Surface-water quality and flow Modeling Interest Group
USGS, Water Resources Division
3916 Sunset Ridge Road
Raleigh, NC 27607
Internet: jdbales@usgs.gov
Phone: (919) 571-4048
FAX: (919) 571-4041
Robbins, J.C., and Bales, J.D., 1995, Simulation of hydrodynamics and solute transport in the Neuse River estuary, North Carolina: U.S. Geological Survey Open-File Report 94-511, 85 p.
Continuous measurements of water level, salinity, water temperature, wind speed and direction, and current speed were obtained; more than one million depth soundings were obtained for each estuary. Additional data from pre-existing gaging stations and meteorological stations also were used in the study. Maximum depths in the estuaries were less than 7 m. Mean daily tidal ranges were about 0.3 meters at the head of each estuary (where the width was less than 1 km), and about 0.15 m at the mouth of the two estuaries, (where widths were about 7 km). Salinities ranged from near zero to nearly 25 ppt, but daily ranges generally were less than 3 ppt. Top-to-bottom differences in salinity existed during some periods, and maximum differences were about 8 ppt. Mixing events could occur very rapidly (within less than an hour). Wind speeds were greatest during the winter months, when winds were from the west to northwest. Maximum measured downstream velocities were less than 50 cm/sec; upstream velocities were of a similar magnitude. Significant lateral asymmetries in longitudinal velocity were present.
The two-dimensional, vertically averaged hydrodynamic and solute transport model SWIFT2D was implemented for each of the estuaries. Model domains were discretized into 200 m by 200 m computational cells. The calibrated models used a resistance coefficient of 0.028; a wind-stress coefficient of 0.001; unadjusted, horizontal momentum-mixing coefficient of 10 sq.m/sec; isotropic mass-dispersion coefficient of 20 sq.m/sec; and coeffient relating mass dispersion to flow properties of 14 sq.m/sec.
The models were tested for vertically stratified and unstratified conditions, and for salinities ranging from about 1 to 22 ppt, and for winds speeds ranging from calm to 9 m/s. The mean difference between simulated and observed water levels was less than 3 centimeters, and the difference between simulated and observed salinities was about 1 ppt.
Simulated results were sensitive to the downstream water level and the value of the wind stress coefficient. Results were insensitive to changes in other model parameters.
Simulated flows ranged from 6,360 cu.m/sec upstream to 7,000 cu.m/sec downstream. Particle tracks showed that under some hydrologic conditions, particles released at various locations in the estuary might not exit the system for more than a month. The models also were used to evaluate mixing and transport of a conservative tracer, such as that in figure 1.
[Click here for a larger
figure.]
Figure 1. Estimated concentration distribution in the Pamlico estuary resulting from actual tidal and wind conditions and a hypothetical release of 25 million gallons/day of water with an initial strength of 1,000 parts per thousand.
Simulated results for the two estuaries for a common one-month period were compared. Water-level variations were greater in the Neuse than the Pamlico. Consequently, currents and transport also were greater in the Neuse. For more information, see the published OFRs.
Back to the SMIG Features Page
Home | Mailing List | Features | Conferences | Classes | Reading | Model Archives | Feedback