During the field work, approximately 75 concurrent hydraulic and vegetation measurements were made in plant communities through which measurable surface-water flows were observed. Measurements of flow depths, flow velocities, and water-surface slopes were made to evaluate flow resistance. Vegetation sampling and analysis are discussed by Carter and others (1999) in this volume. During the 1996 measurements, the distance from the water surface to the firm bottom and the thickness of the litter layer were measured at each measurement site. An acoustic Doppler velocimeter (ADV) equipped with a down-looking probe was used to measure flow velocities that were commonly less than 1 cm/s. The ADV is a point-velocity-measurement device having a sampling volume smaller than 1 cm³. During the 1996 field measurements, two vertical velocity profiles were obtained at 5-cm increments of depth in the water column at each measurement site. Profile velocities collected in November 1996 were referenced to east, north, and up directions. Velocities were sampled at each measurement point in the water column for approximately 2 min at a frequency of five samples per second. These data were later filtered to remove erroneous samples. Points at which average correlations did not exceed 70 percent were not used in subsequent analyses. Mean velocities in the east, north, and up directions were obtained from the last 300 good samples (1 min of data) at each measurement increment. A spreadsheet was developed and used to integrate numerically each velocity profile between the top of the litter layer and the water surface. The mean horizontal flow speed and direction were also calculated in the spreadsheet.

A unique pipe manometer was developed and used to determine local water-surface slopes that are on the order of 1 cm/km. During the November 1996 field efforts, a 2.4-m-long, 7.6-cm-diameter plastic pipe with a short elbow at one end was positioned horizontally about 10 cm below the water surface and parallel to the flow direction with the elbow at the upstream end and pointing down. The centerline flow velocity in the pipe was measured by inserting an ADV, equipped with a side-looking probe, into the downstream end of the pipe. The velocity of water in the pipe is a function of the characteristics of the pipe and the difference in water-surface elevation at the entrance and exit. The pipe manometer was calibrated in a series of measurements in the U.S. Geological Survey tilting flume at Stennis Space Center, Mississippi. During the November 1996 field efforts, the manometer was used to make between one and four water-surface-slope measurements at each measurement site.

Fifteen complete sets of hydraulic and vegetation data were obtained during November 1996 field work at sites NESRS3 and P33 in upper Shark River Slough. The vegetation classes sampled were sparse rushes, medium rushes, medium sawgrass, medium mixed rushes and sawgrass, dense sawgrass, dense cattails, and very dense sawgrass. The distance from the water surface to the top of the litter layer ranged from 0.28 to 0.48 m, and the vertically averaged velocity ranged from 0.21 to 1.22 cm/s. The Manning's n coefficient, an empirical expression commonly used to express flow resistance in open channels, was computed for each measurement site. An approximate inverse relation between Manning's n and velocity was found. This is consistent with results obtained in flume studies (Lee and Carter, 1997). The limited data do not yet reveal a relation between vegetation type or density and the value of Manning's n.

REFERENCES

Carter, Virginia, Rybicki, N.B., Reel, J.T., Ruhl, H.A., and Lee, J.K., 1999, Vegetative characterization for Everglades studies: this volume.

Lee, J.K., and Carter, Virginia, 1997, Vegetative resistance to flow in the Florida Everglades, in Gerould, Sarah, and Higer, A.L., eds., U.S. Geological Survey program on the south Florida ecosystemñProceedings of the technical symposium, Ft. Lauderdale, Florida, August 25-27, 1997: U.S. Geological Survey Open-File Report 97-385, p. 49-50.

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