The Effect of Variable Grain Size Distribution on Beach’s Morphological Response

Abstract

Field studies with in-situ sediment sampling demonstrate the spatial variability in grain size on a sandy beach. However, conventional numerical models that are used to simulate the coastal morphodynamics ignore this variability of sediment grain size and use a uniform grain size distribution of mostly around and assumed fine grain size. This thesis study investigates the importance of variable grain size distribution in a beach’s morphological response. For this purpose, in-situ swash zone grain size distribution, beach profile and accompanying wave and tide time series data were obtained at a field campaign at the USACE Field Research Facility (FRF) in Duck, USA, in the spring of 2014. Using this data, a process based numerical coastal morphology model, XBeach, was chosen to simulate the beach profile response to wave and tidal action. 1-D cross-shore model was run for five major scenarios for a 2-day period with available field data in order to compare beach responses in the cases of measured variable and hypothetical uniform distribution of grain sizes in the swash zone respectively. The data gathered in SABER_Duck, confirmed a highly variable grain size distribution in the swash zone both vertically and horizontally. The numerical model results for alternative grain size distribution scenarios were compared. Computed beach response for uniform grain size distribution differed from that for the original varying distribution, corroborating the importance of grain size variability in beach response. The results for alternative hypothetical bed composition scenarios, demonstrated the importance of the accuracy in spatial distribution of varying grain sizes and the accuracy of the representative grain size. Lastly, the grain size distribution in the computed final bed composition showed layering of different grain sizes similar to those that were observed during field campaign, showing the model’s capability of simulating the sediment grain size sorting.