PE Civil Engineering Exam Prep

Answer: C - the velocity in a rectangular channel with the given dimensions is 3.92 ft/sec.

Solution: Use Manning's Equation and solve for V

V = (K/n) R^2/3 S_f^1/2

Where: K = conversion coefficient (1.486 for English units, 1.0 for SI) n = 0.014, the Manning coefficient
d = depth of flow = 1.5 ft
w = width of channel = 3.0 ft
S_f= channel slope = 0.002 ft/ft

A = Area = d x w = 1.5 ft x 3 ft = 4.5 ft²

P = wetted perimeter = w + 2d = 3 ft + 3 ft = 6 ft

R = hydraulic radius = A/P = (4.5 ft²)/ (6 ft) = 0.75 ft

V = (1.486/0.014) x (0.75 ft)^2/3 x (0.002 ft/ft)^1/2

= 3.92 ft/sec