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Journal ArticleDOI

The Effect of Tidal Asymmetry and Temporal Settling Lag on Sediment Trapping in Tidal Estuaries

TL;DR: In this article, a 2DV model is developed to investigate the changes in relative roughness (friction) and mixing, resulting in fundamental changes in the char- acteristics of the horizontal (velocity) and vertical tides (sea surface elevation).
Abstract: Over decades and centuries, the mean depth of estuaries changes due to sea-level rise, land subsi- dence, infilling, and dredging projects. These processes produce changes in relative roughness (friction) and mixing, resulting in fundamental changes in the char- acteristics of the horizontal (velocity) and vertical tides (sea surface elevation) and the dynamics of sediment trapping. To investigate such changes, a 2DV model is developed. The model equations consist of the width- averaged shallow water equations and a sediment balance equation. Together with the condition of mor- phodynamic equilibrium, these equations are solved analytically by making a regular expansion of the vari- ous physical variables in a small parameter. Using these analytic solutions, we are able to gain insight into the fundamental physical processes resulting in sediment trapping in an estuary by studying various forcings separately. As a case study, we consider the Ems es- tuary. Between 1980 and 2005, successive deepening of the Ems estuary has significantly altered the tidal and sediment dynamics. The tidal range and the surface sediment concentration has increased and the position of the turbidity zone has shifted into the freshwater zone. The model is used to determine the causes of these historical changes. It is found that the increase of the tidal amplitude toward the end of the embayment is the combined effect of the deepening of the estuary and a 37% and 50% reduction in the vertical eddy viscosity and stress parameter, respectively. The phys- ical mechanism resulting in the trapping of sediment, the number of trapping regions, and their sensitivity to grain size are explained by careful analysis of the var- ious contributions of the residual sediment transport. It is found that sediment is trapped in the estuary by a delicate balance between the M2 transport and the residual transport for fine sediment (ws = 0.2 mm s −1 ) and the residual, M2 and M4 transports for coarser sediment (ws = 2 mm s −1 ). The upstream movement of the estuarine turbidity maximum into the freshwa- ter zone in 2005 is mainly the result of changes in tidal asymmetry. Moreover, the difference between the sediment distribution for different grain sizes in the same year can be attributed to changes in the temporal settling lag.

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Citations
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Journal ArticleDOI
TL;DR: In this paper, the authors investigated the effect of channel deepening and port construction on suspended sediment concentration (SSC) levels in the Ems Estuary, using a numerical model of suspended sediment transport forced by tides, waves and salinity.

196 citations


Cites background from "The Effect of Tidal Asymmetry and T..."

  • ...On the short term, maintenance dredging leads to increasing concentration levels in the direct vicinity of the dredging vessel (e.g. Collins, 1995; Pennekamp et al., 1996; Mikkelsen and Pejrup, 2000; Smith and Friedrichs, 2011)....

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Journal ArticleDOI
TL;DR: A transport decomposition is used to present processes that lead to convergent SPM transport, and trapping mechanisms that leading to ETMs at the landward limit of the salt intrusion, in the freshwater zone, at topographic transitions, and by lateral processes within the cross section are reviewed.
Abstract: Estuarine turbidity maxima (ETMs) are generated by a large suite of hydrodynamic and sediment dynamic processes, leading to longitudinal convergence of cross-sectionally integrated and tidally averaged transport of cohesive and noncohesive suspended particulate matter (SPM) The relative importance of these processes for SPM trapping varies substantially among estuaries depending on topography, fluvial and tidal forcing, and SPM composition The high-frequency dynamics of ETMs are constrained by interactions with the low-frequency dynamics of the bottom pool of easily erodible sediments Here, we use a transport decomposition to present processes that lead to convergent SPM transport, and review trapping mechanisms that lead to ETMs at the landward limit of the salt intrusion, in the freshwater zone, at topographic transitions, and by lateral processes within the cross section We use model simulations of example estuaries to demonstrate the complex concurrence of ETM formation mechanisms We also discuss how changes in SPM trapping mechanisms, often caused by direct human interference, can lead to the generation of hyperturbid estuaries

173 citations


Cites background from "The Effect of Tidal Asymmetry and T..."

