Numerical Simulation of Flushing of Trapped Salt Water from a Bar-Blocked Estuary

Results of a numerical simulation investigating the complicated flushing process of an isolated trapped volume of salt water from a bar-blocked estuary are presented. A multiphase model, a part of the commercial code FLUENT 6.2, is applied. The governing equations together with initial and boundary conditions and the numerical scheme are described. The time-dependent salt-wedge position, vertical-density distribution, and proportion of total input kinetic energy converted into potential energy are examined for various incoming flow densimetric Froude number and estuary bed slope. The vertical position and thickness of the interfacial mixed layer between freshwater and salt water as well as the local gradient Richardson number are determined from simulated density profiles and velocity fields. The good agreement between the simulated and measured results indicates that the numerical model can be successfully applied to investigate the complex flushing process involving stratified flow.     

Hydrodynamics of Mesotidal Estuary in Winter

In this article, we quantify the effects of a standing ice cover on the hydrodynamics of a mesotidal estuary. The Portneuf Estuary, Québec, is 5.9?km in length and has a 3-m-deep thalweg. According to our numerical model simulations (using an adapted version of Environment Canada’s ONE-D model) and field measurements, the midwinter 50-cm-thick ice cover produced an attenuation of the neap tidal range (1.9?m) and spring tidal range (4.0?m) of 17 and 37%, respectively, near the upstream end of the estuary. The arrival of low water was also delayed by about 90?min at this location. At the mouth, the cover attenuated peak ebb tide flow (200?m3/s) and flood tide flow (500?m3/s) by approximately 18 and 13%, respectively. Here the peak flood flow was normally delayed by 41?min, while the ebb tide was usually advanced by 8?min. In general, the ice cover attenuated peak velocities by 12 to 20%, although at certain times and locations the ice cover could induce higher velocities than would be present under open water conditions. The ice cover also retarded and diminished the salt wedge intrusion and is expected to dramatically reduce the sediment transport processes, although its presence could cause some sporadic local increases in erosion.     

Field Experiment Study of Transient Stratified Flow in an Estuary

An experimental study of stratified flow was conducted in this study. An electromagnetic measurement instrument, the S4 current meter, was used in field data collections of salinity and currents in the lower Apalachicola River estuary, Florida. The S4 current meter has an advantage in field deployments for long periods of time due to its preprogrammable capability for automatic data sampling and recording of fluid flow. Time series of surface and bottom salinity and currents obtained from field experiments were used to characterize the stratified flow at the measurement location in the Apalachicola River. Analysis of field data indicated that the river was strongly stratified. The stratification was affected by the upstream river flow and the downstream tidal variations. Stratification was stronger at high tide than at low tide. By removing the tidal signal using low-passing filtering, subtidal salinity, and currents were obtained to investigate the salinity stratification and currents responses to the changes of fresh water input. Subtidal vertical salinity and velocity profiles were presented at different flow conditions. At high flow conditions, both surface and bottom subtidal currents were in the seaward direction. At low flow conditions, the bottom subtidal currents were in the upstream direction due to the strong effects of density gradients. Empirical regression equations were obtained to quantify the effects of river flow on the subtidal salinity and the bottom currents. Regression analysis indicated good linear relationship between subtidal salinity stratification and the bottom currents.     

Structure of Turbulent Flow in a Shallow Tidal Estuary

A high-resolution current profiler (HRCP), which belongs to a pulse-to-pulse coherent Doppler sonar, has been used to measure vertical profiles of turbulence parameters, such as the Reynolds stresses, eddy viscosity, production and dissipation rates, etc., and to test the parametrization of dissipation rate and eddy viscosity. The HRCP and automatic ascending/descending CTD are deployed during the autumn of 2001 for 24 h in a tidal estuary. Reliable velocities along the beams with HRCP are collected with 3 s intervals and a vertical resolution as fine as 0.03 m in the range 0.02–0.98 m above the bottom. Density profiles with the CTD are taken nominally every 30 sec. The turbulent velocity variables depend largely on the tidal phase; the variables during the ebb deviate from those in neutral equilibrium boundary layer. This deviation during the ebb presumably arises from the “inactive motion.” The stability function SM in the Mellor–Yamada (M–Y) model is smaller than 0.39 even when the stratification is negligible during the flood. The constant of proportionality B1 in the dissipation model is larger than 16.6 used in M–Y model. There is room for improving some of the mixing parametrizations in estuarine tidal flows.     

