Influencia de la urbanizacion en la dinamica costera, Villa Gesell, provincia de Buenos Aires, República Argentina

Abstract

Las playas de la localidad de Villa Gesell han sufrido un importante grado de erosión relacionado con un mal manejo del ecosistema costero y con un incremento explosivo en la población turística. Se presenta un modelo de variación morfodinámico estacional en playas afectadas por distintos grados de alteración antrópica al medio físico, reconociéndose tres estados de erosión inducida: bajo, mediano, alto. Se determinó un ciclo de recuperación anual para playas naturales, con cambios en la configuración del perfil de playa relacionados a la disposición y presencia de bermas estacionales durante el invierno. En áreas urbanas se detectó una alteración en los ciclos naturales de recuperación. Las bermas estables pueden estar o no presentes en función del grado de deterioro del equilibrio natural. En sectores fuertemente afectados por la urbanización, la berma estable desaparece y pasa a integrar bermas estacionales acíclicas. Si el grado de alteración es separación se mantiene una berma estable que es parcialmente erosionada durante un ciclo anual y pueden o no adosarse bermas transicionales durante el invierno. Asimismo se analizan los parámetros texturales de los sedimentos de los distintos subambientes de playa y su relación con las variaciones temporales en una mesoescala (cambios estacionales) y su relación espacial (variaciones a lo largo de la costa). La playa frontal, es el subambiente que mejor refleja las variaciones temporales y espaciales, debido a la gran influencia de la alternancia entre el lavado y deslizamiento producidos por la acción combinada de las mareas y del oleaje. Los parámetros texturales que evidenciaron una mejor respuesta a las condiciones erosivas son la asimetría, la selección y la media en la playa frontal. Mientras que los parámetros físicos determinantes son disminución en el ancho de playa e incremento en la pendiente. Otro factor textural determinativo es la combinación entre selección y asimetría en los diagramas bivariantes. En playas erosivas, al incrementarse la desviación estándar disminuye la asimetría y en playas naturales la relación es inversa. El análisis univariante de la entropía en los sedimentos de playa frontal puede considerarse como una alternativa apropiada para determinar la selección, aunque en las playas estudiadas la última resalta mejor la selectividad del medio de transporte. La selección y entropía son los únicos parámetros texturales determinativos de las variaciones estacionales. Se detectó una disminución en la desviación estándar y de la entropía durante el verano en las muestras analizadas.

Translated abstract

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.