World Water and Environmental Resource Congress, June 22-26, 2003, Philadelphia, Pennsylvania
Estimating the Volume and Salt Fluxes Through the Arthur Kill and the Kill Van Kull
Imali D. Kaluarachchi1, Michael S. Bruno2, Quamrul Ahsan1, Alan F. Blumberg1,2, Honghai Li1 1HydroQual, Inc., 1 Lethbridge Plaza, Mahwah, NJ 07430; PH 201) 529-5151; FAX (201) 529-5728; email: [email protected] 2Department of Civil, Environmental & Ocean Engineering, Stevens Institute of Technology
Abstract
The hydrodynamic transport characteristics of the Kill van Kull and the Arthur Kill, which connects the New York Bay to the Raritan Bay, have been investigated by using a three-dimensional, time dependent hydrodynamic model, ECOM. The objective of this study is to determine volume and salt transport through the Kill van Kull and the Arthur Kill and to obtain a basic understanding of the physical factors driving the salt transport through these important water bodies. The current model is an enhanced version of the original System Wide Eutrophication Model (Blumberg et al., 1999), which has been re-calibrated and re-validated in the New Jersey tributaries including the Hackensack, the Passaic and the Raritan Rivers. Volume and salt fluxes were determined by the decomposed correlation terms using the model computed salinity, temperature and currents. Results indicate that the net long-tem volume and salt flux is directed west through the Kill van Kull and south through the Arthur Kill. The peak water flux through the Arthur Kill is in excess of 400 m3s-1. Stokes transport term contributed most towards upstream salt transport in Newark Bay and the Arthur Kill. In the Kill van Kull, the upstream salt transport is minimal. Salt flux through the Arthur Kill appears to be dominated by the elevation gradient between entrance to the Kill van Kull (from New York Harbor) and Perth Amboy. The salt flux through the Kill van Kull is influenced to a considerable extent (but not dominated) by the elevation gradient between the entrance to the Kill van Kull (from New York Harbor) and Shooters Island. The density gradient does not appear to be a predominant driving factor for the salt transport through the Kill van Kull and the Arthur Kill.
Introduction
Understanding the movement of substances in an estuary is critical in controlling pollutants within tolerable limits. The transport mechanisms of salt provides a basis for predicting the transport of other soluble conservative substances. Therefore the magnitude and direction of volume and salt flux in an estuarine water body are critical for water quality investigations. The New York Harbor system, Long Island Sound and New York Bight are among the most extensively investigated regions in the world. Hydrodynamic and water quality investigations of this region have been prompted by the desire to improve water quality management and better
1 understand the estuarine circulation and mixing produced by wind, tidal forcing and freshwater flows in the presence of complicated coastline and topographical features. The focus of this study is the Newark Bay-Kill Van Kull-Arthur Kill system. In particular the Kill Van Kull and the Arthur Kill are extremely complicated sub- systems as the straits are connecting two important water bodies, Upper New York Harbor and Raritan Bay. Tides propagate through these two systems from both ends and make the hydrodynamics very complex. The net volume and salt fluxes through Newark Bay, the Kill van Kull and the Arthur Kill are evaluated using a three- dimensional hydrodynamic model. The purpose is to determine the magnitude and direction of the volume and salt transport and to acquire a basic understanding of the physical factors driving the transport.
Hydrodynamic Modeling
The present study uses the model results of the System-Wide Eutrophication Model (SWEM) with enhanced calibration and validation in the New Jersey tributaries for water year 1994-1995. SWEM, developed by Blumberg et al. (1999), is a three-dimensional, time-variable coupled hydrodynamic/eutrophication water quality model of the New York/New Jersey (NY/NJ) Harbor and New York Bight system. The spatial extent of the SWEM domain incorporates the core area of NY/NJ Harbor as defined by the Harbor Estuary Program and extends beyond to include the Hudson River Estuary, up to the Troy Dam, all of Long Island Sound and the New York Bight out to the continental shelf (Figure 1). A 49x84 computational grid employs an orthogonal-curvilinear coordinate system that resolves the complex and irregular shoreline of the NY/NJ Harbor-NY Bight region. In addition, the model uses a 10-layer vertical σ-coordinate system that is scaled on the local water column depth. The enhancement of the SWEM model includes improvements to model geometry (i.e., longitudinal resolution of the model grid segmentation and bathymetry) and adjustments in bottom friction to improve the calibration in the Hackensack, the Passaic and the Raritan Rivers, and Newark Bay. The improvements of the model calibration in these areas have significantly improved the calibration in the Arthur Kill and the Kill van Kull as well. All the boundary conditions and forcing functions used are described in Blumberg et al. (1999). Hence, a detailed description of the forcing functions is not made in this study.
