sheet flow in vegetated wetlands of the everglades
Sheet Flow in Vegetated Wetlands of the Everglades
Poster presented April 2003, at the Greater Everglades Ecosystem Restoration Conference
This project is focused on quantifying the extremely slow flows in Shark River Slough and investigating internal and external forces that affect sheet flow behavior. Insight into the nature of sheet flow behavior in the wetlands, coupled with identification of the factors that influence the flow regime, is essential to the development and evaluation of models being used to assess and compare restoration scenarios for the greater Everglades ecosystem.
Flow velocities, water and air temperatures, and conductivities were variously recorded at eight stations in differing vegetative communities in Shark River Slough. At four sites, flow velocities were measured hourly or bihourly at a fixed point in the water column using acoustic Doppler velocity (ADV) meters (fig. 1A). At two sites, flow velocities were measured bi-hourly throughout the upper water column in 1.6-cm increments using up-looking acoustic Doppler profiling (ADP) meters (fig. 1B). At all eight sites, water and air temperatures were monitored at 5- or 30-minute intervals in 5- or 10-cm increments above the plant litter layer (fig. 1C). Conductivities and water temperatures were measured bi-hourly near the top of the litter layer (fig. 1D) at all four ADV sites. During intermittent visits to all sites vertical velocity profiles were collected at 3-cm depth intervals throughout the water column. The types of measurements and associated equipment at the sites are identified in Table 1.
Site and Data Descriptions
Of the eight continuous monitoring stations, one was established in 1999 (SH1), two in 2000 (GS-33 and GS-203), four in 2001 (X1, GS-202, SQBN and SQUB), and one in 2002 (GS-36). A brief description of the local vegetation and a list of parameters collected at each site are given in Table 2.
Shark River Slough is the dominant path of shallow surface water (sheet) flow through Everglades National Park. It conveys freshwater inflows released from hydraulic control structures and discharged through culverts along Tamiami Trail to the coastal mangrove ecotone bordering the southwest Gulf coast. The low-gradient (10-6) wetlands of Shark River Slough are a mixture of tree islands, sawgrass marshes, and wet prairies as is evident in the photographs below where flow velocities are being measured in this project.
ADV Data Processing
Velocity and Temperature Data
Data collected to date in Shark River Slough illustrate typical flow conditions found in varied vegetative communities and reveal periods during which dynamic factors influence the flow behavior. Examples of the varied types of data collected to quantify sheet flow behavior are illustrated below.
Velocity - continuous point
Flow speeds and directions at SH1, shown as vectors in the horizontal plane relative to magnetic north, are illustrated in Figure 5. Velocities generally ranged from 1.5 to 2.5 cm/s and flow directions averaged approximately 210 degrees. The response of the flow to Hurricane Irene is evident by the higher velocity (4.5 cm/s) and directional change on October 15, 1999.
Velocity - continuous profile
The ADP unit deployed at site GS-33 yields flow velocities at 1.6-cm intervals every 30 minutes from about 15 cm above the litter layer to the water surface. The ADP data are being quality checked and analyzed to gain insight into the vertical flow structure. Preliminary horizontal velocities measured at GS-33 and shown in Figure 6 illustrate the daily range and variability of flow velocities throughout the upper water column.
Velocity - discrete profile
Flow velocities were measured at 3-cm depth increments in two-minute bursts with portable ADVField meters from the litter layer to the water surface during intermittent site visits. A profile collected at SQBN and shown in Figure 7 clearly illustrates the vertical velocity gradient and relatively stable uni-direction flow throughout the water column. Profiles taken throughout the re-growth of vegetation at this fire site have provided insight into the effects of vegetation on flow behavior and structure.
Temperature - continuous profile
Water and air temperatures have been monitored at 5-cm or 10-cm increments above the plant litter layer at all ADV sites. Temperature data measured every 30 minutes at SH1 and shown in Figure 8 illustrate a typical water column thermal pattern. Submerged thermistors (10, 20 and 30 cm above the litter) show the development of vertical stratification during the day and subsequent destratification of the water column during the night. The effects of daily water temperature changes on flow-velocity structure are currently under investigation.
Summary and Conclusions
Data collected at a variety of sites in Shark River Slough since 1999 are yielding insight into the typical range of flow velocities found in various vegetative communities and the dynamic factors that affect flow velocities and sheet flow conditions at and between sites. Measurement techniques and processing methods have been developed to evaluate the extremely low flows which have been found to range from 0 to 4 cm/s. Findings from all data collected and analyzed to date indicate that the dynamics and the magnitude, direction and nature of sheet flow conditions are attributed in varying degrees to both internal and external effects, both locally and regionally driven.
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U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 15 January, 2013 @ 12:42 PM(TJE)