Quantification of dynamic flow conditions within the south Florida ecosystem is vital to understanding implications of the flux and residence time of water--potentially nutrient-enriched (with nitrates or phosphates) or contaminant-laden (with metals or pesticides)--that can alter plant life and affect biological communities. Nutrients, carried by canals draining agricultural areas and directly discharging, over-topping levees, or seeping into neighboring wetlands, are considered to be a major contributor to changes in the types of vegetation found in the Everglades. Freshwater inflows, typically of varying magnitudes and durations, also affect the salinity of Florida Bay and potentially carry toxic substances that can affect the Bay's aquatic biota. Improved numerical techniques are needed, not only to more accurately evaluate flow-forcing functions in the discrete canals and wetlands, but also to simulate their complex interaction, which will facilitate coupled representation of transport processes that affect constituent cycling and exchange mechanisms. This project of the South Florida Ecosystem Program of the U.S. Geological Survey is focused on the development of a generic computer model to simulate the flow of water and analyze the movement of constituents between canals and wetlands. The resultant coupled flow and transport model can be used to investigate the cause-and-effect relation between discharge sources, flow magnitudes, transport processes, and changes in vegetation and biota.

Flow mechanisms and transport processes in low-relief environments, such as south Florida, are complex. Flow velocities in the Florida Everglades are extremely low, which makes the water movement highly susceptible to external forces such as wind that can even cause a wetland area to drain or flood. Wetlands near Florida Bay are also subject to tidal and meteorological effects that further complicate analyses of transport processes. Additional flow complexities are introduced in the south Florida ecosystem by a diversity of natural and man-made controls, such as canals that constitute a major water-delivery component of the system. A complex canal and levee system, designed to control flooding and provide a continuous supply of fresh water for household and agricultural use, has altered natural flow patterns. Flow depths and velocities in the canals are typically more than an order of magnitude greater than in the wetlands. Flows in the typically straight, uniform canals are predominately in the streamwise direction and are well characterized in terms of mean cross-sectional properties. By contrast, flows in the wetlands are highly variable in the horizontal plane in response to varied topographical patterns, vegetative features, and external forces. Flows from canals to neighboring wetlands and reciprocal runoff flows are the combined result of hydraulic, inertial, and meteorological forcing functions.

An area of particular interest, in terms of canal and wetland flow distribution, is the C-111 canal and its associated wetlands between Florida City and Florida Bay in southern Dade County. The C-111 drainage system--a major source of freshwater flow to Florida Bay--is comprised of the C-111 canal, which traverses the wetlands in a southeast direction and subdivides them into northeast and southwest components; a dam and water-control structure, identified as S-197, at the downstream end of the canal, which regulates freshwater outflows and prevents saltwater intrusion from Florida Bay; and a control structure 6.7 miles upstream, identified as S-18C, which regulates inflow to this segment of the canal.

A series of eleven culverts, with removable stop risers, connect the canal to wetlands to the northeast. Spoil mounds along the southwest bank of the canal between the control structures currently contain gaps cut in them in an attempt to distribute overbank flows to wetlands to the southwest. Tree islands and other physiographic features, as examined in aerial photography, clearly indicate that the historical direction of shallow surface-water flow, referred to as sheet flow, to the Bay has been altered by the construction of roads, canals, levees, and hydraulic control structures. Alternative flow-control measures, such as the ongoing effort to entirely remove the spoil mounds, are continually being explored, proposed, and evaluated to restore the C-111 drainage system to more natural flow conditions.

A generic flow-simulation model, formulated using an extended form of the one-dimensional de Saint Venant equations of unsteady flow, is being augmented to include solution of the convection-dispersion transport equation to produce a tool to investigate canal and wetland interactions in complex ecosystem environments such as the C-111 drainage system. The generic model is fully capable of simulating unsteady flow throughout a system of open channels connected in a dendritic or looped pattern. The model solution method accommodates dynamic tributary inflows and controlled diversions as well as lateral overbank flows. The weighted four-point, implicit, finite-difference approximation of the unsteady-flow equations employed in the model permits solutions at large time steps that are consistent with wetland flow rates while also allowing for simulation of dynamic flow-controlled diversions. A mixed Eulerian/Lagrangian approach is being used to solve the one-dimensional convection-dispersion equation for solute transport. The one-dimensional, open-channel model is being coupled to a wetland model that solves the two-dimensional, vertically averaged, conservation of mass, momentum, and constituent transport equations. Highly accurate land-surface elevations and bathymetric soundings, determined using GPS as well as conventional surveying techniques, are being obtained for model implementation on the C-111 drainage system. Synoptic measurements of flow velocities in the canals, wetlands, and culverts, obtained using acoustic Doppler velocity meters, are being collected for model calibration and verification. The generic coupled model is being developed and tested as a means for post assessment of primary cause-and-effect factors affecting the south Florida ecosystem habitat, as well as a developmental tool by which to study the potential effect of remedial restoration plans.

(This abstract was taken from the U.S. Geological Survey Program on the South Florida Ecosystem Proceedings of the Technical Symposium in Ft. Lauderdale, Florida, Open File Report 97-385)

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