U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004-3123

November 2004

Sheet-flow Velocities and Factors Affecting Sheet-flow 

Behavior of Importance to Restoration of the Florida Everglades 

Printed on recycled paper

INTRODUCTION 

The freshwater wetlands of the Everglades are a mosaic of 

tree islands, sawgrass marshes, wet prairies, and sloughs. Flow 

typically is shallow and slow in the wetlands. The shallowdepth, slow-velocity flow, referred to as sheet flow, is controlled 

primarily by the small topographic gradient (~10-6), the high 

internal resistance of submerged aquatic plants and emergent 

vegetation, and to a lesser extent by water levels, microtopography, and meteorological factors. Sheet-flow conditions are vital 

to habitat sustainability. Strong but mostly anecdotal evidence 

suggests that sheet flow helps shape and preserve the ridge and 

slough landscape (Science Coordination Team, 2003). Drainage 

and compartmentalization coupled with decades of managed 

water controls have altered sheet-flow conditions in the 

Everglades.

Significant portions of the Everglades have been compartmentalized with the largest remaining free-flowing part being 

within Everglades National Park (ENP). Shark River Slough 

(fig. 1) is a prominent drainage feature in ENP that conveys 

freshwater inflows to the coastal mangrove ecotone as sheet flow 

through vegetated wetlands. In a recently completed study by the 

U.S. Geological Survey (USGS), sets of continuous flow-velocity, water-temperature, and conductivity data were collected in 

Shark River Slough during wet seasons from July 1999 to July 

2003. The Everglades wet season typically coincides with the 

June through November tropical hurricane season. These flow 

data, documented in Riscassi and Schaffranek (2002, 2003, and 

2004), define the range of flow velocities in a variety of vegetative communities and are yielding insight into factors that 

influence the sheet-flow regime. A summary of the data and an 

overview of study findings are presented in this Fact Sheet. Figure 1. Satellite image showing monitoring sites in southern Florida.

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The Everglades is a vast subtropical ecosystem on the southern tip of the Florida peninsula that is the subject of an extensive restoration effort. A major objective of Everglades restoration, as defined in the Comprehensive Everglades Restoration Plan (CERP) (http://www.evergladesplan.org/), is to restore the ecosystem to more natural predrainage conditions. 

This effort not only requires scientific insight into factors that affect flow behavior for effective restoration planning but 

also requires background knowledge of flow conditions for restoration assessment, identified as a critical need in the 

CERP Monitoring and Assessment Plan (http://www.evergladesplan.org/pm/recover/recover_map_2004.cfm). Knowledge of flow conditions also is vital to the development and use of models to evaluate, interpret, and compare restoration 

scenarios for implementation of the CERP. Moreover, the role of flow as a factor contributing to landscape changes in the 

Everglades is not well understood and needs to be investigated (National Research Council, 2003). In general, the Science 

Coordination Team of the South Florida Ecosystem Restoration Task Force (http://www.sfrestore.org/) has asserted that 

there is an urgent need for flow research in the Everglades to increase the level of understanding and awareness of its role 

in restoration activities (Science Coordination Team, 2003).

MONITORING SITES AND SAMPLING STRATEGY

Over the course of the study, flow velocities, water and air 

temperatures, and (or) conductivities were measured at five sites 

with different vegetative properties. Locations of the five monitoring sites are shown in figure 1. At four sites (SH1, GS-203, GS-33, 

and GS-36), flow velocities were measured at a fixed elevation in 

the water column using acoustic Doppler velocity (ADV) meters. 
At three sites (GS-203, GS-33, and GS-36), conductivities were 

measured at a fixed elevation near the top of the plant litter layer. 

At all five sites, temperatures were monitored in the plant litter, at 

vertical intervals throughout the water column, on the water surface, 

and in the air above the water column using thermally sensitive 

resistors. In addition to considerations of vegetation characteristics 

and regional location, monitoring sites were selected based on their 

proximity to existing hydrologic and meteorological stations. 

