Tidal Prism Model Research Articles

Revisiting ecological carrying capacity indices for bivalve culture

Ecological carrying capacity (ECC) indices for bivalve culture rely on key ecosystem turnover rates: 1. clearance time (CT), the time needed for the cultured bivalves to filter the entire bay volume; 2. renewal time (RT), the time required to replace the entire bay volume with external water; and 3. production time (PT), the time needed for phytoplankton biomass renewal via local primary production. These turnover rates are conceptually straightforward but lack measurement standardizations in the context of ECC assessments. This study compares simple turnover rate methods with more complex approaches designed to address key assumptions and improve accuracy. Method comparisons were performed across multiple embayments (systems) in Prince Edward Island, Canada. When crop aggregation and system-scale refiltration effects were considered, CT increased by a factor of 14 to 22 depending on the system and species under cultivation. Seasonal temperature considerations further impacted CT by a factor of 38 to 142. Regarding RT, validated hydrodynamic models and tidal prism models produced remarkably different outcomes; the tidal prism approach underestimated RT by 77–94% across the studied systems. Conversely, PT was unaffected by contrasting phytoplankton parameterization; pre-aquaculture (1969–1970) and contemporary (2011−2012) datasets led to similar PT outcomes. However, other metrics revealed a contemporary shift towards low phytoplankton biomass and smaller phytoplankton cells (picophytoplankton); these observations suggest that PT provides insufficient granularity regarding microalgae biomass replacement. Overall, the study rejects a common assumption that the bay-scale turnover rates serving the conventional CT/RT and CT/PT indices can be easily and accurately parameterized; these indices should be used cautiously in assessing the sustainability of farming activities.

Submarine groundwater discharge and its implication for nutrient budgets in the western Bohai Bay, China

Submarine groundwater discharge (SGD) supplies substantial quantities of nutrients from land to oceans. However, SGD and associated nutrient fluxes have long been ignored in Bohai Bay, which is subjected to the serious environmental problem. Here, we investigated the concentrations of radon (222Rn) and nutrients in groundwater and surface water in the western Bohai Bay during May 2017. The flushing time in the bay was estimated to be 38.8–58.3 days based on tidal prism model. The SGD flux was estimated to be 7.3 ± 4.8 cm d−1 based on 222Rn mass balance model. The SGD associated nutrient fluxes were estimated to be (6.3 ± 4.1) × 107 mol d−1 for dissolved inorganic nitrogen (DIN), (1.2 ± 0.8) × 106 mol d−1 for dissolved inorganic phosphorus (DIP) and (7.5 ± 4.9) × 107 mol d−1 for dissolved inorganic silicon (DSi). By establishing nutrient budgets, we found that SGD was a major source, contributing 80.8% of all source for DIN, 90.7% of all source for DIP and 78.4% of all source for DSi into the western Bohai Bay. This study shows that SGD associated nutrient fluxes may have significant impact on nutrient budgets in the western Bohai Bay.

Evaluations of submarine groundwater discharge and associated heavy metal fluxes in Bohai Bay, China

Submarine groundwater discharge (SGD) has been recognized as an important source of dissolved heavy metals to the coastal ocean. Bohai Bay, the second largest bay of Bohai Sea in China, is subjected to serious environmental problems. However, SGD and SGD-derived heavy metal fluxes in the bay are seldom reported. In this study, we present mass balance models considering the radium losses caused by recirculated seawater to estimate water age, SGD and SGD-derived heavy metal fluxes in Bohai Bay during May 2017. The water age is estimated to be 56.7–85.0 days based on tidal prism model. By combining water and salt mass balance models, submarine fresh groundwater discharge (SFGD) is estimated to be (3.5–9.3) × 107 m3 d−1. The SGD flux estimated by the radium mass balance models is (3.2–7.7) × 108 m3 d−1, an order of magnitude larger than the discharge of the Yellow River during the sampling period. SGD-derived heavy metal fluxes were estimated to be (0.2–6.0) × 107 mol d−1 for Fe, (1.2–2.7) × 107 mol d−1 for Mn, (3.0–8.2) × 105 mol d−1 for Zn, (2.7–7.4) × 104 mol d−1 for Cr and (0.6–1.8) × 103 mol d−1 for Cd, which are significantly higher than those from local rivers. This study reveals that SGD is a significant source of heavy metals (Mn, Zn and Fe) into Bohai Bay, which may have important influences on the metal budgets and ecological environments in coastal areas.

