(2)
Alternate Bars which occur in somewhat straighter reaches of channels and tend to be
distributed periodically along the reach, with consecutive bars on opposite sides of the
channel. Their lateral extent is significantly less than the channel width. Alternate bars
move slowly downstream;
(3)
Transverse Bars which also occur in straight channels. They occupy nearly the full
channel width. They occur both as isolated and as periodic forms along a channel, and
move slowly downstream; and
(4)
Tributary Bars which occur immediately downstream from points of lateral inflow into a
channel.
In longitudinal section, bars are approximately triangular, with very long gentle upstream
slopes and short downstream slopes that are approximately the same as the angle of
repose. Bars appear as small barren islands during low flows. Portions of the upstream
slopes of bars are often covered with ripples or dunes.
3.4 RESISTANCE TO FLOW IN ALLUVIAL CHANNELS
Resistance to flow in alluvial channels is complicated by the large number of variables and
by the interdependency of these variables. It is difficult, especially in field studies, to tell
which variables are governing the flow and which variables are the result of this flow.
The slope of the energy grade line for an alluvial stream illustrates the changing role of a
variable. If a stream is in equilibrium with its environment, slope is an independent variable.
In such a stream, the average slope over a period of years has adjusted so that the flow is
capable of transporting only the amount of sediment supplied at the upper end of the stream
and by the tributaries. If for some reason a larger or smaller quantity of sediment is supplied
to the stream than the stream is capable of transporting, the slope would change and would
be dependent on the amount of sediment supplied until a new equilibrium is reached.
In the following sections the variables affecting resistance to flow are discussed. The effects
produced by different variables change under different conditions. These changing effects
are discussed along with approximations to simplify the analysis of alluvial channel flow.
The variables that describe alluvial channel flow are:
V
=
velocity
=
depth
yo
Sf
=
slope of the energy grade line
=
density of water-sediment mixture
ρ
=
density of sediment
ρs
=
apparent dynamic viscosity of the water-sediment mixture
g
=
gravitational acceleration
=
representative fall diameter of the bed material
Ds
G
=
gradation coefficient of the bed material
=
shape factor of the particles
Sp
SR
=
shape factor of the reach of the stream
=
shape factor of the cross-section of the stream
Sc
fs
=
seepage force in the bed of the stream
3.21
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