Geomorphic Assessment of Channel Systems
3.1.3.4 Empirical Methods for Stable Channel Design
Three types of stable channel design methods are reviewed in this section: maximum
permissible velocity, tractive force, and regime.
Maximum Permissible Velocities. In 1926, Fortier and Scobey presented a channel
design method based on maximum permissible velocities for uniform flow. An earthen
channel was considered stable if the mean velocity of the channel is less than the maximum
permissible velocity for the channel. The USDA (1977) compiled data from Fortier and
Scobey (1926), Lane (1953), and the Union Soviet Socialist Republic (USSR, 1936) into a
set of design charts. These charts are accompanied by a design procedure found in Technical
Release No. 25 (USDA, 1977).
Tractive Force Design. Lane (1953) developed an analytical design approach for
shear distribution in trapezoidal channels. The tractive force, or shear force, is the force
which the water exerts on the wetted perimeter of a channel due to the motion of the water.
It is the force exerted over an area of the bed or banks. It is equal to and in the opposite
direction from the force which the bed exerts on the flowing water. The average value of the
tractive force per unit wetted perimeter (unit tractive force) is given by the following equation
(Simons and Sentrk, 1992):
o ' RSf
(3.1)
where is the specific weight of water, R is the hydraulic radius, and Sf is the energy grade
line. Simons (1957) provided a detailed process for Lane's tractive stress method.
Regime Relationships for Channel Design. In 1895, Kennedy (Lacey, 1931)
developed an early regime equation in India on the Upper Bari Doab Canal. The equation is
as follows:
V0 ' 0.84 m D 0.64
(3.2)
where: V0 = non-silting, non-scouring velocity, critical velocity;
D = average vertical depth as measured on the horizontal bed of the channel
excluding side slopes; and
m = silt factor.
Simons and Albertson (1963) continued regime development by combining data from
canal studies in India (Punjab and Sind) and the United States (Imperial Valley, San Luis
Valley, and canals in Wyoming, Colorado, and Nebraska). Their motive for additional
development of regime analysis was the inadequacy of previous regime methods.
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