Submarine groundwater discharge and chemical behavior of tracers in Laizhou Bay, China

Naturally occurring radon (222Rn) and radium isotopes are widely used to trace water mixing and submarine groundwater discharge (SGD) in the coastal zones. However, their activities in groundwater are variable both spatially and temporally. Here, time series sampling of 222Rn and radium was conducted to investigate their behavior in intertidal groundwater of Laizhou Bay, China. The result shows that groundwater redox conditions have an important impact on the behavior of tracers. The activities of tracers will decrease under oxidizing conditions and increase under reducing conditions. Radon and radium mass balance models were used to evaluate the flushing time and SGD based on spatial surveys in Laizhou Bay. The flushing time is estimated to be 32.9–55.3 d with coupled models, which agrees well with the result of tidal prism model. The trace-derived SGD in the whole bay ranges from 6.1 × 108 to 9.0 × 108 m3/d and the re-circulated seawater (RSGD) ranges from 5.5 × 108 to 8.5 × 108 m3/d. The average SGD and RSGD fluxes are 22.8 and 21.1 times greater than the Yellow River discharge in April 2014, respectively. The study provides a better understanding of the dynamics of coastal groundwater and behavior of tracers in a well-studied bay system.

Integration of Tidal Prism Model and HSPF for simulating indicator bacteria in coastal watersheds

Coastal water quality is strongly influenced by tidal fluctuations and water chemistry. There is a need for rigorous models that are not computationally or economically prohibitive, but still allow simulation of the hydrodynamics and bacteria sources for coastal, tidally influenced streams and bayous. This paper presents a modeling approach that links a Tidal Prism Model (TPM) implemented in an Excel-based modeling environment with a watershed runoff model (Hydrologic Simulation Program FORTRAN, HSPF) for such watersheds. The TPM is a one-dimensional mass balance approach that accounts for loading from tidal exchange, runoff, point sources and bacteria die-off at an hourly time step resolution. The novel use of equal high-resolution time steps in this study allowed seamless integration of the TPM and HSPF. The linked model was calibrated to flow and E. Coli data (for HSPF), and salinity and enterococci data (for the TPM) for a coastal stream in Texas. Sensitivity analyses showed the TPM to be most influenced by changes in net decay rates followed by tidal and runoff loads, respectively. Management scenarios were evaluated with the developed linked models to assess the impact of runoff load reductions and improved wastewater treatment plant quality and to determine the areas of critical need for such reductions. Achieving water quality standards for bacteria required load reductions that ranged from zero to 90% for the modeled coastal stream.

The effect of tidal exchange on residence time in a coastal embayment

Numerical simulations of an idealized lagoon that is connected to the ocean via a tidal inlet show that the mean residence time is inversely proportional to tidal exchange. In the Delft3D model the tidal exchange is controlled by varying the inlet length, width and depth. These changes in the inlet geometry affect the tidal prism and the ebb/flood flow structure, which are shown to control the exchange of lagoon water with seawater. To map residence time within the lagoon, a new method that implements dye tracer is developed and shows that the tidally averaged residence time exhibits significant spatial variability. For inlet systems in which, as a first approximation, the lagoon can be described by a uniformly fluctuating water level, a simple transport model is developed to elucidate the specific processes that control tidal exchange and their effect on residence time. In this transport model tidal exchange is decomposed into two fractions, an ocean exchange fraction and a lagoon exchange fraction. It is shown that both fractions need to be included to better describe tidal exchange. Specifically, inclusion of a lagoon exchange fraction improves previous tidal prism models that assume complete mixing in the lagoon. The assumption of complete mixing results in an under-prediction of residence time. Relating the spatially averaged residence time results to the exchange fractions for each inlet geometry show that the residence time is inversely proportional to the product of the tidal exchange fractions. For these single inlet systems, Keulegan's 0-D hydrodynamic model shows good agreement with Delft3D in predicting the tidal prism, maximum flow velocity, and exchange fractions. With these parameters, estimates of the mean residence time can be reached through a relationship derived from the simple transport model.

