Transformation of Infragravity Waves during Hurricane Overwash

Katherine Anarde,Damien Sous,Marion Tissier,Jens Figlus

doi:10.3390/jmse8080545

Abstract

Infragravity (IG) waves are expected to contribute significantly to coastal flooding and sediment transport during hurricane overwash, yet the dynamics of these low-frequency waves during hurricane impact remain poorly documented and understood. This paper utilizes hydrodynamic measurements collected during Hurricane Harvey (2017) across a low-lying barrier-island cut (Texas, U.S.A.) during sea-to-bay directed flow (i.e., overwash). IG waves were observed to propagate across the island for a period of five hours, superimposed on and depth modulated by very-low frequency storm-driven variability in water level (5.6 min to 2.8 h periods). These sea-level anomalies are hypothesized to be meteotsunami initiated by tropical cyclone rainbands. Estimates of IG energy flux show that IG energy was largely reduced across the island (79–86%) and the magnitude of energy loss was greatest for the lowest-frequency IG waves (<0.01 Hz). Using multitaper bispectral analysis, it is shown that, during overwash, nonlinear triad interactions on the sea-side of the barrier island result in energy transfer from the low-frequency IG peak to bound harmonics at high IG frequencies (>0.01 Hz). Assuming this pattern of nonlinear energy exchange persists across the wide and downward sloping barrier-island cut, it likely contributes to the observed frequency-dependence of cross-barrier IG energy losses during this relatively low surge event (<1 m).

Highlights

During hurricanes, the nearshore environment extends landward as ocean water levels become elevated by storm surge and wave setup
While a complete analysis of how these very low frequency (VLF) fluctuations influence surf zone processes is beyond the scope of this study, inspection of the wave envelope of the IG wave signal during both time periods of large VLF variability revealed that amplitude modulation of IG waves at VLF was relatively small at this surf zone location (
As energy can go up and down conservatively as waves propagate over different water depths, it is important to assess to what degree cross-barrier decreases in IG wave amplitude and apparent changes in frequency content are due to changing water depths on either side of the barrier island versus losses from dissipation or nonlinear energy transfer

Summary

Introduction

The nearshore environment extends landward as ocean water levels become elevated by storm surge and wave setup. No field studies to date have detailed wave transformation across barrier islands during hurricane impact. On closed sandy beaches (no overwash and inundation), field observations show that infragravity (IG) waves (frequencies f nominally 0.003–0.04 Hz) can dominate shoreline water motions during energetic storm conditions. Given the energetic behavior of IG waves both on closed beaches and in lagoon environments during storms, a general assumption of morphodynamic numerical models—while limited in field observations for validation—is that IG waves contribute significantly to morphological change during overwash and inundation (e.g., [20,21]), and the generation of washover deposits [22]

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Conclusion