Application of a Particle Transport Model in the Vicinity of a Riverine Tidal Boundary (original) (raw)

Effects of tidal and river discharge forcings on tidal propagation along the Guadiana Estuary

Journal of Sea Research, 2019

A numerical model is implemented to explore the effect of the river discharge on tidal propagation along the Guadiana Estuary, a rock-bound estuary located in Southwest Iberia. The MOHID numerical model, in 2D barotropic mode, has been forced by tides at the ocean side and by freshwater at the upstream boundary of the domain. The model was validated using water level and velocity observations at several locations along the estuary. Different scenarios with variable tidal forcings and freshwater discharges were analysed, considering the semi-diurnal constituents and M4 overtide, in order to assess the influence of each external agent on the alongchannel hydrodynamics. The model reproduces the expected general tidal properties along the channel in terms of amplitude (of both elevation and current), asymmetry and phase between horizontal and vertical tides. Three zones along the estuary have been defined based on the overtide patterns. Tidal propagation in Zone I, at the lower estuary, vary essentially with the tidal amplitude at the mouth while changes due to river discharge are minor. Along Zone II (middle estuary), the tidal forcing still predominates, for low discharge values only. Along Zone III (upper estuary), the tidal propagation is primarily controlled by the river discharge. Although the discharge threshold vary with the location and parameter considered, it is estimated that a discharge as little as 100 m3/s has a strong effect on the tidal properties along the system as a whole. It is therefore concluded that tidal properties in rock-bound estuaries may be importantly modified by relatively weak river discharge events. tems (Nash et al., 2014). Most of worldwide estuaries are affected by freshwater inflow

Forces in an Estuary: Tides, Freshwater, and Friction

Oceanography

The goal of this activity is to help environmental science students understand and compare hydrodynamic forces in an actual estuary. The interplay of physical forces in an estuary determines the currents and the amounts of mixing and stratification within the water column. The currents, in turn, are an important control on the distribution of phytoplankton, which form the base of the food web, and suspended sediments, which contain nutrients and pollutants. During this activity, students estimate the relative strengths of the key forces in an estuary, the barotropic and baroclinic pressure gradients, and friction. In addition, students estimate how these values change during flood and ebb and spring and neap tidal phases. Another purpose of this activity is to provide students with experience in analyzing data using spreadsheets, in organizing and collaborating within a fieldwork team, and in producing a scientific report. AUDIENCE This field experiment was designed for an intermediate-to upper-level Introduction to Physical Oceanography class and is also appropriate for upper-level undergraduates or graduate students in marine or environmental studies.

Tidal river dynamics: Implications for deltas

Reviews of Geophysics

Tidal rivers are a vital and little studied nexus between physical oceanography and hydrology. It is only in the last few decades that substantial research efforts have been focused on the interactions of river discharge with tidal waves and storm surges into regions beyond the limit of salinity intrusion, a realm that can extend inland hundreds of kilometers. One key phenomenon resulting from this interaction is the emergence of large fortnightly tides, which are forced long waves with amplitudes that may increase beyond the point where astronomical tides have become extinct. These can be larger than the linear tide itself at more landward locations, and they greatly influence tidal river water levels and wetland inundation. Exploration of the spectral redistribution and attenuation of tidal energy in rivers has led to new appreciation of a wide range of consequences for fluvial and coastal sedimentology, delta evolution, wetland conservation, and salinity intrusion under the influence of sea level rise and delta subsidence. Modern research aims at unifying traditional harmonic tidal analysis, nonparametric regression techniques, and the existing understanding of tidal hydrodynamics to better predict and model tidal river dynamics both in single-thread channels and in branching channel networks. In this context, this review summarizes results from field observations and modeling studies set in tidal river environments as diverse as the Amazon in Brazil, the Columbia, Fraser and Saint Lawrence in North America, the Yangtze and Pearl in China, and the Berau and Mahakam in Indonesia. A description of state-of-the-art methods for a comprehensive analysis of water levels, wave propagation, discharges, and inundation extent in tidal rivers is provided. Implications for lowland river deltas are also discussed in terms of sedimentary deposits, channel bifurcation, avulsion, and salinity intrusion, addressing contemporary research challenges.

