Resumo

ISLA, Federico I  e  BERTOLA, Germán R. Morphodynamics of meso-microtidal beaches between Bahía Blanca and Río Negro. Rev. Asoc. Argent. Sedimentol. [online]. 2003, vol.10, n.1, pp.65-74. ISSN 1853-6360.

Beaches from southern Buenos Aires are dominantly mesotidal. Tidal range increases toward the interior of the Rincón de Bahía Blanca and conditions coastal landforms. Barriers and barrier islands dominate toward the outer bay; tidal flats and marshes toward the inner bay. These effects of the tidal range on coastal landforms are similar to those described at the Georgia and German bights (Hubbard et al., 1979; Nichols and Allen, 1982). Beach morphodynamics also reflects this situation although it is also conditioned by the type of available sediments: sands (dominantly fine sand) toward the outer bay and mud toward the interior of the Rincón. Gravel is limited to the southern beaches where present processes are reworking Holocene beaches, thatbeaches, which are the reworking of pedimentation levels. Within this pattern, the processes of growth and reworking (during Upper-Pleistocene and Mid-Holocene stillstands) of the deltaic lobes of the Colorado River should be considered. At the beaches of the south, there is a littoral sand contribution derived from the Negro River assymetricasymmetric delta. Toward the north, this longshore drift is segmented by ebb-dominated mesotidal inlets. Aeolian sand corridors, driven from the Colorado valley, have also transported sandsized material toward the coast. Open-ocean beaches (Segunda Barranca Lighthouse, El Cóndor Village and La Chiquita Beach) behave as mesotidal with dominant wave effects from the Atlantic Ocean. Along- bay beaches (San Blas Bay and Los Pocitos Beach) the effect of the tide dominates over fetch-limited waves. LonshoreLongshore-drift transport is significant at the coast between the Negro River Inlet and San Blas Bay. Northwards, the longshore drift is segmented by the ebb-oriented tidal inlets of the San Blas, Anegada, Brightman, Falsa and Verde bays. Fine sand dominates at these beaches, although shell accumulations at the backshore and gravel-composed washovers denote the effects of episodic open-ocean storms. The aim of the present paper focused to analyze beach typology in a transitional area: micro and mesotidal regimes; wave-dominated to tide-dominated continental shelf. Beach morphology and physical parameters were also considered to recognized sediment- transport patterns. With this purpose, topographic surveys, grain-size analysis y and breaker-height measurements were performed, in order to calculate the morphodynamic parameters RTR and Ù. In regard to tidal ranges, the coast of the Rincón de Bahía Blanca is mesotidal towards the interior and microtidal to the Outer Bayoffshore. Within the Inner Bay, tidal processes have reworked past delta fans of the Colorado River into tidal flats, marshes and tidal channels. At the Outer Bay, wave energy and the longshore drift toward the north generated barriers and ebb-dominated tidal deltas where there was sand availability, and cliffs, abrassionabrasion platforms and cliff-top dunes where sand or gravel were scarce.

Palavras-chave : Morphodynamics; Beaches; Meso-microtidal transition; Buenos Aires.

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