Geomorphologic evolution of a sand spit located in the mouth of a choked coastal lagoon. Lagoa dos Patos: Southern Brazil

Abstract

The estuarine portion of the Patos Lagoon behaves as a choked lagoon where circulation patterns are driven mostly by wind and freshwater runoff which can reach up to 10,000 m3/s during the rainfall season. Wind direction influences both local and large-scale circulation patterns. Under NE winds pressure gradient raises along the inlet and retreating coastal waters favors flushing of the lagoon water. Contrarily, a SE and SW wind causes inversion of flow raising the water level in the northern part of both the inlet and the main lagoon body. Although being located in a microtidal area with a mean annual daily range of 0.47m, storm surges induced by southern winds can raise the level up to 2 m. As a result of the strong bi-directional flow an ebb tide delta and sand spit developed in the inner portion of the inlet. A historical sequence of aerial photogrammetry combined with bathymetric charts, (82 years) allowed tracking the morphological evolution of the sand spit located at the extreme end of the inlet. During this period, the spit grew 427 m in the northwest direction at the same time that its north to south length was reduced 304.5 m. The origin, evolution and stabilization of these features along the east margin had been quite frequent even after the construction of two convergent jetties which stabilized the inlet mouth.