University of South Carolina Scholar Commons Faculty Publications Geography, Department of 2006 Coherent Structures and Aeolian Saltation Jean Taylor Ellis University of South Carolina - Columbia, [email protected] Follow this and additional works at: https://scholarcommons.sc.edu/geog_facpub Part of the Geography Commons Publication Info 2006. Copyright 2006, Jean Ellis This Book is brought to you by the Geography, Department of at Scholar Commons. It has been accepted for inclusion in Faculty Publications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. COHERENT STRUCTURES AND AEOLIAN SALTATION A Dissertation by JEAN TAYLOR ELLIS Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY December 2006 Major Subject: Geography COHERENT STRUCTURES AND AEOLIAN SALTATION A Dissertation by JEAN TAYLOR ELLIS Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved by: Chair of Committee, Douglas J. Sherman Committee Members, David M. Cairns Steven F. DiMarco Vatche P. Tchakerian Head of Department, Douglas J. Sherman December 2006 Major Subject: Geography iii ABSTRACT Coherent Structures and Aeolian Saltation. (December 2006) Jean Taylor Ellis, B.S., University of Southern California; M.S., University of Southern California Chair of Advisory Committee: Dr. Douglas Joel Sherman Aeolian sand transport models, widely employed by coastal scientists and managers, assume temporal and spatial homogeneity within the saltation field. This research questions that assumption by demonstrating that the saltation field is event-driven, therefore indicating that the saltation field is not temporally steady. The findings from this research may explain a portion of the conclusions from previous studies that indicated inequalities between model-estimated and field-measured aeolian sand transport. The relationship between unsteadiness in a turbulent wind field and pulses in a sand transport field was investigated on a beach near Shoalhaven Heads, New South Wales, Australia. Microphone-based saltation sensors, “miniphones,” and thermal anemometers (both instruments constructed exclusively for this field experiment) were co-located (0.02 m separation on center) and deployed between 0.01 and 0.0225 m above the bed, and sampled at 6000 Hz. Average grain size at the field site was 0.30 mm. Five runs totaling 2050 seconds of wind and saltation data were analyzed. The continuous wavelet transform, using the Morlet wavelet base, was the principle method for analyzing the wind and saltation records. The cross continuous wavelet transform was used to analyze the wind and saltation time series concurrently. Wind, saltation, and cross events were discerned by selecting wavelet power coefficients between wavelet scales of 0.4 and 3.0 seconds and with coefficients exceeding the 95% confidence interval. Average event spacing was 6.10, 6.50, and 6.73 seconds for the wind, saltation, and cross events, respectively. The average event spacing measured in this research was compared to the empirical-based model presented by Rao, Narashimha, and Narayanan (1971). The correspondence between the model and this research strongly suggests that bursting-type coherent structures were present. The durations of average wind, saltation, and cross events were 1.87, 2.10, and 1.73 seconds, respectively. Integral time scales, calculated using normalized auto iv correlation and power spectral density analysis, were approximately two seconds for the wind and saltation systems. The temporal coincidence of the integral time scale estimations and the event durations for the wind and saltation system strongly suggests that wind events are driving sand transport events. v ACKNOWLEDGEMENTS Numerous individuals have contributed to this research, particularly my advisory committee. Douglas Sherman has always been a supportive and enthusiastic advisor and this work was greatly supplemented by our discussions and his suggestions. Steven DiMarco provided critical advice during the data analysis portions of this work. Vatche Tchakerian and David Carins provided useful and thoughtful comments. This research was supported by National Science Foundation Doctoral Dissertation Research Improvement Grant (Award #0425770, co-PI, Douglas Sherman) and a National Science Foundation East Asia Summer Institute for US Graduate Students Grant (Award #0413541), that is also supported by the Australian Academy of Sciences. Texas A&M University, Department of Geography contributed a small portion of funds. During 2005, I was suported by the John A. Knuass National Oceanic and Atmospheric Administration (NOAA) Sea Grant Fellowship (based at NASA Headquarters). Texas Sea Grant, led by Dr. Robert Stickney, should be acknowledged for the supplemental support to this fellowship. The following people partipated in the fieldwork: Eugene Farrell, Wansang Ryu, Barry Preist, Rebecca Morrison, and Diane Horn. The field research was conducted while I was a visiting scholar at the University of Sydney, School of Geosciences, Coastal Studies Institute. Andy Short was my host and was wonderful to me during my stay in Australia, as was his wife Julia. David Mitchell, Nelson Cano, Graham Lloyd, and John Connell, all at the University of Syndey, helped to arrange vehicles, time in the sediment laboratory, and allowed me to borrow University equipment, for example. Rob Brander (University of New South Wales) also assisted me with logisitics during my field experiment and during my stay in Australia. vi TABLE OF CONTENTS Page ABSTRACT ............................................................................................................................ iii ACKNOWLEDGEMENTS .................................................................................................... v TABLE OF CONTENTS ........................................................................................................ vi LIST OF FIGURES................................................................................................................. ix LIST OF TABLES .................................................................................................................. xv 1. INTRODUCTION............................................................................................................... 1 1.1 Research Statement .............................................................................................. 1 1.2 Conceptual Background ....................................................................................... 1 1.3 Research Hypotheses............................................................................................ 2 1.4 Research Objectives ............................................................................................. 3 1.5 Dissertation Outline.............................................................................................. 3 2. BACKGROUND................................................................................................................. 4 2.1 Section Introduction ............................................................................................. 4 2.2 Fluid Flow ............................................................................................................ 4 2.2.1 Boundary Layers......................................................................................... 4 2.2.2 Turbulence .................................................................................................. 5 2.2.3 Coherent Structures..................................................................................... 7 2.2.4 Characterizing Boundary Layer Turbulence............................................... 11 2.3 Aeolian Sediment Transport................................................................................. 12 2.4 Saltation and Wind Interactions During Transport .............................................. 13 2.4.1 Response Time............................................................................................ 13 2.4.2 Unsteadiness in the Saltation Field............................................................. 14 2.5 Event Detection .................................................................................................... 17 2.5.1 Variable Interval Time Averaging (VITA) Method ................................... 18 2.5.2 Continuous Wavelet Transform.................................................................. 19 2.5.2.1 Wavelet Base .................................................................................. 21 2.5.2.2 Wavelet Analysis in Aeolian Research........................................... 22 2.5.3 Cross Continuous Wavelet Transform........................................................ 23 3. STUDY SITE AND FIELD METHODS............................................................................ 24 3.1 Section Introduction ............................................................................................. 24 3.3 Study Site Location .............................................................................................. 24 3.3 Instrumentation..................................................................................................... 26 3.3.1 Thermal Anemometers...............................................................................