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

Abstract

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.

Effects of Locally Generated Wind Waves on the Momentum Budget and Subtidal Exchange in a Coastal Plain Estuary

“…In the present study, the tidally averaged significant wave height ( H s ) mirrors the broad basin topography with maximum values in the deepest regions of the lower bay ( H s ~ 1.5 m, Figure a) and reflects the findings by Chen et al () regarding equilibrium and disequilibrium conditions. Wave heights at shoreline edges in the lower bay were relatively small ( H s ~ 0.25 m), which is indicative of enhanced energy dissipation through bottom friction and whitecapping as waves propagate onshore.…”

“…The study concluded that remotely generated waves cannot deeply propagate into the bay and therefore that waves are predominantly generated by local wind. The latter is in agreement with findings by Chen et al () who concluded that local winds constitute the main source of wave energy in much of the bay. However, these studies did not evaluate the role of waves in the momentum budget, surface stresses, and subtidal exchange.…”

“…The role of waves on drag can be important in systems such as Delaware Bay because of the broad spatial distribution of the wave height and age. The Delaware differs from other coastal systems such as back‐barrier estuaries, tidal inlets, and coral reefs (see, e.g., Beudin et al, ; Olabarrieta et al, , ; Rogers et al, ; Uchiyama et al, ) in that that waves are mainly local and wind‐generated instead of remote (Chen et al, ) and in that the bathymetric distribution is the result of both natural and anthropogenic processes. The latter leads to distinct patterns in the wave‐driven momentum budget.…”

“…Surface mixing accounts for wave breaking according to Craig and Banner (1994) as implemented by Carniel et al (2009). Since previous studies have shown that the bay is sheltered from remote swell (Chen et al, 2018;Kukulka et al, 2017), we neglect wave forcing at the open boundary.…”

“…Wind forcing (with wave generation for the coupled runs) is then prescribed for 5 days, consistent with previous studies of wind-driven circulation in a similar coastal environment (Whitney & Codiga, 2011). We use input parameters and 10.1029/2018JC014585 Journal of Geophysical Research: Oceans advection schemes from a previously validated model setup of the bay (Chen et al, 2018), and further details can be found there. A summary of the model setups is presented in Table 1.…”