Temporal Variability In Cohesive Sediment Dynamics In A Partially Mixed Estuary The York River Estuary Virginia Usa

Fine-grained material such as silts and clays are the predominant sediment type in low energy systems such as micro-tidal embayments and estuaries. Due to its cohesive nature, fine sediment typically moves through marine systems as aggregated particles, or flocs, rather than as individual mineral grains. The particle's components, local hydrodynamics, and concentration influence floc size, density, and fall velocity. These, in turn, impact suspended sediment transport, which complicates predictions of the fate of sediment for water quality, contaminant distribution, and dredging purposes in these systems. This dissertation used a state-of-the-art modeling system and observations to examine the variability in sediment distribution due to cohesive processes along a partially mixed estuary and to determine the role of flocculation on sediment transport for a muddy site within the York River estuary, Virginia. The Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system was used to simulate the hydrodynamics and suspended sediment transport in a muddy estuarine system. The model accounted for flocculation dynamics with a population balance model, FLOCMOD, changes in the erosion of sediment from the bed due to compaction or bed consolidation, and sediment-induced density gradients. The sensitivity of the sediment distribution was performed using an idealized two-dimensional (vertical and longitudinal) model that produced key estuarine features such as salinity-driven circulation and an estuarine turbidity maximum (ETM). The reference model included the effects of flocculation, bed consolidation, and sediment-induced density gradients. Results from the reference model were compared to test cases, each of which removed one of these processes. This showed that the effects of flocculation on suspended sediment concentrations (SSC) were most significant in the surface waters and in the ETM; whereas bed consolidation decreased SSC along the full length of the estuary. Another test case demonstrated that calculations of SSC and median floc diameter (D50) were sensitive to the number of sediment classes used to represent the floc population. The capabilities of the idealized two-dimensional estuary were extended and used to examine the contribution of flocculation compared to other sediment transport mechanisms such as advection, diffusion, settling, and erosion. The dominant processes that impacted the sediment mass balance in the idealized estuary were flocculation, vertical diffusion, and erosion. Next, the D50 produced by FLOCMOD in the idealized estuary was compared to a theoretical equilibrium floc size (Deq) estimated based on the ratio of SSC to the square root of the shear rate (G). This analysis also produced an estimate for a timescale for flocculation. In general, D50 reached Deq in the bottom boundary of the estuary when the flocculation timescales were on the order of minutes. However, immediately above the sediment bed, Deq was very similar to D50 when erosion was minimal or when finer flocs were eroded from the bed. However, the computed D50 most often differed widely from Deq, indicating that equilibrium theory was not appropriate for much of the idealized estuary. To facilitate the direct application of the flocculation model to the York River estuary, a one-dimensional (vertical) model was designed using observations of hydrodynamics and floc properties from the Claybank site for the vertical water column structure. The sensitivity of SSC and floc distribution to the parameterization of FLOCMOD was assessed using a model representing a spring-neap tidal cycle. The SSC was more sensitive to parameterization in the bottom boundary layer, D50 was less sensitive than SSC, and the grain size distribution width (spread) was more sensitive to the fractal dimension. Model results were then compared to observations to choose parameters to represent the floc population in the York River estuary. Parameterization was challenging, but the preferred representation for the York floc population had a low relative error for SSC and acceptable error for the distribution mode and spread. For the spring-neap tidal cycle in general, vertical diffusion, settling, and erosion accounted for more sediment mass transport than flocculation, but flocculation played an important role in the vertical distribution of sediment via changes in floc size.

