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Determining the statistical properties of salt intrusion in estuaries on sub‐tidal time scales is a substantial challenge in environmental modeling. To study these properties, we here extend an idealized deterministic salt intrusion model to a stochastic one by including a stochastic model of the river discharge. In the river discharge model, two types of stochastic forcing are used: one independent (additive noise) and one dependent (multiplicative noise) on the river discharge state. Each type of forcing results in a non‐Gaussian response in the salt intrusion length, which we consider here as the distance of the 2 psu isohaline contour to the estuary mouth. The salt intrusion model including both types of stochastic forcing in the river discharge provides a satisfactory explanation of the multi‐year statistics of observed salt intrusion lengths in the San Francisco Bay estuary, in particular for the skewness of its probability density function. Key Points: A stochastic dynamical model is developed to explain the statistics of salt intrusion in estuariesAdditive (multiplicative) noise in river discharge induces a positive (negative) skewness in the salt intrusion length statisticsBoth additive and multiplicative noise in river discharge are important forthe salt intrusion statistics in the San Francisco Bay estuary [ABSTRACT FROM AUTHOR]

Freshwater pulses (during which river discharge is much higher than average) occur in many estuaries and strongly impact estuarine functioning. To gain insight into the estuarine salinity response to freshwater pulses, an idealized model is presented. With respect to earlier models on the spatiotemporal behavior of salinity in estuaries, it includes additional processes that provide a more detailed vertical structure of salinity. Simulation of an observed salinity response to a freshwater pulse in the Guadalquivir Estuary (Spain) shows that this is important to adequately simulate the salinity structure. The model is used to determine the dependency of the estuarine salinity response to freshwater pulses for different background discharge, tides, and different intensities and durations of the pulses. Results indicate that the change in salt intrusion length due to a freshwater pulse is proportional to the ratio between peak and background river discharge and depends linearly on the duration of the pulse if there is no equilibration during the pulse. The adjustment time, which is the time it takes for the estuary to reach equilibrium after an increase in river discharge, scales with the ratio of the change in salt intrusion length and the peak river discharge. The recovery time, that is, the time it takes for the estuary to reach equilibrium after a decrease in river discharge, does not depend on the amount of decrease in salt intrusion length caused by the pulse. The strength of the tides is of minor importance to the salt dynamics during and after the pulse. Plain Language Summary: The salinity distribution in an estuary, the transition area between river and sea, strongly depends on the river discharge. During periods of low river discharge, salt will move upstream, but when river discharge becomes high, salt is pushed downstream. This study focuses on the effect of freshwater pulses (short periods with sudden high river discharge) on estuarine salt intrusion. When applying an existing model to observed freshwater pulses in the Guadalquivir Estuary (Spain), it turned out that this model was not able to simulate the effect of strong pulses. A new model has been developed that performs well when being applied to the same situations. With this new model, it is shown that the intensity and duration of the pulse control the decrease in salt intrusion. The strength of the tides is found to be of minor importance. The time it takes before the salt intrusion has recovered to its initial location is determined by the river discharge after the pulse and does not depend on how much the salt intrusion moved downstream. Key Points: Modeling of salt dynamics during strong freshwater pulses requires a detailed description of the vertical structure of salinityDependence of salt intrusion length is quantified from the pulse duration and ratio of river discharge before and during a pulseThe recovery time after a freshwater pulse does not depend on the change in salt intrusion length induced by the pulse [ABSTRACT FROM AUTHOR]

This paper presents thickness-weighted averaging (TWA) in generalized vertical coordinates as a unified framework for a variety of existing tidal-averaging concepts in seas and estuaries. Vertical profiles of resulting residual quantities depend on the specific vertical coordinate, which is held fixed during the averaging process. This dependence is demonstrated through the application to one-dimensional analytical tidal flow with sediment transport, to field observations from a tidal channel, and to model results from a two-dimensional estuary. The use of different coordinate systems provides complementary views on the residual dynamics and stresses the importance of a correct interpretation of residual quantities obtained by tidal averaging. [ABSTRACT FROM AUTHOR]

Field observations in estuaries reveal that phytoplankton blooms are strongly affected by advection processes related to river flow. To gain quantitative insight into this dependence, experiments were performed with a new idealised model that couples physical and biological processes. Advection of phytoplankton and nutrient by subtidal flow was explicitly accounted for, as well as longitudinal and vertical mixing processes. Results show that the idealised model is capable of reproducing the observed bloom. The specific spatial distribution of phytoplankton population emerges because the latter is suppressed in the upper reach by the advection processes, and the growth is limited in the lower reach by low nutrient concentrations. A sensitivity study of model results to different river discharges reveals the presence of three regimes. In the low discharge regime, blooms form because growth is faster than decay due to advection processes. In the high discharge regime, the situation is opposite and no blooms form. If time scales of growth and advection are comparable (in moderate discharge regime), phytoplankton population increases significantly slower compared to the low discharge regime. Results of additional model runs, in which water depth and the e -folding length scale of estuarine width convergence were varied, revealed that the three regimes occur in all these cases. [ABSTRACT FROM AUTHOR]

