Question

When the crest of an ogee spillway is designed to be in accordance with the lower nappe of a free-falling water jet over a duly ventilated sharp-crested weir then theoretically

(a) The pressure on the spillway crest will be zero at design head only

(b) The pressure on the spillway crest will always be zero

(c) The pressure on the spillway crest will always be negative

(d) The pressure on the spillway crest will be always positive

The question was asked in unit test.

My enquiry is from Ogee Profile in section Spillways, Energy Dissipators and Spillway Gates of Irrigation Engineering

Answer 1

Correct answer is (a) The pressure on the spillway crest will be zero at design head only

Easy explanation: In actual practice development of negative pressure takes place beneath the nappe due to the removal of air by the falling jet. This causes the danger of cavitation and induces fluctuation and pulsation effects. To control this aeration pipes may be provided along spillway face.