Controls in the development of transgressive dune fields of the Holocene coastal barrier, northern coast of Rio Grande do Sul

No litoral norte do Rio Grande do Sul, ao norte do balneário Xangri-lá, a barreira holocênica encontra-se parcialmente coberta pela fase moderna e relativamente ativa de um campo de dunas transgressivo, enquanto ao sul deste balneário, o campo de dunas cobre completamente a barreira. Este trabalho contribuiu para o conhecimento acerca desse comportamento diferenciado de desenvolvimento da fase moderna do campo de dunas transgressivo sobre a barreira holocênica. Foram utilizadas séries temporais de vento (rumo e velocidade) e precipitação de estações meteorológicas localizadas ao sul e ao norte de Xangri-lá, bem como foram realizadas medições simultâneas de vento, por meio de anemômetros portáteis, imediatamente ao norte e ao sul desse balneário. Potenciais de transporte de areia foram calculados para ambas as estações meteorológicas. Os registros históricos da estação de Tramandaí (mais ao sul) indicaram maiores velocidades de vento e menores de precipitação em relação à estação de Torres (mais ao norte). Devido à maior velocidade dos ventos em Tramandaí, o potencial de transporte de areia nessa localidade foi superior ao de Torres, indicando favorecimento da migração das dunas em suas proximidades. O comportamento desigual de desenvolvimento da fase moderna do campo de dunas transgressivo pode então estar relacionado à maior velocidade e frequência dos ventos NNE, NE, ENE e E em Tramandaí (ou seja, ao sul de Xangri-lá), além da menor precipitação, que também favorece um transporte de areia mais eficiente.In the north coast of Rio Grande do Sul, to the north of Xangri-lá beach, the Holocene barrier is partially covered by the modern and relatively active phase of a transgressive dune field, while to the south of this balneario the dune field completely covers the barrier. This work contributed to the knowledge about this differentiated development behavior of the modern phase of the transgressive dune field on the Holocene barrier. Time series of wind (direction and speed) and precipitation from meteorological stations located to the south and north of Xangri-lá were used, as well as simultaneous wind measurements using portable anemometers, immediately to the north and to the south of this balneario. Sand transport potentials were calculated for both meteorological stations. Historical records of the Tramandaí station (further south) indicated higher wind speeds and lower precipitation relative to the Torres station (further north). Due to the greater speed of the winds in Tramandaí, the potential of sand transport in this locality was higher than Torres, indicating the favor of the dunes migration in its proximities. The uneven developmental behavior of the modern phase of the transgressive dune field can then be related to the higher speed and frequency of the NNE, NE, ENE and E winds in Tramandaí (ie, south of Xangri-lá), as well as the lower precipitation also favors more efficient sand transportation

Variation in the granulometric properties of the coastal barrier of Pinheira (SC) during its progradation in the Late Holocene

