This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Ogee Spillway”.
1. Which of the following spillway is least suitable to earthen dams?
a) Ogee spillway
b) Chute spillway
c) Side-channel spillways
d) Shaft spillway
View Answer
Explanation: It is an improvement over the free over-fall spillway. It is mostly suitable for concrete gravity dams especially when the spillway is located within the body of the dam in the same valley.
2. If the operating head on an ogee spillway is more than the design head then ________________
a) the pressure on the crest will be zero
b) the pressure on the crest will be negative causing cavitation
c) the pressure on the crest will be positive
d) the discharge coefficient of the spillway will be reduced
View Answer
Explanation: The lower nappe of the falling jet may leave the ogee profile when the operating head of the spillway is more than the design head. This generates negative pressure at the point of separation which leads to the formation of cavities in the water resulting in cavitation.
3. In the functioning of an ogee spillway, the operating head ______________________
a) frequently exceeds the design head
b) rarely exceeds the design head
c) never exceeds the design head
d) has no connection with the design head
View Answer
Explanation: An ogee spillway is designed for maximum head up to RL of maximum reservoir level; there are almost no chances of operating head exceeding the design head. Under rare emergency as in excessive floods, it may become necessary to allow the reservoir level to go even above the MRL in such case the operating head may exceed the design head.
4. Which of the following spillway is an improvement over free overflow spillway?
a) Straight drop spillway
b) Open channel spillway
c) Overflow spillway
d) Siphon spillway
View Answer
Explanation: Ogee spillway is an improvement over free overflow spillway and is widely used with concrete, arch and buttress dam. The profile is made such that the shape of the lower nappe of a freely falling jet over a duly ventilated sharp-crested weir.
5. If the head of the water over the spillway is less than the design head, then ________________
a) the pressure on the crest will be zero
b) the pressure on the crest will be negative causing cavitation
c) the discharge coefficient of the spillway is increased
d) the discharge coefficient of the spillway will be reduced
View Answer
Explanation: If the head of water over the spillway is less than the design head, the falling jet would adhere to the crest of the ogee spillway. This causes positive hydrostatic pressure and thereby reduces the discharge coefficient of the weir.
6. The spillway which can be called as an overflow spillway is essentially ___________________
a) an ogee spillway
b) a shaft spillway
c) a chute spillway
d) a syphon spillway
View Answer
Explanation: Ogee spillway is also called as overflow spillway and is an improvement over straight drop spillway. In this spillway, the water jet falls clearly away from the face of the spillway and the gap between the jet and the face is kept ventilated.
7. The W.E.S downstream profile of an ogee spillway can be represented by an equation:
Xn = K Hd n-1. Y
Where x and y are the coordinates of the points on the crest profile and K and n for a vertical upstream spillway would respectively be___________________
a) 2.0 and 1.85
b) 0.5 and 1.85
c) 2.0 and 0.85
d) 0.5 and 0.85
View Answer
Explanation: The value of K and n depends on the upstream slope. For vertical u/s face of the spillway, K is 2.0 and n is 1.85 and for slope 1H : 3V, K = 1.936 and n = 1.836. For slope 1H : 1 1/2V, K = 1.939 and n = 1.810.
8. According to US Army Corps, the u/s profile of ogee spillway extends up to _______________
a) x = 1.27 Hd
b) x = -1.27 Hd
c) x = 0.431 Hd
d) x = -0.431 Hd
View Answer
Explanation: The profile for an Ogee spillway having a vertical upstream face can be determined on the basis of its WES profile. The upstream profile extends up to x = -1.27 Hd Where Hd is the design head.
9. Which of the following plot is useful in determining the clearance for the spillway deck bridge and the top levels of walls on the side of the spillway?
a) The coordinates of the lower nappe
b) The coordinates of the upper nappe
c) Both the coordinates of upper and lower nappe
d) The origin or apex of crest
View Answer
Explanation: The crest profile is determined by the coordinates of the lower nappe. The plotting of the upper nappe is useful in determining the clearance for the spillway deck bridge and the top levels of the training walls on the side of the spillway.
10. What is the satisfactory radius for the reverse bottom curve which is provided at the downstream end of the spillway?
a) One-fourth of the spillway height
b) Equal to the spillway height
c) Half the value of the spillway height
d) One-third of the spillway height
View Answer
Explanation: A smooth gradual reverse curvature is provided at the bottom of the downstream face after the plotting of the profile of the spillway. The curve turns the flow into the discharge channel. The satisfactory radius of the curve is taken as one-fourth of the spillway height.
11. What is Bazin’s profile?
a) Profile for the lower nappe of a free-falling jet
b) The profile obtained for the upper nappe of a free-falling jet
c) Crest profile
d) Profile of both upper and lower nappe
View Answer
Explanation: The theoretical profile obtained for the lower nappe of a free-falling jet is known as Bazin’s profile. Theoretically, this profile causes no negative pressure on the crest under the designed head. But in practice, negative pressure on such a profile seems inevitable.
12. If the upstream face of the spillway is kept vertical, the crest shape should also confirm to the lower nappe of an inclined sharp-crested weir.
a) True
b) False
View Answer
Explanation: If the upstream face of the spillway is kept vertical, the crest shape confirms to the lower nappe of a vertical sharp-crested weir under the maximum head. But if u/s face is kept sloping the crest shape should also confirm to the lower nappe obtained for an inclined sharp-crested weir.
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