Assessing the Influence of the River Discharge on the Minho Estuary Tidal Regime

Tides are an important factor in estuary circulation patterns, contributing for the complexity of these environments. In estuaries, the hydrodynamic components of tides gain importance compared with the constituents of pure astronomical origin that mostly characterize ocean tides. The main purpose of this work was to characterize the tide in the river Minho estuary and compare it with the tide in the nearby coast. Water height measurements were obtained from three pressure gauges deployed along the river in Caminha, Vila Nova de Cerveira and Segadaes. Minho water levels were compared with those from Viana do Castelo in the Atlantic coast. Besides other results, it was observed that the tide is asymmetric, flood dominant and strongly influenced by bathymetric constrains and river flow. The harmonic analysis method was applied to the data and the resulting harmonic constants were analysed. The amplitude of the major lunar semi-diurnal constituent decreases upstream but its first harmonic increases along with compound constituents. Due to the strong influence of river discharge, harmonic predictions did not agree with the tide observations in periods of extreme river flow. In consequence, a prediction equation was derived based on harmonic and wavelet analysis of the hydrometric level in a station upstream of Segaddes and maregraphic observations in the three points of observation. The results improved significantly.