Processes, Morphodynamics, and Facies of Tide-Dominated Estuaries

Principles of Tidal Sedimentology, 2011

As defined in this chapter, an estuary foons during a shoreline transgression and then fills during a progradational phase that is transitional to a delta. The spatial distribu tion of processes, grain sizes and facies within tide-dominated estuaries is predict able in general teons. Tidal currents dominate sedimentation along the axis, with wave-dominated sedimentation occurring along the flanks of the estuary in its outer pan. Tidal energy increases into the estuary but then decreases toward the tidal limit, with a gradual transition to river-dominated sedimentation at its head. The interac tion of the tidal wave with the morphology of the estuary, and with river currents, causes the outer estuary to be flood-dominant, with a net landward movement of sand. By contrast, the inner estuary is ebb-dominant, creating a bedload convergence within the estuary. The axial sandy deposits are typically finest at this location. In transgressive-phase estuaries, the main channel shows a low-high-low pattern of sinuosity, with the tightest bends (sinuosity~2.5) occurring at the bedload conver gence. These bends experience neck cutoff in the transition to the progradational phase of estuary filling. The estuary-mouth region is characterized by cross-bedded sands deposited on elongate sand bars, although wave-generated structures can be imponant in some cases. Estuaries that are down-drift of major rivers have anoma lously muddy outer estuarine deposits. Further landward, upper-flow-regime paral lei lamination can be widespread. The margins of the inner estuary are flanked by muddy salt-marsh and tidal-flat deposits that can contain well-developed tidal rhythmites and evidence of seasonal variations in river discharge.

Tidal inlets and estuaries: Comparison of Bruun, Escoffier, O'Brien and attractors

Coastal Engineering, 2018

The authors have shown previously that over very long time scales a barrier estuary or tidal inlet will tend towards one of two states, called attractors. In this paper it is shown how that analysis represents an extension and generalisation of three earlier procedures. There are three widely recognised quantitative semi-empirical procedures describing the long term equilibrium dimensions of the entrance channel of barrier estuaries and tidal inlets. The best known of these laws is the tidal prism-entrance area relation, often referred to as the O'Brien equation. The second procedure is based on the Escoffier or closure diagram, comprising a simple hydrodynamic relationship between the entrance area and the entrance velocity together with an empirical "equilibrium velocity". The third is the set of rules developed by Bruun that relate the entrance channel stability to the longshore sediment supply and the entrance channel sediment transport capacity. Each of these is based on major simplifications that restrict its utility and range of validity more than is usually recognised. The attractor analysis, while still based on a lumped model, adds sediment transport and deposition/scour equations and enables the use of more realistic entrance hydrodynamics. It predicts the rates of change and presents the results on a practical "attractor map". The predictions of the three empirical laws are compared with the attractor map and their practical application is critically compared. The empirical laws are shown to provide broadly equivalent predictions to the attractor map, but over limited ranges of estuary conditions. In particular, none of the empirical laws identifies the entrance closure state or describes conditions near closure.

Where river and tide meet: The morphodynamic equilibrium of alluvial estuaries

Journal of Geophysical Research: Earth Surface, 2015

We investigate the morphodynamic equilibrium of tidally dominated alluvial estuaries, extending previous works concerning the purely tidal case and the combined tidal-fluvial case with a small tidal forcing. We relax the latter assumption and seek the equilibrium bed profile of the estuary, for a given planform configuration with various degrees of funneling, solving numerically the 1-D governing equation. The results show that with steady fluvial and tidal forcings, an equilibrium bed profile of estuaries exists. In the case of constant width estuaries, a concave down equilibrium profile develops through most of the estuary. Increasing the amplitude of the tidal oscillation, progressively higher bed slopes are experienced at the mouth while the river-dominated portion of the estuary experiences an increasing bed degradation. The fluvial-marine transition is identified by a "tidal length" that increases monotonically as the river discharge and the corresponding sediment supply are increased while the river attains a new morphological equilibrium configuration. Tidal length also increases if, for a fixed river discharge and tidal amplitude, the sediment flux is progressively reduced with respect to the transport capacity. In the case of funnel-shaped estuaries the tidal length strongly decreases, aggradation is triggered by channel widening, and tidal effects are such to enhance the slope at the inlet and the net degradation of the river bed. Finally, results suggest that alluvial estuaries in morphological equilibrium cannot experience any amplification of the tidal wave propagating landward. Hence, hypersynchronous alluvial estuaries cannot be in equilibrium. Estuarine morphology results from a number of forcing factors: tidal motion, riverine (i.e., freshwater) flow, wave action, and, possibly, gravitational circulations driven by salinity and density gradients associated with the progressive admixture of river water and seawater [Hansen and Rattray, 1966]. Sediment characteristics and local geology can also play a role in shaping an estuary. The mutual interplay and feedbacks of all these physical processes make it difficult to provide a simple agreed classification of these landforms [

