We propose an ecomorphodynamic model which conceptualizes the chief land-forming processes operating on the intertwined, long-term evolution of marsh platforms and embedded tidal networks. The rapid network incision (previously addressed by the authors) is decoupled from the geomorphological dynamics of intertidal areas, governed by sediment erosion and deposition and crucially affected by the presence of vegetation. This allows us to investigate the response of tidal morphologies to different scenarios of sediment supply, colonization by halophytes, and changing sea level. Different morphological evolutionary regimes are shown to depend on marsh ecology. Marsh accretion rates, enhanced by vegetation growth, and the related platform elevations tend to decrease with distance from the creek, measured along suitably defined flow paths. The negative feedback between surface elevation and its inorganic accretion rate is reinforced by the relation between plant productivity and soil elevation in Spartina-dominated marshes and counteracted by positive feedbacks in multispecies-vegetated marshes. When evolving under constant sea level, unvegetated and Spartina-dominated marshes asymptotically tend to mean high water level (MHWL), different from multiple vegetation species marshes, which can make the evolutionary transition to upland. Equilibrium configurations below MHWL can be reached under constant rates of sea level rise, depending on sediment supply and vegetation productivity. Our analyses on marine regressions and transgressions show that when the system is in a supply-limited regime, network retreat and expansion (associated with regressions and transgressions, respectively) tend to be cyclic. Conversely, in a transport-limited regime, network reexpansion following a regression tends to take on a new configuration, showing a hysteretic behavior. Copyright 2007 by the American Geophysical Union.

Landscape evolution in tidal embayments: modeling the interplay of erosion, sedimentation, and vegetation dynamics

Waves are fundamentally important to the physical and biological functioning of estuaries. Understanding and predicting contaminant transport, development of sedimentary structures, geomorphological response to changes in external forcings such as rising sea level, and response of estuarine ecosystems to contaminant stressors require understanding of the relative roles of wave- and current-driven sediment transport. We review wave-driven sediment resuspension and transport in estuaries, including generation of bed shear stress by waves, initiation of sediment motion by waves, and the ways waves modulate, add to, and interact with sediment transport driven by currents. A key characteristic of the wave-induced force on the seabed is extreme spatial and temporal variations; simple analytical models are revealing of the way such patterns develop. Statistical methods have been widely applied to predict wave resuspension of intertidal-flat bed sediments, and physically based predictors of resuspension developed from open-coast studies appear to also apply to short-period estuarine waves. There is ample experimental evidence to conclude that over the long term, waves erode and tidal currents accrete intertidal flats. Waves indirectly add to the formation of fluid mud by adding to the estuarine pool of fine sediment, and waves may fluidize subtidal seabeds, changing bed erodibility. Models have been used to explore the dynamic balance between sediment transport by waves and by currents and have revealed the key control of waves on estuarine morphology. Estuarine intertidal flats are excellent natural laboratories that offer opportunities for working on a number of fundamental problems in sediment transport.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013RG000437

Review of wave‐driven sediment resuspension and transport in estuaries

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

Sediment Trapping in Estuaries.

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

The study of the morphology and evolution of tidal flats is particularly well suited in the context of morphodynamics in that characteristics such as profile shape, bed slope, and grain size clearly and systematically vary as a function of sediment supply and wave and tidal forcing, and the nature of wave- and tide-induced velocities across tidal flats is, in turn, a direct function of the tidal flat morphology itself. When averaged over annual or longer timescales, tidal flat morphology typically approximates a dynamic equilibrium with external forcing. Over seasonal and shorter timescales, spatial asymmetries in the hydrodynamic energy associated with temporally alternating tide- and wave-dominance drive sediment landward and seaward, respectively. For flats exhibiting minimal spatial gradients in energy, net sediment transport can still occur in response to local, time-dependent asymmetries associated with finite depth and intertidal storage effects; negative feedback on the scale of entire tidal embayments may favor minimization of these local asymmetries. Assuming that morphological equilibrium is likewise associated with the minimization of spatial asymmetries in energy, dominance by tides results in a convex-up tidal flat profile, whereas dominance by waves results in a concave-up profile. These extremes lead to analytical solutions for tidal flat width, slope, and degree of curvature as a function of tidal range, wave height, and the critical velocity for profile stability. Although a tide- or wave-dominated static equilibrium theoretically exists at each of these extremes, natural tidal flats over annual timescales are better approximated by a dynamic equilibrium somewhere between these two asymptotes. Observations and models indicate that, within this range of morphologies, convex-up profiles are further favored by increased sediment supply, increased bioaggregation/adhesion, and external forcing by faster-rising tides; concave-up profiles are further favored by decreased sediment supply, increased bioturbation, and external forcing by faster-falling tides. Processes/properties associated with evolution toward a convex (vs. concave) profile include shoreward (vs. seaward) sediment transport, net deposition (vs. erosion), decreased (vs. increased) grain size, and a form that progrades seaward (vs. retreating landward). Because surficial grain size responds to energy gradients much more quickly than overall morphology can adjust, the common presence of tides without strong waves (alternating with much shorter periods of intense waves) leads to landward fining of surficial grain size most of the time.

