Assessing water renewal time scales for marine environments from three-dimensional modelling: a case study for Hervey Bay, Australia

We apply the three-dimensional Coupled Hydrodynamical Ecological model for Regional Shelf Seas (COHERENS) to compute water renewal time scales for Hervey Bay, a large coastal embayment situated off the central eastern coast of Australia. Water renewal time scales are not directly observable but are derived indirectly from computational studies. Improved knowledge of these time scales assists in evaluating the water quality of coastal environments and can be utilised in sustainable marine resource management. Results from simulations with climatological September forcing are presented and compared to cruise data reported by Ribbe (2006). A series of simulations using idealised forcing provides detailed insight into water renewal pathways and regional differences in renewal timescales. We find that more than 85 % of the coastal embayment’s water is fully renewed within about 50-80 days. The eastern and western shallow coastal regions are ventilated more rapidly than the central, deeper part of the domain. The climatological simulation yields temperature and salinity patterns that are consistent with the observed situation and water renewal times scales in the range of those derived from idealised model studies. While the reported simulations involve many simplifications, the global assessment of the renewal time scale is in the range of a previous estimate derived for this coastal embayment from a simpler model and observational data

Using a composite grid approach in a complex coastal domain to estimate estuarine residence time

This paper is not subject to U.S. copyright. The definitive version was published in Computers & Geosciences 36 (2010): 921-935, doi:10.1016/j.cageo.2009.11.008.We investigate the processes that influence residence time in a partially mixed estuary using a three-dimensional circulation model. The complex geometry of the study region is not optimal for a structured grid model and so we developed a new method of grid connectivity. This involves a novel approach that allows an unlimited number of individual grids to be combined in an efficient manner to produce a composite grid. We then implemented this new method into the numerical Regional Ocean Modeling System (ROMS) and developed a composite grid of the Hudson River estuary region to investigate the residence time of a passive tracer. Results show that the residence time is a strong function of the time of release (spring vs. neap tide), the along-channel location, and the initial vertical placement. During neap tides there is a maximum in residence time near the bottom of the estuary at the mid-salt intrusion length. During spring tides the residence time is primarily a function of along-channel location and does not exhibit a strong vertical variability. This model study of residence time illustrates the utility of the grid connectivity method for circulation and dispersion studies in regions of complex geometry.W.R. Geyer was supported by the Hudson River Foundation Grant 002/07A,H.G.Arango by the Office of Naval Research,and John Warner was supported by the USGS Community Sediment Modeling Project

Turbulent and numerical mixing in a salt wedge estuary : dependence on grid resolution, bottom roughness, and turbulence closure

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 692–712, doi:10.1002/2016JC011738.The Connecticut River is a tidal salt wedge estuary, where advection of sharp salinity gradients through channel constrictions and over steeply sloping bathymetry leads to spatially heterogeneous stratification and mixing. A 3-D unstructured grid finite-volume hydrodynamic model (FVCOM) was evaluated against shipboard and moored observations, and mixing by both the turbulent closure and numerical diffusion were calculated. Excessive numerical mixing in regions with strong velocities, sharp salinity gradients, and steep bathymetry reduced model skill for salinity. Model calibration was improved by optimizing both the bottom roughness (z0), based on comparison with the barotropic tidal propagation, and the mixing threshold in the turbulence closure (steady state Richardson number, Rist), based on comparison with salinity. Whereas a large body of evidence supports a value of Rist ∼ 0.25, model skill for salinity improved with Rist ∼ 0.1. With Rist = 0.25, numerical mixing contributed about 1/2 the total mixing, while with Rist = 0.10 it accounted for ∼2/3, but salinity structure was more accurately reproduced. The combined contributions of numerical and turbulent mixing were quantitatively consistent with high-resolution measurements of turbulent mixing. A coarser grid had increased numerical mixing, requiring further reductions in turbulent mixing and greater bed friction to optimize skill. The optimal Rist for the fine grid case was closer to 0.25 than for the coarse grid, suggesting that additional grid refinement might correspond with Rist approaching the theoretical limit. Numerical mixing is rarely assessed in realistic models, but comparisons with high-resolution observations in this study suggest it is an important factor.NSF Grant Number: OCE 0926427; ONR Grant Number: N00014-08-1-11152017-07-2

