Flat expansive wetlands are characterized by periods of very sluggish flow during which full turbulence may not be present.  Wetland researchers have previously noted the importance of significant increases to flow resistance—particularly under vegetated conditions—as the flow turbulence and Reynolds number decline, and as flow transitions toward a more laminar condition.  However, typical depth-average analyses of such flow regimes lack well-grounded empirical formulations to describe dynamic resistance. 

 

Basic one-dimensional laboratory experiments into this increasing resistance using traditional flumes have been limited due to the challenge of measuring very low hydraulic gradients.  Bob Jacobsen PE designed, built, and operated a special flume for conducting research into depth-averaged dynamic resistance.  The 0.5+ km flume featured a spiral configurtaion which allowed for measuring 0.01 mm head loss—or gradients of less than 1x10-8.  The flume design and operating conditions allowed for controlled observations of flow Reynolds Number conditions extending well below full turbulence thresholds.  Experiments were performed at steady and unsteady conditions under a range of roughness and obstruction conditions. 

 

The chart illustrates that measurements of one-dimensional, depth-averaged, quadratic resistance—traditionally expressed using Manning’s n—showed significant increases at declining Reynolds Number.