Wave Transformation Near Virginia Coast: the "Halloween" Northeaster

Jerome P. -Y. Maa, David W. -C. Wang


The "Halloween" Northeaster of October 29- 31, 1991 generated severe seas; the maximum wave height and peak wave period reached 8 m and 20 s at an offshore wave station located at the continental shelf break about 100 km off the Virginia coast. Storm waves were also measured shoreward at a nearshore wave station located at the Chesapeake Bay Light Tower, which is about 25 km east of the Chesapeake Bay mouth. The wave spectra at these two stations were quite different. We split the measured offshore wave spectra into seven or eight frequency bands and calculated the representative wave heights, periods, and deepwater wave directions for each band. We then used this offshore wave information, the RCPWAVE wave model, a numerical scheme for calculating wave height attenuation caused by bottom friction, and the given bathymetry to evaluate the following four wave transformation processes: refraction, diffraction, shoaling, and bottom friction. We found that bottom friction is an important factor that affects wave transformation between the two stations. If the effects of bottom friction are excluded, the calculated wave spectra at the nearshore station would be much larger than the measurements. Using small constant wave friction factors (fw = 0.01 for frequency ≤ 0.07 Hz, fw = 0.02 for 0.07 Hz < frequency < 0.08 Hz, and fw = 0.03 for frequency ≥ 0.08 Hz), we can reasonably reconstruct the wave spectra at the nearshore station. This small variation of fw for a rather large frequency band (from 0.045 to 0.115 Hz) might imply that other processes are also responsible for the significant wave damping. The accuracy of deepwater incident wave angles and the directional spreading are critical to reconstruct wave spectra at the nearshore station. The Norfolk Canyon at the continental shelf break has a significant influence on the spectral transformation of these "Halloween" storm spectra because of the long-period wave components.


Storm waves; wave spectra; wave transformation processes; wave station; deepwater waves.

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