Wave Enhanced Overtides in an Idealized Tidal Inlet System

Tides in shallow water environments near estuaries and tidal inlets are subjectto nonlinear interactions, or asymmetries, that produce higher frequency harmonics known as overtides. The tidal asymmetries produced due to the generation of overtides can contribute to long term material transport in coastal regions. The primary mechanisms generating overtides are well known and arise from varying bathymetry, friction, and water depth. However, in systemsdominated by wind waves and barotropic tides other mechanisms may exist. This study utilizes the COAWST modeling system to run process-oriented tide-wavesimulations of an idealized barrier island and tidal inlet system to investigate new mechanisms of overtide generation. The simulations investigate variations in tidal amplitude from micro to meso-tidal coupled with several combinations of wave heights, wave periods, and wave directions. Analysis of overtide amplitudes extracted from 92 numerical simulations indicates that an increase in wave height amplifies overtide current velocities inthe along- and cross-shore directions. Variations in wave period and direction proved to have a lessereffect on elevating overtide amplitudes. A maximum increase of ∼140% in overtide current velocity magnitudes was observed from a tide only to a coupled tide-wave simulation. A decomposition of the depth-averagedmomentum balance of the tide only and tide-wave simulations were used to determine mechanisms responsible for elevating overtide amplitudes. Results identified bottom stress, pressure gradient, andbottom wave streaming as the dominant mechanisms enhancing overtide generation. Accelerations from wave-enhanced pressure gradientsand bottom wave streaming produced a shoreward near-bottom current and an offshore directed surface return flow thatinteracted with tidal currents and enhanced tidal asymmetries.