FLOW-VELOCITY DATA

ADV meters measure the frequency shift between a transmitted 

acoustic pulse and its reflectance from particulate matter within 

a small sample volume (0.25 cm3) to determine flow velocity. An 

ADV meter is shown suspended vertically from a horizontal board 

and centered between two vertical boards at the SH1 site in figure 

2A. The ADV meter senses and records three-dimensional velocity components to an accuracy of ±1 % of the measured velocity 

(SonTek, 2001) with a resolution of 0.01 cm/s. Because vertical velocity components are negligible, only horizontal velocity 

components were used to determine flow speeds and directions. 

A sample plot of velocity vectors showing bihourly flow speeds 

and directions at site GS-203 during the 2000-2001 wet season is 

presented in figure 4.

ADV data collected at four sites indicate a range of sheet-flow 

velocities in typical vegetative communities within ENP. The daily 

means of all ADV data ranged between 0.20 and 5.16 cm/s, with 

an overall mean of 1.15 cm/s (Riscassi and Schaffranek, 2002, 

2003, and 2004). Ninety percent of all daily mean flow velocities 

were between 0.46 and 2.29 cm/s. Mean flow velocities were 1.63, 

0.82, 0.68, and 1.40 cm/s at directions of 225, 243, 200, and 229 

degrees from magnetic north for all data collected at sites SH1, 

GS-203, GS-33, and GS-36, respectively. These mean flow velocities, although representative of sheet-flow conditions in similarly 

vegetated areas within ENP, should not be assumed to be typical 

of flow in compartmentalized regions of the Everglades, such as 

Water Conservation Areas (WCA).

FACTORS AFFECTING FLOW 

Inhomogeneity in the speed, direction, and behavior of sheet 

flow is a function of external and internal forcing mechanisms, 

both locally and regionally driven. Local factors include water 

depth; microtopography; the type, amount, and properties of 

emergent vegetation; and the presence, density, and composition 

of submerged aquatic plants and (or) periphyton. Regional factors Figure 3. Dates of processed and edited flow-velocity, conductivity, and 

temperature data.

Figure 4. Hydrograph of water depths and bihourly flow velocities 

plotted as vectors at site GS-203. Velocity vectors are aligned to 

indicate flow direction relative to magnetic north and plotted in length 

according to y-axis scale to represent flow speed. Negative velocities 

indicate southerly flow.

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Photographs of vegetation at the four sites where flow velocities 

were measured are shown in figure 2. Site SH1 (fig. 2A) is a mediumdense area of spikerush (Eleocharis cellulosa) on the edge of a sawgrass (Cladium jamaicense) stand. Site GS-203 (fig. 2B) is a mediumdense area of sawgrass on the edge of a dense sawgrass stand. Sites 

GS-33 (fig. 2C) and GS-36 (fig. 2D) are sparse and medium-dense 

spikerush areas, respectively, each having various amounts of 

submerged aquatic vegetation and periphyton. Periphyton, a matrix 

of algae found on the top of the litter layer, floating, submerged, or 

attached to plant stems in various concentrations at all sites, can be 

seen in mats floating near the water surface at site GS-36 in figure 2D. 

Un-pictured site NP202 (fig. 1) is a dense cattail (Typha domingensis) 

area. Dates for which valid velocity, temperature, and conductivity 

data were collected, typically bihourly, are identified in figure 3.

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Figure 2. Vegetation at flow-monitoring sites (A) SH1, (B) GS-203, 

(C) GS-33, and (D) GS-36.

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include the water-surface slope; land-surface gradient; vegetative heterogeneity; and the proximity of tree islands, airboat trails, 

hydraulic structures, roads, culverts, canals, and levees. Meteorological factors such as winds variously and intermittently affect flow 

conditions. Fires also have a residual effect on sheet-flow behavior 

(Schaffranek and others, 2003). Several factors affecting sheet-flow 

conditions are discussed in the following sections.

