Question

A launching apron is to be designed at the downstream side of a weir. The structure is designed for a discharge of $8.2m^3/sec/m$. What should be the length of the apron if the tail­ water depth is 3m? (Assume, the scour depth is taken as double as that of Lacey's scour depth, where the silt factor is 1.01)

• A. 8 m
• B. 15 m
• C. 6 m
• D. 20 m

Answer 1

The correct option is A 8 m

$\text{Maximum scour depth}=2\times 1.35{\left(\frac{q^2}{f}\right)}^{1/3}$

$=2\times 1.35{\left(\frac{{8.2}^2}{1.01}\right)}^{1/3}$

$=10.942m$

d = 10.942 - Tail~ water ~depth

$=10.942-3$

$=7.942m$

Length of apron = 1 to 1.5 times of d

$d=7.942to11.913m$