The SWEM model was originally calibrated and validated against a wide spectrum of hydrographic and water quality data across the model domain (Blumberg et al., 1999). Detailed calibration efforts have been described by Blumberg et al. (1999) and HydroQual (2001) and therefore is not discussed here. Comparison of the water level computed through SWEM model was made against the observed data at Sandy Hook, the Battery and Bergen Point.
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Figure 1. SWEM (System Wide Eutrophication Model) domain
Calibration of the model was also performed against an extensive hydrographic data collected in the New Jersey tributary system during a field program conducted in support of SWEM calibration in 1994 and 1995 (HydroQual, 2001). Calibration of the enhanced SWEM model has been described in HydroQual (2002). SWEM has successfully reproduced the salinity temporal gradient observed both during high flow spring and low flow summer conditions. Temperature variations in summer and winter months as well as the vertical stratification in both temperature and salinity are captured very well by SWEM.
Salt Flux Mechanisms
In the present study a comprehensive analysis was performed to examine the transport processes in the Newark Bay system and determine the salt and volume flux across selected cross sections in Newark Bay, the Arthur Kill and the Kill van Kull. The different correlation terms contributing to the overall salt transport and the significance of the transport mechanisms in estuarine systems, including in the New York Harbor region, has been the focus of many previous studies (Hunkins (1981), Oey et al. (1985) and Ahsan et al. (1994)). Based on these studies, the total salt flux can be expressed as
F=+u0S0A0 S0
+
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Table 1. The major salt flux components. Term Expression Description/Physical Process (a) S0u0A0 Steady discharge (Eulerian transport) (b) S0
According to the observations and analysis made by studies Hunkins (1981), Oey et al. (1985) and Ahsan et al. (1994), the steady discharge term (a), representing freshwater inflow, causes downstream salt transport. The rest of the terms contribute to upstream salt transport. The tidal trapping component (c) could be significant due to topographic and bathymetric features. Both the steady transverse component (d) and steady vertical component (e) can be important to estuarine salt transport depending on the location, season and mixing conditions. The contributions made by the unsteady components (f and g) and the triple correlation term (h) towards upstream salt transport is expected to be relatively small.
Volume and Salt Flux Analysis
Three cross sections through Newark Bay, the Kill van Kull and the Arthur Kill (shown in Figure 2) were selected to obtain model-computed fluxes. The fluxes through the Kill van Kull, Newark Bay and the Arthur Kill sections were obtained in two methods: (a) hourly total volume fluxes were computed directly from the model output and (b) monthly-averaged total volume and salt fluxes as well as different salt flux components were obtained using correlation method described in chapter two. In addition, hourly tot al salt fluxes were obtained using the average model-computed salinity at the transects and model computed volume flux. These hourly salt fluxes were used for assessing the forcing mechanisms, discussed later in this section. The model-computed volume fluxes for the simulation period 1994-1995 were filtered to retain only those frequencies below 34 hours. Strong variability in the fluxes appears throughout the simulation periods. Peak fluxes as high as 400 m3/s directed towards the Raritan Bay were evident in the Arthur Kill. From visual inspection the fluxes through the Kill van Kull are predominantly towards Newark Bay – Kill Van Kull juncture (to the west) and in the Arthur Kill and Newark Bay the fluxes are predominantly directed to the south.
Eight different salt flux components (terms a through h of Table 1) were calculated at the three cross sections by using the hourly model output for U and V
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Figure 2. Transects and locations for the flux computations and correlation analysis velocities and salinity. The temporal averaging period used is the actual calendar month. The components were summed to obtain the monthly-averaged total volume and salt fluxes. Values for total volume and salt flux are tabulated in Table 2. The direction of salt flux in the Kill van Kull and the Arthur Kill is similar to its volume flux – to the west and south.
Results from methods (a) and (b) described earlier suggest a transport pattern directed towards south in Newark Bay, towards west in the Kill van Kull and towards south in the Arthur Kill. If the transport pattern is such, the computed volume and salt flux through the Arthur Kill should be balanced by the combined computed volume and salt flux through Newark Bay and the Kill van Kull. This is true within approximately 10% for the computed values from the correlation terms method. The 30-day average volume fluxes calculated directly from the model (method a) agree with this flow balance with an accuracy of 4%-27% depending on the month and year.
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The results for individual salt flux terms are shown in Figure 3. In Newark Bay, the Stokes transport term and the vertical steady shear term are the most significant mechanisms of upstream salt transport. The vertical unsteady shear term is significantly less than its steady counterpart. The tidal trapping term is negative, indicating salt transport in the downstream direction. This is similar to the observation made by Hunkins (1981) for the tidal trapping term in the Narrows and near Yonkers in the Hudson estuary system.
