Shoreline recession rates and sedimentary budget in beaches under very high risk of erosion in Ubatuba County (São Paulo Northern Coast)

Abstract

About 22% of Ubatuba County (Northern Coast of São Paulo State) sandy beaches are under very high risk of erosion. The causes for this phenomenon include natural processes, such as sea-level rise and changes in the coastal dynamics, as well as several anthropogenic interventions along the shoreline and coastal plains. Shoreline recession in the last few decades is one among eleven indicators of coastal erosion that are monitored in the State of São Paulo beaches. This paper presents the historical shoreline recession rates in four beaches under very high risk of erosion located in Ubatuba County – Maranduba, Praia Grande, Barra Seca and Ubatumirim/Estaleiro beaches. Studies were carried out at aerial photographs from 1962, 1973 and 2001. For mapping the beach polygons the shoreline boundaries adopted were the following: superior limit – the permanent vegetation line on the coastal plain or manmade structures; and inferior limit – the high water line. End Point Rate and Average End Point methods were applied in order to calculate recession rates and sedimentary deficit in the time interval analyzed. As expected, the studied beaches presented erosional trends in the last four decades. The stronger retrogradation belongs to Praia Grande beach, which recession rate is about 1.9 m/y. It is followed by Barra Seca beach, with 0.78 m/y, Estaleiro beach, with 0.36 m/y, Ubatumirim beach, with 0.25 m/y, and Maranduba beach, with 0.17 m/y.