Modeling Salt Water Intrusion in Tanshui River Estuarine System—Case-Study Contrasting Now and Then

A vertical (laterally integrated) two-dimensional numerical model was applied to study the salt water intrusion in the Tanshui River estuarine system, Taiwan. The river system has experienced dramatic changes in the past half century because of human intervention. The construction of two reservoirs and water diversion in the upper reaches of the river system significantly reduces the freshwater inflow. The land subsidence within the Taipei basin and the enlargement of the river constriction at Kuan-Du have lowered the river bed. Both changes have contributed farther to the intrusion of tidal flow and salt water in the upstream direction. The model was reverified with the earliest available hydrographic data measured in 1977. The overall performance of the model is in reasonable agreement with the field data. The model was then used to investigate the change in salt water intrusion as a result of reservoir construction and bathymetric changes in the river system. The model simulation study reveals that significant salinity increases have resulted from the combined changes. It has been speculated by ecological researchers that the long-term increase in salinity might be the driving force altering the aquatic ecosystem structure in the lower reach of the estuary and the Kuan-Du mangrove swamp, particularly the enlargement of the mangrove area and the disappearance of freshwater marshes. However, concrete proof has not been available since no prototype salinity data were available prior to the reservoir construction. This case study offers the first quantitative estimate of the salinity changes due to human interference in this natural system.     

Case Study: Mass Transport Mechanism in Kyunggi Bay around Han River Mouth, Korea

The horizontal, two-dimensional Princeton Ocean Model was modified to include the salt and heat-balance equations and wetting-and-drying scheme. It was applied to Kyunggi Bay (Korea) to reproduce mean conditions for one typical year. Extensive data were compiled and analyzed to evaluate input parameters representative of long-term mean conditions for the tide, salinity, and temperature. The model, forced by four major tidal constituents (M2, S2, K1, and O1), daily freshwater discharges, and daily net surface heat exchange, produced a reasonable reproduction of observed tidal elevations, tidal currents, and long-term mean monthly distributions of salinity and temperature. The calculated residual circulation pattern is consistent with previously observed, though limited, data collected in the vicinity of Kanghwa Island and Inchon Harbor. The model was used to study the following mass transport mechanisms: tidal nonlinearity, barotropic pressure gradient associated with freshwater discharge, and baroclinic pressure gradient due to density gradient. The residual circulation pattern, and its variations under different freshwater flow regimes, was examined.     

Quantifying Rainfall and Flooding Impacts on Groundwater Levels in Irrigation Areas: GIS Approach

Landscapes continuously irrigated without proper drainage for a long period of time frequently experience a rise in water-table levels. Waterlogging and salinization of irrigated areas are immediate impacts of this situation in arid areas, especially when groundwater salinity is high. Flooding and heavy rainfall further recharge groundwater and accelerate these impacts. An understanding of regional groundwater dynamics is required to implement land and water management strategies. The purpose of this study is to quantify the impact of flood and rain events on spatial scales using a geographic information system (GIS). This paper presents a case study of shallow water-table levels and salinity problems in the Wakool irrigation district located in the Murray irrigation area with groundwater average electrical conductivity greater than 25,000?μS/cm. This area has experienced several large flood events during the past several decades. Piezometric data are interpolated to generate a water-table surface for each event by applying the Kriging method of spatial interpolation using the linear variogram model. Spatial and temporal analysis of major flood events over the last four decades is conducted using calculated water-table surfaces to quantify the change in groundwater storage and shallow water-table levels. The drainage impact of a subsurface drainage scheme partially covering the area has also been quantified in this paper. The results show that flooding and local rainfall have a significant impact on shallow groundwater. The study also found that postflood climatic conditions (evaporation and rainfall) play a significant role in the groundwater dynamics of the area. The spatial net average groundwater recharge during the flooding events ranges from 0.19 to 0.52?ML/ha. The GIS-based techniques described in this paper can be used for net recharge estimation in semiarid regions where it is important to quantify net recharge impacts of regional flooding and local rainfall. The spatial visualization of the net recharge in a GIS environment can help prioritize management actions by local communities.     