Water Depth 

Data indicate that dynamic variability in flow directions is most 

evident during low water levels and is considerably damped with 

increased water depth. Implications are that when water levels are 

high, regional factors drive flow more uniformly; however, as water 

levels fall, flow velocities decrease, momentum is reduced, and the 

flow becomes more susceptible to local forcing mechanisms, such 

as microtopography and vegetative properties. A graph of flow 

speed plotted against direction for bihourly velocity data collected 

at site GS-203 during the 2000-2001 wet season is shown in figure 

5. Flow velocities greater than about 0.5 cm/s are more consistent 

in direction than flow at slower velocities. The standard deviation 

in flow direction of velocities greater than 0.5 cm/s is less than 

a third of the standard deviation of flow velocities less than 0.5 

cm/s. Near-convergence of flow velocities to a constant direction 

at faster speeds is clearly evident in figure 5. Analyses of velocity 

data in conjunction with water levels recorded at a nearby hydrologic monitoring station revealed that the most significant velocity 

fluctuations occurred during low water depths (see fig. 4). It is 

likely that flow velocities and sheet-flow conditions in WCA of the 

Everglades behave similarly to the slow (< 0.5 cm/s) velocity data 

plotted in figure 5 at nearly all flow depths. However, this hypothesis needs to be tested through concurrent acquisition of synoptic 

time-series of flow data in wetlands of both ENP and WCA.

Vegetation 

The vertical flow structure at all ADV sites was fairly uniform, 

with variations in flow speed and direction corresponding to differences in vegetation composition. Flow velocities at 3-cm depth 

intervals and volumes of organic material, primarily spikerush and 

periphyton, occupying the water column at 10-cm depth increments 

are shown in figure 6 for site GS-33. The reduced flow velocity 

near the water surface is due to the presence of periphyton, which 

was concentrated in mats floating near the water surface. At about 

10 cm above the plant litter layer the flow speed begins to decline 

to zero velocity at the top of the litter layer. Vertical velocity 

profiles at each of the ADV monitoring sites revealed similar 

damped flow speeds within about 5-10 cm above the plant litter 

layer. Relatively uniform flow structure was observed throughout 

the middle to upper part of the water column for uniform vegetation composition. The flow speed and direction measured by the 

self-recording ADV meter deployed at the site are plotted at its 

sampling depth in figure 6 to show the close agreement of the two 

independent velocity measurements.

Meteorological Events 

Intermittent storm and rainfall events appeared to have limited, 

mainly local, influence on flow behavior, but these effects varied 

greatly depending not only on the amount of submerged vegetation 

but also on the amount of emergent vegetation sheltering the water 

surface from wind effects. In general, the relatively deep sample 

depths of the ADV meters inhibited detection and assessment of 

the strength of meteorological effects, which tend to be damped 

with increased distance from the water surface. Some flow data 

Figure 5. Bihourly flow speeds and directions at site GS-203 during 

2000-2001 wet season.

Figure 6. Flow velocities and volumes of organic material in water 

column measured at site GS-33 on October 30, 2002 (1330–1415 

hours). Red symbols identify flow speed and direction measured 

at 1600 hours by self-recording ADV meter deployed at site. Note 

scale of top horizontal axis defining water column volume occupied 

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suggested that strong storms affected vegetation composition, 

which in turn had residual effects on flow behavior, but these 

effects only persisted for a few days. The regional extent of 

storm effects on sheet-flow behavior could not be evaluated 

with any degree of confidence given the limited amount of data 

and few monitoring stations. Temperature-profile data indicate, 

however, that persistently strong winds, rainfall events, or cool 

cloud-covered days have atypical thermal effects on internal 

flow structure.

Temperature

Concurrent temperature-profile and flow-velocity data collected at each of four sites have documented thermal effects on 

internal flow structure. Temperature profiles collected at 5- and 

10-cm depth intervals link large fluctuations in flow velocities measured and recorded after sunset to thermal convection 

(Jenter and others, 2003). Temperature data indicate that the 

water column is typically isothermal at the beginning of each 

day, stratifies during the daytime, and de-stratifies during the 

night due to mixing driven by thermal convection. This mixing 

process has potentially profound implications on understanding processes of importance to successful implementation of 

the CERP, including mercury methylation, evapotranspiration, 

oxidation, and nutrient cycling.