Evaluation of submarine groundwater discharge into the northern Bohai Bay, China using 226Ra

Submarine groundwater discharge (SGD) has been widely recognized as a significant source of water and dissolved material transport from land to ocean. To quantify SGD into the northern Bohai Bay, China, naturally occurring radium isotope (226Ra) was measured in water samples collected along two transects in September 2012. Based on a tidal prism model, two different flushing times of the coastal water were determined to be 9.1 d and 11.5 d with respect to the different return flow factor (b) obtained from a physical model and a mass balance model of 226Ra and salinity, respectively. Using the derived flushing time, we developed a 226Ra mass balance model to estimate the SGD into the bay, which includes mixing, sedimentary input and SGD. The 226Ra budget indicated the 226Ra input from SGD accounted for 99% of the total tracer input to the northern Bohai Bay. We arrived at an average flux from SGD of 4.83 × 107 m3/d. The large volume of SGD confirms its importance in supplying a considerable quantity of nutrients to the bay.

Land use and nitrogen loading in seven estuaries along the southern Gulf of St. Lawrence, Canada

Nitrogen loading from coastal watersheds is a principal factor associated with the decline in eelgrass bed health and cover in estuaries worldwide. We apply the Nitrogen Loading Model (NLM) framework developed in Waquoit Bay, Massachusetts to 7 estuaries in eastern New Brunswick. Using watershed-specific information on human population, wastewater production, atmospheric deposition, and land use in each watershed we estimate annual input of Total Dissolved Nitrogen (TDN) from point and non-point sources. We also estimate flushing time of each estuary using available hydrodynamic and bathymetric data incorporated in a tidal prism model. Finally, we validate the NLM results by testing the link between estimated nitrogen loading, flushing time and nitrogen signals in eelgrass tissue including nitrogen content and stable isotopes. Overall, total nitrogen load (kg TDN yr−1) was strongly dependent on watershed and estuary size, while loading rate per unit watershed area (yield) was linked to watershed population density. Atmospheric deposition was the largest contributor of nitrogen to all estuaries except one, where seafood processing effluent was the greatest source. Stable isotope analysis of eelgrass tissue reflected this distinction, with high δ15N values of 8–10‰ related to high wastewater loading, compared to 2–6.5‰ in the other estuaries that receive proportionally more atmospheric deposition. Tissue nitrogen content was positively related to nitrogen yields and loading rate per volume of estuary, highlighting the influence of variable watershed:estuary size ratio. Multiple regression analysis identified a significant interaction between nitrogen yield and flushing time on eelgrass tissue nitrogen content and isotopes, pointing to the mitigating effect an estuary's quick flushing time can have on the expression of nitrogen enrichment in primary producers. The compilation of new information on nitrogen loading to east Canadian estuaries is a novel contribution from a region where human influences are still at a relatively low level, and hence will add to existing information from cold temperate, mainly forested watershed-estuary environments.

A new formula to calculate residence times of tidal waterbodies

Flushing is a property of each coastal waterbody and is most often described by a timescale characteristic. A number of techniques have been developed to estimate residence times, but some are overly simplistic and not very accurate, while others are difficult to apply due to data requirements. The main objective of this study is the development of a simplified formula relating flushing characteristics to physical properties of tidally flushed estuaries. Numerical models of several dissimilar estuaries were developed and the modelling results were then used to compute average residence times for the waterbodies. Stepwise linear regression was applied to develop relationships between basic physical properties of the estuaries and the computed average residence times. For tidally flushed estuaries, analysis shows that a statistically high correlation exists between average residence times and combined physical properties of the waterbodies. This paper also compares a newly developed residence time formula with other widely used methods. The new method predicts more accurate estimates of average residence times than the tidal prism models frequently used by researchers. It also compares favourably with complex methods such as the jet–sink circulation model and a method based on salinity.

