BIMODAL WAVE SPECTRA IN LOWER CHESAPEAKE BAY, SEA-BED ENERGETICS AND SEDIMENT TRANSPORT DURING WINTER STORMS

The transition zone separating estuarine environments from the coastal ocean is characterized not only by distinctive morphological and sedi mentary trends but by unique hydrodynamic forces as well. Lower Chesap eake Bay, a large coastal estuary within the Mid-Atlantic Eight of the U.S. East Coast, experiences complex wave and current-induced forces produced during winter storms. Wave and current measurements made near Thimble Shoal Light over five winter seasons show that most storms si multaneously produce both ocean and bay-generated wave trains that app ear as distinct bimodal peaks in directional spectra. Analysis of sele cted storm wave records reveal that lower-frequency ocean waves, altho ugh nominally lower in amplitude than higher-frequency bay waves, are roughly equivalent to bay waves in terms of energy expended on beds of fine- to medium-grained sand at either end of the Thimble Shoal Chann el. Grain-friction energy dissipation estimates calculated for waves a nd currents suggest that waves provide more net energy capable of tran sporting bottom sediment than currents, although strong barotropic flo ws briefly encountered during a major storm on 13-14 March 1993, excee ded wave energy expended at the bed by almost an order of magnitude. F rom analyses of wave orbital velocity spectra, it is shown that dual w ave trains characterized by differences in peak frequency and directio n may assist each other through interactions that increase their combi ned contribution to frictional energy dissipation and inferred sedimen t transport at the bed. Copyright (C) 1996 Elsevier Science Ltd