In the Arthur Kill, the only significant salt transport terms are the steady discharge (to the south) and Stokes transport (to the north). In the Kill van Kull, the steady transport and the tidal trapping terms are significant. The tidal trapping term is towards Shooters Island (direction of the net flux) and not towards Upper New York Harbor. Stokes transport in the Kill van Kull is considerably less than the values observed for the other two transects. The triple correlation term is insignificant in all transects.
Table 2. Monthly volume and salt fluxes through the Kill van Kull, Newark Bay and the Arthur Kill (fluxes are positive to the east and north) Kill Van Kull Newark Bay Arthur Kill MONTH Vol. Flux Salt Flux Vol. Salt Flux Vol. Salt Flux (m3/sec) (m3..ppt/sec) (m3/sec) (m3..ppt/sec) (m3/sec) (m3..ppt/sec)
October -16.8 -642.3 -22.6 -313.6 -52.0 -1107.9 November -36.9 -1340.6 -29.3 -243.9 -79.2 -1616.1 December -93.3 -2287.7 -38.9 -134.1 -144.4 -2514.4 January -110.9 -2804.7 -49.3 -88.6 -169.5 -2864.8 February -74.6 -2517.5 -37.5 95.7 -124.0 -2322.1 March -114.0 -2688.4 -43.7 -85.6 -164.6 -2800.7 April -79.4 -2280.8 -22.1 169.4 -108.6 -2011.7 May -43.3 -1591.6 -21.0 60.7 -73.7 -1457.3 June -51.6 -1703.9 -12.9 124.0 -71.8 -1520.2 July -46.7 -1846.8 -29.0 -59.8 -88.9 -1852.8 August -26.8 -1127.5 -8.5 155.3 -40.5 -903.5 September -5.1 -483.0 -18.4 -206.8 -34.0 -714.8
Since the model contains only one grid across each transect, the transverse salt flux term cannot be computed. Based on previous studies (Hunkins, 1981; Oey et al., 1985; and Ahsan et al., 1994), it is evident that both steady and unsteady transverse terms could play an important role in transporting salt. However, it is not possible to confirm those findings in this study.
In an attempt to quantify the main forcing mechanisms, that drive the transport in the Kill van Kull and the Arthur Kill, a linear regression analysis was applied to the fluxes and the major forcing functions. The two major forcing 6
Steady Discharge Stokes Transport Vertical Unsteady shear Tidal Trapping Vertical Steady Shear Triple Correlation 3000 Newark Bay ) -1 s 0 ppt. . 3 ux (m Fl t -3000 Sal
-6000 3000 Kill van Kull ) -1 s 0 ppt. . 3 ux (m Fl t -3000 Sal
-6000 3000 Arthur Kill ) -1 s 0 ppt. . 3 ux (m Fl t -3000 Sal
-6000 Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep
Figure 3. Time series of monthly salt flux components for Newark Bay, the Kill van Kull and the Arthur Kill. Fluxes are positive to the East and North. functions considered for the regression analysis were the elevation difference and the density difference between (1) entrance to the Kill van Kull (from New York Harbor) and Shooters Island (2) Shooters Island and Perth Amboy (3) entrance to the Kill van Kull and Perth Amboy (see Figure 2 for locations). The gradients were computed based on the hourly model output for elevations and salinity. In addition hourly salt flux was computed using the average salinity at the transects and volume flux output from the model.
The elevation at entrance to The Kill van Kull is higher than at Perth Amboy and lower than at Shooters Island. The density at entrance to the Kill van Kull is in general lower than at Perth Amboy but higher than at Shooters Island. A linear regression with the hourly salt flux, as the dependent variable, and the elevation and density gradient, as the independent variables, was performed for the entire 360-day simulation period.
The results showed that the flux through the Arthur Kill is dominated by the elevation gradient between entrance to the Kill van Kull (from New York Harbor) and Perth Amboy, giving an average correlation coefficient (R) of 0.78 (Figure 4). The elevation gradients developed due to the tidal phase lag between the entrance to 7 the Kill van Kull and Perth Amboy drive the flow northwards as well as southwards through the Arthur Kill. For example, a positive elevation gradient in water level results in a negative flux towards the Raritan Bay through the Arthur Kill and similarly a negative elevation gradient results in a positive flux in the Arthur Kill towards the New York Harbor.
The salt flux through the Kill van Kull is influenced to a considerable extent (but not dominated by) the elevation gradient between entrance to the Kill van Kull and Shooters Island, giving an average correlation coefficient (R) of 0.49 (Figure 5). Similar regression analysis of the density gradient against the fluxes suggests that density was not a driving factor for the salt flux through the Kill van Kull or the Arthur Kill. The correlation coefficients (R) were generally low (less than 0.4) for density.
