Regarding the interaction between the submerged seagrass and the surrounding flow, it is well known that the presence of the seagrass canopies attenuate the momentum by the work done on the flow by the stems (Finnigan, 2000) in the velocity field can be differentiated in terms of the scale processes: 1) processes with spatial scales of the order of the stem diameter or spacing between stems; and 2) processes with scales of the order of the drag length scale. The turbulent structures at the scale of the stems are called wake scales and are produced by the shadow zone downstream the stems (Nepf, 2012; Zhang et al., 2018). Turbulent processes at the drag length scale are governed by the density of the canopy and the flow dynamics (Nepf, 2012). These processes modulate the water renewal between the water inside and above the canopy and the amount of suspended sediments along the water column (Luhar and Nepf, 2013). The turbulent processes at the drag length scale can be analysed as a plane mixing layer by two co-flowing streams that present a shear layer at the top of the canopy (Raupach et al., 1996). This shear layer flow is characterized by an inexion point in the velocity profle (two water bodies moving at different velocities), and responsible for the vertical mass exchange at the top of the canopy (Ghisalberti and Nepf,2009). This shear layer facilitates the generation of Kelvin-Helmholtz instability type vortex (Ghisalberti and Nepf, 2002). This Kelvin-Helmholtz type vortex has been widely studied in steady flows (Raupach et al., 1996; Finnigan, 2000; Ghisalberti and Nepf, 2002,0; Nepf, 2012; N., 2012; Mandel et al., 2017), characterizing its effect on the seagrass movement, Reynolds stress, sediment distribution, vertical mixing and free surface; however, the formation and effect of Kelvin-Helmholtz type vortices in oscillatory flows (waves) is still not well understood. Ghisalberti and Schlosser (2013) reported some neccesary conditions in the fow to have Kelvin-Helmholtz instabilities; Abdolahpour et al. (2017) analysed a steady current released by the presence of the shear layer and its relation with the shear layer magnitude and Abdolahpour et al. (2018) used the seagrass-steady flows interaction formulation of Ghisalberti and Nepf (2002) to estimate the Kelvin-Helmholtz frequency range in oscillatory flows. Up to date, the evolution of vortices downstream submerged structures in oscillatory dominant flows is assumed to be dissipated by the viscosity and effects on the wave breaking process have not been yet analysed. To date, assuming non linear wave propagation, it is still not clear in this two-flow problem which are the dominant terms in the Navier-Stokes equations for the oscillatory-seagrass flow interaction. Indeed, a theoretical model to solve the Kelvin-Helmholtz instability modes as a function of the free surface and a general characterization of the turbulent spectra is an important research topic in order to improve the simulation of hydrodynamic processes at coastal scale. The aim of this research is to understand the relation between the free surface frequency and the Kelvin helmtholtz instability modes in seagrass-oscillatory flow interaction by analysing the effect of a vortex by an isolated submerged stem interacting with a surface wave, the development of an analytical model to determine the dominant terms in the momentum equation in seagrass-oscillatory flows interaction, and by solving the possible Kelvin-Helmholtz instability modes in seagrass-oscillatory flow interaction as a function of the free surface wave applying the Piecewise method to a simplifed velocity profle.