projects > freshwater flows into northeastern florida bay > abstract
Estimating Freshwater Flows into Northeastern Florida Bay
Historical changes in water-management practices to accommodate a large and rapidly growing urban population along the Atlantic coast as well as intensive agricultural activities, have resulted in a highly managed hydrologic system with canals, levees, gated control structures, and pumping stations. These structures have altered the hydrology of the Everglades ecosystem, including Florida Bay. The bay, home to several endangered species, is a valuable breeding ground for marine life and an important recreational and sport fishing area. The bay encompasses an area of about 850 square miles with an average depth of less than 4 feet. It is bordered by the mainland portion of Everglades National Park to the north, the Florida Keys to the east and south, and the Gulf of Mexico to the west.
Planning for restoration of the south Florida ecosystem includes the use of hydrologic and hydraulic models that simulate the flow of water through the region based on various water-management practices. The measurement and determination of flow along model boundaries is of great importance for the development and verification of these models. In October 1994, the U.S. Geological Survey, as part of its South Florida Ecosystem Program, began a study to measure flows into Florida Bay from the mainland through the streams along the northeastern coast. This study is providing modelers with essential data along the mangrove zone where data have not been previously available. Flow through the mangrove zone in northeastern Florida Bay is naturally controlled by the wet or dry conditions of the Everglades wetlands, regional wind patterns, and, to some extent, by tidal action on the western part of the bay from the Gulf of Mexico. This flow of freshwater from the mainland into northeastern Florida Bay is mostly confined to several streams or creeks, except during extreme highwater conditions when significant sheetflow can be observed through low-lying mangrove areas between the streams.
Ten stream sites along the southern Florida coastline were selected for the study to determine the magnitude of freshwater flows into northeastern Florida Bay and to describe the general flow characteristics of these streams. Several agencies, including the U.S. Geological Survey, U.S. Army Corps of Engineers, and the South Florida Water Management District, are developing hydrodynamic models to simulate and predict the movement of water in the mainland, flows into Florida Bay, and circulation patterns within coastal waters (Florida Bay). This study provides necessary flow and salinity information for these models along the coastal boundary of northeastern Florida Bay. The data collected for the study include: (1) water level as a variable for the calculation of cross-sectional area and mean channel velocity, (2) acoustic velocity for the calculation of mean channel velocity and discharge, (3) measured discharge and velocity to develop acoustic to mean measured velocity relations, (4) salinity to help qualify the presence of saltwater and to monitor its possible detrimental effects on the acoustic signals, and (5) temperature to monitor its possible detrimental effects on acoustic signals.
In order to estimate total freshwater flows into northeastern Florida Bay, discharge at noninstrumented sites needed to be determined. To estimate mean monthly values of discharge, correlation techniques were used to establish relations between discharge at the noninstrumented sites and discharge at a nearby monitoring station. Flows in the streams along the northeastern coastline of Florida Bay do not present the typical ebb and flood tidal signatures of most estuarine streams. These tidal signatures are either nonexistent or significantly dampened as a result of three factors: (1) the numerous mudbanks that divide Florida Bay into a large number of subbasins, (2) the presence of U.S. Highway 1 along the Florida Keys, and (3) the effects of regional wind forces. Analysis of seasonal flows indicate that about 80 percent of the total freshwater enters northeastern Florida Bay during the wet season (May through October), with a very sharp and distinct transition from brackish to freshwater occurring in the bay at the start of the wet season. Significant effects from the El Niño event on normally dry-season flows were evident based on the data, with flows increasing to a mean dry-season discharge of 273 cubic feet per second, from 41 cubic feet per second during the previous year. El Niño is also the probable cause for flows at McCormick Creek (the westernmost monitoring station) to show negative net discharges for a period starting in August 1998, unlike flow patterns shown by the rest of the monitoring stations in the area during the same time period.
Three main flow signatures also were identified when comparing flows at all monitoring stations, with the most significant one being the magnitude of discharges at Trout Creek, which carries almost 60 percent of the total freshwater entering northeastern Florida Bay. The other two significant flow signatures identified were the shifting of flow patterns at McCormick Creek during the El Niño event and the absence of "net" negative flows at the West Highway Creek site (just west of U.S. Highway 1). The observed flow distribution and especially the magnitude of flows through Trout Creek in comparison with those to the west of the creek suggest that the flow of freshwater in the Everglades wetlands along Taylor Slough may have more of an easterly movement than previously thought.
Trout Creek was selected as the base-gage for correlation analyses and used to determine if one station could be used as the indicator of effects on freshwater flows into northeastern Florida Bay caused by changes in water-management practices in the mainland. As a result of these analyses, it was determined that Trout Creek could be used as the long-term monitoring station provided that remaining questions regarding the flow patterns at McCormick Creek and Long Sound streams were answered. Data from this study will provide water managers with information regarding the distribution of discharges along the coastline of northeastern Florida Bay, which in turn, can be used to determine the magnitude and timing of water releases from the L-31W and C-111 Canal systems.
(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)
|U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology
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Last updated: 15 January, 2013 @ 12:43 PM (KP)