Author(s): Antoine Villefer; Damien Violeau; Michel Benoit; Hubert Branger; Maria Teles; Christopher Luneau
Linked Author(s): Antoine Villefer, Damien Violeau
Keywords: Multimodal wave spectra; Wind-wave growth; Waves interactions; Wind-wave tank; Wave model
Abstract: Although complex sea states composed of at least two different wave systems (i.e. swell and local wind-sea) occur rather frequently in the ocean, their dynamics is not entirely understood. However, it is crucial to model their properties (i.e. significant wave height, period, direction, etc) with precision for many industrial applications like flood forecast in coastal areas. In order to better assess the effect of swell on the generation of local wind-sea and to study the interaction between the two wave systems, physical experiments combined with numerical simulations using a third generation wave model have been carried out. Using the IRPH\'E/Pythéas wind-wave tank facility in Marseilles (France), wind-wave growth is observed with and without the presence of irregular paddle-waves (generated from a JONSWAP-type spectrum) representing the swell system. Considering various combinations of swell and wind conditions, the presence of swell significantly shifts the wind-wave peak frequency towards low frequency, compared to the case without swell. But contrary to what has been observed in the past with a monochromatic swell (Donelan 1987), the irregular paddle-waves do not have a marked damping effect on the wind-wave energy. Thus, irregular paddle-waves significantly modify wind-wave development at laboratory scale in terms of wind-wave steepness due to a frequency downshift. However, only little evidence of interactions between both wave systems is observed at prototype scale (Hwang et al. 2011). We will present the experimental set-up, test conditions, main results and analyses regarding wave spectrum evolution with fetch for different long-waves (swell) conditions (Villefer et al. 2021). Since ideal wind-over-swell conditions are difficult to measure accurately in the ocean, numerical simulations are used as a bridge from laboratory to prototype scale. Thus, the validation of the model has been carried out at both scales. Initially designed for applications from global to coastal scales, the capabilities of a third generation wave model in reproducing bimodal sea states at laboratory scale are investigated. Using the wave model TOMAWAC (Benoit et al. 1996) (part of the TELEMAC-MASCARET hydro-informatic system), different sets of source/sink terms of the action balance equation, describing wind input, whitecapping dissipation and non-linear four-wave interactions, are compared to the obtained experimental results in one hand, and to in situ measurement from the SHOWEX campaign (Ardhuin et al. 2007) on the other hand. A good agreement between measured and simulated synthetic wave parameters as well as full frequency spectra is reached for both cases. Validating a wave model on the simulation of bimodal sea states at laboratory and natural scale allows the numerical study of the frequency and directional evolution of both wave systems' energy in a large number of configurations.