Sediment Flux in Estuaries with an Estuarine Dam : Field Observations and Idealized Modeling

다국어 초록 (Multilingual Abstract)

The objective of this research was to understand sediment fluxes in estuaries with an estuarine dam. This was done using multi-annual field observations in the macrotidal Geum estuary, Korea (estuarine dam completed in 1994), and by studying pre- and post-dam scenarios using idealized numerical modeling. The field observations were based on sediment flux data calculated using suspended sediment concentration (SSC) obtained from calibrated acoustic Doppler current profiler (ADCP) data. It was found that in the inner estuary near the estuarine dam, the sediment fluxes were directed landward mainly due to tidal pumping by flood-dominant tidal asymmetry during spring tides. Freshwater discharges constituted active seaward sediment flux events, however their duration was short (restricted to every few days during ebb tides), and the along-channel extent of their influence, about 7 km seaward of the estuarine dam, was similar to one tidal excursion. This limitation was due to freshwater discharges being restricted to ebb tides to prevent salt intrusion. And as this length was less than the distance to the mouths of the estuary, it did not results in sediments leaving the estuary. It was observed that the estuarine dam discharge could generate periodic stratification in the inner estuary. This was because the discharge generated a strong along-channel salinity gradient which became vertically sheared during the macrotidal ebb tide. At the same time, it was observed that the cohesive sediment flocs were larger during the stratified ebb tide and smaller during the well-mixed flood tide in the periodically stratified inner estuary due to the ebb-flood tidal asymmetry in stratification and turbulence. This implied that sediments were more mobile during the landward flood currents than the seaward ebb currents. In the deeper outer estuary, sediments were moving landward from the shelf and depositing in the estuary predominantly during the spring tides. However, in contrast to the inner estuary, the landward sediment fluxes in the outer estuary were mainly due to the tidally averaged currents.
The Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system was used to investigate the impact of an estuarine dam on a range of estuaries. The model domain was a funnel-shaped estuary, and the range of estuaries covered four scenarios: strongly stratified, partially mixed, periodically stratified, and well-mixed estuaries. The pre- and post-dam outputs were analyzed in terms of the estuarine parameter space and a sediment flux decomposition into five terms (river runoff, tidal pumping, estuarine circulation, tidal straining, and Stokes transport). The results showed that the estuarine dam always reduced the tidal currents and caused a shift from continuous discharge to unsteady discharge. This resulted in the estuaries shifting to being in the more strongly stratified region of the estuarine parameter space during the estuarine dam freshwater discharges. When there was no dam discharge, the estuaries shifted to being in the bay or periodically stratified region of the estuarine parameter space. The estuarine dam changed the sediment flux mechanisms, but each scenario had different responses. The strongly stratified end member was characterized by shift to seaward sediment fluxes due to river runoff and seaward tidal pumping. The well-mixed end member was characterized by a shift to landward tidal pumping and Stokes transport, and reduced river runoff.
Overall this research highlights that estuarine dams can change an estuary’s tidal and river forcing. This has consequences for the sedimentary processes as the currents, suspended sediment concentrations, and sediment fluxes are proportional to the external forcing. In particular, changes to gradients in the external forcing (tidal currents or river currents) result in sediment flux gradients and therefore morphodynamic change. For systems with moderate or large tidal forcing, field observations and numerical modeling provide evidence that estuarine dams can promote deposition by reducing the seaward river runoff, mean flow sediment flux. That is because the river runoff is one of the main seaward sediment flux mechanisms. At the same time, there is a negative gradient in the tidal currents which vanish at the estuarine dam. This can promote landward sediment fluxes by tidal pumping or Stokes transport, in addition to two-layer density related mechanisms such are estuarine exchange flow or tidal straining. However, both field observations and numerical modeling indicated that sediment flux mechanism can vary with estuarine type and even along-channel within an individual estuary, such as the inner estuary near the estuarine dam or the outer estuary influenced by the shelf. Furthermore, this research provided evidence that the sediment flux mechanisms are modified by cohesive sediment processes.

