Development and Application of an Infragravity Wave (InWave) Driver to Simulate Nearshore Processes

Infragravity waves are key components of the hydro-sedimentary processes in coastal areas, especially during extreme storms. Accurate modeling of coastal erosion and breaching requires consideration of the effects of infragravity waves. Here, we present InWave, a new infragravity wave driver of the Coupled Ocean-Atmopshere-Waves-Sediment Transport (COAWST) modeling system. InWave computes the spatial and temporal variation of wave energy at the wave group scale and the associated incoming bound infragravity wave. Wave group-varying forces drive free infragravity wave growth and propagation within the hydrodynamic model of the coupled modeling system, which is the Regional Ocean Modeling System (ROMS) in this work. Since ROMS is a three-dimensional model, this coupling allows for the combined formation of undertow currents and infragravity waves. We verified the coupled InWave-ROMS with one idealized test case, one laboratory experiment, and one field experiment. The coupled modeling system correctly reproduced the propagation of gravity wave energy with acceptable numerical dissipation. It also captured the transfer of energy from the gravity band to the infragravity band, and within the different infragravity bands in the surf zone, the measured three-dimensional flow structure, and dune morphological evolution satisfactorily. The idealized case demonstrated that the infragravity wave variance depends on the directional resolution and horizontal grid resolution, which are known challenges with the approach taken here. The addition of InWave to COAWST enables novel investigation of nearshore hydro-sedimentary dynamics driven by infragravity waves using the strengths of the other modeling components, namely the three-dimensional nature of ROMS and the sediment transport routines.Plain Language Summary Infragravity waves have periods between 25 and 250 s and are the result of wind-wave groups, or "sets." When wind-waves of similar periods travel together, they group, resulting in varying wave heights within the groups. This wave height variation at the group scale forces ocean surface infragravity waves. Coastal circulation, flooding, sand transport, and erosion are strongly influenced by these infragravity waves, especially during extreme storms. Therefore, it is important that we are able to model infragravity waves. We present a novel infragravity wave component of the Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system: InWave. By coupling InWave and a circulation model, COAWST is now able to account for the main processes needed to predict coastal hazards due to extreme storms. This new coupled system is verified by reproducing observations from idealized numerical cases, a laboratory experiment on dune erosion, and a field experiment. Results show a good agreement with observations.