Abstract The aim of this study is to assess the morphological evolution of the Ararapira (Brazil) barrier-inlet system, at different time scales. Based on satellite imagery, elevation data, and in-situ observations, we quantify the morphological evolution of the region. Results show that the Ararapira inlet migrated continuously southwards, moving updrift, with erosion at its southern margin and intercalated erosion and accretion at the northern margin. At approximately 5.5 km north of the old inlet, the gradual narrowing of the sandy barrier due to channel meandering and coastal erosion resulted in its breaching, in August 2018. We document the initial stages of the new inlet, which after opening, presents intense erosion at its southern margin, resulting in the channel widening to ~1 km. After the barrier breaches, the system begins to adjust to a new equilibrium condition, with the widening of the new inlet being balanced by the gradual closure of the old inlet. These drastic environmental changes control the functioning of such systems, and our results provide important background information for their use and management.

O papel desempenhado pela atividade humana nas mudanças do litoral indica uma tendência geral de recuo da linha de costa, especialmente em ambientes deltaicos, como resultado da recente tendência de aumento do nível do mar, bem como do bloqueio do aporte de sedimentos em direção ao litoral, principalmente devido à construção de barragens. Isto é particularmente importante em ambientes deltaicos que sofreram uma perda dramática de área nas últimas décadas. Em contraste, no presente trabalho, relatamos a origem e evolução de um delta antropogênico, o Delta do Valo Grande, na costa sudeste do Brasil, cuja origem está relacionada com a abertura de um canal artificial e desvio do fluxo principal do rio Ribeira de Iguape. A metodologia incluiu a análise de mudanças de costa, levantamentos batimétricos e testemunhagem, esta última utilizada para determinar as taxas de sedimentação e mudanças na granulometria dos sedimentos ao longo do tempo. Os resultados permitiram reconhecer as diferentes facies do delta antropogênico e estabelecer suas tendências deposicionais, laterais e verticais. Apesar de não serem muito frequentes, deltas antropogênicos representam um ambiente favorável para o registro de mudanças naturais e antrópicas em tempos históricos e, portanto, merecem mais atenção dos pesquisadores de diferentes áreas do conhecimento.

The role played by human activity in coastline changes indicates a general tendency of retreating coasts, especially deltaic environments, as a result of the recent trend of sea level rise as well as the blockage of the transfer of sediments towards the coast, especially due to the construction of dams. This is particularly important in deltaic environments which have been suffering a dramatic loss of area in the last decades. In contrast, in this paper, we report the origin and evolution of an anthropogenic delta, the Valo Grande delta, on the south-eastern Brazilian coast, whose origin is related to the opening of an artificial channel and the diversion of the main flow of the Ribeira de Iguape River. The methodology included the analysis of coastline changes, bathymetry and coring, which were used to determine the sedimentation rates and grain-size changes over time. The results allowed us to recognize the different facies of the anthropogenic delta and establish its lateral and vertical depositional trends. Despite not being very frequent, anthropogenic deltas represent a favorable environment for the record of natural and anthropogenic changes in historical times and, thus, deserve more attention from researchers of different subjects.