Influence of Physical Forcing on Bottom-Water Dissolved Oxygen within Caloosahatchee River Estuary, Florida

Environmental Fluid Dynamics Code, a numerical estuarine and coastal ocean circulation hydrodynamic and eutrophication model, was used to simulate the distributions of dissolved oxygen (DO), salinity, water temperature, and nutrients in the Caloosahatchee River Estuary. Modeled DO, salinity, and water temperature were in good agreement with field observational data from the Florida Department of Environmental Protection and South Florida Water Management District. Sensitivity analyses identified the effects of river discharge, atmospheric winds, and tidal forcing on the spatial and temporal distributions of DO. Simulation results indicated that vertical mixing due to wind forcing increased the bottom DO concentration. River discharge enhanced stratification in deep locations but propagated vertical mixing in the shallow upper estuary. Finally, tidal forcing heavily influenced bottom layer DO concentrations throughout the whole river estuary.     

Hydraulic Study of the Onset of Hypoxia in the Tone River Estuary

Field measurements were conducted to investigate the onset and growth of hypoxia in the Tone River Estuary. Vertical profiles of dissolved oxygen (DO), salinity, and temperature were measured along the deepest line of the estuary. Rates of oxygen consumption by water and sediment in a salt wedge were obtained using laboratory tests. The measurements showed that hypoxia frequently occurred in the front part of the salt wedge and expanded its area toward the river mouth during the summer when the river flow rate was small. The data also suggested that the onset of hypoxia was delayed by the estuarine circulation which supplied oxygen-rich seawater to the salt wedge. To simulate this phenomenon, a two-dimensional flow model in the vertical-longitudinal plane was constructed by transversely integrating the k–ε model equations. The results of model simulation for three months in the summer of 1997 closely matched the field data. The model simulation proved that DO degradation is highly correlated with the residence time of salt water in the estuary.     

Data Requirements for Load Estimation in Well-Mixed Tidal Channels

Load estimation is an important component of watershed analysis and management, particularly in light of the Total Maximum Daily Load program set forth by the Clean Water Act. This paper addresses load estimation in tidally driven channels that are dominated by advection and well mixed over the channel cross section. Such systems are common in Southern California, and represent an important link between watersheds and the coastal ocean. The oscillating flow field of tidal channels calls for a time series of data (volumetric flow rate and scalar concentrations) sampled at a sufficiently high frequency to resolve the net load of each scalar. Results of an experiment conducted in the Talbert Channel in Huntington Beach, California, to quantify freshwater loads suggest that sampling should be conducted at a minimum frequency of 0.5/h. In this system, traces of freshwater were detected in the channel by salinity depressions that ranged from 1–7 ppt and lasted 4–6 h. The freshwater load was computed as the integral of the volumetric flow rate multiplied by the freshwater fraction of the water column. Based on the duration of these events and the 0.5/h sampling frequency, these results suggest that loads will be resolved to a reasonable level of accuracy when each event is captured by at least three or four data points. Strategies to estimate the flow rate and scalar concentrations are discussed.     

Quantifying Uncertainty in Estuarine and Coastal Ocean Circulation Modeling

A methodology is developed to describe the effect of errors (or uncertainty) in the specification of certain drivers (bathymetry, river inflow, and wind speeds) on the circulation computed by a three-dimensional estuarine and coastal hydrodynamic circulation model. The methodology is based on first order variance analysis. Two analytical examples are used to illustrate the method and to provide a context for interpreting real world settings. An application of the method to a model of the N.Y./N.J. Harbor Estuary shows that current predictions are considerably sensitive to the accurate specification of bathymetry, and are usually more sensitive than water level predictions to errors in bathymetry. Estuarine properties with a strong seasonal (or spatial) component such as temperature or salinity did exhibit a sensitivity to driver accuracy that shifted from one season (or regime) to another. Bathymetry appears to control the circulation of the N.Y./N.J. Estuary more than the dynamic forcing of the winds and the Hudson River inflow, perhaps due to the fact that the estuary is primarily tidally driven.     