Natural, numerical and structure-induced mixing in dense gravity currents: idealised and realistic model studies

Diese Arbeit befasst sich mit der Untersuchung von natürlicher, numerischer und durch vertikale Strukturen verursachter Vermischung dichter Bodenströmungen mit Hilfe von Modellsimulationen der westlichen Ostsee. Zusätzlich wurde das Ausbreitungsverhalten dichter Bodenströmungen in der westlichen Ostsee mit passiven Tracern im Modell analysiert. Ebenfalls konnte gezeigt werden, dass die durch Diskretisierungsfehler der Advektionsschemen hervorgerufene numerische Vermischung ähnliche Größenordnungen wie die rein natürliche Vermischung, aber eine andere räumliche und zeitliche Verteilung aufweist

Using tracer variance decay to quantify variability of salinity mixing in the Hudson River Estuary

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Warner, J. C., Geyer, W. R., Ralston, D. K., & Kalra, T. Using tracer variance decay to quantify variability of salinity mixing in the Hudson River Estuary. Journal of Geophysical Research: Oceans, 125(12), (2020): e2020JC016096, https://doi.org/10.1029/2020JC016096.The salinity structure in an estuary is controlled by time‐dependent mixing processes. However, the locations and temporal variability of where significant mixing occurs is not well‐understood. Here we utilize a tracer variance approach to demonstrate the spatial and temporal structure of salinity mixing in the Hudson River Estuary. We run a 4‐month hydrodynamic simulation of the tides, currents, and salinity that captures the spring‐neap tidal variability as well as wind‐driven and freshwater flow events. On a spring‐neap time scale, salinity variance dissipation (mixing) occurs predominantly during the transition from neap to spring tides. On a tidal time scale, 60% of the salinity variance dissipation occurs during ebb tides and 40% during flood tides. Spatially, mixing during ebbs occurs primarily where lateral bottom salinity fronts intersect the bed at the transition from the main channel to adjacent shoals. During ebbs, these lateral fronts form seaward of constrictions located at multiple locations along the estuary. During floods, mixing is generated by a shear layer elevated in the water column at the top of the mixed bottom boundary layer, where variations in the along channel density gradients locally enhance the baroclinic pressure gradient leading to stronger vertical shear and more mixing. For both ebb and flood, the mixing occurs at the location of overlap of strong vertical stratification and eddy diffusivity, not at the maximum of either of those quantities. This understanding lends a new insight to the spatial and time dependence of the estuarine salinity structure.This study was funded through the Coastal Model Applications and Field Measurements Project and the Cross‐shore and Inlets Project, US Geological Survey Coastal Marine Hazards and Resources Program. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government

Freshwater Composition and Connectivity of the Connecticut River Plume During Ambient Flood Tides

The Connecticut River plume interacts with the strong tidal currents of the ambient receiving waters in eastern Long Island Sound. The plume formed during ambient flood tides is studied as an example of tidal river plumes entering into energetic ambient tidal environments in estuaries or continental shelves. Conservative passive freshwater tracers within a high-resolution nested hydrodynamic model are applied to determine how source waters from different parts of the tidal cycle contribute to plume composition and interact with bounding plume fronts. The connection to source waters can be cut off only under low-discharge conditions, when tides reverse surface flow through the mouth after max ambient flood. Upstream plume extent is limited because ambient tidal currents arrest the opposing plume propagation, as the tidal internal Froude number exceeds one. The downstream extent of the tidal plume always is within 20 km from the mouth, which is less than twice the ambient tidal excursion. Freshwaters in the river during the preceding ambient ebb are the oldest found in the new flood plume. Connectivity with source waters and plume fronts exhibits a strong upstream-to- downstream asymmetry. The arrested upstream front has high connectivity, as all freshwaters exiting the mouth immediately interact with this boundary. The downstream plume front has the lowest overall connectivity, as interaction is limited to the oldest waters since younger interior waters do not overtake this front. The offshore front and inshore boundary exhibit a downstream progression from younger to older waters and decreasing overall connectivity with source waters. Plume-averaged freshwater tracer concentrations and variances both exhibit an initial growth period followed by a longer decay period for the remainder of the tidal period. The plume-averaged tracer variance is increased by mouth inputs, decreased by entrainment, and destroyed by internal mixing. Peak entrainment velocities for younger waters are higher than values for older waters, indicating stronger entrainment closer to the mouth. Entrainment and mixing time scales (1–4 h at max ambient flood) are both shorter than half a tidal period, indicating entrainment and mixing are vigorous enough to rapidly diminish tracer variance within the plume

