Hydrodynamic Modeling of a Reef-Fringed Pocket Beach Using a Phase-Resolved Non-Hydrostatic Model

Johan Risandi,Ryan J Lowe,Dirk P Rijnsdorp,Jeff E Hansen

doi:10.3390/jmse8110877

Abstract

The non-hydrostatic wave-flow model SWASH was used to investigate the hydrodynamic processes at a reef fringed pocket beach in southwestern Australia (Gnarabup Beach). Gnarabup Beach is a ~1.5 km long beach with highly variable bathymetry that is bounded by rocky headlands. The site is also exposed to large waves from the Southern Ocean. The model performance was evaluated using observations collected during a field program measuring waves, currents and water levels between June and July 2017. Modeled sea-swell wave heights (periods 5–25 s), infragravity wave heights (periods 25–600 s), and wave-induced setup exhibited moderate to good agreement with the observations throughout the model domain. The mean currents, which were highly-spatially variable across the study site, were less accurately predicted at most sites. Model agreement with the observations tended to be the worst in the areas with the most uncertain bathymetry (i.e., areas where high resolution survey data was not available). The nearshore sea-swell wave heights, infragravity wave heights and setup were strongly modulated by the offshore waves. The headlands and offshore reefs also had a strong impact on the hydrodynamics within the lagoon (bordered by the reefs) by dissipating much of the offshore sea-swell wave energy and modifying the pattern of the nearshore flows (magnitude and direction). Wave breaking on the reef platforms drove strong onshore directed mean currents over the reefs, resulting in off-shore flow through channels between the reefs and headlands where water exchanges from the lagoon to ocean. Our results demonstrate that the SWASH model is able to produce realistic predictions of the hydrodynamic processes within bathymetrically-complex nearshore systems.

Highlights

IntroductionReefs (including coral and rocky) are common features in nearshore environments worldwide
Reefs are common features in nearshore environments worldwide
We focused on swell events (Tp > ~10 s, see Figure 2a) that occurred between 30 June to 3 July 2017 during which time the significant wave heights Hs recorded by AWACs ranged between 2.4 and 5.0 m and the hourly-mean water levels ranged from −0.15 to 0.19 m

Summary

Introduction

Reefs (including coral and rocky) are common features in nearshore environments worldwide. The physical mechanism underlying the hydrodynamic process on reef beaches can be similar to sandy beaches, the presence of the reefs introduces key differences. Infragravity (IG) waves (with periods of 25–600 s) have been shown to often account for a considerable portion of the total wave energy, e.g., [1,2] and are key to developing an understanding of the hydrodynamic processes. Reefs are often detached from the shoreline and/or are discontinuous in the alongshore introducing two-dimensionality in the waves and the resulting wave-driven mean currents. This two-dimensionality is often accompanied by large bottom roughness and steep slopes formed by the reef morphology. While an increasing body of research has been conducted on the hydrodynamics of coral reefs (e.g., see [3] and [4] for reviews), less research has been conducted at sites that include smaller scale (O(100 m)) reef structures that are common along temperate rocky coastlines, e.g., [5]

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