Launching Apron

Q. 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

1. $\sqrt{5}m$
2. $4.7m$
3. $5m$
4. $5\sqrt{5}m$

Q. 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)

1. 8 m
2. 15 m
3. 6 m
4. 20 m