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U.S. Department of the Interior
Vegetation Affects Water Movement in the Florida Everglades
The Florida Everglades is a vast, diverse wetland ecosystem characterized by small ground-surface slopes, slowly moving surface waters, and dense aquatic vegetation. Both submersed and emergent vegetation are found in sawgrass and cattail marshes (fig. 1), wet prairies, and sloughs. The south Florida ecosystem has been greatly altered during the last 100 years. A complex water-management system that includes levees, canals, pumps, and water-control structures now regulates flooding and provides a steady supply of fresh water to urban areas and agriculture. Drainage projects have diverted much of the water that originally flowed slowly southward from Lake Okeechobee through the Everglades. Restoration and management of the Everglades ecosystem requires understanding and manipulating the amount and timing of water flows throughout the ecosystem.
The aquatic vegetation of the Everglades affects both the depth of water and the rate at which it moves. The presence of living and dead plant material in the water column creates drag forces on the moving water. Water flows most slowly and the water-surface slope is largest in areas where vegetation is most dense.
Plan of Study
Both indoor flume and field measurements are being conducted to develop methods for evaluating the resistance of relatively uniform stands of vegetation and to identify the most appropriate parameters for representing resistance due to vegetation types typically found in the Everglades. By using computer models, remote-sensing techniques, and geographical-information-system tools, empirical expressions will be developed to define flow resistance in areas of mixed vegetation of variable density. Such expressions will quantify flow resistance at scales appropriate for large-scale hydrodynamic models or regional hydrologic models. These expressions for flow resistance will be tested by comparing results from computer flow models with measurements in selected areas of the Everglades.
Indoor Flume Experiments
During each experimental series, the vegetation in the flume is sampled to determine, as a function of distance from the bed, biomass per unit area, the number of stems and leaves per unit area, and leaf and stem width. The number of culms (basal stems composed of many closely packed leaves) and leaves per square foot are shown for each of four horizontal layers for the experiments of January 1996 in figure 3. The relation between the vegetation characteristics and flow resistance is being explored.
Field measurements are being made at several locations in the Everglades where sawgrass is the dominant plant. Factors such as the presence of periphyton (a thick floating mat of algae anchored to plants in the water column) and a litter layer (dead plant material at the bottom of the water column) are best studied in the field because they are difficult to reproduce in the laboratory.
Vegetation is sampled at sites where hydraulic measurements are made to explore the dependence of flow resistance on vegetation characteristics. In contrast to this direct technique for sampling vegetation, another USGS project is using indirect methods, including videography, to determine whether vegetation can be sufficiently characterized by remote-sensing techniques to estimate flow resistance.
Manning's n, an empirical coefficient commonly used to express flow resistance in open channels, is plotted as a function of velocity for the flume measurements of January 1996 in figure 4. The plot shows that for a uniform stand of sawgrass with no litter layer, the value of n increases with flow depth. At a fixed depth, Manning's n is relatively constant for velocities between 0.05 and 0.15 foot per second but increases as the flow velocity approaches zero.
Results of field efforts are not yet available. However, the first field measurements have shown that instruments are available with the capabilities needed to determine flow resistance for small, relatively uniform stands of vegetation in the field.
Quantifying the effects of vegetation on the movement of surface water in the Everglades is necessary for the development of accurate numerical flow and water-quality models that can be used to evaluate restoration alternatives. In undertaking laboratory, field, and theoretical studies to define the flow-resistance characteristics of Everglades vegetation, the USGS is playing a key role in providing reliable model parameters to Federal and State agencies involved in managing the Everglades ecosystem.
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For more information contact:
SOFIA Project: Vegetative Resistance to Flow in the Everglades
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 15 January, 2013 @ 12:43 PM(TJE)