Question

For medium silt whose average grain size is 0.16 mm, Lacey’s silt factor is likely to be
1. 0.30
2. 0.45
3. 1.32
4. 0.90
5. 0.70

Answer 1

Correct Answer - Option 5 : 0.70

Concept:

According to Lacey’s, the design formulas to build canal is as follows:

1) Silt factor ⇒ \(\rm{f = 1.76√ {{d_{mm}}}}\)

2) Velocity of flow ⇒ \(\rm{V = {\left[ {\frac{{Q{f^2}}}{{140}}} \right]^{\frac{1}{6}}}}\)

3) Hydraulic mean depth ⇒ \(\rm{R = \frac{{5{V^2}}}{{2f}}}\)

4) Wetted perimeter ⇒ \(\rm{P = 4.75√ Q}\)

5) Bed slope ⇒ \(\rm{S = \frac{{{f^{\frac{5}{3}}}}}{{3340 \times {Q^{\frac{1}{6}}}}}}\)

Calculation:

Lacey’s silt factor:

 f \(= 1.76\sqrt {{d_m}}\)

\(= 1.76\sqrt {0.16} = 0.70\)