Coherence between infragravity waves and ambient water motions over cape-associated shoals

Abstract

Cross-shelf water motions influence fluxes of nutrients, larvae and sediments, which in turn affect nearshore morphodynamics. Among these motions are infragravity waves, which typically exhibit periodicities from 20 to 200 s, and originate from multiple sources including the superposition of incident wave fields. To examine how infragravity waves co-vary with water motions in an area of complex bathymetry, we analyzed measurements from acoustic Doppler current profilers around cape-associated shoals near Cape Canaveral, on the Florida (USA) Atlantic coast. Observations of water motions and infragravity wave heights at the outer and inner swales of two isolated shoals (Shoal E and Shoal D), located approximately 15 km offshore in ∼ 14 m water depth, were subjected to squared coherency and wavelet coherence analyses. Coherences between infragravity wave heights and flow conditions were unsteady, i.e. variable over time, and exhibited differences between outer and inner swales. Subtidal flows (periods > 0.5 days) were sporadically coherent with the total and bound infragravity wave heights at both inner and outer swales. Tidal flows (∼ 2 cycles/day) were coherent with the total infragravity wave heights only at outer swale locations. These results indicate that tidal motions may modulate the generation of free (directed both oppositely to and in the direction of short-wave propagation) infragravity waves by short-wave groups shoaling and by refractive trapping at shoals. Subtidal flows, on the other hand, may influence triad interactions that generate infragravity motions.