Question

An ogee spillway of a concrete gravity dam having FRL of 328 m and MWL of 340 m is provided with vertical gates between piers erected on the spillway. The effective length of the spillway is 60 m. The discharge through the spillway when gates are opened up to the actual reservoir level of 331.0 m will be_____________

(a) 850 cumecs

(b) 1700 cumecs

(c) 2800 cumecs

(d) 1000 cumecs

This question was posed to me during an online exam.

The doubt is from Types of Spillway Gates in division Spillways, Energy Dissipators and Spillway Gates of Irrigation Engineering

Answer 1

Right answer is (b) 1700 cumecs

Easiest explanation: The discharge is given by the equation –

Q = Cd. A. (2gH)^1/2

Where Cd is the coefficient of discharge through the orifice

A is the area of the opening = 60 x 6 =360 m^2

H is the water head over the centre line of the opening = RL (331 – 328) = 3m

Q = 0.62 x 360 x (2 x 9.81 x 3)^1/2 = 1700 cumecs.