Question

An earthen channel has been designed on Lacey formulae to carry a full supply discharge of $30m^3/s.$ The mean velocity of flow at this discharge is:

• A. 0.98 m/s
• B. 0.76 m/s
• C. 2.2 m/s
• D. 1.36 m/s

Answer 1

The correct option is B 0.76 m/s

Silt factor is generally in the range of 0.8-1.2

Let silt factor be f = 1.0

According to Lacey;

$V={\left(\frac{Q{f}^2}{140}\right)}^{1/6}$

V = Velocity of flow
Q = Discharge
f = Silt factor

Given
$Q=30m^3/sec$

$V={\left(\frac{30\times 1^2}{140}\right)}^{1/6}$

$=0.77m/sec$