SUMMARY AND CONCLUSIONS 

A study of sheet flow in ENP from July 1999 to July 2003 has 

produced data defining flow velocities in varied vegetative communities and yielded insight into factors that affect flow behavior 

of importance to Everglades restoration. The mean of all daily 

mean flow velocities for the four sites was 1.15 cm/s. Ninety 

percent of all daily mean flow velocities were between 0.46 and 

2.29 cm/s. Mean flow directions at all sites were southwesterly 

ranging between 200 and 243 degrees relative to magnetic north. 

Flows at velocities greater than 0.5 cm/s were more consistent in 

direction than flows at slower velocities. Vertical flow structure 

was fairly uniform and correlated to vegetation composition. 

Meteorological effects on flow were limited mainly to significant 

storm events, although data limitations precluded more definitive 

analyses. Thermal-driven vertical mixing was documented to 

occur nearly daily in the absence of storm events and passage of 

weather fronts. More such flow data would be desirable to define 

and evaluate sheet-flow conditions in other areas of ENP and in 

other regions of the Everglades to aid restoration efforts. Comparable flow data in WCA would enable comparison of sheet-flow 

behavior in compartmentalized regions with free-flowing conditions in ENP and provide a means to assess the role of flow in 

shaping and preserving the Everglades landscape.

Any use of trade, pr

For more information contact:

Raymond W. Schaffranek

U.S. Geological Survey

12201 Sunrise Valley Drive

National Center, Mail Stop 430

Reston, VA 20192

(703) 648-5891 (voice)

(703) 648-5484 (fax)

rws@usgs.gov (E-mail)

ACKNOWLEDGMENTS 

Support for this study was provided by the USGS Priority 

Ecosystems Science Initiative. Nancy Rybicki and Alfonso Lombana, USGS, provided assessments of vegetative properties at the 

monitoring sites.

REFERENCES CITED

Jenter, H.L., Schaffranek, R.W., and Smith, T.J., 2003, Thermally driven vertical mixing in the Everglades, Joint Conference on the Science and Restoration of the Greater Everglades and Florida Bay, Palm Harbor, FL, April 13-18, 2003, 

pp 290-292, accessed November 3, 2004 at http://sofia.usgs.

gov/projects/vege_resist/vertmix_geer03abs.html

National Research Council, 2003, Does water flow influence Everglades landscape patterns, Washington, D.C., The 

National Academies Press, 41 p, accessed November 3, 2004 

at http://books.nap.edu/catalog/10758.html

Riscassi, A.L., and Schaffranek, R.W., 2002, Flow velocity, 

water temperature, and conductivity in Shark River Slough, 

Everglades National Park, Florida: July 1999 – August 2001, 

U.S. Geological Survey Open-File Report 02-159, 32 p.

Riscassi, A.L., and Schaffranek, R.W., 2003, Flow velocity, 

water temperature, and conductivity in Shark River Slough, 

Everglades National Park, Florida: August 2001 – June 2002, 

U.S. Geological Survey Open-File Report 03-348, 37 p.

Riscassi, A.L., and Schaffranek, R.W., 2004, Flow velocity, 

water temperature, and conductivity in Shark River Slough, 

Everglades National Park, Florida: June 2002 – July 2003, 

U.S. Geological Survey Open-File Report 04-1233, 56 p.

Schaffranek, R.W., Riscassi, A.L., Rybicki, N.B., and Lombana, A.V., 2003, Fire effects on flow in vegetated wetlands 

of the Everglades, Joint Conference on the Science and 

Restoration of the Greater Everglades and Florida Bay, 

Palm Harbor, FL, April 13-18, 2003, pp 470-472, accessed 

November 3, 2004 at http://sofia.usgs.gov/projects/dynamics/fireeffects_03geerab.html

Science Coordination Team, 2003, The role of flow in the 

Everglades ridge and slough landscape, South Florida Ecosystem Restoration Working Group, 62 p, accessed November 3, 2004 at http://www.sfrestore.org/sct/docs/.

SonTek, 2001, SonTek ADV acoustic Doppler velocimeter 

technical documentation: San Diego, CA, 202 p.