A trapezoidal channel of 2:1 side slope and 5 ft bottom width is to be used to discharge a flow of 220 ft3/sec. If the channel slope is 1.5% and the Manning coefficient is 0.015, determine the minimum depth required for the channel. Is the flow supercritical or subcritical?
What will be an ideal response?
Using the graphical solution of Manning’s equation for a trapezoidal channel with
a side slope of 2:1 and bottom width of 5 ft, provided in Figure 16.6,
Qn = flow × manning coefficient
Qn = (220)(0.015)
Qn = 3.3
From Figure 16.6, depth of flow = 1.8 ft.
The intersection of the discharge flow line and the 1.5% slope line lies above
(barely) the critical curve, thus, the flow is supercritical.
Allowing a free board of 1 ft, the minimum depth required for the channel is 2.8
ft.
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