Estuarine, Coastal and Shelf Science 93 (2011) 305e319 Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss Tidally averaged circulation in Puget Sound sub-basins: Comparison of historical data, analytical model, and numerical model Tarang Khangaonkar a,*, Zhaoqing Yang a, Taeyun Kim a, Mindy Roberts b a Pacific Northwest National Laboratory, Marine Sciences Division, 1100 Dexter Avenue North, Suite 400, Seattle, WA 98109, USA b Washington State Department of Ecology, PO Box 47600, Olympia, WA 98504-7600, USA article info abstract Article history: Through extensive field data collection and analysis efforts conducted since the 1950s, researchers have Received 25 September 2010 established an understanding of the characteristic features of circulation in Puget Sound. The pattern Accepted 25 April 2011 ranges from the classic fjordal behavior in some basins, with shallow brackish outflow and compensating Available online 10 May 2011 inflow immediately below, to the typical two-layer flow observed in many partially mixed estuaries with saline inflow at depth. An attempt at reproducing this behavior by fitting an analytical formulation to Keywords: past data is presented, followed by the application of a three-dimensional circulation and transport modeling numerical model. The analytical treatment helped identify key physical processes and parameters, but fjords fi fi partially mixed estuaries quickly recon rmed that response is complex and would require site-speci c parameterization to analytical solution include effects of sills and interconnected basins. The numerical model of Puget Sound, developed using 3-D hydrodynamic model unstructured-grid finite volume method, allowed resolution of the sub-basin geometric features, unstructured grid including presence of major islands, and site-specific strong advective vertical mixing created by Puget Sound bathymetry and multiple sills. The model was calibrated using available recent short-term oceanographic FVCOM time series data sets from different parts of the Puget Sound basin. The results are compared against 1) recent velocity and salinity data collected in Puget Sound from 2006 and 2) a composite data set from previously analyzed historical records, mostly from the 1970s. The results highlight the ability of the model to reproduce velocity and salinity profile characteristics, their variations among Puget Sound sub- basins, and tidally averaged circulation. Sensitivity of residual circulation to variations in freshwater inflow and resulting salinity gradient in fjordal sub-basins of Puget Sound is examined. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Fraser River in Canadian waters, and substantial regional runoff help create stratified two-layer conditions where the tidally Puget Sound, the Strait of Juan de Fuca, and Georgia Strait, averaged circulation consists of outflow mixed brackish water in recently defined as the Salish Sea, compose a large and complex the surface layer and inflow of saline water through the lower estuarine system in the Pacific Northwest portion of the United layers. States (U.S.) and adjacent Canadian waters. Pacific tides propagate Much of our present understanding of tidally averaged circula- from the west into the system via the Strait of Juan de Fuca tion in Puget Sound is based on analysis and interpretation of around the San Juan Islands, north into Canadian waters through considerable data collected since the 1950s and insights gained the Georgia Strait. Propagation of tides into Puget Sound occurs from the application of a physical scale model of Puget Sound at the primarily through Admiralty Inlet (see Fig. 1(a)). This is a glacially University of Washington (Rattray and Lincoln, 1955). The historical carved fjordal estuarine system with many narrow long and records of moored current meter and salinity profile observations relatively deep interconnected basins. The freshwater discharged are extensive and date back to 1930. Cox et al. (1981) tabulated by 19 gaged rivers including those in Puget Sound, the inflows to known current observations, including periods of intensive moni- the Strait of Juan de Fuca, the freshwater discharge from the toring from 1951 to 1956 and in the 1970s and 1980s. Ebbesmeyer et al. (1984) and Cox et al. (1984) provided a synthesis and inter- pretation of these current measurements in Puget Sound. Using this information, Ebbesmeyer and Barnes (1980) developed a concep- * Corresponding author. Pacific Northwest National Laboratory, Marine Sciences Division, 1100 Dexter Avenue North, Suite 400, Seattle, WA 98109, USA. tual model of Puget Sound which describes circulation in the main E-mail address: [email protected] (T. Khangaonkar). basin of Puget Sound as that in a fjord with deep sills (landward sill 0272-7714/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2011.04.016 306 T. Khangaonkar et al. / Estuarine, Coastal and Shelf Science 93 (2011) 305e319 Fig. 1. (a) Oceanographic regions of Puget Sound and the Northwest Straits (Salish sea) including the inner sub-basinsdHood