When the bed configuration is dunes, field and laboratory studies indicate that resistance to
flow may increase or decrease with an increase in depth, depending on the size of bed
material and magnitude of the depth. Additional studies are needed to define the variation of
resistance to flow for flow over dune beds.
When the bed configuration is antidunes, resistance to flow increases with an increase in
depth to some maximum value, then decreases as depth is increased further. This increase
or decrease in flow resistance is directly related to changes in length, amplitude, and activity
of the antidunes as depth is increased.
3.4.2 Slope
The slope, Sf, is an important factor in determining the bed configuration which will exist for a
given discharge. The slope provides the downstream component of the fluid weight, which in
turn determines the fluid velocity and stream power. The relation between stream power,
velocity and bed configuration has been illustrated in Figure 3.7.
Even when bed configurations do not change, resistance to flow is affected by a change in
slope. For example, with shallow depths and the ripple-bed configuration, resistance to flow
increases with an increase in slope. With the dune-bed configuration, an increase in slope
increases resistance to flow for bed materials having fall velocities greater than 0.06 m/s
(0.20 ft/s). For those bed materials having fall velocities less than 0.06 m/s (0.20 ft/s), the
effect is uncertain.
3.4.3 Apparent Viscosity and Density
The effect of fine sediment (bentonite) on the apparent kinematic viscosity of the mixture is
shown in Figure 3.11. The magnitude of the effect of fine sediment on viscosity is large and
depends on the chemical make up of the fine sediment.
Figure 3.11. Apparent kinematic viscosity of water-bentonite dispersions
(Simons and Richardson 1966).
3.23
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