Estuarine Dispersion from Tidal Trapping: A New Analytical Framework

MacVean, L. J.; Stacey, Mark T.

doi:10.1007/s12237-010-9298-x

Cited by 25 publications

(35 citation statements)

References 17 publications

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“…We present results of field observations and hydrodynamic modeling designed to examine in detail the behavior of flows and mixing at a Delta junction. We will show that tidal river junction dynamics affect dispersion in a fashion similar to that described by MacVean and Stacey (2011), in that water masses are separated because of phase differences in junction inflows and outflows. The phase differences result from tidal modification of the riverine inflows over different time scales and magnitudes.…”

Section: Introductionmentioning

confidence: 69%

Dispersion Mechanisms of a Tidal River Junction in the Sacramento–San Joaquin Delta, California

Gleichauf¹,

Wolfram²,

Monsen³

et al. 2014

SFEWS

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In branching channel networks, such as in the Sacramento-San Joaquin River Delta, junction flow dynamics contribute to dispersion of ecologically important entities such as fish, pollutants, nutrients, salt, sediment, and phytoplankton. Flow transport through a junction largely arises from velocity phasing in the form of divergent flow between junction channels for a portion of the tidal cycle. Field observations in the Georgiana Slough junction, which is composed of the North and South Mokelumne rivers, Georgiana Slough, and the Mokelumne River, show that flow phasing differences between these rivers arise from operational, riverine, and tidal forcing. A combination of Acoustic Doppler Current Profile (ADCP) boat transecting and moored ADCPs over a spring-neap tidal cycle (May to June 2012) monitored the variability of spatial and temporal velocity, respectively. Two complementary drifter studies enabled assessment of local transport through the junction to identify small-scale intrajunction dynamics. We supplemented field results with numerical simulations using the SUNTANS model to demonstrate the importance of phasing offsets for junction transport and dispersion. Different phasing of inflows to the junction resulted in scalar patchiness that is characteristic of MacVean and Stacey's (2011) advective tidal trapping. Furthermore, we observed smallscale junction flow features including a recirculation zone and shear layer, which play an important role in intra-junction mixing over time scales shorter than the tidal cycle (i.e., super-tidal time scales). The study period spanned open-and closed-gate operations at the Delta Cross Channel. Synthesis of field observations and modeling efforts suggest that management operations related to the Delta Cross Channel can strongly affect transport in the Delta by modifying the relative contributions of tidal and riverine flows, thereby changing the junction flow phasing. KEYWORDS Junction dispersion, flow phasing, tidal trapping, super-tidal time scales, Delta Cross Channel Hydrodynamic processes in the Sacramento-San Joaquin River Delta affect ecosystem function through the transport of salt, sediments, heat, contaminants including selenium and ammonium, and,

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Section: Introductionmentioning

confidence: 69%

Dispersion Mechanisms of a Tidal River Junction in the Sacramento–San Joaquin Delta, California

Gleichauf¹,

Wolfram²,

Monsen³

et al. 2014

SFEWS

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“…In estuaries, shoal/channel interactions and transverse circulation can contribute to altered horizontal dispersion via trapping (Okubo 1973;Fischer et al 1979;MacVean and Stacey 2010) and/or via altered cross-sectional mixing times (Dronkers and Zimmerman 1982;Fischer 1972;Smith 1976Smith , 1980Smith , 1996. Additionally, these processes can alter vertical mixing (e.g., Lacy et al 2003).…”

Section: Introductionmentioning

confidence: 99%

Frontogenesis and Frontal Progression of a Trapping-Generated Estuarine Convergence Front and Its Influence on Mixing and Stratification

Giddings

Fong

Monismith

et al. 2011

Estuaries and Coasts

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Estuarine fronts are well known to influence transport of waterborne constituents such as phytoplankton and sediment, yet due to their ephemeral nature, capturing the physical driving mechanisms and their influence on stratification and mixing is difficult. We investigate a repetitive estuarine frontal feature in the Snohomish River Estuary that results from complex bathymetric shoal/ channel interactions. In particular, we highlight a trapping mechanism by which mid-density water trapped over intertidal mudflats converges with dense water in the main channel forming a sharp front. The frontal density interface is maintained via convergent transverse circulation driven by the competition of lateral baroclinic and centrifugal forcing. The frontal presence and propagation give rise to spatial and temporal variations in stratification and vertical mixing. Importantly, this front leads to enhanced stratification and suppressed vertical mixing at the end of the large flood tide, in contrast to what is found in many estuarine systems. The observed mechanism fits within the broader context of frontogenesis mechanisms in which varying bathymetry drives lateral convergence and baroclinic forcing. We expect similar trapping-generated fronts may occur in a wide variety of estuaries with shoal/channel morphology and/or braided channels and will similarly influence stratification, mixing, and transport.