Canal, Whidbey Basin, Central Basin, and South Sound. (b) Intermediate scale FVCOM grid along with composite current and salinity profile stations from Cokelet et al. (1990). zone at Tacoma Narrows and a seaward sill zone at Admiralty Inlet) branched system, consisting of seven, two-layered boxes (Box Model defining a large basin, outflow through the surface layers, and of Puget Sound) was developed by Babson et al. (2006) based on the inflow at depth. Analysis of long-term current meter records by work by Li et al. (1999). The basin characteristics, such as depth of Cannon (1983) showed a departure from the classic fjordal signa- zero flow to determine the thickness of boxes representing specific tures seen in Hood Canal and Saratoga Passage to one where depth basins, were adopted from the composite information of Puget Sound of zero flow (where tidally averaged velocity crosses zero between developed by Cokelet et al. (1990). A key simplification in these and outgoing surface layer and inflowing deeper layer) was deeper in prior Puget Sound box models, including those by Friebertshauser the water column in the main basindabout 25% of depth, lower and Duxbury (1972) and Hamilton et al. (1985),isthefixed and than conventional fjords (10e15%) in the main basin. The depth of constant surface and bottom layer depth in each basin. Only a limited peak inflow varied from near 50e75% of depthddeeper than classic number of hydrodynamic modeling studies exist that cover the entire fjords, but higher in the water column than typical shallow estu- Puget Sound, including a laterally averaged, vertical two-dimensional aries. This behavior may be recognized as a transition between model used to study density intrusion into Puget Sound (Lavelle et al., a fjord and partially mixed estuary and is a characteristic feature of 1991), and a three-dimensional (3-D), structured grid model devel- Puget Sound circulation. oped using Princeton Ocean Model (POM) code to study the vari- Nutrient pollution is considered a potential threat to the ecolog- ability of currents with a focus on complexities of the triple junction ical health of Puget Sound. There is considerable interest in under- site at the confluence of Admiralty Inlet, Possession Sound and the standing the hydrodynamics and the effect of nutrient loads entering Main (or Central) Basin of Puget Sound (Nairn and Kawase, 2002). Puget Sound and, given climate change and sea-level rise possibili- Water quality in Puget Sound, as indicated by conventional ties, how this balance may be altered in the future. This concern is not parameters such as dissolved oxygen, nutrients (nitrate þ -nitriteeni new. Recognizing that pollutant build-up problems and climatic trogen (NO3 þ NO2) and phosphateephosphorus (PO4)), algae, and changes are longstanding and require long-term application of fecal coliform bacteria, is generally considered to be good. However, models, Cokelet et al. (1990) developed a numerical model consisting there are several specific locations where water quality appears of a branched system of two-layered reaches separated by exchange reduced due to low dissolved oxygen and fecal coliform bacteria zones to calculate refluxing, salinity concentrations time series, and contamination. The areas with lowest dissolved oxygen levels include annual volume transports in nine sub-basins of Puget Sound. The southern Hood Canal, Budd Inlet, Penn Cove, Commencement Bay, technique used estimates of annual freshwater runoff and composite Elliott Bay, Possession Sound, Saratoga Passage, and Sinclair Inlet. profiles of long-term mean currents and salinity profiles from historic Historical observations of primary production suggest that phyto- measurements in nine reaches to specify conservative mass trans- plankton growth in Puget Sound is closely coupled to the circulation ports for the Strait of Juan de Fuca and Puget Sound domain. A similar characteristics. This was demonstrated through the early work by T. Khangaonkar et al. / Estuarine, Coastal and Shelf Science 93 (2011) 305e319 307 Winter et al. (1975). Subsequent studies and review of historic data vu vw þ ¼ 0; (1) show that although spring and summer blooms occur regularly, the vx vz potential for eutrophication impacts in the main basin is mitigated by the presence of strong residual circulation and water renewal from vu vu 1 vP v vu fl fi u þ w ¼ þ Km ; (2) freshwater discharges and in ow of water from the Paci c Ocean. vx vz r vx vz vz However, poorly flushed inner basins and shallow embayments, particularly in the southern end, show depleted surface nitrate P ¼ r$g$ðh À zÞ: (3) concentrations during the summer and very low oxygen concentra- tions at depth (Harrison et al., 1994; Newton and Van Voorhis, 2002). Equations (1) and (2) are continuity and momentum equations in Therefore to correctly simulate nutrient, algae and dissolved oxygen the x direction, assuming that the longitudinal momentum diffu- balance for water quality management in Puget Sound, the ability to sion terms are small for long, narrow estuaries.