Generation of overtides and compound tides in Amazon estuary

Abstract

Tidal propagation in estuaries is affected by friction and fresh water discharge, besides changes in the depth and morphology of the channel. Main distortions imply variations in the mean water level and asymmetry. Tidal asymmetry can be important as a mechanism for sediment accumulation and turbidity maximum formation in estuaries, while mean water level changes can affect navigation depths. Data from several gauges stations from the Amazon estuary and the adjacent coast were analyzed and a 2DH hydrodynamic model was configured in a domain covering the continental shelf up to the last section of the river where the tidal signature is observed. Based on data, theoretical and numerical results, the various influences in the generation of estuarine harmonics are presented, including that of fresh water discharge. It is shown that the main overtide, M 4 , derived from the most important astronomic component in the Amazon estuary, M 2 , is responsible for the tidal wave asymmetry. This harmonic has its maximum amplitude at the mouth, where minimum depths are found, and then decreases while tide propagates inside the estuary. Also, the numerical results show that the discharge does not affect water level asymmetry; however, the Amazon river discharge plays an important role in the behavior of the horizontal tide. The main compound tide in Amazon estuary, Msf, generated from the combination of the M 2 and S 2 , can be strong enough to provoke neap low waters lower than spring ones. The results show this component increasing while going upstream in the estuary, reaching a maximum and then slightly decaying.

Tidal and sub-tidal sea level variability at the northern shelf of the Brazilian Northeast Region

“…The NBC flowing westward at the continental margin produces rings (Philander and Pacanowski 1986), and the passage of these rings could affect somewhat the shelf sea level. At the shore of the Amazon, the synodical constituent MSf is relatively important (Gallo and Vinzon 2005). The SubTF oscillations recorded in Fortaleza (~ 1,000 km eastward) may be associated with both effects, although with the energy spanned from the harmonic frequency due to non-linear interactions with the NBC and shelf morphology.…”

The M2 and M4 Tides in the Pará River Estuary

“…3). As it was found in the Amazon River estuary the importance of M4 (Gallo and Vinzon, 2005), this overtide was also considered a shallow-water component of greater tidal significance in the Pará river estuary.…”

“…In addition to high discharge, highlighted by several authors (e.g. Beardsley et al, 1995;Gabioux et al, 2005;Gallo and Vinzon, 2005;Geyer et al, 1996;Le Bars et al, 2010), tidal effect is the main forcing which acts on the Amazon continental shelf, with strong oscillations in semidiurnal, diurnal, and fortnightly tidal frequencies.…”

Estuaries — a biological point of view

“…the Amazon River, Volga River, Yangtze River (e.g. Lie, Cho 2002;Gallo, Vinzon 2005) Estuaries characteristically include a transition between freshwater and marine (saline) realms and, from an ecological point of view, should be treated not as an isolated entity but rather as a natural continuum from river to sea with no clearly defined boundaries. Another distinguishing attribute of estuaries is the elevated hydrodynamics that, combined with the tidal influence, makes these systems exceptionally variable in space and over time.…”