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“…The front trapping mechanism identified here caused by the shoaling and the subsequent vertical mixing of the salinity front can serve as an important mechanism for interfacial exchanges in estuarine systems where the main channel is flanked by shallow shoals. This type of trapping with a baroclinic nature differs from tidal trapping caused by barotropic processes, such as the differential phasing between the main channel and its side embayments or braching channels (Fischer et al, ; Giddings et al, ; MacVean & Stacey, ). For estuaries with wide shoals that are subject to strong buoyancy forcing, this process of generation and the subsequent shoalward movement of the trapped patch of salt as illustrated in Figure repeats itself in each tidal cycle, resulting in a series of local salinity maxima on the shoal.…”

Section: Lateral Baroclinic Circulations Across the Channel‐shoal Intmentioning

confidence: 99%

Numerical Investigation of Baroclinic Channel‐Shoal Interaction in Partially Stratified Estuaries

et al. 2020

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The internal, density-driven channel-shoal interaction in partially stratified estuaries is numerically investigated. Idealized General Estuarine Transport Model (GETM) simulations with the rigid-lid assumption are performed in a two-dimensional cross-sectional mode forced by a constant longitudinal salinity gradient and an M2 tidal current. Simulation results show that within each tide cycle, the abrupt flattening of bathymetry at the channel-shoal interface leads to the trapping of a patch of high-salinity water mass on the shoal. The resulting local salinity maximum near the interface interacts with the differential advection by the gentle slope of shoal, driving complex density-driven on-shoal lateral circulations. Next, it is found that the presence of the shoal in turn enhances the longitudinal residual circulation in the channel. As a storage of estuarine water that is partially isolated from the main stream, the shoal traps saltier water that originates from the channel during slack after ebb (thus a less stratified flood), while injects fresher water into the surface of the channel during slack after flood (thus a more stratified ebb). These lateral baroclinic exchanges across the channel-shoal interface give rise to a new mechanism for generating tidal asymmetry of eddy viscosity and shear in the channel (i.e., tidal straining circulation), which has the same orientation with the classical estuarine circulation. The interfacial salt trapping and the accompanied shoal-induced modification of channel residual circulation are demonstrated to possess a highly localized nature, with their characteristics largely independent of shoal bathymetry far away from the channel-shoal interface.Plain Language Summary Drowned-river estuaries are frequently characterized by a deep channel laterally bounded by shallow shoals with a gentle slope. Many such estuaries are experiencing impairment from nutrient over-enrichment such as low dissolved oxygen level, massive phytoplankton blooms and water quality degradation. Previous field and modeling studies have indicated that the shallow shoals with higher light availability allow for faster phytoplankton growth and that its exchange with the channel can influence the occurrence, timing and extent of large-scale phytoplankton blooms. We perform idealized numerical simulations of straight estuaries with distinct channel-shoal morphology to explore the hydrodynamic processes driven by differences in fluid density inside the water body. We focus on how the salt is exchanged between channel and shoal and how the water on the shoal moves perpendicular to the main axis of the estuary. We also highlight that the presence of the shoal tends to enhance the net along-estuary transport of water in the central channel when the oscillating tide is filtered out. The insights gained are expected to expand our knowledge of the physics of channel-shoal estuaries at various timescales, which can aid us in making management decisions regarding estuarine ecosystems.

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Estuarine Dispersion from Tidal Trapping: A New Analytical Framework

Cited by 25 publications

References 17 publications

Dispersion Mechanisms of a Tidal River Junction in the Sacramento–San Joaquin Delta, California

Dispersion Mechanisms of a Tidal River Junction in the Sacramento–San Joaquin Delta, California

Frontogenesis and Frontal Progression of a Trapping-Generated Estuarine Convergence Front and Its Influence on Mixing and Stratification

Numerical Investigation of Baroclinic Channel‐Shoal Interaction in Partially Stratified Estuaries

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