The effect of the joint action ofM2andM4tidal flow, residual flow and spatial settling lag on the lateral entrapment of sediment is examined in tidally dominated estuaries with an idealized model that assumes along-estuary uniform conditions. Approximate solutions are obtained for arbitrary cross-channel bed profiles by scaling and perturbation analysis. The hydrodynamics include externally drivenM2tidal flow, externally and internally drivenM4tidal flow and residual flow driven by horizontal density gradient, river discharge and nonlinear advection. The sediment concentration includes a mean component, anM2component driven by bed erosion and anM2component driven by both bed erosion and inertial terms. Sediment availability is calculated by imposing a morphodynamic equilibrium condition. The model is applied to a transect in the James River estuary where data of flow and suspended sediment concentration are available. Two types of sediment are separately considered, viz., fine silt and coarse silt. Residual advective transport of sediment by the lateral flow induces trapping of sediment over the left shoal (looking landward). Model results also show that the incorporation ofM4tidal flow and spatial settling lag leads to a second sediment trapping region over the right shoal. Model results are qualitatively in good agreement with the observations. [ABSTRACT FROM AUTHOR]

Residual currents induced by asymmetric tidal mixing were examined for weakly stratified, narrow estuaries using analytical and numerical models. The analytical model is an extension of the work of R. K. McCarthy, with the addition of tidal variations of the vertical eddy viscosity in the longitudinal momentum equation. The longitudinal distribution of residual flows driven by asymmetric tidal mixing is determined by the tidal current amplitude and by asymmetries in tidal mixing between flood and ebb. In a long channel, the magnitude of the residual flow induced by asymmetric tidal mixing is maximum at the estuary mouth and decreases upstream following the longitudinal distribution of tidal current amplitude. Larger asymmetry in tidal mixing between flood and ebb produces stronger residual currents. For typical tidal asymmetry, mixing is stronger during flood than during ebb and results in two-layer residual currents with seaward flow near the surface and landward flow near the bottom. For reverse tidal asymmetry, mixing is weaker during flood than during ebb and the resulting residual flow is landward near the surface and seaward near the bottom. Also, the residual flow induced by tidal asymmetry has the same order of magnitude as the density-driven flow and therefore is important to estuarine dynamics. Numerical experiments with a primitive-equation numerical model [the Regional Ocean Modeling System (ROMS)] generally support the pattern of residual currents driven by tidal asymmetry suggested by the analytical model. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model with a prognostic salinity field is used to investigate the effect of a partial slip bottom boundary condition on lateral flow and sediment distribution in a transect of a tidally dominated channel. The transect has a symmetrical Gaussian cross-channel bottom profile. For a deep, well-mixed, tidally dominated channel, partial slip decreases the relative importance of Coriolis deflection on the generation of cross-channel flow patterns. This has profound implications for the lateral distribution of residual salinity that drives the cross-channel residual circulation pattern. Transverse sediment transport, however, is always found to be governed by a balance between advection of residual sediment concentration by residual lateral flow on the one hand and cross-channel diffusion on the other hand. Hence, the changes in the cross-channel distribution of residual salinity modify the lateral sediment distribution. For no slip, a single turbidity maximum occurs. In contrast, partial slip gives a gradual transition to a symmetrical density distribution with a turbidity maximum near each bank. For a more shallow, partially mixed tidal channel that represents the James River, a single turbidity maximum at the left bank is found irrespective of the near-bed slip condition. In this case, semi-diurnal contributions to sediment distribution and lateral flow play an important role in cross-channel sediment transport. As vertical viscosity and diffusivity are increased, a second maximum at the right bank again exists for partial slip. [ABSTRACT FROM AUTHOR]

The dynamics of finite-amplitude bed forms in a tidal channel is studied with the use of an idealized morphodynamic model. The latter is based on depth-averaged equations for the tidal flow over a sandy bottom. The model considers phenomena on spatial scales of the order of the tidal excursion length. Transport of sediment mainly takes place as suspended load. The reference state of this model is characterized by a spatially uniform M2 tidal current over a fixed horizontal bed. The temporal evolution of deviations from this reference state is governed by amplitude equations: these are a set of non-linear equations that describe the temporal evolution of bed forms. These equations are used to obtain new morphodynamic equilibria which may be either static or time-periodic. Several of these bottom profiles show strong similarity with the tidal bars that are observed in natural estuaries. The dependence of the equilibrium solutions on the value of bottom friction and channel width is investigated systematically. For narrow channels (width small compared to the tidal excursion length) stable static equilibria exist if bottom friction is slightly larger than rcr. For channel widths more comparable to the tidal excursion length, multiple stable steady states may exist for bottom friction parameter values below rcr. Regardless of channel width, stable time-periodic equilibria seem to emerge as the bottom friction is increased. [ABSTRACT FROM AUTHOR]

Estuaries are often characterised by a complex network of branching channels, in which the water motion is primarily driven by tides and fresh water discharge. For both scientific reasons and management purposes, it is important to gain more fundamental knowledge about the hydrodynamics in such networks, as well as their implications for turbidity and ecological functioning. A generic 2DV estuarine network model is developed to study tides and river water transport and to understand the dependence of their along-channel and vertical structure on forcings, geometry characteristics and sea level changes. The model is subsequently applied to the Yangtze Estuary to investigate tides and the distribution of river water over channels during dry and wet season, spring tide, as well as prior to and after the formation of Hengsha Passage and the construction of the Deep Waterway Project and sea level rise. Increasing river discharge enhances the friction for tides by increasing both internal and bottom stresses. Changes in tidal forcing are correlated with the friction for both tide and river. A shortcut channel reduces the water level difference in adjacent channels, as well as tidal amplitudes difference. Sea level rise results in larger friction parameters and faster propagation of tides. The distribution of river water transport is hardly affected by above mentioned changes. Model results and current vertical structure are consistent with observations. • We developed a 2DV semi-analytical estuarine network model. • Various modifications are explained by the interaction between friction parameters. • Impacts of local geometric changes can be either local or global. [ABSTRACT FROM AUTHOR]