Anthropogenic Influences on the Tidal Prism and Water Exchanges in Jiaozhou Bay, Qingdao, China

The tidal-driven flow field and average residence time for water in Jiaozhou Bay in the years 1966, 1988, 2000, and 2008 were investigated using the EFDC (Environmental Fluid Dynamics Code) with a coupled dye module. Jiaozhou Bay (JZB) in northeastern China is a semi-enclosed shallow bay that has undergone large-scale land reclamation over the last four decades, especially over the extensive intertidal flats. Data from field observations were used to calibrate and verify the EFDC model for JZB. The verified JZB model was used to study spatial variations of flow field and water exchanges from 1966 to 2008. The overwhelming influence of human activities, especially land reclamation, is the main cause of the significant changes in hydrodynamic conditions and water exchange in JZB. The human-induced changes of the coastline position-configuration and nearshore bathymetry have resulted in substantial changes in the residual current patterns, especially in Qianwan Bay, Haixi Bay, and northeastern Jiaozhou Bay. The overall tidal prim of JZB has been reduced by 26% as compared to that in 1928. This is considerably less than the 35% reduction obtained by other studies. The decreasing water-exchange ability corresponds to an increasing average residence time (ART) over the past several decades, particularly after the 1980s. In addition, the influences of the return flow of the bay water from the open sea back into the estuary were quantified by determining the return flow factor for each year. An existing tidal prism model was revised by introducing a mixing factor κ, and a simplified formula was developed for JZB. The revised tidal prism model suggests continued deterioration in water quality and exchange ability of Jiaozhou Bay in the near future.

Modeling estuarine-shelf exchanges in a deltaic estuary: Implications for coastal carbon budgets and hypoxia

The export of wetland-derived materials to the coastal ocean (i.e., the “Outwelling” hypothesis) has received considerable attention over the past several decades. While a number of studies have shown that estuaries export appreciable amounts of nutrients and carbon, few studies have attempted to estimate the importance of estuarine sources for the coastal carbon budgets in river-dominated coastal ecosystems. A novel tidal prism model was developed to examine estuarine-shelf exchanges in the Barataria estuary, a deltaic estuary located in the north-central Gulf of Mexico. This estuary has been the site of a massive wetland loss, and it has been hypothesized that carbon export from the eroding coastal wetlands supports the development of a large hypoxic zone in the coastal Gulf of Mexico. The model results show that the Barataria estuary receives nitrogen through the tidal passes and releases carbon to the coastal ocean. The mean calculated tidal water discharge of 6930 m 3 s −1 is equivalent to about 43% of the lower Mississippi River discharge. The annual total organic carbon (TOC) export is 109 million kg, or 57 gC m 2 yr −1 when prorated to the total water area of the estuary. This carbon export is equivalent to a loss of 0.5 m of wetland soil horizon over an area of 8.4 km 2, and accounts for about 34% of the observed annual wetland loss in the estuary between 1978 and 2000. Compared to the lower Mississippi River, the Barataria estuary appears to be a very small source of TOC for the northern Gulf of Mexico (2.7% of riverine TOC), and is unlikely to have a significant influence on the development of the Gulf's hypoxia.

