U.S. Geological Survey (USGS) hydrologists and ecologists are conducting studies to quantify vegetative flow resistance in the Florida Everglades. These studies are needed to improve the mathematical representation of the physical processes that influence flow resistance and the values of associated parameters in numerical models of surface-water flow used to evaluate alternative proposals for restoration of the Everglades. For use in numerical models, vegetative flow resistance must be expressed in terms of parameters that describe the flow and the vegetation. These parameters include the flow velocity through the vegetation, the water depth, the slope of the water surface, and the type, geometric characteristics, and density of the vegetation. Both indoor flume and field measurements were made to develop methods for evaluating the resistance of nearly uniform stands of vegetation and to identify the most appropriate parameters for representing resistance due to vegetation types that are typically found in the Everglades.

Indoor flume measurements were made in the USGS tilting flume at Stennis Space Center, Mississippi. The flume was used to determine the flow resistance of a uniform stand of sawgrass (Cladium jamaicense). Uniform, dense stands of sawgrass were grown in pans that were fit snugly into the flume to form a 61-m-long, 1.8-m-wide artificial sawgrass ecosystem. The depth of water in the flume was controlled by adding or removing metal plates (stop logs) at the downstream end. Experiments were conducted for five flow depths between 0.15 and 0.76 m and for mean cross sectional velocities between 0.03 and 4.6 cm/s. Series of measurements were made between September 1995 and April 1997. As the plants matured, their height increased from about 1 m to more than 2 m, their density decreased, and culms (basal stems that are composed of many closely packed leaves) and leaves became wider and less flexible. Also, the amount of dead plant material in the water column and near the bed increased. The measurement program provided the data needed to evaluate the effects of changes in plant maturity on flow resistance. To simulate the flow-resistance effects of periphyton, which is a thick floating mat of algae, the water surface in the final series of experiments was partially covered with sponges.

For each flume experiment, the flow rate, flow depths, and water-surface elevations were measured. Because the water-surface slope (equivalent to the energy loss per unit length of channel), which is calculated from the water-surface elevations, is very small, on the order of 1 cm/km, it was important to measure water-surface elevations as accurately and precisely as possible. Water-surface elevations were measured 0.46 m from each wall of the flume at each of five longitudinal positions. Hook gages were used for accuracy, and readings were made with calipers precise to 0.01 mm. A level water surface, which was attained with no flow in the flume, was used as the horizontal reference datum. Both a small amount of leakage from the flume and evapotranspiration had to be accounted for to obtain an accurate measurement of the water surface. During each experimental series, vegetation in the flume was sampled to determine biomass per unit area, number of stems and leaves per unit area, and leaf and stem width as a function of distance from the bed. The hydraulic and vegetation data are being analyzed to determine and quantify the relation between vegetation characteristics and flow resistance.

In addition to the flume experiments, field experiments were conducted to obtain information on the relation between flow and vegetation characteristics. Multiple measurements were made at two sites in the Everglades National Park where sawgrass is the dominant plant. Measurement of flow depth, flow velocity, and water-surface slope was necessary to evaluate flow resistance. Vegetation was sampled wherever hydraulic measurements were made. The same vegetation characteristics were measured in the field as in the flume. An acoustic Doppler velocity meter (ADV) was used to measure flow velocities that are often less than 1 cm/s. Because the sampling volume of the ADV is smaller than 1 cm3, the meter is considered a point-velocity-measurement device. The meter is mounted on a tripod and lowered in increments of 5 or 10 cm to obtain a vertical velocity profile. The current meter is equipped with a compass/tiltmeter, and east, north, and vertical velocity components are plotted in real time on the screen of a notebook computer connected to the meter.

A method was developed for the local measurement in the field of water-surface slopes on the order of 1 cm/km. A 2.4-m-long, 7.6-cm-diameter plastic pipe with a short elbow at one end is positioned horizontally just below the water surface and parallel to the flow direction with the elbow at the upstream end and pointing down. 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 centerline flow velocity in the pipe is measured by inserting an acoustic Doppler velocity meter that is equipped with a side-looking probe into the downstream end of the pipe. The pipe was calibrated in the flume at the hydraulics laboratory at Stennis Space Center, Mississippi, and has proven to be an efficient, accurate method for the local measurement of water-surface slopes for the low-velocity, small-gradient flows of the Everglades.

Preliminary analysis was performed for the flow-resistance and vegetation data that were collected in the flume between September 1995 and January 1996. The January 1996 vegetation measurements characterized the vegetation when the plants were about a year old and had not been thinned. There were, on the average, 11, 9, 6, and 1 sawgrass culms in four 20-cm layers counted from the bed. An average of 13, 31, 52, and 67 leaves and 55, 37, 32, and 19 percent dead biomass was measured in the same layers. For each experiment, the water-surface slope was calculated by use of linear regression from the average water-surface-elevation values that were obtained at the 10 water-surface-elevation measurement points. The Manning's n coefficient and the Darcy-Weisbach friction factor, which are empirical expressions that are commonly used to express flow resistance in open channels, were computed from the flow rate, mean depth, and water-surface slope. For the January 1996 measured flow data, Manning's n was found to be nearly constant at a fixed depth of flow for velocities between 1.5 and 4.5 cm/s, but increased as the flow velocity approached zero. For flow velocities between 1.5 and 4.5 cm/s, Manning's n averaged 0.33, 0.43, 0.50, 0.55, and 0.61 for flow depths of about 0.15, 0.30, 0.46, 0.61, and 0.76 m, respectively. On the other hand, at a depth of 0.61 m and a flow velocity of 0.19 cm/s, a Manning's n of 1.14 was calculated, and at a depth of 0.76 m and a flow velocity of 0.14 cm/s, a Manning's n of 1.78 was calculated. Because flow velocities in the Everglades are frequently 1 cm/s or less, these results show that the Manning's n coefficient for Everglades marsh depends on both the flow depth and the flow velocity as well as on the characteristics of the vegetation. Similar results were obtained for the Darcy-Weisbach friction factor.

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