Tidal flat morphodynamics: a synthesis

In this review we discuss the morphodynamics of tidal inlet systems that are typical of barrier coasts formed during a period of continuous sea-level rise during the Holocene. The morphodynamics concerns feedbacks between three major components: the hydrodynamics of tidal currents and wind waves; the erosion, deposition, and transport of sediment under the action of the former hydrodynamic agencies; and the morphology proper, which results from the divergence of the sediment transport. We discuss the morphodynamics of the different units that characterize a tidal inlet system: the overall system, the ebb-tidal delta, the tidal channels, channel networks, tidal bars and meanders, and finally the intertidal zone of tidal flats and salt marshes. In most of these units, stability analysis is a major guide to the establishment of equilibrium structures.

Morphodynamics of Tidal Inlet Systems

Etude du courant rectifie, induit par la divergence du flux de vorticite dans une couche limite laterale, oscillatoire, visqueuse, situee le long de la paroi verticale d'un bassin semi-ferme. Elle est conduite en fonction du nombre de Strouhal (equivalent au nombre de Rossby de la couche limite interieure, oscillatoire, visqueux) et du nombre de Stokes. Le rapport au carre, de ces nombres, definit un autre nombre de Reynolds mesurant la force de l'auto-advection par le courant residuel. Pour une forte auto-advection, ce courant-ci decroit vers zero sur une frontiere exterieure, sa largeur etant grande comparee a celle de la couche interieure. Analyse des regimes d'auto-advection, faible, modere et fort

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Tidal rectification in lateral viscous boundary layers of a semi-enclosed basin

A nonlinear Stommel model of the ocean circulation on the beta plane, driven by a time periodic wind stress, is investigated in order to study symmetry properties of the observed time-mean ocean gyres. Due to the presence of vorticity advection terms the model will have a steady or rectified response to fluctuating wind fields. In this paper a small inverse Ekman number, “the small beta regime”, is considered. It is demonstrated that for this case all qualitative features of the residual circulation, obtained numerically by Veronis (1970). are reproduced in an analytical way. They include the dipole character of the gyre, its maximum symmetry breaking around the north-south axis for intermediate Reynolds numbers, measuring the ratio of vorticity forcing and dissipation, and the maximum residual response for intermediate forcing frequencies.

Rectification of the wind-driven ocean circulation on the beta plane

A series solution approach to the forced and damped quasi-geostrophic barotropic vorticity equation is considered in order to examine the strongly forced and inertial limits of ocean gyre dynamics. The strongly forced limit is the limit investigated numerically by Veronis (1966b). It is shown that this limit, although superficially having the same symmetry properties as the inertial limit, is distinguishably different from the latter. After isolating the inertial limit in an appropriate way it is shown that our series solution method is able to find the ‘free mode’ and its ‘almost free correction’, that the ‘free mode’ obeys the integral criteria of Niiler (1966) and Pierrehumbert & Malguzzi (1984) and that the relationship between the streamfunction and the absolute vorticity is in general a nonlinear one.

http://electronicsandbooks.com/eab1/manual/Magazine/J/Journal%20of%20Fluid%20Mechanics/1995%20Volume%20295/S0022112095001893.pdf

A series solution of the nonlinear wind-driven ocean circulation and its inertial limit

Tidal flats, i.e. areas in tidal embayments that fall dry during part of the tidal cycle, have a significant impact on the nonlinear ”shallow water” tides, which are asymmetries of the tidal signal in sea surface elevation and tidal velocity. Tidal asymmetry is important for net sediment transport and thereby also for the morphological stability of the channel. Indeed, it has been shown in previous studies that the importance of tidal flats as a temporal storage of mass is non-negligible.
In this contribution the focus lies on a new aspect of tidal flats, viz. that they also act as a net sink term for the along-channel momentum. This momentum loss term is a nonlinear function of sea surface elevation and velocity, and is significant in areas where the tidal flat width, bf, is of comparable or larger size than the width bc of the main channel.
The process of momentum loss will be analyzed and its efficiency in generating nonlinear tides and net sediment transport will be compared with other sources of nonlinearities on the basis of results from a numerical model.

J. T. F. Zimmerman

Papers

Morphodynamics of Tidal Inlet Systems

Tidal rectification in lateral viscous boundary layers of a semi-enclosed basin

Rectification of the wind-driven ocean circulation on the beta plane

A series solution of the nonlinear wind-driven ocean circulation and its inertial limit

Tidal asymmetries in estuaries due to channel-flat interactions, a simple model