Wave Generation, Dissipation, and Disequilibrium in an Embayment With Complex Bathymetry

Heterogeneous, sharply varying bathymetry is common in estuaries and embayments, and complex interactions between the bathymetry and wave processes fundamentally alter the distribution of wave energy. The mechanisms that control the generation and dissipation of wind waves in an embayment with heterogeneous, sharply varying bathymetry are evaluated with an observational and numerical study of the Delaware Estuary. Waves in the lower bay depend on both local wind forcing and remote wave forcing from offshore, but elsewhere in the estuary waves are controlled by the local winds and the response of the wavefield to bathymetric variability. Differences in the wavefield with wind direction highlight the impacts of heterogeneous bathymetry and limited fetch. Under the typical winter northwest wind conditions waves are fetch-limited in the middle estuary and reach equilibrium with local water depth only in the lower bay. During southerly wind conditions typical of storms, wave energy is near equilibrium in the lower bay, and midestuary waves are attenuated by the combination of whitecapping and bottom friction, particularly over the steep, longitudinal shoals. Although the energy dissipation due to bottom friction is generally small relative to whitecapping, it becomes significant where the waves shoal abruptly due to steep bottom topography. In contrast, directional spreading keeps wave heights in the main channel significantly less than local equilibrium. The wave disequilibrium in the deep navigational channel explains why the marked increase in depth by dredging of the modern channel has had little impact on wave conditions. Plain Language Summary Heterogeneous, sharply varying bathymetry is common in many coastal regions, and complex interactions between the bathymetry and wavefield affect the amplitude of wave energy that impinges on the shoreline. The manuscript has focused on the formation of waves over complex bathy under different fetch conditions. In addition, the role of channel deepening on wave energy will be of particular interest and discussed in this manuscript. We found that bathymetric change due to dredging has had minimal impact on wave climate because the directional spreading keeps wave heights in the main channel significantly less than local equilibrium.