Channel and nearbed flow environments

The biota of running waters dwell in a highly variable environment from the standpoint of current regime. This is apparent to anyone mesmerized by the delicate swirls on the surface of the smallest stream, or the awesome power of a storm-swollen river. Three fundamental types of flow characterize moving fluids: laminar, turbulent, and transitional. In laminar flow, fluid particle movement is regular and smooth, and particles can be thought of as "sliding" in parallel layers with little mixing. Turbulent flow is characterized by irregular movement with considerable mixing. Intermediate conditions are described as transitional. In fact, laminar flow conditions are so rare in aquatic environments that they are relevant primarily as a theoretical reference point. Although laminar flow can occur in pipes, and over smooth mud surfaces, even beds of sand produce complex flow. This complexity increases with increasing roughness of the channel bottom and with mean velocity.

At the interface between a fluid and a solid, the velocity of the two is identical (the "no-slip" condition, Vogel 1994), which means that water in contact with noneroding substrate has zero velocity. Because surface water can move quite rapidly, there must be a gradient in velocity as one approaches the bottom and sides of streams Figure 2.8). This decrease of velocity with depth produces a region of shear, known as the boundary layer. The upper limit of the boundary layer occurs where the speed of the current is no longer influenced by the presence of the stream bottom. The boundary layer may extend to the surface in a shallow stream. Very close to the stream bottom, there may be a viscous sublayer where shear stress is zero and flow is greatly reduced.

The possibility that a thin layer of low flow exists very near the stream bottom, perhaps functioning as a refuge from the turbulence and high velocities of the water column just above, has attracted the attention of stream ecologists since at least the turn of the last century (e.g., Steinmann 1908, Ambuhl 1959). This idea gained credibility from the dorsally compressed body shapes exemplified by the water penny Psephenus and a number of mayflies, and the expectation that current must be reduced at the water-substrate interface. However, as Vogel (1994) puts it, "most biologists have the fuzzy notion that [the boundary layer] is a discrete region rather than the discrete notion that it's a fuzzy region.'' Moreover, the terms boundary layer and viscous sublayer should not be used interchangeably. Strictly speaking, the region of greatly reduced flow is the viscous sublayer, which is found very close to the streambed or other surface. It now appears that as flow becomes more turbulent and more typical of natural streams, the viscous layer is thinned to the point that most benthic invertebrates likely experience a turbulent, three-dimensional flow microenvironment (Nowell and Jumars 1984, Hart and Finelli 1999). This shift in perspective heightens the need for a better understanding of the hydrodynamic conditions that organisms actually experience.

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