Conferência sobre o Sistema de Distribuição de Sedimentos do rio São Francisco

Na próxima sexta-feira (28 de maio de 2021) o prof. José Maria Landim Dominguez, coordenador do GT1.1 do inctAmbTropic (Deltas e erosão da linha de costa) proferirá uma palestra no webinário “2021 World River and Delta Systems – Source-to-Sink Webinar Series” organizado pelo professor Paul Liu, da Universidade Estadual da Carolina do Norte, Estado Unidos.

A palestra apresentará uma síntese dos principais resultados alcançados para o delta do São Francisco, no âmbito do inctAmbTropic.

A palestra poderá ser acompanhada no canal do YouTube: tinyurl.com/s2stalks , com inicio previsto para as 10:00hrs (horário de Brasília), dia 28/05/2021.

Mudanças Climáticas e a Evolução Holocênica do Delta do rio São Francisco

Um trabalho recentemente publicado na revista Marine Geology (IF: 3.04) pelo GT1.1 (Deltas e Erosão da Linha de Costa) intitulado “Effects of Holocene climate changes and anthropogenic river regulation in the development of a wave-dominated delta: The São Francisco River (eastern Brazil)” de autoria de José Maria Landim Dominguez e Junia Kacenelenbogen Guimarães, discute os efeitos das mudanças climáticas durante o Holoceno e da regulação antropogênica do rio São Francisco no desenvolvimento do seu delta e conclui que a construção do delta ocorreu em pulsos associados a períodos mais úmidos na bacia hidrográfica. O artigo também discute as causas da erosão severa que afeta a desembocadura fluvial nas última décadas.

Modelo evolutivo do delta do São Francisco, durante o Holoceno

Erosão da linha de costa durante o periodo 1960-2020 no delta do São FranciscoABSTRACT

The São Francisco River is the fourth longest river in South America and one of the most regulated. Severe coastalerosion has affected the delta shoreline since 1985, leading to the complete destruction of Cabeço village be- tween 1997 and 1999. In this study, we mapped and radiocarbon dated the beach ridge sets occurring on the delta plain and performed a detailed analysis of the delta shoreline changes since 1960. During the Holocene, the delta plain construction was punctuated and took place during episodes of higher river discharges coincident with Bond events 4, 2 and 1 and periods of higher precipitation in the river basin, as reconstructed by δ18O measurements in cave speleothems. The last major episode of delta construction apparently ended at approxi- mately 1.0 ka cal. BP. Since that time, riverine sediment input has been just sufficient to maintain the shoreline. A comparison of historical maps and aerial photographs showed that from 1853 to 1960, the shoreline at the river mouth remained in approximately the same position. A decrease in rainfall in combination with river regulation, particularly after 1985, triggered extensive erosion at the delta shoreline. This erosion was not caused by sediment retention behind the major dams but instead resulted from changes in the backwater/drawdown effects deriving from river regulation. Shoreline erosion mostly affected the river mouth. The mobilized sediments caused progradation of the downdrift shoreline. Updrift of the river mouth, the shoreline remained stable, as it had already reached an equilibrium orientation in which the net longshore transport was zero

O trabalho completo pode ser acessado em: https://doi.org/10.1016/j.margeo.2021.106456

Erosão Costeira no delta do Parnaíba

Saiu publicado no periódico Ocean and Costal Management (IF=2.482) um novo trabalho no âmbito do inctAmbTropic Fase II intitulado “Evaluation of decadal shoreline changes along the Parnaíba Delta (NE Brazil) using satellite images and statistical methods” de autoria de Thiago Augusto Bezerra Ferreira , André Giskard Aquino da Silva, Yoe Alain Reyes Perez, Karl Stattegger e Helenice Vital, que investiga o comportamento da linha de costa do delta do Parnaíba no período 1984-2017. O trabalho concluiu que a redução na descarga fluvial do rio Parnaíba entre 1995-1999 e 2009-2012 foi a causa principal do recuo erosivo experimentado pelo delta.

Comportamento da linha de costa do delta do Parnaíba entre 1984-2017. Vermelho: erosão. Verde: progradação.

