Restoration and management decisions regarding the south Florida ecosystem are based in part on the results of numerical surface-water flow models. These model results are sensitive, in turn, to the expressions used to account for the resistance effects of vegetation on flow and to the values of the coefficients that appear in those expressions. U.S. Geological Survey (USGS) hydrologists and ecologists are conducting studies to quantify vegetative flow resistance in order to improve the models.

Expressions for vegetative flow resistance include coefficients that must be evaluated in terms of measurable parameters that describe the flow conditions and the vegetation characteristics. These parameters include the flow velocity through the vegetation, water depth, slope of the water surface, and the type, physical characteristics, and density of the vegetation. Water-surface slope is perhaps the most difficult of the flow-resistance parameters to measure in the Everglades due to the very low-gradient characteristics of the topography and flow. Conventional surveying methods do not provide the level of precision needed to accurately determine water-surface slopes in such wetland

Figure 1. Pipe manometer, laboratory configuration. Not drawn to scale. Click for larger image.

environments. A unique pipe manometer (fig. 1) has been developed by the authors to evaluate these very small water-surface slopes that are typically on the order of 1 cm per 1 km ^(10-5) .

The pipe manometer is a 2.4m long, 7.6cm diameter PVC pipe with a short elbow of the same internal diameter at one end. In application, the pipe is positioned fully submerged near the water surface with its long axis parallel to the direction of flow. It is oriented with the elbow opening downward at the upstream end of the pipe. For low Reynolds number flow, water velocity in the pipe is theoretically a function of only the pipe geometry, water viscosity and the head difference between the ends of the pipe (that is, Streeter and Wylie, 1979). The relationship, either theoretical or empirical, between the flow velocity in the pipe and the head difference can be used as a surrogate for measurement of the water-surface slope. It is under this assumption that the pipe manometer is developed to determine water-surface slope.

A series of steady-state controlled flows, conducted in the tilting flume at the USGS Hydrologic Instrumentation Facility at Stennis Space Center in Bay St. Louis, Mississippi during February and March of 1999, were used to establish the relationship between the pipe-centerline velocity and water-surface slope. During this time period, eleven separate flow conditions were replicated, representing a variety of water depths and slopes. Each controlled condition consisted of two water-surface slope measurements and approximately twenty pipe-centerline velocity measurements.

A single water-surface slope was calculated from six independent stage measurements collected using hook gages equippedwith digital calipers at each of five locations along a side of the 60m long, 2m wide flume (fig. 2). Pipe-center-line velocity was measured by inserting the side-looking probe of an acoustic Doppler velocity meter into the downstream end of the pipe manometer at five locations along each side of the flume using the configuration shown in figure 1.

Analyses of the observations collected under the controlled flow conditions show a strong empirical relationship between pipe-centerline velocity and water-surface slope. The relationship between pipe-centerline velocity and the square root of water-surface slope is nearly linear within the range of flow conditions observed in the Everglades. Efforts are currently underway to reconcile the observations with pipe flow theory in order to establish design criteria for pipe manometers with geometries different than the pipe manometer used in this study.

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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