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. 2 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 <ut (t)At (t) >+A0 <ut (t)St (t) >+usv (y,z)Ssv (y,z)A0 +ust (y)Sst (y)A0 +<udv (y,z,t)Sdv (y,z,t)A0 >+<udt (y,t)Sdt (y,t)A0 >+<ut (t)St (t)At (t)> 3 Table 1. The major salt flux components. Term Expression Description/Physical Process (a) S0u0A0 Steady discharge (Eulerian transport) (b) S0<utAt> Stokes wave transport (tidal pumping) (c) A0<utSt> Tidal trapping (d) usv(y,z) Ssv(y,z)A0 Vertical steady shear dispersion (e) ust(y) Sst(y)A0 Transverse steady shear dispersion (f) <udv(y,z,t) Vertical unsteady shear dispersion Sdv(y,z,t)A0> (g) <udt(y,t) Sdt(y,t)A0> Transverse unsteady shear dispersion (h) <utStAt> Triple tidal correlation 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 4 Figure 2.