Neuse River
Stratification Project
Background
The dramatic fish kills that occurred during 1995 in the Neuse River
estuary have been closely linked to low levels of dissolved oxygen in
the water column. This condition was caused by the decay of excessive
algal blooms in the river (a process that consumes oxygen) combined
with the presence of stratification (a layer of fresher, less dense
water overlying saltier, denser water) that isolated the lower part of
the water column and restricted the replenishment of oxygen from the
atmosphere. In the short term, reducing nutrient inputs into the Neuse
will likely minimize the severity of future algal blooms. However, if
we are to effectively manage this system into the next century, a
quantitative understanding of the system must be developed. This
understanding will be the foundation for predicting the behavior of the
critical processes controlling water quality in the river.
Understanding the circulation in the system is fundamental to any such
effort since the hydrodynamics are responsible for delivering and
distributing nutrients and algae, and ultimately flushing these
materials, from the river system.
The Neuse River is located near New Bern, North Carolina, and has
been the site of several recent fish kills.
Project Members
Dr. Francisco E. Werner,
UNC-CH, Department of Marine Sciences
-
Dr. Rick Luettich,
UNC-CH, Institute of Marine Sciences, Morehead City, NC.
-
Brian O. Blanton,
UNC-CH, Department of Marine Sciences
-
John A. Quinlan,
UNC-CH, Department of Marine Sciences
The stratification of the Neuse River is a function of several physical
variables which include up-river fresh-water discharge and duration of
weak wind periods. These two parameters are the focus of the current
stratification project and are being investigated by using a
state-of-the-art 3-dimensional prognostic Finite Element model that
solves the relevant hydrodynamic equations in Shallow-Water Wave Equation form
and incorporates heat and salt transport and turbulence closure in
tidal time. This numerical model was (and is being) developed by Drs.
D.R. Lynch,
C.E. Naimie,
and
J.T.C. Ip,
at
Numerical Methods Laboratory, Thayer School of Engineering,
Dartmouth College, and
Dr. F.E. Werner here at OPNML, UNC-CH.
Simulation Domain
This implementation of the Finite Element Method involves
representing the geographic region of the water body of interest
(the "domain") as discrete points joined together in a matrix of
triangles (or "elements"), called a "mesh". The computer then
solves the equations of motion governing fluid flow and
calculates results (e.g., sea surface level, water velocity,
salinity concentration, temperature distribution) at each
intersection (or triangle vertex) in the grid. A major benefit
of using the Finite Element Method is the ability to describe the
domain with triangles of irregular spacing. This allows the
concentration of computational effort in particluar regions of
interest and/or in regions where the dynamics of the flow are
likely to be quite complicated. This "concentration" appears as
regions where the element vertices are close together (i.e., the
mesh appears black) Notice that our model is able to simulate
both the exchange of water between the sounds and the coastal
ocean and the details of the circulation in the Neuse River
Estuary.
The Neuse River model mesh, shown within the red boundary in the
following images, is actually a section of a larger mesh
encompassing Albemarle and Pamlico Sounds and the associated
continental shelf sea. The computational domain (mesh) is shown
on the left, and the bathymetries (depths) are on the right.
Simulation Parameters
Length: 15 days
Forcing:
-
River discharge: Water with salinity of 10 PSU is discharged at
100 cubic meters per second into ambient water with
no momentum and 32 PSU salinity. The discharge location is
indicated by the red dot on the model mesh image above and lasts
for the duration of the simulation.
-
Wind: A wind blowing about 20 knots from Northeast to Southwest
is "turned on" at the start of day 7 and "turned off" at the start
of day 10.
Results
The first set of images shows the development of stratification
and movement of the lighter, fresher water down river. Each image
represents the instantaneous distribution of salinity at the
END of days 1, 3, 5, 7, 9, 11, 13, and 15. The
x-axis is distance downstream in kilometers from the discharge location,
and the y-axis is
depth below the river water surface in meters. This transect is made up
of 4 sections that approximately follow the bends in the river; these sections
are the red lines on the Neuse
River "model mesh" close-up image above.
Note the following:
In this sequence of images, the saltier, heavier water is
represented by red and the fresher, lighter water, the river
discharge, is represented by blue. Intermediate colors represent
the result of mixing processes that act to homogenize the waters
of different characteristics. The strength of vertical mixing is
a function of differences in vertical velocity, called shear.
-
Through day 7, the water is stratified, evidenced by the
discrete color bands that overlay each other in the horizontal.
Between day 7 and day 9, the
imposed wind stress adds momentum to the system, increasing the
vertical shear, and consequently the vertical mixing. The water
column becomes well-mixed, evidenced by the color bands being
oriented vertically, instead of horizontally as was the case previous to the
application of the wind. Note the end of Day 9.
-
The winds are "turned off" after day 9 and the system
starts to return to a stratified state, particularly in the lower end
of the river (15-20 kilometers down stream).
|
Animations
Below is an animation of the above sequences, with better resolution in
time. In this animation, the timing of the application of the model forcings (buoyancy
discharge and winds) is shown in the lower right.
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Click here for a 1. Mb .FLC
to the OPNML home page.
Maintained by: Brian Blanton
TO : blanton@marine.unc.edu