A study of non-linear tidal propagation in shallow inlet/estuarine systems Part II: Theory

Estuarine Coastal and Shelf Science, 1985

The offshore tide becomes strongly distorted as it propagates into shallow estuarine systems. Observations of sea surface elevation and horizontal currents over periods ranging from three days to one year, at nine stations within Nauset inlet/estuary, document the non-linear interaction of the offshore equilibrium tidal constituents. Despite strong frictional attenuation within the estuary, the overtides and compound tides of M,, S, and N,, in particular, reach significant amplitude, resulting in strong tidal distortion. High frequency forced constituents in sea surface are phase-locked, consistently leading the forcing tides by 60-70", resulting in a persistent distortion where falling tide is longer than rising tide. Forced constituents in currents are more nearly in phase with equilibrium constituents, producing flood currents which are shorter but more intense than ebb currents. A compound fortnightly tide, MS,, modulates the mean water level such that lowest tides occur during neap phase instead of spring phase. This fortnightly tide can be contaminated by storm surge, changing the phase characteristics of this constituent. Implications of the overtides, compound tides, and lower frequency tides on near-bed, suspended and dissolved material transport are profound.

Some features of the dynamic structure of a deep estuary

Estuarine, Coastal and Shelf Science, 1983

A boundary layer formulation for the dynamic structure of a deep estuary is developed. Cross-stream averages are used, but the boundary layer structure is shown to depend on the cross-stream geostrophic constraint. A similarity transformation and a weighted residual method are used to derive an approximate solution for the velocity and salinity structure of the upper layer. This solution indicates that, in the central regime of the estuary, outflow extends through the entire halocline. Inflow takes place in a much less stratified lower layer, and mass exchange between the layers is by upwelling. This structure is modified in the outer regime of the estuary, where mixing between the layers develops, and in the inner regime, where a sharp halocline develops and where the dynamics are dominated by river runoff. The implications of the dynamics for the flushing process and for pollutant movement and dispersion are discussed.

Development of the Turbidity Maximum in a Coastal Plain Estuary

1973

A study of the turbidity maximum in the Rappahannock EstuarY' Virginia was conducted to determine how high concentrations of suspended sediment accumulate to form a maximum. Time-series observations of current velocity, salinity an? suspended sediment over 8 to 18 tidal cycles reveal that the maX~~~~on forms in a convergence of bottom residual currents near the trans: ~ r between fresh and salty water. Sediment supplied mainly by the rlve is transported into the convergence by density currents and accum~-•ng. lates since velocity is nearly zero and settling exceeds upward m~X~ The maximum forms in the middle estuary after freshet or ~lood ing and shifts upstream with a landward shift of the salt intrus~on head and diminished river inflow. At the same time, its intensitYd is reduced by settling out, reduced strength of the convergence an increased mixing. Prime prerequisites for development are a strong convergence and high river inflow. One of the main difficulties in studying partially-mixed estuaries is that river inflow, tidal currents, salinity and sediment distributions are continuously changing and therefore never in a iv steady-state condition. They are subject to wide variations with time due to meteorological disturbances, tidal inequalities and inflow fluctuations. Therefore, to overcome these variations and to detect relatively small differences representing the magnitude of net flow and residual transport, synoptic time-series observations over many tidal cycles are required. By computing net velocities and resultant transport over 8 or more tidal cycles, the variaions can be averaged out and the tidal motions eliminated. The remaining net-non-tidal components of the current then can be related to density effects, bottom geometry and river inflow.