The background for the Workshop on Cohesive Sediment Dynamics - . !!!!!. Special Reference to Physical Processes in Estuaries is briefly outlined in Chapter I. Here I wish to acknowledge those whose support I consider to be pivotal to this under taking. My deepest appreciation goes to Cynthia Vey, whose organizational skills and dedicated effort made the completion of this volume possible. Thanks are also due to Gail Terry for workshop organization, Jean Branson for word processing and Lillean Pieter for helping with drawings. Finally, I must express my sincere appreciation to Arthur Ezra 9f the National Science Foundation for providing support (through Grant No. CEE-8401185) for the workshop, and to Hsiang Wang for depart mental encouragement. With deepest regret, I must note the untimely death of Ranjan Ariathurai, 39, on June 5, 1985, before this volume could be published. He was a guiding force to many within the small group of researchers in cohesive sediment dynamics, and his professional brilliance and inspirational personal qualities constituted the true spirit . behind the workshop. I trust this volume will serve, albeit in a small way, as a fitting memory to this spirit, and to the remarkable professional contributions Ranjan made during his short career. Professor Ray B. Krone Professor Emmanuel Partheniades Department of Civil Engineering Department of Engineering Sciences University of California University of Florida Davis, California Gainesville, Florida TABLE OF CONTENTS CHAPTER PAGE I. INTRODUCTION Ashish J. Mehta •••••••••••••••••••••••••••••••••••••••••••••••••••• 1 II.

This book is a printed edition of the Special Issue "Selected Papers from the 13th Estuarine and Coastal Modeling Conference" that was published in JMSE

A reference volume discussing the dynamics, mixing, sediment regimes and morphological evolution in estuaries for researchers, students and engineers.

Many estuaries are located in urbanized, highly engineered environments. Cohesive sediment plays an important role due to its link with estuarine health and ecology. An important ecological parameter is the suspended sediment concentration (SSC) translated into turbidity levels and sediment budget. This study contributes to investigate and forecast turbidity levels and sediment budget variability at San Francisco Bay-Delta system at a variety of spatial and temporal scales applying a flexible mesh process-based model (Delft3D FM). It is possible to have a robust sediment model, which reproduces 90% of the yearly data derived sediment budget, with simple model settings, like applying one mud fraction and a simple bottom sediment distribution. This finding opens the horizon for modeling less monitored estuaries. Comparing two case studies, i.e. the Sacramento-San Joaquin Delta and Alviso Slough, a classification for estuaries regarding the main sediment dynamic forcing is proposed: event-driven estuary (Delta) and tide-driven estuary (Alviso Slough). In the event-driven estuaries, the rivers are the main sediment source and the tides have minor impact in the net sediment transport. In the tide-driven estuaries, the main sediment source is the bottom sediment and the tide asymmetry defines the net sediment transport. This research also makes advances in connecting different scientific fields and developing a managerial tool to support decision making. It provides the basis to a chain of models, which goes from the hydrodynamics, to suspended sediment, to phytoplankton, to fish, clams and marshes.

The Estuary as a Filter contains the proceedings of the Estuarine Research Federation’s seventh biennial conference at Virginia Beach, Virginia, in late October, 1983. In five invited sessions, scientists and managers considered the physical, geological, chemical-geochemical, and biological processes involved in the ""filtering"" role of estuaries and reflected on management implications of these matters. Most of their presentations and reflections are included in this book in order to demonstrate what is known and what needs to be explored further. The papers in this volume are grouped as they were presented at the conference. Thus, physical oceanographers begin the work by considering turbulence, mixing, and circulation processes in estuaries. Geologists then examine estuarine sedimentation, including the roles of flocculation and bioturbation in accelerating this process. Chemists and geochemists describe the interactions among and effects of inputs of nutrients, metals, and organic matter into estuaries, and the fate of radionuclides in these systems. Biological and biochemical processes involving surface foam, microbes, sea grasses, and wetlands are considered, along with carefully derived nutrient budgets of selected estuarine regions. Finally, some of the problems facing managers of estuarine ecosystems in three areas of the United States are described, along with the success story of the ongoing rehabilitation of the Thames Estuary in England.

This collection of papers offers a new approach to nearshore and estuary studies, with an emphasis on multidisciplinary techniques and data integration. The important results of these studies are accompanied by full color images.