Mixing Processes in Tidally Pulsed River Plumes: Mechanisms, Significance, and Variability

River plumes form at the river-ocean interface when fresh, buoyant river water merges with salty, dense ocean water and can significantly modify coastal water properties and circulation. It is important to understand how plumes physically mix into the ocean to inform predictive modeling of river-borne tracers to coastal seas. In tidally energetic regions such as New England, river plumes can form and evolve with each new tide and are referred to as “tidally pulsed”. In this dissertation, we explore the numerous mechanisms which can contribute to mixing tidally pulsed plumes (i.e., frontal, stratified shear [interfacial], and bottom-generated tidal mixing) their spatiotemporal variability, controlling processes, and the relative importance of each to plume dilution by utilizing numerical modeling and field observation techniques. The contributions of frontal, interfacial, and bottom-generated tidal mixing are first investigated using an idealized numerical model broadly inspired by the Connecticut River plume. A mixing budget is applied, and river discharge and tidal amplitude are varied between experiments to isolate the influence of each forcing on the budget. Results indicate bottom-generated tidal mixing can dominate the mixing budget for large tide, small discharge events, when the product of the nondimensional Estuarine Richardson number and inverse Rossby number ( ) exceeds 1. When the nondimensional parameter is below 1, interfacial mixing dominates. Frontal mixing was found to never exceed 10% of total mixing in the budget. This is the first study to identify the potential for bottom-generated tidal mixing to dominate mixing in surface-advected river plumes. Wind controls on stratified shear mixing in tidal plumes is investigated using a realistic model of the Merrimack River plume system. A salinity variance approach is applied, allowing for the quantification of stratifying and de-stratifying processes (straining, mixing, advection) throughout the tidal plume. Winds countering the right-turning tendency of the plume are found to be most effective at increasing plume mixing. During the wind events, ambient shelf stratification is advected offshore, which creates a saltier shelf condition beneath the plume and increases the vertical salinity gradient. Simultaneously, plume layer velocities are enhanced, increasing shear and straining. The larger salinity gradient between plume and ambient coupled with increased shear leads to enhanced stratified shear mixing in the near and mid-field plume. The wind mechanism was found to be effective at modulating mixing at short, tidal time scales. The evolution of stratified shear mixing throughout the interior Merrimack River plume is characterized using observational data. Three source-to-front transects were conducted over a ~6-hour tidal pulse during low wind conditions. Data collection on each transect included continuous sampling of current magnitude and direction supplemented by profiles of turbulent kinetic energy dissipation rates and conductivity, temperature, and depth (CTD). Analysis shows stratified shear mixing transforms spatially and temporally over a tide and is characterized by three distinct regimes: plume layer mixing, nearfield interfacial mixing, and tidal interfacial mixing. Plume layer mixing is confined within the plume and decreases offshore of the nearfield as the tide progresses. Nearfield interfacial mixing facilities exchange between the plume and underlying ambient shelf throughout the tidal pulse. Tidal interfacial mixing mixes plume with ambient waters offshore of the nearfield at the end of ebb tide when shelf currents reverse direction beneath the plume. These observations provide some of the most robust spatiotemporal plume mixing estimates to date. This dissertation highlights the highly variable nature of mixing in tidally pulsed river plumes and the oftenimportant influence of the ambient shelf condition on mixing. Winds and tides impact the collective plumeshelf system to varying degrees which subsequently modulates mixing in a spatiotemporally varying manner. Analyses of static locations or times likely omit essential processes contributing to mixing. This research provides important context for future coastal model development

