posted on its Web site. Though swell information can be gleaned from the wave energy spectral, many mariners do not have the time or experience to do so. NDBC developed a method based on wave steepness that requires only nondirectional wave data. This method determines a period to separate the wind seas from the swell based on the knowledge that seas are steeper than swell and that maximum steepness occurs near the peak period of the wind waves. However, the method underestimates the swell when winds are light or abating, as compared to the Navy's operational wave model (WAM). To improve performance, the steepness method was modified to limit the maximum allowable separation period based on the observed wind speed. Since peak frequencies of fully developed seas generated by a given wind speed are well known, this relationship can be used to set an upper limit on the separation period. Positive results were obtained in tests of the modified method using measurements from directional buoys where swell and wind seas can be easily identified by differences in propagation direction. The modified method also compared much more favorably with wind sea and swell estimates from the WAM.