ABSTRACT

This study investigated changes of the shoreline position in the Parnaíba River Delta (PRD), which is in an underdeveloped and sparsely populated area. The general behavior of the shoreline between 1984 and 2017 was analyzed using 12 Landsat satellite images and the Digital Shoreline Analysis System (DSAS) software. DSAS applied statistical methods, such as Weighted Linear Regression (WLR) and End Point Rate (EPR), to define whether the shoreline was retreating or accreting. Data analysis revealed that, on a decadal time scale (33 years), 52% of the PRD shoreline retreated under erosional conditions, while 48% accreted. Accretion mainly occurred at the vicinities of the river mouth, especially eastward where the highest progradation rate was observed (maximum of 29 m/year). Shoreline retreat, up to 9 m/year, occurred predominantly in the western part of the PRD and away from the river mouth. However, changes of the general behavior of the shoreline (retreat/accretion) were observed at an inter- decadal time scale, which were positively related to variations in river discharge. Variations of river discharge are directly dependent on the rainfall amount along the drainage basin that, in this region, is controlled by climatic conditions such as El Niño and La Niña. Therefore, the reduced river discharge observed between 1995- 1999 and 2009–2012 (El Niño periods), was the major cause the triggered the observed coastal retreat (>6 m/ year) on PRD shoreline during these periods. Consequently, the variability of natural factors, such as river discharge and rainfall, are the main drivers of shoreline changes on PRD, since anthropogenic impacts have hitherto only marginal effects on the delta region and on the drainage basin of the Parnaíba river.

O trabalho completo pode ser acessado em: https://doi.org/10.1016/j.ocecoaman.2020.105513

Cânion do São Francisco: nova publicação do inctAmbTropicII

Saiu publicado na revista Geomorphology (IF = 3.819) um novo trabalho no âmbito do inctAmbTropic Fase II intitulado “Continuous canyon-river connection on a passive margin: The case of São Francisco Canyon (eastern Brazil)” de autoria de Rafael Fonseca Ribeiro, José Maria Landim Dominguez, Alana Aderne Santos, e Adriane Gonçalves de Araújo Nunes Rangel. Este trabalho investiga a origem do cânion do São Francisco à partir da integração de levantamentos com batimetria multifeixe, sísmica 3D e trabalhos anteriores publicados por pesquisadores do inctAmbtropic, além de aprofundar o conhecimento sobre a região do delta do São Francisco. Também apresenta pela primeira vez um mapeamento detalhado do talude superior na região.

Localização do cânion do São Francisco e sua relação com o delta homônimo.

Principais feições mapeadas no cânion do São Francisco e regiões vizinhas do talude superior.

ABSTRACT

Submarine canyons play a major role in sediment transfer to the deep sea. When connected with fluvial systems, they can produce large submarine fans, whose sandy bodies have a high potential as petroleum reservoirs. Although canyon-river connections are common on passive margins during lowstands, they are rare during highstands. So far, the only canyon in the Western South Atlantic to have exhibited continuous river connection throughout the Quaternary, regardless of sea level position, is the São Francisco Canyon (SFC) in eastern Brazil. In this study, we used multibeam and 3D seismic records integrated with previously published data to investigate the reasons for the continuous canyon-river connection in the area of the São Francisco River. The SFC comprises two compartments: The Upper and the Mid Canyon. The Upper Canyon indents 15 km of the shelf and acts as an erosive feeder trunk to the São Francisco Submarine Fan. When it emerges on the continental slope (Mid Can- yon), depositional processes dominate; the canyon now begins to meander and the thalweg is bordered by nu- merous terraces and levees. Five stratigraphic units have accumulated in the region of the canyon head since the beginning of the Oligocene. Initially, sedimentation had a uniform aggradational character (Unit 1), which later became more discontinuous in association with the development of reef build-ups (Unit 2) and of a low-lying area around the canyon head. This depression was partially infilled by siliciclastics (Unit 3) and later incised by a precursor of the SFC (Unit 4). The depression still has a morphological expression in the shelf nowadays as a bathymetric low (BL), within which the São Francisco River has built its delta (Unit 5). We hypothesize that the origin and evolution of the SFC is associated with three main factors: (i) breaching by the canyon head of thick carbonates rimming the shelf break; (ii) development of the shelf depression referred above; and (iii) major continental drainage reorganization. Continuous canyon-river connection was ensured by this shelf de- pression, which helped funnel siliciclastic sediments to the canyon and precluded lateral changes to the lower river trunk. Morphological features, such as axial incision of mass movement deposits is suggestive of some, although very limited, modern canyon activity. This case study contributes to a better understanding of river- basin sediment pathways, particularly on passive margins, which are of great importance for the successful ex- ploration of submarine fan systems.

