Dinâmica da desembocadura do corpo lagunar do Ararapira

Abstract

The Ararapira lagoonal mouth connects the ocean to Ararapira lagoon (Ararapira Sea), and has a SSW-NNE orientation, parallel to the coastline. The mouth has had a southwestward continuous lateral migration. The dominant direction of longshore currents is NE, related to the passage of cold fronts. Lasting absence of this meteorological effect causes temporary changes on wave features, leading to inversion of longshore currents. Considering this inversion, two circulation models of tidal currents and sediments are suggested acting alternately to induce migration to SW, opposite to longshore currents. Under the influence of NE longshore currents the acting processes on the erosive margin during ebb tide are similar to those on the concave margins of meandering rivers; sand is deposited on the longshore drift delta. However when longshore currents are inverted, the sands of the longshore drift delta are eroded through wave action and deposited, during flood tides, on the growing margin. This sand is here deposited because of drift blocking by ebb tide currents. The Ararapira lagoonal margins show critical erosion portions, mainly on the internal margins of the sandy barrier that separates coastal lagoon from ocean; this erosional process can lead to a new lagoonal mouth. This, together with progressive elongation of the coastal lagoon caused by lateral sandy barrier growth would lead to the opening of a new mouth to the NE of the present one, and to the filling of the portion of the coastal lagoon between the two mouths.