Question

Which of the following formula is valid when the downstream depth is more than 80% of the upstream depth?

(a) Unwin’s formula

(b) Broad-crested weir formula

(c) Orifice formula

(d) Molesworth formula

I had been asked this question in an international level competition.

I'm obligated to ask this question of Small Road Bridges Construction in section Culverts and Small Road Bridges Construction of Irrigation Engineering

Answer 1

Correct option is (c) Orifice formula

The explanation is: The weir formula is not valid when the afflux is less than 1/4 of the downstream depth i.e. the downstream depth is more than 80% of the upstream depth. In such a case, the discharge is calculated by the Orifice formula which is given by-

Q = Co (2g) ^1/2. L. ϒd. [h + (1+e)Va^2 / 2g] where, Co = coefficient for the orifice formula, ϒd = downstream depth, Va^2 / 2g = head due to velocity of approach.