MORPHODYNAMIC CHANGES OF THE “SACO DO LARANJAL” BEACHES ON THE NORTHWESTERN COAST OF THE PATOS LAGOON ESTUARY, RS

Abstract

Analysis of the morphological characteristics of the “Saco do Laranjal”, an embayment located at the northwest portion of the Patos Lagoon estuary allowed to evaluate recent and actual changes. The embayment is subdivided in four beaches: Laranjal, Barro Duro, Totó and Colônia de Pescadores Z3. Coastline changes analysis was based on aerial photographs obtained in 1980 and 1995. Beach profiles surveys and grain size analysis both at the subaerial and subaqueous portions of the beach were performed at monthly intervals. Hydrodynamic characteristics related to wave height and period were obtained through modeling and visual observations. Shoreline changes demonstrate that between 1980 and 1995, erosion rates for Barro Duro beach reached 1.04 m/year. Colônia Z3 beach displays some erosion spots and Totó beach presents both erosive and accretion stretches. In contrast, Laranjal beach displays accretion and stability. The studies allowed inferring that the main erosion causes can be attributed to short period waves generated by local winds, the associated littoral drift and the frequent increase of the water level on the margin during periods of floods associated with winds from the southern quadrant. The evaluation of the morphometric parameters obtained from beach profiles showed high, medium and low mobility index respectively for Barro Duro, Totó and Colonia Z3 beaches and Laranjal beach. Based on the morphological evolution, the sedimentology and hydrodynamics characteristics, actions of erosion mitigations which combines beach nourishment and light coastal structures is proposed.