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projects > vegetative resistance to flow in the everglades > abstract
Determination of Resistance Coefficients for Flow through Submersed and Emergent Vegetation in the Florida EvergladesJonathan K. Lee, Lisa C. Roig, Harry L. Jenter, Hannah M. Visser 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
Fluid moving through an array of erect objects is commonly characterized by the stem Reynolds number, ReD = 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 ReD < 150. For 6 < ReD < 44, separation occurs behind the cylinder creating a recirculation zone in the lee of the object. For 44 < ReD < 150, organized vortex shedding is observed. For 150 < ReD < 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 ReD 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 ReD < 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 Mannings 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 Mannings n for this purpose. An approximation of Mannings 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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U.S. Department of the Interior, U.S. Geological Survey, Center for Coastal Geology This page is: http://sofia.usgs.gov /projects/vege_resist/detresabgeer00.html Comments and suggestions? Contact: Heather Henkel - Webmaster Last updated: 11 October, 2002 @ 09:30 PM (KP) |