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Results of a two-dimensional, vertically averaged, computer simulation model of the Loxahatchee River estuary show that under typical low freshwater inflow and vertically well-mixed conditions, water circulation is dominated by freshwater inflow rather than by tidal influence. The model can simulate tidal flow and circulation in the Loxahatchee River estuary under typical low freshwater inflow and vertically well-mixed conditions but is limited, how- ever, to low-flow and well-mixed conditions. Computed patterns of residual water transport show a consistent seaward flow from the northwest fork through the central embayment and out Jupiter Inlet to the Atlantic Ocean. A large residual seaward flow was computed from the North Intracoastal Waterway to the inlet channel. Although the tide produces large flood and ebb flows in the estuary, tide-induced residual transport rates are low in comparison with freshwater-induced residual transport. Model investigations of partly mixed or stratified conditions in the estuary need to await development of systems capable of simulating three-dimensional flow patterns. INTRODUCTION Growing concern for the environmental health of the Loxahatchee River estuary by citizens and local, State, and Federal agencies led to an agreement in 1979 whereby the U.S. Geological Survey would conduct a cooperative inves- tigation to provide information on physical, chemical, biological, and hydrologic conditions within the Loxahatchee River estuary (McPherson and Sabanskas, 1980). This report documents tidal flows and horizontal circula- tion patterns of the estuary. Results from other aspects of the investigation have been reported in several previous publications (McPherson and Sonntag, 1983; McPherson and Sonntag, 1984; McPherson and others, 1984; Sonntag and McPherson, 1984a; Sonntag and McPherson, 1984b; Wanless and others, 1984). Water circulation is one of the most important physical processes that occurs in estuaries. It is also one of the most difficult processes to mea- sure. Water circulation controls, or greatly affects, the distribution and flushing of dissolved and suspended waterborne constituents including salt, nutrients, dissolved oxygen, plankton, sediment, and trace elements and compounds. Purpose and Scope This report presents results from one objective of the overall study, which is to determine the patterns of two-dimensional tide-induced circu- lation in the Loxahatchee River estuary. The information is useful in explaining the distribution of bottom sediments and waterborne constituents within the estuary. The study area extends from nearshore regions of the Atlantic Ocean near Jupiter Inlet--the ocean entrance to the Loxahatchee River estuary--to the approximate upstream limit of tidal influence in the tributaries to the es- tuary (fig. 1). Parts of the Intracoastal Waterway, both north and south of Jupiter Inlet, are also within the study area. Approach The primary tool used to define water circulation in the Loxahatchee River estuary is a two-dimensional, vertically averaged, hydrodynamic numerical model that utilizes a high-speed computer to solve the system of equations. The model is capable of closely approximating water-level and water-velocity data in the prototype (real-world) system. Once the model is adjusted to adequately simulate the prototype, controlled numerical experiments can be conducted with the model. On the basis of experimental results, inferences can be made regarding how the prototype would respond to simulated field conditions. A graphical comparison between model-computed data and observed stage, velocity, and tidal-flow distribution data from the prototype is used to determine the degree of model-to-prototype similarity. Prototype data were available from a one-dimensional model study by Chiu (1975) and from data collected as part of this study. These data include bathymetry, tidal stage, and tidal velocity in the estuary. Bathymetric data were obtained from the National Ocean Survey for depth information obtained in 1969. The U.S. Geo- logical Survey resurveyed selected areas in 1980 and 1981 using depth-sounding equipment (McPherson and others, 1982). No extensive bottom changes were detected since the National Ocean Survey assessment except for dredged chan- nels in the central embayment. Tidal stage and velocity data were obtained on February 26, 1975, by Chiu (1975), and by the U.S. Geological Survey on March 2-3, 1981. Simulation of water circulation in the Loxahatchee River estuary was based on assumptions of vertically well-mixed conditions. Partly mixed or density-stratified conditions were excluded from simulation analysis. Overall circulation patterns were displayed in computer-prepared maps that show water- transport vectors at computation points within the model. Description of Study Area The Loxahatchee River estuary is tributary to the Atlantic Ocean at Jupiter Inlet (fig. 1). The estuarine system comprises three forks: the southwest fork, north fork, and northwest fork. At a point 4.26 river miles upstream from the ocean (fig. 1), the northwest fork widens to form the west- ern part of an embayment. At river mile 2.58, the north and southwest forks join to complete the irregularly shaped embayment that is about 3 miles long and averages about 0.5-mile wide. The eastern end of the embayment narrows at the location of the Florida East Coast Railroad (FECRR) bridge. A 1-mile section connecting the estuary to the ocean is a nearly straight channel culminating at Jupiter Inlet. The Intracoastal Waterway (fig. 1) is another 8 0 ° 0 9 * 0 7 ' 0 5 ' 8 0 ° 0 3 ' 2 7 ° 00 * S T U D Y A R E A H o b 0 \G ro v 9 D lt c A L -- v ^ 7

Simulation of tidal flow and circulation patterns in the Loxahatchee River Estuary, southeastern Florida