Tidal Variability of Infragravity Waves Over Cape-Associated Shoals

Abstract:

Particulate transport at cape-related shoals is generally driven by water circulation that is derived from interactions among tides, waves, and wind. At inner-shelves with alongshore uniform slopes, it has been shown that gravity waves produce offshore-directed transport by means of infragravity (IG) motions. However, the influence of IG waves on the spatial and temporal patterns of particulate transport is not yet understood at cape-related shoals (i.e. inner-shelves characterized by non-uniform bathymetry). To analyze the connection between IG waves and tides, cross-spectral and cross-wavelet analyses were performed on time series data of current profiles and pressure that were measured at both sides of a shoal (east swale and west swale) near Cape Canaveral, Florida. Overall, IG wave heights were coherent with water levels at ~2 cycles/day with a 95% statistical confidence at both locations. However, the coherence at the west swale (closer to shore) was lower than at the east swale. High coherence squared (>0.8) between tidal motions and IG energy could be explained by changes in water depth that produced IG energy losses to sea-swell frequencies during low tide. The 1-m difference in mean depth between east and west locations may explain the difference in coherence as water is not shallow enough at the west swale to produce IG energy losses. This may highlight the sensitivity of IG waves to changes in water depth within this shoal complex. Our results agree with previous studies regarding tidal variability of IG energy in nearshore and inner-shelf environments and could be applied to improve understanding of the role of complicated bathymetry in particulate transport at cape-associated shoals.