Resistance to flow due to antidunes depends on how often the antidunes form, the area of
the stream they occupy, and the violence and frequency of their breaking. If the antidunes
do not break, resistance to flow is about the same as that for flow over a plane bed. If many
antidunes break, resistance to flow is larger because the breaking waves dissipate a
considerable amount of energy. With breaking waves, Manning's n may range from 0.012 to
0.020.
3.3.8 Chutes and Pools
At very steep slopes, alluvial-channel flow changes to chutes and pools (Figure 3.5). In the
2.4 m (8-foot) wide flume at Colorado State University, this type of flow and bed configuration
was studied using fine sands. The flow consisted of a long chute 3 to 9 m (10 to 30 ft) in
which the flow was rapid and accelerating followed by a hydraulic jump and a long pool. The
chutes and pools moved upstream at velocities of about 0.3 to 0.6 m (1 to 2 ft) per minute.
The elevation of the sandbed varied within wide limits. Resistance to flow was large with
Manning's n of 0.018 to 0.035.
3.3.9 Regime of Flow, Bed Configuration, and Froude Number
The change from lower to upper regime flow or the reverse (that is a change from dune bed
to a plane bed or plane bed to a dune bed) is not related to the Froude number. However,
standing wave and antidune bed configuration in the upper flow regime only occurs with a
Froude number greater than 1.0 (Fr > 1.0), and ripples and dunes only occur in the lower
flow regime at a Froude number less than 1.0 (Fr < 1.0) (Vanoni 1977).
The misconception that the lower flow regime shifts to the upper flow regime at a Froude
number of 1.0 (Fr = 1.0) results from studies made in small flumes where the depth is
shallow and large velocities are needed in order to shift from a dune bed to a plane bed. In
larger flumes and in rivers the shift occurs at Froude numbers as low as 0.2 (Simons and
Richardson 1966, Richardson and Simons 1967, Nordine 1964, Richardson 1965, Dawdy
1961). Figure 3.8 illustrates the relation between flow depth, Froude number and regimes of
flow and Figure 3.9 conceptualizes the crossover from lower to upper flow regime in natural
rivers.
3.3.10 Bars
In natural channels, some other bed configurations are also found. These bed configurations
are generally called bars and are related to the plan form geometry and the width of the
channel (Figure 5.14).
Bars are bed forms having lengths of the same order as the channel width or greater and
heights comparable to the mean depth of the generating flow. Several different types of bars
are observed. They are classified as:
(1)
Point Bars which occur adjacent to the inside banks of channel bends. Their shape
may vary with changing flow conditions and motion of bed particles but they do not
move relative to the bends;
3.19
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