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Geomorphological framework control on beach dynamics
Publication . Silveira, Tanya Mendes, 1978-; Andrade, César Freire de, 1955-; Psuty, Norbert P.
Beaches occur when sediment is available and where accommodation space exists for the sediments to settle and accumulate. Beaches that occur within one coastal cell share the same oceanographic forcing climate, essentially related to waves and tides, and the same distribution and intensity of sources and sinks of sediment. However, contrasting beach forms and dynamics are frequent along the same coastal cell, leading to the questioning of what other factors are controlling beach morphodynamics. Conceptual models have been developed and are very useful in describing and classifying beaches according to the characteristics of the incoming waves and beach sediments or morphology. Still, several studies suggest that site-specific characteristics, such as the geology, geomorphology or even the human-induced framework of beaches, can play a determinant role in beach systems’ behavior and evolution. However, the majority of studies focusing on the role of geomorphological framework on beach dynamics, address cases sharing the same general characteristics. This thesis focused on how the geomorphological framework interacts with the available coastal sediments and controls the subaerial beach configuration and responses. It investigates a myriad of beach geomorphological conditions along a high energy coastal stretch that is under the influence of the same general deepwater wave regime. By comparing the morphodynamic response of adjacent constrained and unconstrained, platform and no-platform beaches, exposed to the same offshore forcing conditions, the present work provides new insights on the role of the geomorphological settings on beach dynamics. The central hypothesis of this study was formulated as follows: the geomorphological constraints and the local settings of beaches are the primary drivers for the varying temporal and spatial groupings of morphological responses. In order to verify this, work was carried out at the seasonal scale, under modal process-response conditions, and over a two and a half year period, along 14 selected beach sites representative of a 200 km coastal stretch. The study was divided in three major components: 1) geomorphological framework; 2) beach response; and 3) hydrodynamic forcing. Geomorphological framework was analyzed in terms of planform geometry and description of physical boundaries, and type of nearshore and backshore features present at each study site. Beach response investigation relied upon regular field sampling planned to ensure accurate and representative data collection on subaerial beach morphology and sediments. A total of 52 beach profiles were monitored quarterly along the study area. The hydrodynamic forcing description included characterization of the deepwater and nearshore wave regimes at each study site and made use of numerical modelling of wave propagation. In addition, total water levels were computed for each site. The overall results were further explored to derive metrics and investigate and detect spatial organizations related to the geomorphological settings. The geomorphological framework analysis attests to the high variability of settings in which the studied beaches occur. These include several combinations between nearshore and backshore type of features, physical boundaries and subsequent wave obliquities and degrees of indentation. Beach response was analyzed in terms of geoindicators, which were used to evaluate and describe the study sites’ seasonal behavior. Results show that the magnitude of seasonal change between study sites varied considerably, in relative as well as absolute terms, especially regarding the morphological geoindicators. The hydrodynamic forcing included the analysis of a 36-year time series, used to describe the typical modal and storm regimes, and of a two and a half years subset, synoptic of the sediment and morphological field surveys. Results show that the study period was appropriate to characterize the modal process-response conditions, descriptive of a period with no extreme events. They also highlight the differences between sites exposed to higher and lower waves and total water level regimes. The exploratory analyses of the data showed no linear relationship between forcing, controlling and response variables. However, it put in evidence the existence of clusters of beaches sharing similarities in types of boundaries. Beaches with rocky platforms experienced low volumetric variation, consistently lower than beaches with no rocky platform. In addition, within the no platform beaches, those with the higher degree of embaymentization, varied the most. A conceptual model of the subaerial beach dynamics as a function of the geomorphological framework is put forth, in which the magnitude of beach variation is controlled by: 1) the presence of a rocky platform; and 2) in no-platform beaches, by the degree of embayment and its impact on beach circulation. It is suggested that the presence of a permanent and rigid obstacle in the surf zone, such as a rocky platform, limits the range of broken wave characteristics reaching the subaerial beach, and thus the amount of volumetric variation. On the other hand, beaches characterized by a mobile substrate (no-platform beaches) can present a variety of morphological features in the surf zone, and are expected to allow for a wider range of broken wave conditions and subsequent effects on the subaerial beach, including higher volumetric changes. Within the no-platform beaches, subaerial volume variations increase with the degree of embaymentization. Unconstrained beaches have open lateral boundaries, and therefore see their cross-shore morphology and volume vary mostly with the cross-shore sediment exchanges, that are expected to be small under modal conditions. The constrained beaches (with higher degree of embaymentization) on the other hand, are bounded laterally and therefore are prone to beach rotation processes (and subsequent higher volume variation) promoted by longshore sediment transport that occurs under modal conditions. The conceptual model herein proposed departs from existing morphodynamic models that apply only to unconstrained beaches. It incorporates both unconstrained and constrained beaches and relates the constraining boundaries (both lateral and vertical) to the beach subaerial dynamics. It provides a first description on how the geomorphological framework controls subaerial beach dynamics, and hopefully can be developed and evaluated further.
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Fundação para a Ciência e a Tecnologia
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SFRH
Número da atribuição
SFRH/BD/72555/2010