Tidal-to-seasonal variability in the parameters of the carbonate system in a shallow tidal creek influenced by anthropogenic inputs, Rio San Pedro (SW Iberian Peninsula)

The main objective of the present study is to assess the temporal variability of the carbonate system, and the mechanisms driving that variability, in the Rio San Pedro, a tidal creek located in the Bay of Cadiz (SW Iberian Peninsula). This shallow tidal creek is affected by effluents of organic matter and nutrients from surrounding marine fish farms. In 2004, 11 tidal samplings, seasonally distributed, were carried out for the measurement of total alkalinity (TA), pH, dissolved oxygen and Chlorophyll- a (Chl- a) using a fixed station. In addition, several longitudinal samplings were carried out both in the tidal creek and in the adjacent waters of the Bay of Cadiz, in order to obtain a spatial distribution of the carbonate parameters. Tidal mixing is the main factor controlling the dissolved inorganic carbon (DIC) variability, showing almost conservative behaviour on a tidal time scale. The amplitude of the daily oscillations of DIC, pH and chlorophyll show a high dependence on the spring–neap tide sequence, with the maximum amplitude associated with spring tides. Additionally, a marked seasonality has been found in the DIC, pH and oxygen concentrations. This seasonality seems to be related to the increase in metabolic rates with the temperature, the alternation of storm events and high evaporation rates, together with intense seasonal variability in the discharges from fish farms. In addition, the export of DIC from the Rio San Pedro to the adjacent coastal area has been evaluated using the tidal prism model, obtaining a net export of 1.05×10 10 g C yr −1.

Embayment characteristic time and biology via tidal prism model

Transport time scales are often offered by scientists, and accepted by ecologists, as qualitative indicators of the susceptibility of ecological components within an embayment. However, rigorous quantitative methods were never presented to confirm this intuition. Transport time scales in water bodies are classically based on their physical and chemical aspects rather than their ecological and biological character. The direct connection between a physical time scale and an ecological effect has to be investigated in order to quantitatively relate a transport time scale to ecology. This concept is presented here with some general guidelines and clarifying examples. To be able to relate physical time scales to biological processes, a simple tidal prism model is developed that calculates temporal changes in concentration and the related exposure. This approach provides a quick method to calculate the characteristic time for transport in a large number of embayments, which can also help in classification endeavors.

Effects of Hurricane Ivan on water quality in Pensacola Bay, Florida

Pensacola Bay, Florida, was in the strong northeast quadrant of Hurricane Ivan when it made landfall on September 16, 2004 as a category 3 hurricane on the Saffir-Simpson scale. We present data describing the timeline and maximum height of the storm surge, the extent of flooding of coastal land, and the magnitude of the freshwater inflow pulse that followed the storm. We computed the magnitude of tidal flushing associated with the surge using a tidal prism model. We also evaluated hurricane effects on water quality using water quality surveys conducted 20 and 50 d after the storm, which we compared with a survey 14 d before landfall. We evaluated the scale of hurricane effects relative to normal variability using a 5-yr monthly record. Ivan's 3.5 m storm surge inundated 165 km2 of land, increasing the surface area of Pensacola Bay by 50% and its volume by 230%. The model suggests that 60% of the Bay's volume was flushed, initially increasing the average salinity of Bay waters from 23 to 30 and lowering nutrient and chlorophylla concentrations. Additional computations suggest that wind forcing was sufficient to completely mix the water column during the storm. Freshwater discharge from the largest river increased twentyfold during the subsequent 4 d, stimulating a modest phytoplankton bloom (chlorophyll up to 18 μg l−1) and maintaining hypoxia for several months. Although the immediate physical perturbation was extreme, the water quality effects that persisted beyond the first several days were within the normal range of variability for this system. In terms of water quality and phytoplankton productivity effects, this ecosystem appears to be quite resilient in the face of a severe hurricane effect.

