Quantcast HYDROLOGY/MATERIAL TRANSPORT (Cont.)

 
  
 
The equation for the Reynold's number may be represented as:
qd
NR'
Reynold's numbers greater than 2800 indicate turbulence or low viscosity and are much more prevalent
than numbers for laminar flow, i.e less than 2000.
Laminar flow is significant in streams to plants and animals (e.g., refuge) and is found as a thin
layer on river substrate such as gravel/rocks and woody debris. Turbulent flow is effective at eroding
the stream channel and transport of materials. Zones of maximum turbulence are associated with
changes between forward flow (river bends) and the friction of the stream channel (riffle zones). The
Austausch (German meaning exchange) coefficient (A) is a measure of turbulence or describes the
mixing in addition to the mixing associated only with molecular diffusion. It is an attempt to quantify all
the aimless movements that are counter to the main direction of flow (Cole 1983). Turbulence and
laminar flow impact the concentration of dissolved gases and dissipation of heat (both increase with an
increase in turbulence).
Flow pattens downstream from a hydropower project are presented in Figure 1.3.4 which is a
photograph of the release from a scaled, physical model with markers used to depict flow patterns. As
represented by the light markers, there are distinct flow patterns and areas of different velocities that
become less defined as flow proceeds downstream. High velocity, mixing, and turbulence are indicated
in the lower right corner, just downstream from the powerhouse. There is a well-established, counter-
clockwise flow established further downstream that creates increased turbulence and a smaller
clockwise flow in the vicinity of the flood control gates (lower left corner). These patterns are
representative for reservoir tailwaters but would vary as a function of release regimes and channel
morphometry.
Velocity (distance/unit time) is determined by the quantity of water, channel shape, bottom
texture/bank structure, and gradient or slope (Table 1.3.1). Wide, deep rivers with a slight slope would
have a greater velocity than a small, shallow stream on a much greater slope. Flow also proceeds at a
faster rate on smooth channels than it does on rough-bottom channels and increases at constrictions.
Velocity varies in a stream channel with greater velocities occurring on the surface in the center of the
stream (Figure 1.3.5).
Discharge is the quantity of water that passes a particular point in a unit of time and is
represented by the following equation:
Discharge (m3 s-1) = channel width (m) * channel depth (m) * water velocity (m s-1)
1.3-5

 


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