Numerical study on salinity stratification in the Pamlico River Estuary

Abstract

The variations of current circulation, salt intrusion, and vertical stratification under different river flow and wind conditions in the Pamlico River Estuary (PRE) were investigated in this paper using a three-dimensional numerical model. The model was calibrated and verified against water level variation, temperature, and salinity variations during 2003 and 2001, respectively. Eight sensitivity tests were conducted with different river flow and wind conditions specified in the model. Model results show that salinity intruded further upstream under scenarios with low flow, downriver local wind, and remote-wind-caused water level set-up conditions. In contrast, the responses of salinity stratification to different environmental forcing functions were different in different portions of the estuary. Salinity stratification was enhanced under high flow condition at the lower part of the estuary, under upriver wind near the river mouth, under downriver wind at the upstream to middle portion of the estuary, and under remote-wind-caused water level set-up condition at the majority of the estuary except near the river mouth. Model results also show that across-channel wind tended to reduce salt intrusion and salinity stratification in the PRE through increased vertical mixing.

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.