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The Effect of Variable Grain Size Distribution on Beach's Delft University of Technology Faculty of Civil Engineering and Geosciences Department of Hydraulic Engineering MSc Thesis THE EFFECT OF VARIABLE GRAIN SIZE DISTRIBUTION ON BEACH’S MORPHOLOGICAL RESPONSE Graduation committee: Prof.dr.ir. A.J.H.M. Reniers Author: Dr.ir. Matthieu de Schipper Melike Koktas Ir. Tjerk Zitman Dr. Edith L. Gallagher May 2017 P a g e 2 | 67 EXECUTIVE SUMMARY 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, first a field experiment campaign was conducted at the USACE Field Research Facility (FRF) in Duck, USA, in the spring of 2014. This experiment campaign was called SABER_Duck as an acronym for ‘Stratigraphy And BEach Response’. During SABER_Duck, in-situ swash zone grain size distribution, the prevailing hydrodynamic conditions and the time-series of the cross-shore bathymetry data were collected. The data confirmed a highly variable grain size distribution in the swash zone both vertically and horizontally. Additionally, the two trench survey observations showed the existence of continuous layers of coarse and fine sands comprising the beach stratigraphy. Secondly, a process based numerical coastal morphology model, XBeach, was chosen to simulate the beach profile response to wave and tidal action. A 1D cross-shore profile model was built and tested with the bathymetry data and accompanying boundary conditions that were collected during SABER_Duck. The default model settings were not an adequate match to the observed beach response; thus, a site-specific calibration was needed. Before proceeding with calibration, sensitivity analysis was performed to determine the dominant parameters. Here, the computation results were found to be sensitive to the model parameters, hmin, form, turb, facua and eps. Then the model was calibrated using the bathymetry, the initial varying grain size distribution in the swash zone and the hydrodynamic boundary conditions that were recorded during a 2- day storm event between 25-27 March 2014 at Duck Beach, USA. After calibration, the model reproduced the measured cross-shore profile evolution sufficiently by using varying grain size distribution in the swash zone. Modeling study continued with computation of beach responses for four alternative initial bed compositions. Two of these scenarios had uniform grain size distribution along the cross-shore profile, one with fine and one with coarse sand as the mean sediment grain size. The following two scenarios were defined with fine sandy bed composition with coarse sand patches at lower swash and around shoreline, respectively. All scenarios were tested for the same 2-day period with the same hydrodynamic conditions and model results were compared with that of the original scenario. 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 model results for alternative scenarios also demonstrated the importance of the accuracy in spatial distribution of varying grain sizes and the accuracy of the representative grain size in numerical modeling of coastal morphodynamics. Additionally, the grain size distribution in the computed final bed compositions showed layering of different grain sizes similar to those that were observed P a g e 3 | 67 during trench survey and from the core samples, showing that the model was capable of simulating the sediment grain size sorting. This thesis study clearly showed the importance of variability in grain size distribution in a beach’s morphological response. Furthermore, the numerical model was able to compute the beach response using variable grain size distribution and showed promising results related to the capabilities for the future pursuit of this research topic. Knowledge on the bed sediment composition and its temporal evolution in response to hydrodynamic forcing would advance with continuing experimental and numerical research. P a g e 4 | 67 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 using variable grain size distribution in numerical modeling of morphological beach 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. P a g e 5 | 67 P a g e 6 | 67 PREFACE With this thesis, I finalize my Master of Science degree in Hydraulic Engineering at the Delft University of Technology. I would like to express my gratitude to those who supported me during my graduation study. First of all, I would like to thank my committee members, Ad Reniers, Edith Gallagher, Matthieu de Schipper and Tjerk Zitman. Their guidance throughout the research and reporting was highly valuable in getting the insight into details of my research topic, developing my thesis and my skills on scientific research and reporting. I would also like thank Arnold van Rooijen, for his valued guidance and help as my former committee member during the first half of my graduation work. I would also like to thank Heidi Wadman for providing data and insight into data interpretation, and to Shoshan Abrahami for her guidance with my report. Furthermore, I would like to give special thanks to Aliya van den Brink Sarayeva and Bert Valkenburg for evaluating my research and my report, and for their support. Your support and friendship have been precious to me during this period. I would like to give another special thanks to John Stals and to my professor Ad Reniers, for their support during my graduation studies, for which I am grateful. Lastly, I would like to express my gratitude towards my family as well as my devoted friends for their support and believing in me, and for making this possible. P a g e 7 | 67 P a g e 8 | 67 TABLE OF CONTENTS EXECUTIVE SUMMARY ................................................................................................................ 3 ABSTRACT ................................................................................................................................... 5 PREFACE ..................................................................................................................................... 7 1 INTRODUCTION ................................................................................................................ 11 1.1 BACKGROUND .............................................................................................................. 11 1.2 RESEARCH OBJECTIVE .................................................................................................... 12 1.3 APPROACH AND OUTLINE OF THE THESIS REPORT .............................................................. 12 2 SEDIMENT GRAIN SIZE CLASSIFICATIONS AND SEDIMENT CLASSIFICATIONS .................... 14 3 SABER_DUCK FIELD EXPERIMENT ..................................................................................... 17 3.1 INTRODUCTION ............................................................................................................ 17 3.2 FIELD SURVEY METHODS ................................................................................................ 18 3.2.1 Walking survey ........................................................................................................................18 3.2.2 Depth of disturbance measurements .....................................................................................20
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