Morpho-sedimentological dynamics along the beach system of Ilha Comprida - SP

O objetivo deste trabalho é avaliar as variações espaço-temporais da dinâmica morfo-sedimentar do sistema praial de Ilha Comprida-SP. Assim, para reconhecer os padrões de onda encontrados na região, resultados do modelo global de ondas WAVEWATCH III foram utilizados. Da mesma forma, levantamentos de campo periódicos foram realizados em cinco diferentes setores com obtenção de dados morfológicos e sedimentares. Simulações com o modelo numérico MIKE21 SW foram realizadas para reconhecer as tendências de transporte e distribuição de força de onda. Os resultados mostraram uma tendência geral de transporte para NE, com maiores valores para a região sul em relação a norte, com dois setores principais de divergência residual: centro-sul e norte. A divergência centro-sul apresenta um aumento e uma migração para sul nos meses mais energéticos, enquanto que a divergência norte apresenta uma manutenção espacial ao longo das estações. A distribuição de força de onda mostrou-se mais elevada nos setores centro-sul e norte, coincidindo com os pontos de divergência. Dados granulométricos mostraram baixa variabilidade temporal, mas possuem diferenças espaciais que refletem a hidrodinâmica local. Os dados volumétricos apresentam maiores valores médios nos setores centro-norte e menores no centro-sul e norte. De modo geral, os resultados mostram que ao longo de uma linha de costa exposta a um mesmo padrão de ondas, sua variabilidade é um reflexo de alterações hidrodinâmicas locais. Tais conclusões são importantes para o conhecimento do estado atual das praias e de sua evolução.The aim of this study is to evaluate the spatial-time variations on the beach system of Ilha Comprida-SP. Therefore, the wave reanalysis database from the global wave generation model WAVEWATCH III was used to recognize the wave pattern for the region. Furthermore, periodic field works at five different sectors have been conducted to collect morphological and sedimentological data. The numerical model MIKE 21 SW has been applied to propagate waves onshore and recognize the transport tendency and the nearshore wave power distribution. Results show a transport trend to the NE, with the southern sector being larger than the northern sector in magnitude, with two main residual longshore drift divergence spots: in the central-southern and northern regions. Moreover, the central-southern divergence spot become larger and migrated to the south during the most energetic months, while the northern divergence spot kept its position throughout the year. Wave power results show two main areas with higher values that coincide with the observed longshore divergence spots. Sediment data presents low temporal variability, although spatial variations have been found reflecting the hydrodynamic conditions. The volumetric data shows largest values in the central-northern sector, being smaller in the central-southern and northern regions. In summary, the findings show that along this wide open stretch of coastline, exposed to the same offshore wave regime, its variability is a result of local hydrodynamic conditions. These results help in further understanding the island\'s long term evolution and current state of its beaches

On the Relation between Beach-Dune Dynamics and Shoal Attachment Processes: A Case Study in Terschelling (NL)

Inlet-driven processes are capable of modifying the adjacent shoreline. However, few studies have attempted to understand how these changes affect coastal dunes. The present study aims to understand how shoreline changes induced by shoal attachment affect coastal dunes. A barrier island in the Netherlands is used as a case study. Both bathymetric and topographic annual data were analysed, together with the application of a cellular automata model for dune development. The objective of the model is to explore idealised scenarios of inlet-driven shoreline movements. With the model, ten different scenarios were examined regarding beach width increase and rate of alongshore spreading of the shoal. Field data showed that, for the case study, dune volume and shoal attachments could not be directly linked. Instead, rates of dune volume change differed significantly only due to long-term ebb-tidal delta evolution. Such morphological evolution oriented the beach towards the main wind direction, increasing overall aeolian transport potential. Modelling results showed that shoals significantly increased dune volumes only on three out of ten scenarios. This suggests that beach width increase, and rate of alongshore sediment spreading, determine whether the shoal will influence dune growth. Therefore, within the studied time-scale, local rates of dune growth are only increased if shoals are capable of increasing the beach width significantly and persistently

Longshore transport gradients and erosion processes along the Ilha Comprida (Brazil) beach system

The aim of this study is to assess the longshore transport gradients and wave power distribution along the Ilha Comprida beach system and relate it to the distribution of the current erosion process along this barrier island. The study is based on quantitative analysis of the potential longshore drift and the wave power distribution, as well as on the morpho-sedimentary seasonal variations in the beach system. Therefore, the 30-year wave reanalysis database from the global wave generation model WAVEWATCH III (NOAA/NCEP) has been extracted and analyzed for the region, as well as field surveys with topographic measurements and sediment samples. The numerical model MIKE 21 SW has been applied to propagate waves onshore and recognize the longshore transport tendencies and the nearshore wave power distribution. Results show an overall transport trend to the NE, being larger in the southern sector than in the northern sector of the island. Varying transport magnitudes prove to generate gradients in longshore drift. Two positive gradients in the longshore drift, resulting in local sediment losses, are observed. One is found in the central-southern area and another in the northern part of the island. Both areas coincide with erosive spots, as observed through field surveys. The central-southern positive gradient becomes larger and migrates to the south during the most energetic months, while the northern gradient presents only variations in magnitude, being relatively stable in position throughout the year. Nearshore wave power results show two main areas with higher values that coincide with the positive longshore transport gradients. Sediment data presents low temporal variability, although spatial variations have been found reflecting the local hydrodynamic conditions, while the volumetric data shows largest values in the central-northern sector, being smaller in the central-southern and northern regions. Moreover, the central portions are more stable than the extreme portions regarding its seasonal variability. Our findings show that along this wide open stretch of coastline, exposed to the same offshore wave regime, an alternated nearshore wave regime results in areas with hydrodynamic conditions which lead to erosion or accretion. Erosion is caused by negative sediment balance as a function of higher wave power and positive gradients in longshore transport, and accretion due to lower wave power and negative gradients in longshore transport. Our findings help in further understanding the island’s long-term evolution and current state of its beaches