Playas reflectivas y disipativas macromareales del Golfo San Jorge, Chubut

Abstract

El Golfo San Jorge tiene una forma semicircular abierta a las olas del Atlántico, con diferencias en la orientación de sus playas y por lo tanto en el efecto de los vientos dominantes del oeste. Su régimen macromareal semidiurno lo hace ideal para analizar modelos morfodinámicos de playas. La costa es de acantilados de variada altitud conformados en sedimentitas marinas del Terciario superior (Formación Patagonia) y playas fósiles cuaternarias. En una rápida descripción morfológica de las playas del golfo, se pueden distinguir de grava y arena, de considerable pendiente en los sectores intermareales más altos (10%), y playas de muy baja pendiente (1,5%) compuestas casi exclusivamente por arena fina. En el presente trabajo se caracterizaron morfológica y granulométricamente las playas de Caleta Córdova (CC) y de El Límite Chubut - Santa Cruz (Ch-SC) como representantes de los extremos reflectivo y disipativo, respectivamente. Las diferencias reconocidas no obedecen a cuestiones dinámicas sino principalmente a factores heredados (morfología y disponibilidad de aportes). La playa de CC es originada por la erosión de acantilados. La playa Ch-SC está en una zona baja y es mantenida por aportes eólicos del oeste (blowouts) y por el reciclado de acumulaciones medanosas de arena fina. Debido a que ambas playas poseen normalmente rompientes de ola de sólo 0,2 m durante la bajamar, se concluye que las diferencias morfodinámicas obedecen exclusivamente a la disponibilidad de sedimento (tamaño de grano y volumen) que es distribuido en el perfil de la playa. Se destaca en estas playas de arena fina (playas de La Herradura y Alsina) la necesidad de mantener sus aportes sedimentarios del oeste de modo de no inducir balances erosivos involuntarios.

The morphodynamics of macrotidal beaches is not well known. The tide controls the action of waves, and the swash, backwash and surf are very important processes only at shallow water. Bedforms occur in relation to these shallowwater processes and to grain-size variations. The San Jorge Gulf has a semicircular shape open to waves entering from the Atlantic, with differences in beach orientations and therefore in the significant effects of the dominant westerly winds. The macrotidal semidiurnal regime makes it ideal to test models about beach morphodynamics. The coast is constituted by cliffs carved into Upper Tertiary marine sediments (Patagonia Formation) and gravel beach-ridge systems belonging to Quaternary highstands. Open-coast beaches (Camarones, Cabo Blanco) are composed of gravel, while towards the headlands of the gulf there are beaches composed dominantly by sand. Pocket beaches along the coastline between Rada Tilly and Playa Alsina are characterised by an eolian supply of fine sand. In regard to the morphology of these beaches, it should be distinguished between beaches composed of gravel and sand with steep intertidal slopes (>10%), and sandy beaches of very gentle slopes (<2%). Both extremes, reflective and dissipative beaches, were characterised in morphology and grain-size distribution, and were considered represented by Caleta Córdova (CC, Chubut) and El Límite (EL, Chubut- Santa Cruz), respectively. The differences do not obey directly to dynamic causes, but to inherited factors: bathymetry (that induce wave-refraction patterns and sediment sources). The CC beach originated by the erosion of cliffs. The EL beach is on a low-lying coast, supported by aeolian sources from the west, and the recycling of fine sand foredunes. As both beaches have wave heights at breakers of only 0,2 m during low tide, it is here concluded that their morphodynamics differ exclusively by the sediment availability (grain size and volume) that control the spatial and temporal distribution of wave energy along the beach profile, and the refraction pattern of waves coming from the Atlantic. Waves incoming from the ENE, E and ESE concentrate at the southwestern coast of the gulf due to a submerged platform located at its southeastern portion. Only waves from the SE converge at the northern coast of the gulf. It is stressed here that at the fine-sand beaches (El Límite, Herradura, Alsina) sediment input rates should be preserved in order to avoid negative budgets.