Implementation of a 1D k-ε model for studying the vertical mixing in Comau fjord, Chile

Abstract:

Comau fjord,located in southern Chile (42º22' S, 72º25' W), is a narrow and deep semi-enclosed basin of 68 km-long, 4 km-width and a maximum depth of 500 m. The fjord has a semi-diurnal tidal regime dominated by the M2 component, with maximum oscillations that reach 7,5 m. The stratification and mixing in the Comau fjord was examined in a field campaign during January 2014, using temperature and conductivity sensors and a down-looking ADCP. A very shallow salinity-controlled stratification was detected with a very strong and sharp density change at about 8 m-depth. Furthermore, a surface current of magnitude 5 cm·s^-1 due to river discharge was measured and found to interact with both the tide and the wind forcing. A description of the fjord's hydrodynamic was performed using a general 1D vertical k-ε model, in which the stratification was forced by exchanges of energy and momentum with the atmosphere. In addition, the river currents and the effects of tidal influence were incorporated through adding terms in the momentum equation that account for non-linear interaction between current and tide, and a periodic barotropic pressure gradient due to the tide. A decomposition in vertical modes were carried out to analyze the velocity data and the results of the model, which showed that the energy is concentrated in the first barotropic mode, in agreement with the fact that the aspect ratio of stratification h₁/(h₁+h₂) is close to 0. Field data and numerical simulations suggest that the energy injected to the system by external forcings is dissipated in the surface region as a consequence of the strong stratification, which acts as a buffer of the energy transfer, controlling the vertical mixing. The model implemented calculates the vertical dissipation rate of turbulent kinetic energy, allowing the estimation of turbulent diffusion parameter, thus connecting the system's hydrodynamics response with the vertical transport of water quality parameters of the system.