Question

A launching apron is to be designed at downstream of a weir for discharge intensity of $6.5m^3/s/m.$ For the design of launching aprons, the scour depth is taken two times of Lacey scour depth. The silt factor of the bed material is unity. If the tailwater depth is 4.4 m, the length of the launching apron in the launched position is

• A. $\sqrt{5}m$
• B. $4.7m$
• C. $5m$
• D. $5\sqrt{5}m$

Answer 1

The correct option is C $5m$

Lacey's scour depth,

$R=1.35{\left(\frac{q^2}{f}\right)}^{1/3}$

$Here,q=6.5m^3/s/m$
Silt factor, f = 1

$R=1.35{\left\{\frac{{\left(6.5\right)}^2}{1}\right\}}^{1/3}=4.702m$

So, scour depth = 2R
= 2 × 4.702 = 9.404 m
Tail water depth = 4.4 m
Length of launching appron
= 9.404 - 4.4
= 5 m