O trabalho completo pode ser acessado em: https://doi.org/10.1016/j.geomorph.2020.107549

Participação do inctAmbTropicII no PGGM

Saiu agora em novembro o número especial da Geo-Marine Letters, comemorativo dos 50 anos do PGGM (Programa de Geologia e Geofísica Marinha). Este número especial reune 23 trabalhos de pesquisadores brasileiros principalmente na área da geologia e geofísica marinha. Oito destes trabalhos são contribuições dos GTs 1.1 e 2.1 do inctAmbTropic. Os trabalhos e respectivos links encontram-se listados abaixo:

Shallow sedimentation of Natal shelf and coastal erosion implications, NE Brazil

https://link.springer.com/article/10.1007/s00367-019-00594-y

Structural controls on the morphology of an extremely narrow, low-accommodation, passive margin shelf (Eastern Brazil)

https://link.springer.com/article/10.1007/s00367-019-00605-y

Organic matter in the deltaic clinoform of the São Francisco River (Eastern Brazil)

https://link.springer.com/article/10.1007/s00367-019-00607-w

Quantifying the high coastal dynamics of tropical mesotidal barrier island-spit systems: case study in Northeast Brazil

https://link.springer.com/article/10.1007/s00367-019-00610-1

Geodiversity as an indicator to benthic habitat distribution: an integrative approach in a tropical continental shelf

https://link.springer.com/article/10.1007/s00367-019-00614-x

Antecedent topography controls preservation of latest Pleistocene-Holocene transgression record and clinoform development: the case of the São Francisco delta (eastern Brazil)

https://link.springer.com/article/10.1007/s00367-019-00609-8

The Santo Antônio Bank: a high-resolution seismic study of a deflected ebb-tidal delta located at the entrance of a large tropical bay, eastern Brazil

https://link.springer.com/article/10.1007/s00367-019-00624-9

Statistical approach on mixed carbonate-siliciclastic sediments of the NE Brazilian outer shelf

https://link.springer.com/article/10.1007/s00367-019-00625-8

Panorama da erosão costeira

A linha de costa é uma região extremamente instável, sujeita a grandes mudanças em função de variações no padrão de ondas, no balanço de sedimentos, no nível do mar, ou como consequência de obras costeiras. Apesar dessas mudanças ocorrerem em diferentes escalas de tempo e espaço, estratégias simples de gestão para a ocupação da linha de costa, baseadas em estudos de longo prazo, podem minimizar prejuízos econômicos e ambientais.

É o que sugere o estudo Panorama da Erosão Costeira realizado sob os auspícios do Ministério do Meio Ambiente. Um grupo de pesquisadores do Instituto de Geociências da UFBA, alguns dos quais integrantes do inctAmbTropic, foram responsáveis pelo estudos dos litorais dos estados de Alagoas, Sergipe e Bahia. Segundo esse estudo, embora a tipologia de comportamento da linha de costa dominante nos três estados seja a de aparente “equilíbrio”, 26% do litoral alagoano (75 km), 50% do litoral sergipano (90 km) e quase 20% do litoral baiano (200 km) foram considerados como de elevada variabilidade e portanto instáveis, oferecendo assim um alto risco para ocupação humana – essas regiões estão principalmente associadas a desembocaduras de rios e canais de maré.

A recomendação é clara, nos trechos em que a linha de costa apresenta uma elevada variabilidade, a ocupação deve ser evitada, ou até mesmo proibida. Estas áreas são as mais instáveis e, na maior parte das vezes, é muito difícil prever o seu comportamento a longo prazo. Áreas como estas que ainda não estão ocupadas, poderiam ser destinadas à criação de unidades de conservação.

Nos trechos em aparente “equilíbrio”, onde o comportamento da linha de costa é mais previsível, o estabelecimento de faixas de recuo “non aedificandi” minimizaria em grande parte os problemas futuros de erosão. Embora essas áreas sejam mais estáveis, ainda assim, estão sujeitas a variações na posição da linha de costa decorrentes, por exemplo, de eventos extremos associados a tempestades.

Quer conferir a situação da sua praia? Acesse o trabalho completo no link.