3D Analytic Model for Testing Numerical Tidal Models

A 3D analytic solution is presented for tides in channels with arbitrary lateral depth variation. The solution is valid for narrow channels in which the lateral variation of the amplitude of tidal elevation is small. The error introduced by the solution is on the order of a few percent in a tidal channel of a few kilometers in width. The solution allows an arbitrary lateral depth variation and thus provides a wide choice of depth functions, especially those with large bottom slopes. The largest amplitude of the along-estuary velocity appears on the surface in the deepest water. The depth-averaged velocity is the largest in the deepest water. The time of flood (ebb) in deep water lags that in shallow water. The time of flood (ebb) on the surface lags that at the bottom. Since this solution is simple and allows arbitrary lateral depth variations, it can be used to demonstrate the first-order tidal flow in narrow tidal channels of variable depth, and to test high-resolution numerical models with large depth gradients.     

Mitigation of Salinity Intrusion in Well-mixed Estuaries by Optimization of Freshwater Diversion Rates

The diversion of fresh water from estuaries for agricultural and municipal uses leads to an upstream shift in the brackish water zone that can disrupt ecosystems and deteriorate water quality at downstream points. Models are routinely used to predict hydrodynamic and water quality conditions in estuaries, and presented here is a modeling approach that ultimately could prove helpful in designing strategies to divert fresh water while mitigating changes in salinity further downstream. An optimization problem is formulated whereby a least-squares function of the salinity distribution predicted by the model is minimized by optimizing a parameter vector describing the diversion rate as a function of time. The optimization method is shown to rapidly identify diversion schedules in a range of test systems. Optimization is performed by a quasi-Newton method that utilizes a Broyden–Fletcher–Goldfarb–Shanno update, and an adjoint sensitivity method is formulated and applied to evaluate the gradient of the objective function with respect to the parameter vector. The sensitivity of salinity levels to diversion rates is predicted to have both intratidal and intertidal variability, giving insight into the potential for diversions at any given time and any location along an estuary to have either a rapid or longer-term effect on the salinity distribution. The controllability of salinity levels by fresh-water diversions is directly related to these sensitivities.     

Adjoint Sensitivity Analysis for Shallow-Water Wave Control

An adjoint sensitivity method based on the shallow-water equations is developed for water wave control in river and estuarine systems. The method is used to compute the gradient of a user-defined objective function in the N-dimensional parameter space consisting of system control settings with just one solution of the basic problem and one solution of the associated adjoint problem. Characteristic equations are derived for the adjoint problem and a new formalism is proposed for the sensitivity of shallow-water flow to boundary changes in depth and discharge. New adjoint boundary conditions are developed for river and estuarine forecasting models with open-water inflow and outflow sections. This gives rise to new expressions for sensitivities at these sections. Characteristic analysis of the adjoint and basic problems shows that sensitivities propagate in the reverse time direction along the characteristic paths of the basic problem. The Riemann variables of the adjoint problem are shown to precisely describe the sensitivity of the objective function to changes in depth and discharge at system boundaries. The method is extended to two space dimensions by bicharacteristic analysis.     

Development of Management Models for Sustainable Use of Coastal Aquifers

A number of nonlinear optimization-based multiple-objective management models for sustainable utilization of coastal aquifers are formulated and solved. The management objectives represent plausible scenarios for planned withdrawal and salinity control in coastal aquifers. The first multiple-objective management model is developed for spatial and temporal control of aquifer salinity through planned pumping (withdrawal) from locations closest to the ocean boundary. The second multiple-objective management model is useful for maximizing sustainable water withdrawal from the aquifer for beneficial uses, while limiting the maximum salinity in the aquifer. The third multiple-objective management model is developed for maximizing sustainable water withdrawal from the aquifer for beneficial uses and minimizing the total pumping at locations adjacent to the ocean boundary to control the salinity in the aquifer. The nonlinear finite-difference form of the steady-state density-dependent miscible flow and salt transport model for seawater intrusion in coastal aquifers is embedded within the constraints of the management model. The constraint method of generating noninferior solutions is used to solve the multiple-objective management problems. The management models are solved for a hypothetical unconfined coastal aquifer system. The projected augmented Lagrangian method of nonlinear programming is used to solve the resulting large-scale optimization problem. The solution results demonstrate the feasibility of the developed optimization models and also the conflicting nature of the various objectives of coastal aquifer management.     