Tracer and Timescale Methods for Passive and Reactive Transport in Fluid Flows

Geophysical, environmental, and urban fluid flows (i.e., flows developing in oceans, seas, estuaries, rivers, aquifers, reservoirs, etc.) exhibit a wide range of reactive and transport processes. Therefore, identifying key phenomena, understanding their relative importance, and establishing causal relationships between them is no trivial task. Analysis of primitive variables (e.g., velocity components, pressure, temperature, concentration) is not always conducive to the most fruitful interpretations. Examining auxiliary variables introduced for diagnostic purposes is an option worth considering. In this respect, tracer and timescale methods are proving to be very effective. Such methods can help address questions such as, "where does a fluid-born dissolved or particulate substance come from and where will it go?" or, "how fast are the transport and reaction phenomena controlling the appearance and disappearance such substances?" These issues have been dealt with since the 19th century, essentially by means of ad hoc approaches. However, over the past three decades, methods resting on solid theoretical foundations have been developed, which permit the evaluation of tracer concentrations and diagnostic timescales (age, residence/exposure time, etc.) across space and time and using numerical models and field data. This book comprises research and review articles, introducing state-of-the-art diagnostic theories and their applications to domains ranging from shallow human-made reservoirs to lakes, river networks, marine domains, and subsurface flow

Modelling the Water Quality of the Patos Lagoon, Brazil

A two-dimensional depth integrated finite element modelling suite comprising the flow model TELEN4AC-2D and the water quality model WQFLOW-2D has been calibrated to simulate the physics and chemistry of the Patos Lagoon and Estuary system in southern Brazil for the investigation of nutrients, primary production and faecal bacteria. The model has been evaluated for use as a predictive tool to aid the decision making process for the rehabilitation and management of the shallow embayment of Saco da Mangueira adjacent to the city of Rio Grande in the lower Patos Estuary. This bay is one of several shallow areas bordering the city which suffers from the water quality pollution problems associated with eutrophication due to the influence of multiple and conflicting human impacts including the disposal of waste water from domestic and industrial sources such as the fertiliser industry, fish processing, and petroleum refining. The validated flow model indicated a very weak circulation in the Saco da Mangueira with velocities an order of magnitude lower than in the estuary. Simulations conducted to evaluate transport and mixing time scales demonstrated limited water exchange between the bay and the estuary principally controlled by wind direction and duration, with efolding flushing times between 21 and 45 days using observed wind and river flow data. The water quality modelling undertaken in this research represents the first reported application of WQFL0W-2D to the Patos lagoon and estuary system, and the first water quality modelling exercise of any kind reported to date for the Saco da Mangueira. Calibration and validation processes demonstrated that WQFLOW-2D could simulate annual average observed concentrations of water quality variables consistently and confirmed the eutrophic nature of the waters within the Saco da Mangueira. The model was used as a comparative tool to evaluate the predicted performance of hypothetical engineering schemes designed to improve the water quality within the bay and water exchange at the mouth. A number of recommendations were made including an imperative requirement for the collection of pollutant input and process data to reduce the level of uncertainty associated with the water quality model.HR Wallingford Limite

The atmospheric carbon sequestration potential of man-made tidal lagoons

Understanding sequestration of carbon by coastal ecosystems is central to addressing the role they play in climate change mitigation. To quantify this process, accurate measurements of CO2 fluctuation, coupled with variations in residence time of coastal water-bodies are required. Nearshore ecosystems, including coastal lagoons, may provide an effective sink for atmospheric carbon dioxide, particularly those containing productive biota such as seagrass. However, the rate and pattern of carbon sequestration in seagrass meadows across a range of environmental settings is still poorly constrained. In this study, we utilize a robust physical tidal model, along with biogeochemical dissolved inorganic carbon (DIC) assessment, to estimate water residence time and net sequestration of atmospheric CO2 in an intertidal lagoon containing a seagrass (Zostera noltii) meadow. Total alkalinity and pH measurements taken from advected water mass exchanged with the open ocean at inlet boundaries are used to calculate DIC and pCO2. A predictive model of hydrodynamics provides good approximation of mean water residence time to within 6 h (±3 s.d). Results indicate that during the daytime study period the lagoon is a sink for carbon, having a mean net ecosystem productivity (NEP) of 3.0 ± 0.4 mmol C m−2 hr−1. An equivalent diel NEP range of between 15.23 and −9.24 mmol C m−2 d−1 is calculated based on reported shallow water pelagic respiration rates. Moreover, approximately 4% of DIC availability occurs from atmospheric CO2 transfer to lagoon water. However, a negative diel rate of −82 ± 81 mmol C m−2 d−1 is found, assuming overnight respiration ascertained from converted Zostera noltii O2 utilization. We hypothesize that analogous regional nearshore ecosystems provide baseline study sites suitable to elucidate the carbon capture potential of planned, nearby tidal range energy schemes