Irrigation Engineering Multiple Choice Questions on “Ogee Profile”.
1. The discharge passing over an ogee spillway per unit length of its apex line is proportional ___________________
a) H
b) H2
c) H1/2
d) H3/2
Answer: d
Clarification: The discharge passing over the ogee spillway is given by the formula –
Q = C. Le. H3/2 where Le is the effective length of the spillway crest, C is the coefficient of discharge and H is the total head over the crest including the velocity head.
It is clear from the discharge equation that the discharge is proportional to H3/2.
2. 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
Answer: a
Clarification: 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.
3. When the gated sluices are provided through the body of the dam spillway then the ogee spillway structure shall normally be of Corbel type.
a) True
b) False
Answer: b
Clarification: The extra concrete is required for thickening of the section at the d/s end which can be saved by shifting the curve of the nappe in a backward direction until it becomes tangential to the d/s face. It can be affected by providing a corbel on u/s face.
4. The velocity of approach has found to be negligible effect upon discharge if ____________
a) the ratio H/Hd is more than 1.33
b) the ratio H/Hd is less than 1.33
c) the ratio H/Hd is equal to zero
d) the ratio H/Hd is more than 1
Answer: a
Clarification: The velocity of approach has been found to have a negligible effect upon discharge when the height of the weir is more than 1.33 times the design head. When this ratio is less than 1.33 as in low spillways, the velocity is having an appreciable effect.
5. If a flood enters a dam reservoir at F.R.L, the efficiency of its ogee spillway will ______________
a) increase with the increasing head
b) reduce with the increasing head
c) remain constant with the increasing head
d) vary with the increasing head
Answer: a
Clarification: The coefficient of discharge increases with the increasing head above the full reservoir level till it becomes about 2.2 at full design head i.e. MRL. Since the spillway starts working when the water level just crosses FRL, the coefficient of discharge is about 77% at full design head. As the water level increases from F.R.L to M.R.L, the efficiency increases from 77% to 100%.
6. The downstream apron of the ogee spillway is found to have a negligible effect on the coefficient of discharge when the value of ________________
a) (Hd + d) / He is less than 1.7
b) (Hd + d) / He exceeds 1.7
c) (Hd + d) / He is equal to 1
d) (Hd + d) / He is less than 1.33
Answer: b
Clarification: The d/s apron is found to have a negligible effect on the coefficient of discharge when the value of (Hd + d) / He exceeds 1.7. where d is the tailwater depth, Hd is the design head and He is the Design head including the velocity head. There can be a decrease in the coefficient due to tailwater submergence.
7. What is the value of the pier abutment coefficient for pointed nose piers?
a) 0.1
b) 0.02
c) 0.01
d) 0.0
Answer: d
Clarification: The value of pier contraction coefficient depends upon the shape of the piers.
| S.NO. | Pier Shape | Contraction Coefficient |
|---|---|---|
| 1 | Square nosed piers without any rounding | 0.1 |
| 2 | Rounded nose piers and 900 cut water nosed piers | 0.01 |
| 3 | Pointed nose piers | 0.0 |
8. What is the value of abutment contraction coefficient for square abutment with headwall perpendicular to the direction of flow?
a) 0.2
b) 0.1
c) 0.02
d) 0.01
Answer: a
Clarification: For square abutment with headwall at 90° to the direction of the flow, the abutment contraction coefficient is 0.2. For rounded abutment with headwall at 90° to the direction of flow, the value is taken as 0.1.
9. What is the assumption that is taken during the entire design of an overflow spillway?
a) The upper and lower nappe is subjected to negative pressures
b) The upper and lower nappe is subjected to full atmospheric pressure
c) The upper and lower nappe is subjected to zero atmospheric pressure
d) The upper and lower nappe is subjected to both positive and negative pressure
Answer: b
Clarification: In actual practice, the development of negative pressure due to insufficient aeration takes place beneath the nappe due to the removal of air by the falling jet. But the entire design has been done with the assumption that the upper and lower nappe is subjected to full atmospheric pressure.
10. The greater is the divergence from the streamline flow, greater is the contraction coefficient.
a) True
b) False
Answer: a
Clarification: The greater is the divergence, the greater is the contraction coefficient and lesser is the effective length of the crest. A 90° cut water nose pier is generally preferred as it has quite a low value of pier contraction coefficient and is most efficient.
11. Calculate the effective length of the spillway which consists of 6 spans having a clear width of 10 m each. The thickness of each pier may be taken as 2.5 m and the total design head on the crest including velocity head is 16.3 m. Assuming 90° cut water nose piers and rounded abutments.
a) 55 m
b) 60 m
c) 65 m
d) 75 m
Answer: a
Clarification: The effective length of the spillway is given by-
Le = L – 2[N.Kp + Ka].H
Where L = clear length of the spillway crest = 6 x 10 = 60, N = Number of piers = 5 and H = 16.3 m
For 90° cut water nose piers and rounded abutments; Ka = 0.1 and Kp = 0.01
Le = 60 – 2[5 x 0.01 + 0.1] x 16.3 = 55.1 m.