Indoor flume experiments and field surveys have been conducted to yield unique data sets describing flow through submersed and emergent vegetation at low Reynolds numbers (Lee and Carter, 1999; Lee and Carter, 1997). Hydraulic measurements were conducted concurrently with vegetation sampling surveys to provide the data needed to

Figure 1. Location of southwest estuarine river stations. Click for larger image.

determine the correlation between frictional resistance and vegetative characteristics (Carter and others, 1999; Rybicki and others, 1999). In addition, an innovative method for measuring the extremely small water-surface slope has been developed (Lee and others, 2000). The objectives of the present effort are as follows: (1) to determine the flow resistance due to vegetation for each of the plant communities sampled in the laboratory and field surveys; (2) to derive equivalent Manning’s n functions to quantify frictional resistance for the specific vegetation types surveyed; and (3) to examine the role of upscaling on the derived Manning’s n values. Future USGS research will correlate the flow resistance to specific physical characteristics of the plants. This future work will permit flow resistance to be predicted from generalized functions, rather than requiring physical surveys in each plant community.

Fluid moving through an array of erect objects is commonly characterized by the “stem” Reynolds number, Re_D = DV/v, where D is the average spacing of the objects, V is the discharge velocity, and v is the kinematic viscosity. Preliminary analyses indicate that the laboratory and field data have stem Reynolds numbers in the range 10 to 400. Let us consider what is known about the flow regime in this range (Churchill, 1982). For an isolated erect cylinder in a horizontal flow field, flow is laminar for Re_D < 150. For 6 < Re_D < 44, separation occurs behind the cylinder creating a recirculation zone in the lee of the object. For 44 < Re_D < 150, organized vortex shedding is observed. For 150 < Re_D < 30000, a turbulent wake forms. In a multi-cylinder array, the limits of these different regimes are different because of wake interference, sheltering, and tortuosity. Nonetheless, the range of Re_D experienced in the Everglades data suggests that the flow regime varies from laminar to transitional, but does not become fully developed turbulent flow, as is commonly assumed for open channel flow. Other researchers suggest that laminar flow in a multi-cylinder array occurs for Re_D < 200 (Nepf, 1999).

The conceptual model of a multi-cylinder array is useful for advancing the analysis of flow through submersed and emergent vegetation to a certain point. Yet the vegetation array is much more complex. The vertical variation of the plant form and the vertical variation of the plant population density affect the flow field. The velocity profiles observed in the laboratory and field studies are very different than what is typical for open channel flows, and than what has been suggested for uniform multi-cylinder arrays. These observations indicate that the historical use of Manning’s n to describe the flow resistance of heavily vegetated environments is inappropriate. One goal of this work is to identify a simple and useful function for specifying the resistance factor for each plant community sampled in the field survey. Resistance coefficients such as the Darcy-Weisbach friction factor or the average stem drag coefficient may be more suitable than Manning’s n for this purpose. An approximation of Manning’s n for use in Everglades hydraulic routing models can be derived for either of these.

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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