Abstract
The Simulating WAves Nearshore (SWAN) model has been extended with an infragravity module to predict the Wave-Group-Forced (WGF) infragravity response to a frequency-directional sea-swell spectrum at a mildly sloping alongshore uniform beach. To that end the SWAN model has been extended with an WGF-infragravity source term denoted Ssb where the subscript denotes surfbeat. The corresponding WGF infragravity energy model has been verified with a set of benchmark tests using the infragravity amplitude model of Reniers et al. (2002). Next the implementation of the energy balance in SWAN has been validated with both prototype-scale laboratory experiments and field observations, showing a good comparison with observations not affected by the nodal structure of the (partially) standing infragravity waves. This suggests that the model is capable of providing improved infragravity boundary conditions in relatively shallow water compared to the typical assumption of equilibrium forcing conditions using for instance Hasselmann’s equilibrium theory (Hasselmann, 1962). These infragravity boundary conditions can subsequently can be used by other more sophisticated models to compute runup, overtopping and dune erosion.
Highlights
The frequency-directional spectrum of the incident sea-swell waves controls the wave interference patterns forming wave groups with periods between approximately 25 s to 250 s
The implementation of the energy balance in Simulating WAves Nearshore (SWAN) has been validated with both prototype-scale laboratory experiments and field observations, showing a good comparison with observations not affected by the nodal structure of the standing infragravity waves
This suggests that the model is capable of providing improved infragravity boundary conditions in relatively shallow water compared to the typical assumption of equilibrium forcing conditions using for instance Hasselmann’s equilibrium theory (Hasselmann, 1962)
Summary
The frequency-directional spectrum of the incident sea-swell waves controls the wave interference patterns forming wave groups with periods between approximately 25 s to 250 s. This results in a modulation of the radiation stress, forcing bound infragravity waves that propagate with the wave groups (Biesel, 1952; Longuet-Higgins and Stewart, 1962; Hasselmann, 1962). On mildly sloping beaches this can result in a dominance of the infragravity wave energy at the waterline (Holman, 1981; Guza and Thornton, 1982; Ruessink, 1998a,b; De Bakker et al, 2014), controlling the run-up and potential overtopping at the beach For a comprehensive review on infragravity waves refer to Bertin et al (2018)
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