  • ...The importance of barotropic, tide-induced SPM transport and trapping was later discussed for many other estuaries (see, e.g., Uncles & Stephens 1993, Dyer 1997, Brenon & Le Hir 1999, Chernetsky et al. 2010, Yu et al. 2014)....

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  • ...If the tidally averaged B is time independent, the system is said to be in morphodynamic equilibrium (Friedrichs et al. 1998, Huijts et al. 2006, Chernetsky et al. 2010)....

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Journal ArticleDOI
TL;DR: In this paper, an exploratory, semi-analytical model is calibrated to simulate the equilibrium SPM distribution in the upper estuary during five time periods from 1965 to 2005, using archival bathymetric and tidal data.
Abstract: Active deepening of tidal channels usually results in the alteration of the vertical and the horizontal tide. This may lead to concurrent significant increases in mean suspended matter concentrations (SPM) in coastal plain estuaries, the turbidity maximum (ETM) included. This is exemplified by an extensive analysis of the Ems estuary, a prototypical heavily stressed estuary in the Dutch-German border area. Measurements show that the SPM concentrations in the lower reaches of the estuary have increased an average of 2- to 3-fold between 1954 and 2005, with a 10-fold increase observed in the upper estuary (tidal river). Longitudinal profiles of surface SPM demonstrate that the ETM has moved upstream by up to 25 km and has broadened into a zone 30 km in length which extends into the freshwater tidal river. On an annual scale, variations in freshwater discharge significantly influence the formation and breakdown of the ETM: during low river discharge the ETM approaches equilibrium over 2–3 months, whilst elevated river discharges relocate the ETM downstream over several weeks. An exploratory, semi-analytical model is calibrated to simulate the equilibrium SPM distribution in the upper estuary during five time periods from 1965 to 2005, using archival bathymetric and tidal data. Results suggest that the deepening of tidal channels and a reduction in hydraulic drag have most likely resulted in a landward shift of the SPM trapping location. The measured increase in SPM concentrations and the development of fluid mud around the 1990s likely contributed to reduced mixing and bottom drag, creating a feedback loop that further altered tidal and SPM dynamics. It is argued that the removal of some non-erodible (consolidated) layers in the lower reaches of the estuary has created new internal sediment sources that may be responsible for feeding the observed high SPM concentrations, rather than increased sediment input from the boundaries. All findings are based on and supported by measured short-term seasonal fluctuations, as well as long-term developments of yearly averaged concentrations in the longitudinal SPM distribution.

168 citations

Journal ArticleDOI
TL;DR: In this article, the authors highlight six main factors that can cause changes in measured tidal statistics on local scales, and a further eight possible regional/global driving mechanisms, while only a few studies have combined observations and models, or modelled at a temporal/spatial resolution capable of resolving both ultra-local and large scale global changes.
Abstract: Scientists and engineers have observed for some time that tidal amplitudes at many locations are shifting considerably due to non-astronomical factors. Here we review comprehensively these important changes in tidal properties, many of which remain poorly understood. Over long geological time-scales, tectonic processes drive variations in basin size, depth, and shape, and hence the resonant properties of ocean basins. On shorter geological time-scales, changes in oceanic tidal properties are dominated by variations in water depth. A growing number of studies have identified widespread, sometimes regionally-coherent, positive and negative trends in tidal constituents and levels during the 19th, 20th and early 21st centuries. Determining the causes is challenging because a tide measured at a coastal gauge integrates the effects of local, regional, and oceanic changes. Here, we highlight six main factors that can cause changes in measured tidal statistics on local scales, and a further eight possible regional/global driving mechanisms. Since only a few studies have combined observations and models, or modelled at a temporal/spatial resolution capable of resolving both ultra-local and large-scale global changes, the individual contributions from local and regional mechanisms remain uncertain. Nonetheless, modelling studies project that sea-level rise and climate change will continue to alter tides over the next several centuries, with regionally coherent modes of change caused by alterations to coastal morphology and ice sheet extent. Hence, a better understanding of the causes and consequences of tidal variations is needed to help assess the implications for coastal defense, risk assessment, and ecological change.

143 citations


Cites background from "The Effect of Tidal Asymmetry and T..."

  • ...Resonance and Reflection The reflection of a tide wave at a river boundary, and the partial reflection that occurs due to abrupt changes in width and depth, tends to cause local amplification in tides (Chernetsky et al., 2010; Jay, 1991)....