Effect of Flood Recession on Scouring at Bed Sills

The effect of the flood recession time on the local scour depth at bed sills in gravel deposits is examined. Experiments were carried out to study the development of scour holes under time-varying hydraulic conditions with no upstream sediment feed. Triangular-shaped hydrographs, having recession times up to three times the duration of the rising limb, were used. Traditionally, the peak water discharge in any flood event is used as a design value in estimating the final depth of scour formed by a flood. This approach is overly conservative when the flow hydrograph is steep, i.e., during the occurrence of flash floods. The actual reduction of the scour depth from this estimated value is dependent on both the characteristics of the flood event and the characteristics of the stream. The results show that the maximum potential scour depth can be achieved only for hydrographs with long recession times, while the rate of this process can be estimated as a function of the ratio between a characteristic flood time and the steady-state temporal scale of scour development. A method is proposed for the prediction of the scouring process under unsteady flows in terms of two dimensionless temporal parameters. Results obtained for clear-water boundary conditions can be extended to sediment-supply tests if specific supply input conditions hold.     

Turbulent Stresses and Secondary Currents in a Tidal-Forced Channel with Significant Curvature and Asymmetric Bed Forms

Acoustic Doppler current profilers are deployed to measure both the mean flow and turbulent properties in a channel with significant curvature. Direct measurements of the Reynolds stress show a significant asymmetry over the tidal cycle where stresses are enhanced during the flood tide and less prominent over the ebb tide. This asymmetry is corroborated by logarithmic fits using 10?min averaged velocity data. A smaller yet similar tendency asymmetry in drag coefficient is inferred by fitting the velocity and estimated large-scale pressure gradient to a one-dimensional along-channel momentum balance. This smaller asymmetry is consistent with recent modeling work simulating regional flows in the vicinity of the study site. The asymmetry in drag suggests the importance of previously reported bed forms for this channel and demonstrates spatial and temporarily variations in bed stress. Secondary circulation patterns observed in a relatively straight section of channel appear driven by local curvature rather than being remotely forced by the regions of significant curvature only a few hundred meters from the measurement site.     

Growth and Decay of Estuary Ice Cover

This paper presents a new vertical 1D finite-element model to simulate the growth and decay of an ice cover (columnar ice, snow ice, slush, and snow cover consolidation) over a winter season in a partially mixed mesotidal estuary. In this study, we present, discuss, and validate boundary conditions as well as test and optimize the numerical model. Predictions from the model were assessed against field observations of ice thicknesses and internal temperatures taken over 3?years. This paper explains why ice in the estuary is almost completely composed of freshwater. We demonstrate how a columnar ice sheet grows over the first half of the winter season and is then melted from underneath during the second half of winter, especially where currents are strong. We also demonstrate that significant snow ice may form over the second half of winter if the ice cover survives long enough by resisting mechanical breakup of spring tides in March.     

Confounding Effect of Flow on Estuarine Response to Nitrogen Loading

The total maximum daily load (TMDL) concept provides the basis for regulating pollution load from riverine sources to impaired water bodies. However, load is comprised of two components: flow and concentration. These two components may have confounding, or even conflicting, effects on waterbody attributes of concern. This is particularly the case for dynamic, advective systems, such as estuaries. Resolving these components is critical for properly predicting the response of impaired systems to watershed management actions. The Neuse River Estuary in North Carolina is an example of such an impaired system. Nitrogen has been identified as the pollutant of concern, and the process of developing a TMDL for nitrogen is underway. We, therefore, analyze the extensive data that have been collected for the Neuse River and estuary to investigate spatiotemporal relationships between river flow, riverine total nitrogen (TN) inputs, water temperature, dissolved inorganic nitrogen concentration, algal density, and primary productivity. Results support the belief that phytoplankton in the estuary are under substantial riverine control. However, the riverine TN concentration alone has only a minor role in determining estuarine chlorophyll concentration. River flow has a stronger influence, likely through its effects on down-estuary nitrogen delivery, residence time, salinity, and turbidity. These results imply that using riverine nitrogen load as the metric to evaluate watershed nutrient management may not be appropriate. While nitrogen controls should reduce loads in the long term, in the short term, river flow is the dominant component of load and has the opposite effect of nitrogen on algae at the up-estuary locations.