Question

If a flood enters a dam reservoir at F.R.L, the efficiency of its ogee spillway will ______________

(a) increase with the increasing head

(b) reduce with the increasing head

(c) remain constant with the increasing head

(d) vary with the increasing head

The question was asked during an interview.

My question is taken from Ogee Profile in chapter Spillways, Energy Dissipators and Spillway Gates of Irrigation Engineering

Answer 1

Right option is (a) increase with the increasing head

The explanation: The coefficient of discharge increases with the increasing head above the full reservoir level till it becomes about 2.2 at full design head i.e. MRL. Since the spillway starts working when the water level just crosses FRL, the coefficient of discharge is about 77% at full design head. As the water level increases from F.R.L to M.R.L, the efficiency increases from 77% to 100%.