Quantification of Water, Salt and Nutrient Exchange Processes at the Mouth of A mediterranean Coastal Lagoon

Vassova lagoon is a typical Mediterranean (small, shallow, micro-tidal, well-mixed) coastal lagoon, receiving limited seasonal freshwater inflows from direct precipitation and underground seepage. An intensive study was carried out in order to quantify the mechanisms responsible for the intra-tidal and residual transport of water, salt, nutrients and chlorophyll at the mouth of this lagoon and to assess the lagoon's flushing behavior. Results indicated that although the system is micro-tidal, tidal effects appeared to be the dominant factor for the longitudinal distribution of physical and chemical parameters, while the associated residual flow is also important and serves as a baseline measure of overall circulation. However, analysis of the net longitudinal currents and fluxes of water, salt and nutrients revealed the importance of non-tidal effects (wind effect and precipitation incidents) in the mean tidal transport. It is shown that the Eulerian residual currents transported water and its properties inwards under southern winds, while a seaward transport was induced during precipitation incidents and northern winds. The Stokes drift effect was found an order of magnitude lower than the Eulerian current, directed towards the lagoon, proving the partially-progressive nature of the tide. Nutrients and chlorophyll-alpha loads are exported from the lagoon to the open sea during the ebb phase of the autumn and winter tidal cycles, associated with the inflow of nutrient-rich freshwater, seeped through the surrounding drainage canal. The reverse transport occurs in spring and early summer, when nutrients enter the lagoon during the flood tidal phase, from the nutrient-rich upper layer of the stratified adjacent sea. Application of a tidal prism model shows that Vassova lagoon has a mean flushing time of 7.5 days, ranging between 4 to 18 days, affected inversely by the tidal oscillation.

The calculation of estuarine turnover times using freshwater fraction and tidal prism models: A critical evaluation

Freshwater fraction and tidal prism models are simple methods for estimating the turnover time of estuarine water. The freshwater fraction method prominently features flushing by freshwater inflow and has sometimes been criticized because it appears not to include flushing by seawater, but this is accounted for implicitly because the average estuary salinity used in the calculation reflects all the processes that bring seawater into the estuary, including gravitational circulation and tidal processes. The model relies on measurable salinity differences among water masses and so must be used for estuaries with substantial freshwater inflow. Tidal prism models are based on flushing by flood tide inflow and ignore seawater inflow due to gravitational circulation. These models should only be applied to estuaries with weak or nonexistent gravitational circulation, which are generally those with little freshwater inflow. Using a framework that is less ambioguous and more directly applicable to the estimation of turnover times than those used previously, this paper critically examines the application of tidal prism models in well-mixed estuaries with complete tidal exchange, partial ebb return, or incomplete flood mixing and in partially mixed estuaries. Problems with self-consistency in earlier versions of these models also apply to the budgeting procedure used by the LOICZ (Land-Ocean Interactions in the Coastal Zone) program. Although freshwater fraction and tidal prism models are different approaches to estimating turnover times in systems with very different characteristics, consistent derivation shows that these models have much in common with each other and that they yield equivalent values that can be used to make comparisons across systems.

Development of the Fecal Coliform Total Maximum Daily Load Using Loading Simulation Program C++ and Tidal Prism Model in Estuarine Shellfish Growing Areas: A Case Study in the Nassawadox Coastal Embayment, Virginia

In this study, a linked model system including the Loading Simulation Program C+ + (LSPC) and the tidal prism water quality model (TPWQM) was proposed as an alternative tool for total maximum daily load (TMDL) studies. The feasibility of the model system was tested by a case study in the Nassawadox Creek, a Virginia tidal water shellfish growing area. The watershed model, driven by hourly precipitation, simulates hydrology and fecal coliform accumulation and transport processes in the watershed. The simulated surface runoff and subsurface flow as well as fecal coliform loads from the watershed are discharged to the tidal creek. The tidal prism model simulates fecal coliform transport in the Creek. The model results demonstrate the effectiveness in simulating hydrology and fecal coliform concentration in the watershed and its embayment. A series of sensitivity runs was conducted to estimate the load reduction necessary for fecal coliform concentration to meet the water quality standards. The model application to the Nassawadox Creek indicates that the model system is useful in developing fecal coliform TMDLs for estuarine shellfish growing areas.