The coastal barrier of Pinheira is located in the South-Central coast of Santa Catarina State. Its morphology and stratigraphy are typical of a regressive barrier. During the Late Holocene (last 6-5 cal ka) the barrier has prograded approximately 5,500 meters. Its progradation was determined by both a sea-level fall of 2 meters and an expressive transference of sands from the adjacent continental shelf into the beach system of the barrier. In the last 3,000 meters of progradation, corresponding to the last 3 cal ka, approximately 60 foredune ridges were formed, each ridge corresponding in average to a time interval of 50 years. The study of the granulometric properties of the modern beach-dune system, and of two old foredune ridges called intermediate and inner ridges, respectively formed at 750 and 3,000 cal years BP, has demonstrated that in the last 3 ka cal these properties have not changed significantly. This behavior is being explained by the policyclic nature of the sands consumed in the progradation, and/or by a constant or even a little change on the general dynamic conditions of the beach-dune system of the Pinheira embayment.The comparative study among the sub-environments (beach face, berm and foredune) has shown that standard deviation and skewness are potentially important sediment properties on the distinction between aeolian (foredunes) and beach (beach face) deposits.A barreira costeira da Pinheira está localizada no litoral centro-sul do Estado de Santa Catarina. Sua morfologia e estratigrafia são típicas de uma barreira regressiva. Durante o Holoceno Superior (últimos 6-5 ka cal), a barreira progradou cerca de 5.500 m. Sua progradação foi determinada por um abaixamento de aproximadamente 2 m do nível do mar e por um expressivo aporte de sedimentos arenosos em seu sistema praial, provenientes da plataforma continental adjacente. Nos últimos 3.000 m de progradação, ocorridos nos últimos 3 ka cal, cerca de 60 cordões de dunas frontais foram formados. O intervalo de tempo decorrido entre a formação de dois cordões sucessivos foi de aproximadamente 50 anos. O estudo das propriedades granulométricas do sistema praia-duna atual e de dois cordões antigos de dunas frontais, denominados cordões intermediário e interno, formados, respectivamente, há cerca de 750 e 3.000 anos cal AP, revelou que, nos últimos 3 ka cal, não ocorreram variações significativas destas propriedades.  Este comportamento é atribuído à natureza policíclica do estoque de areia consumido na progradação e/ou a uma relativa constância, nos últimos 3 ka cal, das condições dinâmicas gerais do sistema praia-duna da enseada da Pinheira.O estudo comparativo entre os diferentes subambientes (face da praia, berma e duna frontal) mostrou que o desvio-padrão e a assimetria da distribuição granulométrica são potencialmente importantes na distinção entre depósitos eólicos (dunas frontais) e praiais (face da praia) da barreira

A critical evaluation of coastal erosion in Rio Grande do Sul, Southern Brazil

Evidences of coastal erosion in Rio Grande do Sul have been obtained by three methods: (a) analysis of the long-term morphodynamics and stratigraphy of coastal barriers, (b) annual shoreline mapping using the Differential Global Positioning System (DGPS), and (c) local beach profile measurements. The first method reflects coastal erosion as continuity of the geological evolution in the last 5 ka, taking place mainly along the southern half of gentle coastal projections. The second method represents a shorter temporal scale and indicates that approximately 80% of the coast is eroding. Beach profiling has been measured in very few places that are distant from each other since the early 1990s; consequently, their results reflect local and very short time shoreline behavior. A critical evaluation of published data addressing coastal erosion in Rio Grande do Sul strongly suggests that short and long term negative balance on the sediment budget is the main cause of erosion along this coastline

Seasonal and interannual influences on the patterns of shoreline changes in Rio Grande do Sul, southern Brazil

Changes in shoreline positions along the Rio Grande do Sul coast, southern Brazil, are analyzed to determine the spatial and temporal variability of shoreline movements at a regional scale. Using a kinematic Differential Global Positioning System (DGPS) survey method, the 618-km long shoreline of Rio Grande do Sul has been mapped on five occasions between November 1997 and April 2002. These data show that the coastline responds differently to hydrodynamic and meteorological forcing along the three major coastal sectors. The observed differences occur especially in the magnitude of changes, in the time interval in which the shoreline returns to a previous position, and in the length scale of dominant shoreline changes. Here changes in shoreline position are examined with respect to grain size, shoreline orientation, storms, El Nino-Southern Oscillation (ENSO) events, and gradients of longshore sediment transport. Data analyses and modeling indicate that the alongshore variability in the patterns of shoreline change through time is closely related to shoreline orientation and the associated gradients of longshore sediment transport. Results indicate also that variability in wave energy and storminess influence seasonal changes, whereas ENSO events influence interannual changes. This study demonstrates the importance of understanding the drivers of shoreline change at a regional scale and has applications in studies concerned with coastal engineering and shoreline response to climate change

Parâmetros morfodinâmicos e deriva litorânea da praia de Tramandaí - RS

Parâmetros morfodinâmicos e deriva litorânea da praia de Tramandaí - R