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

Modelling Study of Transport Time Scales for a Hyper-Tidal Estuary

Water, 2020

This paper presents a study of two transport timescales (TTS), i.e., the residence time and exposure time, of a hyper-tidal estuary using a widely used numerical model. The numerical model was calibrated against field measured data for various tidal conditions. The model simulated current speeds and directions generally agreed well with the field data. The model was then further developed and applied to study the two transport timescales, namely the exposure time and residence time for the hyper-tidal Severn Estuary. The numerical model predictions showed that the inflow from the River Severn under high flow conditions reduced the residence and exposure times by 1.5 to 3.5% for different tidal ranges and tracer release times. For spring tide conditions, releasing a tracer at high water reduced the residence time and exposure time by 49.0% and 11.9%, respectively, compared to releasing the tracer at low water. For neap tide conditions, releasing at high water reduced the residence time and exposure time by 31.6% and 8.0%, respectively, compared to releasing the tracer at low water level. The return coefficient was found to be vary between 0.75 and 0.88 for the different tidal conditions, which indicates that the returning water effects for different tidal ranges and release times are all relatively high. For all flow and tide conditions, the exposure times were significantly greater than the residence times, which demonstrated that there was a high possibility for water and/or pollutants to re-enter the Severn Estuary after leaving it on an ebb tide. The fractions of water and/or pollutants re-entering the estuary for spring and neap tide conditions were found to be very high, giving 0.75-0.81 for neap tides, and 0.79-0.88 for spring tides. For both the spring and neap tides, the residence and exposure times were lower for high water level release. Spring tide conditions gave significantly lower residence and exposure times. The spatial distribution of exposure and residence times showed that the flow from the River Severn only had a local effect on the upstream part of the estuary, for both the residence and exposure time.

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.

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

Estuarine Coastal and Shelf Science, 1985

On tide propagation in convergent estuaries

Journal of Geophysical Research: Oceans, 1998

We revisit the problem of one-dimensional tide propagation in convergent estuaries considering four limiting cases defined by the relative intensity of dissipation versus local inertia in the momentum equation and by the role of channel convergence in the mass balance. In weakly dissipative estuaries, tide propagation is essentially a weakly nonlinear phenomenon where overtides are generated in a cascade process such that higher harmonics have increasingly smaller amplitudes. Furthermore, nonlinearity gives rise to a seaward directed residual current. As channel convergence increases, the distortion of the tidal wave is enhanced and both tidal wave speed and wave lenght increase. The solution loses its wavy character when the estuary reaches its "critical convergence"; above such convergence the weakly dissipative limit becomes meaningless. Finally, when channel convergence is strong or moderate, weakly dissipative estuaries turn out to be ebb dominated. In strongly dissipative estuaries, tide propagation becomes a strongly nonlinear phenomenon that displays peaking and sharp distortion of the current profile, and that invariably leads to flood dominance. As the role of channel convergence is increasingly counteracted by the diffusive effect of spatial variations of the current velocity on flow continuity, tidal amplitude experiences a progressively decreasing amplification while tidal wave speed increases. We develop a nonlinear parabolic approximation of the full de Saint Venant equations able to describe this behaviour. Finally, strongly convergent and moderately dissipative estuaries enhance wave peaking as the effect of local inertia is increased. The full de Saint Venant equations are the appropriate model to treat this case. 30,793 30,794 LANZONI AND SEMINARA: TIDE PROPAGATION IN ESTUARIES ies also differs significantly from the typical frictionless scale e x/•, with e ratio between the characteristic tidal amplitude a* and a typical flow depth D•). The balance imposed by flow continuity in the case of a constant width gives u;-T* (2) The picture changes considerably when the effect of estuary convergence is significant. About 160 years ago, Green [1837] employed an energy argument to treat tide propagation in estuaries with slowly varying width and depth in the absence of friction. The resulting Green's law predicts that tidal amplitude increases landward as B*-•/2D *-1/4, having denoted by B* and D* the local width and depth of the channel, respectively. However, both assumptions, slow channel convergence and frictionless propagation, are commonly nonrealistic, as the spatial scale of channel convergence is often much smaller than tidal wavelength while, most often, friction plays a nonnegligible or even dominant role in tide propagation. Let us then focus on two contributions that have recently attempted to remove the latter restrictions (but see Jay [1991] and Friedrichs and Aubrey [1994] for a detailed review of the previous literature). Jay [1991] considered tide propagation in estuaries characterized by channel convergence, accounting for the presence of a steady river flow and for the retarding effect of tidal fiats adjacent to the main channel treated as storage areas. Some finite amplitude effects were also accounted for, but the effect of overtides was not considered. As a result, the treatment of nonlinear terms of the momentum equation led to linear contributions and, not surprisingly, the resulting wave equation was indeed linear. Jay I1991] was then able to derive two analytical solutions: the former applies to weakly dissipative estuaries either strongly or weakly convergent, the latter concerns strongly dissipative estuaries. By analyzing the main features of his results, Jay [1991] was able to clarify how the classical picture associated with Green's solution is modified. Jay's [1991] discussion centered on how the competing effects of local inertia, friction, and topography act to control the real and imaginary parts of the tidal wavenumber, hence of the wave speed and the rate of spatial growth or decay of tidal amplitude. In particular, it turned out that the topographic funneling effect predicted by Green may be significantly reduced or even overcome by damping associated with friction and us

Reviews of Geophysics Tidal river dynamics: Implications for deltas

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.