Coastal structures along canals in eastern Dade, Broward, and Palm Beach Counties, Florida, are used to maintain higher water levels upstream, preventing saltwater intrusion into the Biscayne aquifer. Higher surface-water levels induce higher ground-water levels, which prevent saltwater movement inland through the aquifer. Excess stormwater is also drained through these coastal structures. These freshwater discharges not only affect the amount of water available to the wetland areas and for water supply in the lower east coast, but also affect the biota in Biscayne Bay and the Intracoastal Waterway.

Quantifying these freshwater discharges to the east coast is an important component in computation of accurate water budgets for the inland and wetland areas, calibration and use of regional water-management models, and computation of nutrient loadings to Biscayne Bay, the Intracoastal Waterway, and associated water bodies. The U.S. Geological Survey began a study in 1994 to measure freshwater flows through coastal structures in southeastern Florida and to develop discharge-coefficient ratings for these coastal structures. Flows through the 16 coastal structures in Dade County, the 7 coastal structures in Broward County, and the 3 coastal structures in Palm Beach County are presently computed by theoretical discharge-coefficient ratings developed from scale modeling and theoretical flow coefficients, whose accuracies for specific sites are unknown. To create more accurate discharge-coefficient ratings for the coastal structures, field flow measurements were taken with an Acoustic Doppler Current Profiler at each coastal structure under a variety of structure operations. The Acoustic Doppler Current Profiler is capable of measuring low velocities and rapidly varying flow patterns that occur at the coastal structures. These field measurements were used to develop computed discharge-coefficient ratings for the coastal structures under different flow regimes: submerged orifice flow, submerged weir flow, free orifice flow, and free weir flow. The computed-coefficient ratings were compared to the theoretical-coefficient ratings for each coastal structure.

The theoretical- and computed-coefficient ratings for submerged orifice flow were similar at structures G-56, S-13, S-22, S-25B, S-26, S-27, S-28, and S-123, but were remarkably different at structures G-57, S-20F, and S-21. The theoretical- and computed-coefficient ratings for submerged weir flow were similar at structures G-57, G-93, S-20F, S-27, S-29, S-33, and S-123; however, significant differences were apparent at structures S-20, S-20G, S-21, S-21A, S-25B, S-28, S-36, and S-37A. The closest match to the theoretical-coefficient rating for the submerged orifice- and submerged weir-flow regimes was at structure S-33 in Broward County and structure S-123 in Dade County; the worst match was at structure S-21 in Dade County and structure S-36 in Broward County. Discharge-coefficient ratings for the free orifice- and free weir-flow regimes were determined at the three coastal structures (S-40, S-41, and S-155) in Palm Beach County. The theoretical- and computed-coefficient ratings for free orifice and free weir flows were similar at these structures. Some differences between the theoretical- and computed-coefficient ratings for the four flow regimes could be better defined with more data because the ratings were based on 30 or fewer points. This study shows the usefulness of the Acoustic Doppler Current Profiler in rating these coastal structures and the value of discharge-coefficient ratings developed from field measurements in evaluating the accuracy of the theoretical discharge-coefficient ratings.

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