Nonlinear tide-surge interactions in the Rio de la Plata Estuary

The nonlinear tide-surge interaction (NTSI) can significantly affect the levels associated with storm surges that threaten coastal areas. In this work, water level observations and numerical simulations are used to investigate those interactions in the large, mighty and socio-economically important Rio de la Plata (RdP) estuary. We introduce a novel objective approach to seek evidence of nonlinear interactions in hourly water level records from six tide gauges collected along the coast. Results indicate that NTSI occurs in the RdP and suggest that it becomes more important upstream and along the southern coast of the estuary. CROCO ocean numerical model is used to quantify the interactions, to determine the areas where they are stronger, and to identify their sources. The amplitude of the interaction accounts for 16% of the total water level at the upper RdP, being comparable to tidal amplitude. The quadratic bottom friction is the principal cause of NTSI, and is modulated by the tidal current; therefore the nonlinearity is present all the time and is independent of the wind speed and direction. This is a particularity of this estuary and occurs because, as a consequence of its large width, surge currents are weak in spite of the big volume of water mobilized by the storms. Due to its nature, the interaction attenuates and smoothes the level anomalies due to the surge. Results highlight the need of utilizing a complete NTSI model for the forecast of the surge in the RdP; otherwise, the estuarine dynamic would be misrepresented generating significant forecasts errors (10%-20% at the upper RdP).