250+ TOP MCQs on Shaft Spillway and Answers

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Irrigation Engineering Multiple Choice Questions on “Shaft Spillway”.

1. A shaft spillway is located _______________________
a) inside the body of a gravity dam
b) inside the upstream reservoir
c) inside the downstream reservoir
d) on the side flanks of the main dam
Answer: b
Clarification: The shape of the shaft spillway is just like a funnel. The lower end of the funnel is turned at right angles and then taken out below the dam horizontally. Water spills over the circular crest and then enters the vertical shaft and comes out of the dam through the horizontal tunnel.

2. The siphons installed within a gravity dam to spill the surplus reservoir water are known as ______________________
a) hooded type siphon spillway
b) tilted outlet type siphon spillway
c) both hooded and titled type siphon spillway
d) shaft spillway
Answer: b
Clarification: The siphon pipe is installed within the body of the dam and an air vent may be connected with it. The outlet is submerged to avoid the entry of the air in the siphon from the d/s end. When the air enters the siphon through the vent, siphoning action cannot take place and the flow will stop.

3. The siphons installed over overflow dams constitute what are known as __________________
a) hooded type siphon spillway
b) tilted outlet type siphon spillway
c) discharge carriers
d) baby siphons
Answer: a
Clarification: In hooded type spillway, a reinforced concrete hood is constructed over an ordinary overflow section of a gravity dam. The inlet is kept submerged to prevent the entry of debris, ice, etc. A small de-priming hood is kept above the main hood and they are connected by an air vent.

4. The only spillway among the following through which the discharge does not increase as fast as it increases in all others is _________________
a) chute spillway
b) side-channel spillway
c) ogee spillway
d) shaft spillway

Answer: d
Clarification: A shaft spillway is generally adopted when the possibility of an overflow spillway and a trough spillway has been ruled out. The discharge does not increase at such a high rate as in weir type spills. Hence, it may not prove as useful as a weir type spills if unestimated high floods occur.

5. The only spillway among the following through which the discharge is almost at its capacity rate even from the start of its functioning is ________________
a) chute spillway
b) side-channel spillway
c) ogee spillway
d) siphon spillway
Answer: d
Clarification: Since the change in the effective head is small as compared to the corresponding change in head over an ogee spillway. When once the water level has risen above the normal pool level, the discharge through the siphon is always at its capacity. This makes it advantageous in disposing of sudden surges of water.

6. An air vent is provided at FRL to break the siphoning action at that level in a ___________________
a) hooded type of siphon spillway
b) tilted outlet type of siphon spillway
c) saddle siphon spillway
d) volute siphon spillway
Answer: c
Clarification: When the water level falls to FRL an air vent is provided on the crown to break the siphoning action. Air enters the lower limb through the air vent and siphonic action is stopped.

7. Which of the following spillway is designed in India by Ganesh Iyer?
a) Saddle siphon spillway
b) Saddle spillway
c) Glory hole spillway
d) Volute Siphon spillway
Answer: d
Clarification: Volute siphon spillway consists of a vertical shaft circular in the section provided with a cap covering. It has low priming depth and higher discharge efficiency. It can be provided with an earth dam also.

8. The crest of a siphon spillway is fixed at ________________
a) full reservoir level
b) dead storage level
c) maximum water level
d) top of the dam

Answer: a
Clarification: The crest of a siphon spillway is fixed at FRL. When the water level in the reservoir rises above FRL the sheet of water starts spilling over the crest. When the waterfalls to FRL, air enters the vent and siphonic action is stopped.

9. A siphon spillway is sufficiently independent of the water surface elevation of the reservoir.
a) True
b) False
Answer: a
Clarification: The discharge through the siphon is given by the equation-
Q = Cd. A. (2gH)1/2 where Cd is the coefficient of discharge, A is the area of the cross-section and H is the effective head i.e. the difference of the water level in the reservoir and the tailwater level for a submerged outlet. The discharge through the spillway is affected to a very less extent is the water surface in the reservoir rises.

10. The allowable maximum negative head on an average is equal to __________
a) 2.5 m or so
b) 5 m or so
c) 7.5 m or so
d) 10 m or so
Answer: c
Clarification: The vertical distance from the crown of the siphon down to the discharging point or HGL should not exceed a value of 7.5 m under average conditions. This is necessary to avoid cavitation.
Limiting vaccum pressure = Atmospheric pressure – Vapour pressure = 10 – 2.5 = 7.5 m.

11. The structure which is not used in a shaft spillway is ________________
a) tunnel
b) bridge
c) radial gates
d) radial piers
Answer: c
Clarification: The radial gates consists of steel plates with struts which are pinned at the center of the circular plate. When the water level rises above FRL the gates are raised by hoisting arrangement to allow the flood water to discharge freely. Each gate is separately housed between two piers constructed on the crest.

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12. Morning glory is the _____________________________
a) special flared inlet of the shaft spillway of a dam of very small height
b) special flared inlet of the shaft spillway of a large dam project
c) a horizontal tunnel constructed in a shaft spillway across the body of a gravity dam to carry surplus reservoir water to the d/s river
d) a horizontal tunnel constructed in a shaft spillway of an earthen dam through its foundation to carry surplus reservoir water to the d/s river
Answer: b
Clarification: In shaft spillway of large heights, RCC may be used and for smaller heights, no special inlet design is necessary. A flared inlet called morning glory is often used in large projects. The horizontal tunnel is either taken through the dam body or below the foundations.

13. For a saddle siphon, the maximum operative head is 4.53 m. The width and height of the throat of the siphon are 5 m and 2.25 m respectively. The coefficient of discharge is 0.90. How many units are required to pass a flood of 350 cumecs? (Assume g = 10 m/s2)
a) One
b) Two
c) Three
d) Four
Answer: d
Clarification: The discharge through the siphon is given by the equation:
Q = Cd. A. (2gH)1/2 Where Cd = 0.9, H = 4.53 m and A = 5 x 2.25 = 11.25 m2
Q = 0.9 x 11.25 x (2 x 9.81 x 4.53)1/2 = 95.45 m3/sec
No. of units of saddle siphon = 350 / 95.45 = 3.66 units. = 4 units.

14. A siphon spillway in a concrete gravity dam is in the shape of ________________
a) U-shape
b) Inverted U-shape
c) Horizontal bend through the abutment
d) Any straight line
Answer: b
Clarification: A siphon spillway is a closed conduit of the shape of an inverted U-tube with unequal legs and works on the principle of siphonic action. It consists of a siphon pipe one end of which is in u/s side and the other end discharges water on the d/s side.

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