Análise multitemporal da linha de costa e indicadores de erosão na praia da Ponta D’Areia, ilha do Maranhão: diagnóstico dos impactos de obras costeiras

Abstract

The coastal zone is defined as the geographical transition space between the ocean and the continent. In the state of Maranhão, it comprises five sectors, including the Maranhão Golfão. Ponta D’Areia beach is located northwest of the Maranhão Island, making up this sector where tides can reach up to 7.2 meters in height. The beach is approximately 2.5 km long, bounded by the Anil River and São Marcos Beach. In 2014, the construction of a coastal groin was completed at Ponta D’Areia beach, aimed at containing erosion and preventing sedimentation towards the Anil River. In this context, the research addressed the following questions: (a) How did the shoreline change over 27 years? (b) What are the erosional, depositional, and stable sectors from a morphodynamic perspective, considering the period before and after the construction of the coastal groin at the beach? It was believed that the rigid engineering structure was intended to stop the sedimentation at the mouth of the Anil river, which did not occur. As a result, it was necessary to extend the groin to an "L" shape to attempt to curb sedimentation, but this led to continued erosional activity at the northeastern end of the beach. Therefore, the objective of this research was to conduct a multitemporal analysis of the Ponta D’Areia shoreline from 1996 to 2022 and assess its current vulnerability to erosion. The methodology included: (1) on-site observational analysis to fill out pre-defined tables related to coastal erosion geoindicators and collection of surface beach sediments in November 2022 and April 2023, during the dry and rainy seasons, respectively; (2) beach topography survey and orthophoto acquisition through drone flights in April 2023; (3) multitemporal shoreline analysis from 1996 to 2022 using Landsat satellite images, ArcGIS software, and the Digital Shoreline Analysis System (DSAS) extension, as well as shoreline projections for 10 and 20 years into the future; (4) application of the Coastal Erosion Vulnerability Index (CEVI) in three coastal sectors, through the evaluation of natural and anthropogenic parameters; and (5) assessment of the impacts of coastal engineering works on Ponta D’Areia beach. Results showed variations in the shoreline from -64.63 meters (-3.46 m/year: erosion) to 32.15 meters (2.39 m/year: acretion) from 1996 to 2022, with a projected advance of 157.76 meters (4.94 m/year) and retreat of -123.26 meters (-3.68 m/year) by 2032, and a projected advance of 101.93 meters (1.48 m/year) and retreat of -141.35 meters (-1.63 m/year) by 2042. The morphodynamic state of the beach was identified as dissipative through topographic mapping with a drone, and moderate vulnerability to coastal erosion in Sector I, the marina sector, which had the lowest CEVI of 4. Sector II, the groin sector, had CEVI of 6.37 (moderate vulnerability), while Sector III, the Lighthouse sector, had a CEVI of 6.8, indicating high vulnerability to coastal erosion. It was possible to observe how meteo-oceanographic processes (waves, littoral drift, tidal currents, winds, and estuarine discharge) are influencing shoreline variation, as well as human interventions (coastal occupation and construction of rigid structures). Coastal accretion intensified at the beach after the anthropogenic intervention with the groin construction. It is concluded that the multitemporal analysis of the shoreline in the study area between 1996 and 2022 revealed significant variations influenced by natural and anthropogenic factors. Even with human interventions to alter the resulting sedimentation caused by meteo-oceanographic agents, these natural processes continue to intensely shape the region's coastal dynamics and are mainly responsible for variations in the shoreline.