Large Eddy Simulation of Cross‐Shore Hydrodynamics Under Random Waves in the Inner Surf and Swash Zones

• Published in: Journal of geophysical research. Oceans Vol. 129; no. 9
• Main Authors: Tsai, Benjamin, Hsu, Tian‐Jian, Lee, Seok‐Bong, Pontiki, Maria, Puleo, Jack A., Wengrove, Meagan E.
• Format: Journal Article
• Language: English
• Published: 01.09.2024
• Subjects: coastal hydrodynamics; inner surf zone; large eddy simulation; Sleath parameter; swash zone; wave‐backwash interaction
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2024JC021194

A 3D large eddy simulation coupled with a free surface tracking scheme was used to simulate cross‐shore hydrodynamics as observed in a large wave flume experiment. The primary objective was to enhance the understanding of wave‐backwash interactions and the implications for observed morphodynamics. Two simulation cases were carried out to elucidate key processes of wave‐backwash interactions across two distinct stages: berm erosion and sandbar formation, during the early portion of a modeled storm. The major difference between the two cases was the bathymetry: one featuring a berm without a sandbar (Case I), and the other, featuring a sandbar without a berm (Case II) at similar water depth. Good agreement (overall Willmott's index of agreement greater than 0.8) between simulations and measured data in free surface elevation, wave spectrum, and flow velocities validated the model skill. The findings indicated that the bottom shear stress, represented by the Shields parameter, was significant in both cases, potentially contributing substantial sediment transport. Notably, the occurrence of intense wave‐backwash interactions were more frequent in the absence of a sandbar. These intense wave‐backwash interactions resulted in a pronounced horizontal pressure gradient, quantified by high Sleath parameters, exceeding the criteria for momentary bed failure. Additionally, a more vigorous turbulence‐bed interaction, characterized by near‐bed turbulent kinetic energy, was observed in the case lacking a sandbar, potentially augmenting sediment suspension. These insights are pivotal in understanding the mechanisms underlying berm erosion and how sandbar formation serves to protect further beach erosion. Plain Language Summary Computer simulations that mimic real‐world nearshore waves in a large wave flume were used to understand how waves and the beach interact, especially for beach shape during storm impact. Simulations accuracy was verified by comparing output with the experiment data, including changes of the water surface, wave patterns, and the speed of flow. We studied the interactions between waves and the beach for two different beach shapes. First, we studied the beach before a storm characterized by a raised area called a berm at the shoreline. Then, we studied the beach during a storm, where the berm disappears but an underwater sandbar forms seaward from the shoreline. Comparing the two scenarios helped us understand how beaches change during different stages of a storm. We found that between the two, the way that the water moved along the bottom was different, which may cause distinct ways of sand movement. When there is no sandbar, the offshore‐directed flow following after wave breaking interacts more strongly with incoming waves. This process leads to significant changes in horizontal pressure and turbulent activity, possibly causing more movement of sand. Our findings help us better understand why berms erode and how sandbars might help protect the beach from erosion. Key Points The 3D large eddy simulation is validated by near‐prototype scale wave flume data for random waves Strong wave‐backwash interaction leads to a large Sleath parameter and near‐bed turbulence The absence of a sandbar favors the occurrence of strong wave‐backwash interaction events