Residual Exchange Flows in Subtropical Estuaries |
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| Authors: | Arnoldo Valle-Levinson Guillermo Gutierrez de Velasco Armando Trasviña Alejandro J Souza Reginaldo Durazo Ashish J Mehta |
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| Institution: | 1.Civil and Coastal Engineering Department,University of Florida,Gainesville,USA;2.Centro de Investigación Científica y Educación Superior de Ensenada,La Paz,Mexico;3.Proudman Oceanographic Laboratory,Liverpool,UK;4.Universidad Autonoma de Baja California,Ensenada,Mexico |
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| Abstract: | Observations of residual exchange flows at the entrance to four subtropical estuaries, two of them semiarid, indicate that
these flows are mainly tidally driven, as they compare favorably with theoretical patterns of tidal residual flows. In every
estuary examined, the tidal behavior was that of a standing or near-standing wave, i.e., tidal elevation and tidal currents
were nearly in quadrature. The pattern of exchange flow that persisted at every estuary exhibited inflow in the channel and
outflow over the shoals. Curiously, but also fortuitously, this pattern coincides with the exchange pattern driven by density
gradients in other estuaries. The tidal stresses and the residual elevation slopes should be the dominant mechanisms that
drive such tidal residual pattern because the Stokes transport mechanism is negligible for standing or near-standing waves.
Time series measurements from the semiarid estuaries showed fortnightly modulation of the residual flow by tidal forcing in
such a way that the strongest net exchange flows developed with the largest tidal distortions, i.e., during spring tides.
This modulation is opposite to the modulation that typically results in temperate estuaries, where the strongest net exchange
flows tend to develop during neap tides. The fortnightly modulation on tidal residual currents could be inferred from previous
theoretical results because residual currents arise from tidal distortions but is made explicit in this study. The findings
advanced herein should allow the drawing of generalities about exchange flow patterns in subtropical estuaries where residual
flows are mainly driven by tides. |
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