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A stable channel has three degrees of freedom in the sense that the depth, width and slope can adjust to the given discharge and sediment conditions. In this sense which of the following methods of canal design gives a true regime channel for silt laden flow in fine alluvium?
According to Lacey’s theory the dimensions of bed width, depth and slope of canal attain a state of equilibrium with time which is called true regime state. Lacey defined a regime channel as a stable channel transporting a minimum bed load consistent with fully active bed.
If it is required to design a stable channel for a given discharge in an alluvial material of known median size, it can be done by least number of assumptions concerning the important variables by using
Lacey, in his regime equations, has adjusted his silt factor to be unity for the standard silt used by Kennedy. It is known that Kennedy used BariDoab alluvium as the standard silt in his equation. Then, the median size, in mm, of BariDoab alluvium and the critical velocity ratio for this silt are, respectively
Average bed material load of a regime channel conforming to Lacey equations will be of the order of
The total number of independent equations that form the Lacey's regime theory is
Lacey collected a large number of data of stable channels in IndoGanetic plains. Analyzing the data he gave the following equations of regime channel relating regime velocity Vc, silt factor f, hydraulic radius R, area A, sediment size in mm and bed slope S,
These four equations are the Lacey’s basic regime flow equations.
The velocity of flow in a channel with a depth of 1 mt calculated by Kennedy’s theory with a critical velocity ratio as 1.2 is
A regime channel has a discharge of 100 m^{3}/s. It will have a perimeter of
When an alluvial channel attains its regime it will have side slopes
For medium silt whose average grain size is 0,16 mm, Lacey’s silt factor is likely to be
Lacey’s silt factor,
The Lacey’s silt factor for a particular alluvium is 2.0. This alluvium would comprise
Lacey’s silt factor,
∴
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