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  • ...For example, construction of a weir in 1900 altered the length of the Ems estuary and produced an amplification in upriver tides (Chernetsky et al., 2010; Keller, 1901; Talke & Jay, 2013)....

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  • ...Furthermore, a number of modeling studies at local (e.g., Chernetsky et al., 2010; Familkhalili & Talke, 2016; Holleman & Stacey, 2014; Lee et al., 2017; Orton et al., 2015), regional (e.g., Arns et al., 2015, 2017; Devlin et al., 2018; Greenberg et al., 2012; Idier et al., 2017; Luz Clara et al.,…...

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  • ...Depth of Channels and Flats Multiple analytical, field, and numerical studies have now established that depth changes can change estuary tides, primarily through the frictional term in the momentum balance (Chernetsky et al., 2010; DiLorenzo et al., 1993; Familkhalili & Talke, 2016)....

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  • ...Consequently, even small changes in tidal range and tidal current velocities can influence coastal morphology (e.g., Chernetsky et al., 2010; Idier et al., 2017)....

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Journal ArticleDOI
TL;DR: In this paper, the authors recover archival tide gauge data back to 1844 and evaluate the trajectory of the annual maximum storm tide, finding that approximately half of longterm variance is anticorrelated with decadal-scale variations in the North Atlantic Oscillation, while long-term trends explain the remainder.
Abstract: Three of the nine highest recorded water levels in the New York Harbor region have occurred since 2010 (March 2010, August 2011, and October 2012), and eight of the largest twenty have occurred since 1990. To investigate whether this cluster of high waters is a random occurrence or indicative of intensified storm tides, we recover archival tide gauge data back to 1844 and evaluate the trajectory of the annual maximum storm tide. Approximately half of long-term variance is anticorrelated with decadal-scale variations in the North Atlantic Oscillation, while long-term trends explain the remainder. The 10 year storm tide has increased by 0.28 m. Combined with a 0.44 m increase in local sea level since 1856, the 10 year flood level has increased by approximately 0.72 ± 0.25 m, and magnified the annual probability of overtopping the typical Manhattan seawall from less than 1% to about 20–25%.

137 citations

References
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Journal ArticleDOI
TL;DR: In this article, the authors adapted the energy considerations used in the surface heating problem to describe the competition between the stabilizing effect of fresh water and the vertical mixing brought about by tidal and wind stirring.
Abstract: Buoyancy input as fresh water exerts a stratifying influence in estuaries and adjacent coastal waters. Predicting the development and breakdown of such stratification is an inherently more difficult problem than that involved in the analogous case of stratification induced by surface heating because the buoyancy input originates at the lateral boundaries. In the approach adopted here, we have adapted the energy considerations used in the surface heating problem to describe the competition between the stabilizing effect of fresh water and the vertical mixing brought about by tidal and wind stirring. Freshwater input induces horizontal gradients which drive the estuarine circulation in which lighter fluid at the surface is moved seaward over heavier fluid moving landward below. This contribution to stratification is expected to vary in time as the level of turbulence varies over the tidal cycle. The density gradient also interacts directly with the vertical shear in the tidal current to induce a periodic input to stratification which is positive on the ebb phase of the tide. Comparison of this input with the available stirring energy leads to a simple criterion for the existence of strain-induced stratification. Observations in a region of Liverpool Bay satisfying this criterion confirm the occurrence of a strong semidiurnal variation in stratification with complete vertical mixing apparent around high water except at neap tides when more permanent stratification may develop. A simulation of the monthly cycle based on a model including straining, stirring, and the estuarine circulation is in qualitative agreement with the main features of the observations.

776 citations


"The Effect of Tidal Asymmetry and T..." refers background in this paper

  • ...The importance of tidally varying processes on the formation of residual (tidally averaged) flows and sediment fluxes is discussed in many publications (Simpson et al. 1990; Geyer 1993; Jay and Musiak 1994; Burchard and Baumert 1998)....

