Resumen

MARCOMINI, Silvia C  y  LOPEZ, Rubén A. Influencia de la urbanizacion en la dinamica costera, Villa Gesell, provincia de Buenos Aires, República Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 1997, vol.4, n.2, pp.79-96. ISSN 1853-6360.

The beaches of Villa Gesell have suffered  important rates of erosion related to antrhopogenic activities which altered the natural coastal ecosystem. The man induced erosion was mainly recognized on the beaches located in the center of the town. A morphodynamic model that shows the beach configuration response to man induced erosion and seasonal  variations is proposed in the paper. Three stages of man induced erosion were described: high, moderate and low .The model is useful to detect low rates of erosion based on the monitoring of the stable berm and transitional berms behavior in the beach profile. The coastal area of Buenos Aires is represented by an accumulative coast characterized by an active  dune field of 0.05 km to 3.5 km width with dunes height that vary between 2 to 25 m. The beaches are 60 m to 140 m width and present slopes below 2°. They are composed of medium to fine sand. The beach profiles show seasonal changes. It was observed that only one berm is stable and is the most sensitive parameter to evaluate the natural recover to erosion. It is preserved during storm erosion episodes and is recovered naturally except in those sectors where the beach is affected by urbanization.  Two or more berms attach to the stable berm in winter and disappear in summer during a year cycle. This reflects the natural evolution of the beach profile under natural conditions. In urbanized areas, the transitional berms disappear and the stable berm is cut by scarps during surge storms. The stable berm has higher altitudes and  width (30-40 m), than the seasonal berms (20-25 m). The mean grain-size of the stable berm varies from 1.10 to 1.88 phi and the sorting between 0.56 to 1.28. Grain-size diminished from summer to winter while the sorting increased. The backshore between the stable berm crest and the foredune also suffers seasonal changes. During winter the slope is reduced. The natural recovery is related to the supply of sand from the dune. In sectors where the dune was destroyed by urbanization the profile can not be naturally recovered resulting in erosion  of the backshore. Textural analysis of beach sediments from different subenvironments were analyzed as well as their relation with: temporal variations in a mesoscale (seasonal changes), spatial changes along the coast and across the beach profile and human-induced erosion. Mean grain-size, phi standard deviation and skewness were adequate parameters to identify man-induced erosional processes in the foreshore of the city of Villa Gesell. An increase in the standard deviation, a coarser grain size and a trend to a negative skewness characterised the beach sectors affected by erosional processes. Diagrams comparing skewness and standard deviation representations were useful tools to discriminate shore erosion. It was observed a positive linear regression for erosional beaches and a negative regression for natural beaches. The univariate entropy analysis was used because it can treat a complex grain size distribution as readily a simple one and does not ignore limitation such as normality of distribution. The comparison of the results are quite well correlated with the standard deviation. The standard deviation and the relative entropy analysis of the foreshore sediments mainly represent the seasonal changes. During summer the standard deviation decreases while during winter it increases in response to the incoming of seasonal berms with higher contents of shells. The presence, location, and morphological conservation of the stable berm during a year cycle is proposed to be used as a field indicator to determine the beach vulnerability to erosion.

Palabras clave : Coastal morphodynamic; Urbanization; Erosion; Berms; Beach seasonal changes.

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