Pollution Flushing In Cascading Tidal Basins

Pollution flushing from an idealized tidal embayment has been extensively studied both analytically and numerically. The analytical tidal prism model of Barber [1] results in a simple and elegant set of equations giving the pollution concentration at the nodes in the flushing cycle. It is limited in its ability to handle the bathymetry of real embayments, complex pollution loading functions and return flow conditions. The MSB model is a numerical approach written in Stella[2], (Stella is a software product and trademark of High Performance Systems, Inc. Hanover, USA). This model is not limited by bathymetry or in its ability to handle variable pollution loading functions. In this paper we extend the MSB model to include cascading basins. The model is developed using the concept of coupled chemical reactors, a well-known chemical engineering problem involving chemical productions. As an illustration, the issue of return pollutant flow to a single embayment is studied. The results are used to calibrate a return flow parameter that is often used to overcome the difficulty of dealing with this problem in analytical frameworks. Providence, USA Providence, USA

A Simplified Model for Predicting the Pollution Exchange Coefficient of Small Tidal Embayments

In recent years there has been a significant increase in the development and application of ever more sophisticated multi-dimensional models for solving the hydrodynamic and constituent transport equations which govern the flushing of pollution. However, advanced numerical modelling techniques can sometimes be augmented by alternative mathematical approaches which use simplified analytical solutions to predict the dispersion of contaminants. In the present article, a novel analytical tidal prism model is described for predicting the pollution flushing characteristics of small tidal embayments. The model relates the water quality response of the basin to the external forcing effects of the tide, the initial pollutant loading and the freshwater inflow rate. In general, the pollution flushing not only depends upon the geometry of the embayment and the tidal range but also on the proportion of effluent water which leaves the basin on an ebb tide, mixes with the surrounding coastal water and returns on subsequent flood tides. This effect has been taken into account in the mathematical model by the inclusion of a ‘pollution-return’ parameter. The analytical approach offers a viable and computationally inexpensive alternative to conventional multi-dimensional pollutant transport simulations and, more importantly, provides an increased understanding of the flushing characteristics of semi-enclosed tidal basins. The efficiency of the tidal flushing can be expressed in terms of the pollution exchange coefficient which measures the proportion of water exchanged with the sea each tidal cycle. To demonstrate the accuracy of the proposed mathematical formulation, analytical water quality predictions are compared against experimental pollution data from a 1:400 scale laboratory model of a generic square harbour. The results demonstrate that the analytical approach provides a simple and robust method of determining the water quality response of well-mixed tidal embayments.

A comparison of residence time calculations using simple compartment models of the Altamaha River estuary, Georgia

The residence and flushing times of an estuary are two different concepts that are often confused. Flushing time is the time required for the freshwater inflow to equal the amount of freshwater originally present in the estuary. It is specific to freshwater (or materials dissolved in it) and represents the transit time through the entire system (e.g., from head of tide to the mouth). Residence time is the average time particles take to escape the estuary. It can be calculated for any type of material and will vary depending on the starting location of the material. In the literature, the term residence time is often used to refer to the average freshwater transit time and is calculated as such. Freshwater transit time is a more precise term for a type of residence time (that of freshwater, starting from the head of the estuary), whereas residence time is a more general term that must be clarified by specifying the material and starting distribution. We explored these two mixing time scales in the context of the Altmaha River estuary, Georgia, and present a comparison of techniques for their calculation (fraction of freshwater models and variations of box models). Segmented tidal prism models, another common approach, have data requirements similar to other models but can be cumbersome to implement properly. Freshwater transit time estimates from simple steady-state box models were virtually, identical to flushing times for four river-flow cases, as long as boxes were scaled appropriately to river flow, and residence time estimates from different box models were also in good agreement. Mixing time estimates from box models, were incorrect when boxes were imporperly scaled. Mixing time scales vary nonlinearly with river flow, so characterizing the range as well as the mean or median is important for a thorough understanding of the potential for within-estuary processing. We are now developing an imporved box model that will allow the calculation of a variety of mixing time scales using simulations with daily variable river discharge.