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01 Jan 1990
TL;DR: In this article, the authors adapted the energy considerations used in the surface heating problem to describe the competition between the stabilizing effect of fresh water and the vertical mixing brought about by tidal and wind stirring.
Abstract: Buoyancy input as fresh water exerts a stratifying influence in estuaries and adjacent coastal waters Predicting the development and breakdown of such stratification is an inherently more difficult problem than that ivolved in the analogous case of stratification induced by surface heating because the buoyancy input originates at the lateral boundaries In the approach adopted here, we have adapted the energy considerations used in the surface heating problem to describe the competition between the stabilizing effect of fresh water and the vertical mixing brought about by tidal and wind stirring Freshwater input induces horizontal gradients which drive the estuarine circulation in which lighter fluid at the surface is moved seaward over heavier fluid moving landward below This contribution to stratification is expected to vary in time as the level of turbulence varies over the tidal cycle The density gradient also interacts directly with the vertical shear in the tidal current to induce a periodic input to stratification which is positive on the ebb phase of the tide Comparison of this input with the available stirring energy leads to a simple criterion for the existence of strain-induced stratification Observations in a region of Liverpool Bay satisfying this criterion confirm the occurrence of a strong semidiurnal variation in stratification with complete vertical mixing apparent around high water except at neap tides when more permanent stratification may develop A simulation of the monthly cycle based on a model including straining, stirring, and the estuarine circulation is in qualitative agreement with the main features of the observations

737 citations

Journal Article
TL;DR: In this article, a coupled set of partial differential equations and associated boundary conditions for estuaries in which turbulent mixing results primarily from tidal currents is described, where the circulation is separated into modes analogous to the barotropic, baroclinic and Ekman modes of oceanic circulation.
Abstract: A coupled set of partial differential equations and associated boundary conditions is written to describe circulation and salt-flux processes for estuaries in which turbulent mixing results primarily from tidal currents. Similarity solutions, motivated by characteristic salinity distributions observed in estuaries, are obtained for this set of equations and are compared with observational data. The circulation is separated into modes analogous to the barotropic, baroclinic, and Ekman modes of oceanic circulation. The salinity distribution, although coupled to the velocity distribution, is found to vary independently of it as well. The theoretical results are discussed in regard to: (i) correlation between the vertical variations of mean velocity and salinity, (ii) the role of this correlation in maintaining the steady-state salinity distribution in estuaries, and (iii) some implications for computations of flushing and dispersion of contaminants.

629 citations


"The Effect of Tidal Asymmetry and T..." refers methods in this paper

  • ...For example, using a tidally averaged numerical model, Festa and Hansen (1978) investigated the convergence zone of sediment due to the balance between gravitational circulation (Hansen and Rattray 1965; Officer 1976) and freshwater discharge....

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Journal ArticleDOI
TL;DR: In this article, the generation of tidal asymmetries is clarified via numerical integration of the one-dimensional equations for channel geometries characteristic of shallow estuaries, which are illustrated by modeling idealized representations of tidal channels at Nauset Inlet and Wachapreague Inlet, VA.
Abstract: The generation of tidal asymmetries is clarified via numerical integration of the one-dimensional equations for channel geometries characteristic of shallow estuaries. Channels without tidal flats develop a time asymmetry characterized by a longer falling than rising tide. This behavior is enhanced by strong friction and large channel cross-sectional area variability over a tidal cycle. Resulting tidal currents have a shorter, intense flood and a longer, weak ebb (flood-dominant). Addition of tidal flats to the channels can produce a longer rising tide and stronger ebb currents (ebb-dominant), if the area of tidal flats is large enough to overcome the effects of time-variable channel geometry. Weaker friction with flats can also produce this asymmetry. Despite the physical complexity of these systems, essential features of estuarine tidal response can be recovered from one-dimensional models. Shallow estuaries are shown to have a system response leading to stable, uniform senses of tidal asymmetry (either flood- or ebb-dominated, due to phase-locking of forced tidal constituents), with down-channel development in magnitude of asymmetry. These concepts are illustrated by modeling idealized representations of tidal channels at Nauset Inlet, MA, and Wachapreague Inlet, VA, which have flood- and ebb-dominance, respectively.

441 citations

Journal ArticleDOI

393 citations


"The Effect of Tidal Asymmetry and T..." refers methods in this paper

  • ...For example, using a tidally averaged numerical model, Festa and Hansen (1978) investigated the convergence zone of sediment due to the balance between gravitational circulation (Hansen and Rattray 1965; Officer 1976) and freshwater discharge....

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