The role of morphology and wave‐current interaction at tidal inlets: An idealized modeling analysis

Abstract

The outflowing currents from tidal inlets are influenced both by the morphology of the ebbtide shoal and interaction with incident surface gravity waves. Likewise, the propagation and breaking of incident waves are affected by the morphology and the strength and structure of the outflowing current. The 3-D Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system is applied to numerically analyze the interaction between currents, waves, and bathymetry in idealized inlet configurations. The bathymetry is found to be a dominant controlling variable. In the absence of an ebb shoal and with weak wave forcing, a narrow outflow jet extends seaward with little lateral spreading. The presence of an ebb-tide shoal produces significant pressure gradients in the region of the outflow, resulting in enhanced lateral spreading of the jet. Incident waves cause lateral spreading and limit the seaward extent of the jet, due both to conversion of wave momentum flux and enhanced bottom friction. The interaction between the vorticity of the outflow jet and the wave stokes drift is also an important driving force for the lateral spreading of the plume. For weak outflows, the outflow jet is actually enhanced by strong waves when there is a channel across the bar, due to the ''return current'' effect. For both strong and weak outflows, waves increase the alongshore transport in both directions from the inlet due to the wave-induced setup over the ebb shoal. Wave breaking is more influenced by the topography of the ebb shoal than by wave-current interaction, although strong outflows show intensified breaking at the head of the main channel.

Wave-, wind-, and tide-driven circulation at a well-mixed ocean inlet

“…Although the wave radiation-stress gradient and cross-shore wave-driven changes to the pressure gradients are negligible inside the inlet (Figures 2.7b and c, green and blue curves, and Figure 2.9a, solid red and blue curves are flat, cross-shore distance > 900 m), there is a reduction in ebbdominance during large waves owing to enhanced onshore mass flux (Orescanin et al 2014;Wargula et al 2014) and the partial blocking of the ebb jet (Olabarrieta et al, 2014). Large waves (H sig > 1.2 m) increase the water levels more on ebb (the cross-sectional area inside the inlet is ~200 m 2 larger than without waves) than on flood (the cross-sectional area inside the inlet is ~100 m 2 larger) (Figure 2.9), decreasing the tidal prism by ~20%.…”

“…Spatial and tidal variability in water depth can lead to complex wave breaking patterns (Kang and Di Iorio, 2006;Zippel and Thomson, 2015) that drive asymmetric changes to the water levels and flows (Piedracoba et al, 2005;Olabarrieta et al, 2011Olabarrieta et al, , 2014Chen et al 2015). In …”

“…Numerical simulations suggest that the processes governing the hydrodynamics of shallow, unstratified tidal inlets vary over short spatial and temporal scales (Hench and Luettich, 2003). However, the effects of waves and winds are not understood well, especially over and near complex natural shoals, channels, and shorelines (Blanckaert and de Vriend, 2003;Olabarrieta et al, 2014) and during storms with large waves and strong winds (Sanay and Valle-Levinson, 2005;Bertin et al, 2009). Field observations that resolve the spatial and temporal variability in flows and dynamics are essential to improve understanding of the interactions and feedbacks between tides, waves, wind, and bathymetry at inlets.…”

“…Wave heights and wave breaking are tidally modulated at inlets owing to changing flows and water depths (Kang and Di Iorio, 2006), leading to spatially and temporally varying wave effects on the flows and dynamics. Model simulations and field observations suggest wave momentum flux (radiation-stress) gradients owing to dissipation across the ebb shoal can decrease the offshore extent of the ebb jet (Olabarrieta et al, 2014) and drive fluxes into the inlet (Bertin et al 2009;Malhadas et al, 2009;Orescanin et al, 2014;Wargula et al, 2014;Chen et al, 2015). Enhanced influx may result in increased bay water levels (Olabarrieta et al, 2011;Dodet et al, 2013).…”

“…If the shoals are asymmetric and waves break primarily on one side of an inlet connected to an enclosed bay, flows into the inlet driven by the wave breaking may be balanced by a return flow on the opposite side of the inlet (Piedracoba et al, 2005). In addition, wave-forced flows may constrict the ebb current jet, causing it to narrow and intensify in the main inlet channel, depending on the jet outflow rate, wave energy, and inlet morphology (Olabarrieta et al, 2011(Olabarrieta et al, , 2014Chen et al, 2015).…”