Response Modes of the Lower Chesapeake Bay Wave Field

An extensive wave monitoring program was conducted in the lower Chesapeake Bay from fall 1989 through spring 1995. From October 23, 1992 - April 20, 1993 the Virginia Institute of Marine Science deployed an aluminum tetrapod containing wave and current sensing and recording equipment in the Thimble Shoal region of the lower Chesapeake Bay. Measurements were made in half hour bursts every three hours to document the area’s dynamic wave characteristics. It was shown previously by Boon et al. (1990 - 1996) that a bimodal frequency distribution of waves in the Thimble Shoal region existed, which was not seen in other areas of the lower Chesapeake Bay. This study examines the temporal changes in the wave energy spectra at Thimble Shoal, fall 1992 - spring 1993, providing a more complete description of the wave climate of the lower Chesapeake Bay. I used Q-mode Factor Analysis to examine the temporal changes in the wave energy spectra. This method of analysis reduced the dimensionality of the large data set by decomposing the data into its basic modal components. Using this technique, four primary modes of spectrum shape at this site were described. These modes include calm, bimodal, local and a mode that is modulated by non-local events. Identification of these components (modes) and the systematic variation between them provides important insights about the wave climate of the lower Chesapeake Bay. The onset of a local wind event starts a systematic variation between the modes. The dominance of the calm mode is quickly reduced by the local storm waves. The bimodal mode is seen as a transition from dominance in the previous two. The fourth mode is dominated by very low frequency waves and appears to be modulated by non-local events. This variation is seen for every local wind event, regardless of type or size