Nonlinear interaction between the tide and river flow in the Maule estuary, Chile

The hydrodynamics features of an estuary are the resulting of two main driving forces, the tide and the river flow, interacting one each other and with the bathymetry. This periodic modulation may interact with the bathymetry such that exciting new tide harmonics, as a result of the nonlinear effect of bottom friction and advection terms. This nonlinear coupling between the tide and the estuary change when the river flow increases, which explains early theoretical studies that showed that when the river flow increases, there is a shift between M4 and M6 tide harmonic in the estuary. The objective of this study is to analyze field observation where this phenomenon is observed, for which numerical simulations conducted with FVCOM are used for fully understanding this nonlinear interactions between river inflow and tide harmonic excited in an estuary. Field measurements were conducted in June of 2011 in Maule estuary located in central Chile (°35.3S,°72.4W). During this period, the water surface and current were measured along the Thalweg. Also, river flow was continuously recorded at 29km upstream the estuary mouth, and these observations shows a river flow change that rapidly increased in a factor of two, changing the hydrodynamics conditions. Field observations were analyzing based on a least squares analysis, Fourier transforms and continuous wavelet transforms methodologies for processing measured time series. It is obtained that the dominant modes in the estuary are the M4 and M6 tide harmonic. Furthermore, the FVCOM numerical model was implemented to the estuary to study the response of the semi-enclosed system forced by a synthetic time series of the monochromatic M2 tide constituent, to see the excitation of M4 and M6 tide constituents. The results at the mouth of the estuary show that the magnitude of the current induced by the M4 and M6 tide constituents are higher at the bottom after the flood event by a factor of 1.5 and 2.3 respectively, with respect to conditions before the flood. In the contrary, M4 and M6 in the surface decreased because of the flood in a factor of 0.46 and 0.92. Finally, numerical simulation where the estuary was forced with a monochromatic M2 shows similar tendency with the anterior analysis. These results are coherent with the theoretical studies showed that bottom friction shift the magnitude of these harmonics; however, our observations in the surface indicates that the interaction between river flow and the tide depends on the vertical location. 1D numerical models or simple Fourier decomposition of the harmonics in the estuary are not enough to understand the real interaction of the river inflow with the induced current tides. Consequently, vertical variations in the flow velocity are required to be included.