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SF6 Tracer Evaluation of Modeled Currents

SF6 Tracer Experiment and Supporting Numerical Simulations in New York Harbor

Color-coded plot of model simulated near-surface residence time in Newark Bay.   
Model simulated near-surface residence time in Newark Bay.

The hydrodynamic model of the Port of New York/New Jersey Operational Forecast System (NYOFS)  has been used to simulate tracer dispersion in New York Harbor. With the addition of a concentration model to the NYOFS, the model can simulate the movement of the passive tracer sulfur hexafluoride (SF6). SF6 was deliberately released in the Newark Bay in July 2002 and in the East River in June 2003 by researchers from Lamont Doherty Earth Observatory at Columbia University. The June 2003 experiment consisted of two releases: one before the flood tide (flood injection), and one before the ebb tide (ebb injection) The tracer field experiments are used to study the dispersive characteristics of the inland waterways in the New York Harbor estuary, particularly at the Newark Bay, Arthur Kill, Kill van Kull, and East River.

The hydrodynamic model was set-up for simulating the tracer movement released in the field experiments. The model is forced with observed water levels at lateral open boundaries at Sandy Hook, NJ and Kings Point, NY, and with observed winds on the surface. Observed discharges at Hudson and Passaic Rivers are used as the flow input to the model (instead of climatology as used in NYOFS). A one-dimensional outflow and constant spatial gradient inflow are specified for the concentration boundary condition at the northern end of the East River near Kings Point. The simulated water levels and currents are verified with observations to ensure model accuracy. Model-simulated tracer concentration distribution and mass at the model surface layer are compared with daily, tidally-synchronized measurements. The comparisons include: longitudinal tracer concentration distribution, tracer concentration vertical profile, tracer mass inventory, tracer center of mass, and tracer residence time.

The simulated longitudinal tracer concentration distributions are qualitatively in agreement with observations. The simulated flushing rate is slower than the observations at the Arthur Kill and East River due to slower modeled current velocity associated with coarse grid resolution. Since the model is barotropic (lacking in salinity and temperature), there is no baroclinic structure in the simulated tracer vertical profile similar to observations found at Hudson River and northern East River.

For the July 2002 field experiment, the residence time from the mass inventory, within the inland waterways of Newark Bay, Arthur Kill, and Kill van Kull was estimated about 3.4 days for the data versus 4.5 days for the model. For the June 2003 field experiment, the residence time within the East River was estimated 3.8 (flood injection) and 1.7 (ebb injection) days from the data, and 3.2 (flood injection) and 3.3 (ebb injection) days from the model, respectively.

SF6 Tracer Experiment and Supporting Numerical Simulations in the Houston Ship Channel
NOAA’s National Ocean Service (NOS) and the Lamont Doherty Earth Observatory at Columbia University have designed and conducted a sulfur hexafluoride (SF6) experiment in the Houston Ship Channel (HSC). SF6 was released at the confluence of the HSC and Patrick Bayou, an EPA superfund site, to measure the dispersion characteristics of the upper Houston Ship Channel within the Port of Houston.

The Bay and Channel hydrodynamic models within the Galveston Bay Operational Forecast System (GBOFS) were used to simulate the transport and dispersion of SF6 over the ten day period of November 2-11, 2004, by adding two concentration algorithms to each model to simulate the movement of the passive tracer with and without surface gas transfer. The simulated water levels, currents, and density were compared with PORTS observations to quantify model accuracy. Model simulated tracer concentration distributions and the total tracer mass balance were studied. Residence time, turnover times of the injection cells, and exposure level and duration were computed. Areal extents of tracer concentration above background levels of 2 fmol/L were determined to assist in the planning of the tracer release experiment. In addition, surface gravity wave algorithms were incorporated into the models to simulate short period waves with and without wave-current interaction and indicated the feasibility of including the wave algorithms within GBOFS.

Approximately 2 moles of SF6 was released on 17 May at slack tide before ebb. Surface and vertical profiles of SF6 concentration were measured with an automated high-resolution measurement system using a gas chromatograph over the period 17-26 May 2005. Data were corrected for tidal movement and the first order loss rate was ~0.13 day-1, which yielded an e-folding residence time of ~8 days for an injection at Patrick Bayou. Areal extents of tracer concentration were in general agreement with those simulated for a November 2004 simulation period and indicated the utility of the numerical model in designing field tracer experiments.

 Color-coded SF6 concentration profiles for transects conducted on Days 0-10 in the navigation channel.
SF6 concentration profiles for transects conducted on Days 0-10 in the navigation channel.

 Color-coded map showing near surface residence times in  Galveston Bay.
 Map showing residence times in Houston Galveston area.

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