Estimación del caudal a partir de la evolución del nivel del agua en un estuario con onda de marea estacionaria

A method to calculate the freshwater discharge in an estuary and the net exchange flow with the sea measuring the evolution of the water level in fixed monitoring stations is presented. It rises from the need to obtain reliable data of discharges under the particular morphological conditions that determine the hydrodynamic behaviour of the coastal plain Quequén Grande River estuary (Buenos Aires, Argentina), in which the tide behaves as a standing wave. The technique is calibrated with direct measurements of discharges performed with an acoustic Doppler current profiler in field studies during periods of poor and abundant rains. It is found that the exchange of water with the sea is proportional to the time derivative of the height of the free surface, and the fluvial discharge can be estimated from the height difference between monitoring stations. The results make it possible to quantify the river contribution during dry and wet seasons, and the flows generated by occasional and unusual tides. The method proposed is simple, does not require complex instruments to continuously measure velocity, and takes advantage of existing monitoring nets to provide the information of the estuarine flows in real time and of historic files to evaluate the changes produced in the past.