목차 (Table of Contents)

• ABSTRACT 5
• TABLE OF CONTENTS 8
• LIST OF TABLES 12
• LIST OF FIGURES 13
• ACKNOWLEDGEMENTS 19
• CHAPTER I: INTRODUCTION AND SYNOPSES 21
• 1. Introduction 21
• 2. Synopses of chapters 24
• 2.1. Effects of an estuarine dam on sediment flux mechanisms in a shallow, macrotidal estuary 24
• 2.2. Evaluation of along-channel sediment flux gradients in an anthropocene estuary with an estuarine dam 25
• 2.3. Impact of estuarine dams on the estuarine parameter space and sediment flux decomposition: idealized numerical modeling study 27
• References 29
• Figures 30
• CHAPTER II: EFFECTS OF AN ESTUARINE DAM ON SEDIMENT FLUX MECHANISMS IN A SHALLOW, MACROTIDAL ESTUARY 35
• 1. Introduction 35
• 2. Regional setting 39
• 3. Materials and methods 40
• 3.1. Data collection 40
• 3.2. Data processing and analysis 43
• 3.2.1. Sediment flux decomposition 43
• 3.2.2. Flux gradient and bed level change estimation 44
• 3.2.3. Evaluation of river and tidal forcing on flux mechanisms 45
• 3.2.4. Evaluation of salinity structure following dam discharge 46
• 4. Results 47
• 4.1. Spatiotemporal variation of external forcing and flux mechanisms 47
• 4.2. Flux gradient and bed level change 51
• 4.3. Factors controlling sediment flux 52
• 4.3.1. Discharge volume and tidal skewness 53
• 4.3.2. Longitudinal salinity gradient 54
• 5. Discussion 56
• 5.1. Sediment flux mechanisms in a shallow, macrotidal estuary with an estuarine dam 56
• 5.2. Effect of estuarine dam on sediment flux mechanisms in a shallow, macrotidal estuary 57
• 6. Conclusions 60
• References 62
• Tables 66
• Figures 67
• CHAPTER III: EVALUATION OF ALONG-CHANNEL SEDIMENT FLUX GRADIENTS IN AN ANTHROPOCENE ESTUARY WITH AN ESTUARINE DAM 76
• 1. Introduction 76
• 2. Regional setting 81
• 3. Materials and methods 83
• 3.1. Data collection 83
• 3.2. Data processing and analysis 85
• 3.2.1. Conversion to channel coordinates and ADCP backscatter calibration 85
• 3.2.2. Sediment flux decomposition and calculation of cumulative sediment fluxes 86
• 3.2.3. Evaluation of proportion of cumulative sediment fluxes by mechanism and forcing 88
• 3.2.4. Evaluation of factors controlling sediment fluxes 89
• 4. Results 90
• 4.1. Environmental conditions 90
• 4.2. Currents and suspended sediment concentrations 91
• 4.3. Sediment fluxes and cumulative sediment fluxes 94
• 4.4. Along-channel cumulative sediment flux gradient 96
• 4.5. Factors controlling sediment fluxes 98
• 5. Discussion 100
• 5.1. Along-channel sediment flux gradients in an estuary with an estuarine dam 100
• 5.2. Comparison of sediment fluxes in an estuary with and without an estuarine dam 103
• 5.3. Global perspective of estuarine dams 106
• 6. Conclusions 108
• References 110
• Tables 116
• Figures 118
• CHAPTER IV: IMPACT OF ESTUARINE DAMS ON THE ESTUARINE PARAMETER SPACE AND SEDIMENT FLUX DECOMPOSITION: IDEALIZED NUMERICAL MODELING STUDY 128
• 1. Introduction 128
• 2. Materials and methods 131
• 2.1. Data collection 131
• 2.1.1. Model description 131
• 2.1.2. Model domain and setup 132
• 2.1.3. Model scenarios 136
• 2.2. Data processing and analysis 139
• 2.2.1. Estuarine parameter space 139
• 2.2.2. Change in bed level and surficial sediment grain size 139
• 2.2.3. Sediment flux decomposition 140
• 3. Results 142
• 3.1. Pre- and post-dam time series of tide, current velocity, salinity, and SSC 142
• 3.2. Change in along-channel profiles of tide, current velocity, salinity, and SSC 145
• 3.3. Shifts in the estuarine parameter space 146
• 3.4. Bathymetric and surficial mud content maps 148
• 3.5. Along-channel decomposed sediment fluxes 150
• 4. Discussion 153
• 4.1. Trends in the impact of estuarine dams 153
• 4.2. Comparison with other modeling studies and field observations 155
• 4.3. Implications of estuarine dams 159
• 5. Conclusions 162
• References 165
• Tables 171
• Figures 174
• CHAPTER V: CONCLUSIONS 182
• VITA 186