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projects > vegetative resistance to flow in the everglades > abstract
Field Measurement of Flow Resistance in the Florida EvergladesJonathan K. Lee and Virginia Carter Standing vegetation and bottom litter in the Florida Everglades create resistance (drag and shear) forces that slow the movement of water southward from Lake Okeechobee to Florida Bay. Gravity (represented as ground-surface slope), flow resistance, and wind, together with boundary inflows and outflows (at sloughs, creeks, and manmade structures) and areal inflows and outflows (of ground water, precipitation, and evapotranspiration), determine the depth and velocity of the water. Better representation of vegetative effects on flow resistance, in terms of parameters that describe the flow and vegetation, and better quantification of the parameters that appear in the resulting mathematical expressions are needed to improve numerical models of surface-water flow. Parameters that describe the flow and vegetation include flow velocity through the vegetation; water depth; slope of the water surface; and type, geometric characteristics, and density of the vegetation. Surface-water models with improved representation of vegetative effects on flow resistance, together with field measurements of flow resistance, can be used to study, for example, how changes in vegetative density affect changes in flow patterns. Indoor flume measurements were made to identify the most appropriate mathematical expressions and parameters for representing flow resistance due to nearly homogeneous stands of sawgrass, one of the most common plants of the Everglades. Field measurements were made in April and November 1996 in the Shark River Slough and in September and November 1997 and July 1998 in Taylor Slough to obtain information on the relation between flow and vegetation characteristics. In this synopsis, we focus on the methods used to measure hydraulic parameters in the field and on preliminary results of the field measurements. Preliminary results from the indoor flume measurements were reported in Lee and Carter (1997). 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.
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.
(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)
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U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology This page is: http://sofia.usgs.gov /projects/vege_resist/vegeabsfrsf2.html Comments and suggestions? Contact: Heather Henkel - Webmaster Last updated: 11 October, 2002 @ 09:30 PM (KP) |