Conclusions
An enhanced version of the previously calibrated and validated SWEM, a three-dimensional, time-variable coupled hydrodynamic/eutrophication water quality model of the New York/New Jersey Harbor and New York Bight system, was used to determine the volume and salt flux through Kill Van Kull and Arthur Kill in the Newark Bay system. The hydrodynamic model, ECOM, developed by Blumberg and Mellor (1980, 1987) and Blumberg and Herring (1987) provided the modeling framework.
Hourly and mean-monthly volume and salt fluxes through Newark Bay, Kill Van Kull and Arthur Kill for the simulation period 1994-1995 were obtained directly as model output and from the decomposed correlation terms. A principal observation of this study was that the mean-monthly volume and salt fluxes appear to be directed to the west through Kill van Kull and to the south through Arthur Kill. In Newark Bay the volume flux is directed to the south. Fluxes through Arthur Kill can be balanced by the flux through Newark Bay and Kill van Kull within 10% accuracy. Volume flux in Arthur Kill reached a peak of 400 m3/s towards the Raritan Bay.
The Stokes transport term proved to be the most significant towards upstream salt transport in Newark Bay and Arthur Kill transects. Upstream (towards Upper New York Harbor) salt transport was not evident in Kill Van Kull. In Newark Bay, the vertical shear terms also contributed towards upstream salt transport but the steady term was more prominent than the unsteady term. The triple correlation term is insignificant in all transects. Since the model contains only one grid across each transect, the transverse salt flux terms cannot be computed. However, previous studies (Hunkins, 1985; Oey et al., 1991; and Ahsan et al., 1995) have confirmed that both steady and unsteady transverse terms could play an important role in transporting salt. It is recommended to refine the grid in the cross channel direction in
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Figure 4. Regression analysis for salt flux through the Arthur Kill (AK) and elevation gradient between entrance to the Kill van Kull (EKVK) and Perth Amboy (PA)
Figure 5. Regression analysis for salt flux through the Kill van Kull (KVK) and elevation gradient between entrance to the Kill van Kull (EKVK) and Shooters Island (SI)
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Kill van Kull and Arthur Kill to perform a more comprehensive decomposed correlation term analysis for salt flux.
This study shows that the salt flux through Arthur Kill is dominated by the elevation gradient between entrance to Kill van Kull and Perth Amboy and the salt flux through Kill van Kull is influenced to a considerable extent (but not dominated by) the elevation gradient between Kill van Kull and Shooters Island. Density gradients between entrance to Kill van Kull, Perth Amboy and Shooters Island was not found to be a driving factor for the salt transport through Kill van Kull or Arthur Kill.
References
Ahsan, Q., M.S. Bruno, L-Y. Oey and R. I. Hires, 1994. “Wind-Driven Dispersion in New Jersey Coastal Waters,” Journal of Hydraulic Engineering, Vol. 120, No. 11. Blumberg, A.F., L.A. Khan and J.P. St. John. 1999. Three-Dimensional Hydrodynamic Model of New York Habor Region, Jorunal of Hydraulic Engineering, 125, 799-816. Blumberg, A.F. and H.J. Herring. 1987. Circulation Modeling Using Orthogonal Curvilinear Coordinates. IN: Three-Dimensional Models of Marine and Estuarine Dynamics, J.C.J. Nihoul and B.M. Jamart, Eds., Elsevier Pub. Company, 55-88. Blumberg, A.F. and G.L. Mellor. 1987. A Description of a Three-Dimensional Coastal Ocean Circulation Model. IN: Three-Dimensional Coastal Ocean Models, Coastal and Estuarine Sciences, 4, N. Heaps, Ed., American Geophysical Union, Washington, D.C., 1-16. Blumberg, A.F. and G.L. Mellor. 1980. A Coastal Ocean Numerical Model,” In: Mathematical Modelling of Estuarine Physics, Proceedings of an International Symposium, Hamburg, August 24-26, 1978. J. Sundermann and K.P. Holz, Eds., Springer-Verlag, Berlin. Hunkins, K. 1981. “Salt Dispersion in the Hudson Estuary,” Journal of Physical Oceanography, Vol. 11, 729-738. HydroQual, Inc. 2002. Calibration Enhancement of the System-Wide Eutrophication Model (SWEM) in the New Jersey Tributaries, prepared for the NJDEP under Agreement with the Passaic Valley Sewerage Commissioners. HydroQual, Inc. 2001. Newtown Creek Water Pollution Control Project East River Water Quality Plan Task 10.0 System-Wide Eutrophication Model (SWEM) Sub-Task 10.6 Validate SWEM Hydrodynamics, prepared for City of New York Department of Environmental Protection under subcontract to Greeley and Hansen. Oey, L.Y., G.L. Mellor and R.I. Hires. 1985. A Three Dimensional Simulation of the Hudson-Raritan Estuary. Part III: Salt Flux Analyses, Journal of Physical Oceanography, 15, 1711-1720.
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