Marine Ecosystem Dynamics Modeling Laboratory

Buoyancy-Driven Flow due to River Discharges

Freshwater discharges into Mt. Hope Bay (MHB)/Narragansett Bay (NB) are mainly from three major rivers: the Taunton River (TR) at the northeastern head of MHB, and the Blackstone and Pawtuxet Rivers (BR and PR) at the northwestern head of NB (note that the PR usually refers to the combination of BR and PR). Annual average discharge rate (based on outflow data from 1929-2003) is about 14 m3/s for the TR, 22 m3/s for the BR, and 10 m3/s for the PR (Fig. on the right). The peak of individual river discharge usually occurs in December and March, with a monthly-averaged discharge rate of about 40 m3/s.  The river discharge rate exhibits a significant interannual variability. In the extreme large discharge year, for example in 1972, the maximum discharge rate in the BR exceeded 60 m3/s in December and 80 m3/s in March. Unlike other years, the additional discharge peak occurred in June in 1972, with a maximum rate of > 40 m3/s. In an extremely low discharge year, for example 1965, the maximum discharge rate in the BR was only about 20 m3/s and occurred in March.process-oriented experiment was made to examine the impact of river discharges on the s
Although the total river discharge rate into NB/MHB is much smaller than the major rivers in the western Gulf of Maine and southeastern US coast, it is significant due to the limited size of NB/MHB (~3.7´108 m2), its semi-enclosed geometry, and shallow mean depth (~ 5.5 m).  The river discharge has a direct impact on the seasonal variation of the near-surface stratification and nutrient loading into the bay (Weisberg, 1976; Weisberg and Sturges, 1976; Kremer and Nixon, 1978). The buoyancy-induced flow in NB/MHB is driven mainly by freshwater discharges from rivers. This flow is characterized with a surface-bottom mixed frontal circulation in the shallow region where the water is vertically well mixed and a surface-intensified frontal current in the region where the water is vertically stratified (like the PR).

A process-oriented experiment was made to examine the impact of river discharges on the sub-tidal circulation in MHB/NB. We ran the model using the real time tidal and river discharge conditions for the selected period of March 15 to April 14, 2001: a period with large daily river discharges. In the PR (BR and PR combined), two peaks occurred during that period:  one on March 14 and another on April 1. The maximum discharge rate was over 350 m3/s. The temporal variation of the river discharge in the TR is in the same phase as the PR, except the maximum discharge rate was about 50% less.  To separate the temperature and freshwater driven flows in this system, we ran the model with a constant salinity background field in which no water temperature was included. A value of 35 psu is specified uniformly before the freshwater is added into the computational domain.

The freshwater input generates a buoyancy-driven sub-tidal current: flowing southwestward around the river mouth, westward in the upper MHB region, and then southward along the western coast.  This current split into two branches around the downstream headland coast. The first flows southwestward and enters NB on the western coast. The second moves southeastward, then rotates anti-cyclonically and eventually flow into NB on the eastern coast. After 30 days, the entire MHB shows the outflow to NB and SR near the surface.  As a result, most of eddies, which are observed in the residual field in the case with the only tidal forcing, disappear or considerably weaken (See the picture below). The outflow (MNB to NB), which is characterized by low-salinity water, occupies the water column throughout the western and eastern coastal regions of the NB/MHB channel. The lowest salinity is in the upper 5 m in the western coastal region, which corresponds to the location of strongest outflow. In the interior of the channel, the current is characterized by a two-layer flow system: a weak outflow near the surface and a relatively strong, inflow beginning 5m from the surface that intensifies with depth.

The river discharge induced sub-tidal buoyancy flow may vary in intensity and direction with the amount of river discharge rate and background stratification. During August 18-19 2005 survey, we did observe the low-salinity profile on both western and eastern coastal side on the cross-channel section near the MHB Bridge, which is consistent with the model-predicted pattern described here.

Posted on January 16, 2014