Historial shoreline changes near lagoonal and river stabilized inlets in Rio Grande do Sul state, southern Brazil

Abstract

Aerial photography time series of six ocean beaches (Cassino, Mar Grosso, Tramandaí, Imbé, Praia Grande and Passo de Torres beaches) adjacent to lagoonal and river inlets of southern Brazil were analyzed in order to verify the influence of jetties on its shoreline evolution and configuration. Digital processing and data standardization were performed in order to eliminate distortions. In a GIS database the changeable positions of the coastline were vectorized and its differences calculated. Two beach features: the High Water Line (HWL) and the Base of the Foredunes were used in order to represent the shoreline. Cassino beach displayed the higher accretion rates (4.10m/y) showing highly distinct accretion rates for two different time frames. For Mar Grosso beach, shoreline recession rate was -1.44m/year starting in a sector distant 3km from the inlet. The beaches of Tramandaí and Imbé displayed a homogeneous recession rate along the coastline, due to human impacts associated to urbanization and sand exploitation. For Praia Grande beach an intense accretion rate followed by a stabilization period was observed. At the same time Passo de Torres beach displayed an accretion rate adjacent to the jetty which was followed by a stabilization period. At a greater distance of the inlet coastline recession was noticed. Highly accretion versus low erosion rates respectively to the south and north of the two main inlets suggest a bi-directional littoral sediment transport process. The coastline suffered a reorientation process due to the bathymetric changes of the adjacent shoreface, reduction of beach exposure and interference on the littoral drift impinged by the jetties construction. The anthropogenic influence on these beaches and on its morphological evolution process emphasizes the urgent needs of setback lines, which represents a logical and rational urban development.