Abstract

ISLA, Federico I; BERTOLA, Germán R  and  SCHNACK, Enrique J. Morphodinamic of meso- and macrotidal beaches of Buenos Aires, Río Negro and Chubut. Rev. Asoc. Argent. Sedimentol. [online]. 2001, vol.8, n.1, pp.51-60. ISSN 1853-6360.

In order to analise and compare the morphologic and grainsize characteristics of the sandy meso- and macrotidal beaches of Buenos Aires, Río Negro and Chubut, topographic surveys were executed and samples collected at the bathing villages of San Cayetano, Orense, Claromecó, and beaches from Baliza San Matías, Playa Larralde and Puerto Pirámide. In some of these beaches, surveys from 1972 and 2000 were compared in order to recognise differences in morphology in the last 28 years that could be assigned to dynamics or grain-size changes. The three beaches of Buenos Aires are mesotidal and subject to the action of incoming waves from the Atlantic Ocean. The macrotidal beaches of northern Patagonia are placed within gulfs (San Matías, San José and Nuevo). They are therefore fetch-limited, and oriented to the south (Baliza San Matías, Puerto Pirámide) or to the north (Playa Larralde). All the mesotidal beaches are mainly composed of fine sand. Only at the intertidal sectors of the Balneario San Cayetano and Claromecó medium sand was sampled. Dealing with macrotidal beaches of Patagonia, gravel was collected at the berm crest of Playa Larralde, and shells were sampled at the storm berm of Baliza San Matías beach. At the lower intertidal sectors of Baliza San Matías beach very fine sands, probably related to the distal parts of ebb-tidal delta of San Antonio Bay, were recognised. The three beaches of Buenos Aires Province extend 180- 200 m during low tide, and have wave heights of 0,4-0,7 m. On the other hand, those from northern Patagonia extend 300-400 m, with wave heights of less than 0,3 m during low tide (fetch-limited). The beaches of Baliza San Matías may have reflective dynamics during high tide.The sand would be partly supplied by the reworking of fossil beaches (from Peninsula Villarino) and is located close to the distal portions of the ebb-tidal delta of San Antonio Bay. A similar behaviour is expected at the gravel berm of Playa Larralde; the gravel was supplied by the reworking of fossil beaches. Puerto Pirámide beach has an intermediate behaviour with a stable multiparallel sand-bar system. Sand is supplied from the high foredunes at the headlands of this bay. These macrotidal sandy beaches fit within the models proposed by Short (1991) and Masselink y Short (1993). However, these models do not consider the processes that occur on bimodal beaches (gravel and sand), where breaker wave height varies along the tide and seepage affects dynamics and slope stability of the saturated media.

Keywords : Beaches; Grain size; Morphodynamics.