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TAXAS DE TRANSPORTE SELETIVO PARA DIFERENTES TAMANHOS DE PARTÍCULAS AO LONGO DE UMA PRAIA: REPERCUSSÃO NO REGISTRO SEDIMENTAR

1991; Sociedade Brasileira de Geologia; Volume: 21; Issue: 2 Linguagem: Inglês

10.25249/0375-7536.1991121126

2177-4382

A.C.S.P. Bittencourt, G.S. Vilas Boas, ANTÓNIO E. G. AZEVEDO, FÉLIX F. FARIAS,

Geography and Environmental Studies

REPERCUSSION ON THE SEDIMENTARY RECORD.Monitoring of two beaches profiles along the Caixa-Pregos spit (Todos os Santos Bay, Bahia) extending in time from June/1987 to August/1988 has shown that coarse, medium and very fine sands move faster than fine sand under the action of the longshore drift.This selective longshore transport is related to differences in hyclrodynamic behavior of the various sand size fractions.Coarse and medium sand move continuously as bedload on the beach face, whereas very fine sand goes in suspension.Fine sand lags behind probably because it is transported in saltation.Because the construction phase at the Caixa-Pregos split temporally coincides with an increase in the longshore component of the wave energy flux, thus enhancing the selective grain size transport, such phase if preserved in the stratigraphic record, should be characterized by a relative increase in the beach-face deposits, of the coarse, medium and very fine sand fractions in comparision to the fine sand fraction.These grain size trends however, have not been observed at the Caixa-Pregos spit deposits, because the erosion of these sediments during the subsequent destructional phase of the beach cycle.The preservation in the stratigraphic record of the grain size trends reported herein requires a positive balance between constructional and destructional phases of beach cycle, thus resulting in progradation of the shoreline.In addition it is also required that, during the constructive phase there is a considerable reduction in the frequency of small erosional events, that affects the beach-face.Such scenario will be feasible in this low latitude sand spit if for example, the periodic advances in the Polar Atlantic Front, which cause true erosional S and SSE winds, are reduced by some atmospheric barrier like the El Niño phenomenon.The effectiveness of such phenomenon in controlling longhshore sediment transport has already been demonstrated for the Armação Beach (Bahia).