Numerical study on salinity stratification in the Pamlico River Estuary
Introduction
Estuary acts as a bridge between riverine and marine system, transporting dissolved substances and suspended particles from river to costal area. In the past several decades, coastal environment becomes more stressed by pollutants introduced from land. The fates of the pollutants are strongly affected by physical processes, which interplay with biogeochemical processes in estuaries (Boyer et al., 1994, Lin et al., 2008a). In micro-tidal, partially mixed estuaries, a typical estuarine gravitational circulation mode, with net seaward current near surface and net landward current near bottom, is often observed. Its driving mechanism is commonly attributed to longitudinal baroclinic pressure gradients and the viscosity acting against it (Pritchard, 1956, Hansen and Rattray, 1965, Goodrich and Blumberg, 1991). The gravitational circulation is further influenced by factors such as earth's rotation, local topography, river flow, salinity intrusion, tide and wind forcing (Friedrichs and Hamrick, 1996, Kasai et al., 2000, Wong and Valle-Levinson, 2002, Guo and Valle-Levinson, 2007). In addition, lateral circulation can sometimes play important roles in material transport in estuaries (Valle-Levinson et al., 2003, Reynolds-Fleming and Luettich, 2004). A three-dimensional approach is often required to examine the basic transport fields in estuaries with complex geometry.
Pamlico River Estuary (PRE) is a micro-tidal, partially mixed estuary in the east of North Carolina (NC), USA (Fig. 1). Extending from Washington, NC to Pamlico Sound, the PRE is about 60 km long and 3 m deep in average. It gradually widens from 0.5 km at Washington to 6.5 km at its river mouth. In this shallow estuary, bottom water hypoxia occurs frequently during warm seasons (Hobbie et al., 1975, Stanley and Nixon, 1992, Lin et al., 2008b). Close correlations between bottom DO concentration and salinity stratification have been frequently observed, especially at the upper to middle portions of the estuary (Stanley and Nixon, 1992, Lin et al., 2008b). Accurate prediction of salinity stratification is critical in predicting bottom water hypoxia in this shallow estuary.
Within the PRE, mean astronomical tidal range is usually less than 0.1 m (Stanley and Nixon, 1992, Lin et al., 2007, Reed et al., 2008). In contrast, wind-induced tides in the Pamlico Sound area can reach 0.5–0.7 m. Luettich et al. (2002) identified a 13.2 h-period wind seiching in the Pamlico Sound system. The general circulation pattern of the PRE is dominated by wind, freshwater discharge and salinity-induced currents (Weisberg and Pietrafesa, 1983, Stanley and Nixon, 1992). Salinity stratification is mainly controlled by freshwater discharge and wind (Stanley and Nixon, 1992, Lin et al., 2008b).
In order to have a better prediction of salinity stratification (which correlates well with bottom hypoxia) in the PRE, a three-dimensional hydrodynamic model is applied to examine current circulation, salinity intrusion and stratification responses to different river flow and wind conditions. The model was first calibrated and verified using field data during 2003 and 2001, respectively. Model experiments were then conducted, which were driven by different environmental forcing.
Section snippets
Model description
A three-dimensional hydrodynamic model, EFDC (Environmental Fluid Dynamics Code) is used in this study. EFDC was developed by Hamrick (1996) and has been successfully employed in many water bodies such as estuaries, lakes, and costal bays (Kuo et al., 1996, Shen et al., 1999, Lin and Kuo, 2003, Shen and Haas, 2004, Park et al., 2005, Shen and Lin, 2006, Lin et al., 2007, Lin et al., 2008a). EFDC solves the Navier–Stokes equation for a water body with free surface and incorporates a modified
Base case
The model results of vertically averaged salinity and current distributions are presented in Fig. 5. Under the base case (Fig. 5a), the model-simulated salinity reached near the mouth of Bath Creek, which is approximately 35 km upriver from the mouth of the PRE. The model results also show that an across-channel salinity gradient exists, with fresher water hugging the southern shore. No wind was applied under the base case and this across-channel asymmetry of salinity is primarily due to the
Discussion
Based on Hansen and Rattray's central region theory, the magnitudes of both the two-layer flow and salinity anomaly (stratification) are functions of water depth. One would expect that gravitational flow and salinity stratification are usually weak in shallow estuaries. The average depth of PRE is about 3 m (channel depth 4.5 m), much shallower than many estuaries in the east coast of US (e.g., tributaries of Chesapeake Bay, Hudson River, Delaware Bay). However, salinity stratification often
Conclusion
The most important findings of this research work are perhaps that the response of salinity distribution in the PRE to various environmental forcing is very different in different portions of the estuary. In addition, in the PRE, salinity stratification tends to be most sensitive to changes of river discharge, while the distance of salinity intrusion is very sensitive to additional factors such as water level set-up/set-down at the estuary mouth and along-river wind.
Compared to some other
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