Irrigation Engineering Multiple Choice Questions on “Energy Dissipation Below Overflow Spillway – 1”.
1. The energy dissipation at the toe of the spillway is affected basically by the use of hydraulic jump in _______________________
a) roller bucket
b) a ski-jump bucket
c) a sloping apron below the downstream river bed
d) both roller and ski-jump bucket
Answer: c
Clarification: Most of the kinetic energy is destroyed by creating a condition suitable for the formation of a hydraulic jump. Sometimes the depth of tail-water may be more than that necessary to create the hydraulic jump. The depth of water can be reduced to create a hydraulic jump by providing a sloping apron.
2. The most ideal condition for energy dissipation in the design of spillways is the one when the tail-water rating curve coincides with the jump rating curve at all discharge.
a) True
b) False
Answer: a
Clarification: The most ideal condition for jump formation is when TWC coincides with JHC at all discharge. To ensure protection in the region of a hydraulic jump, a simple concrete apron of apron length 5 (y2 – y1) is provided.
3. When the tail-water depths in the river downstream of a spillway are quite low such that the tail-water curve at all discharges lies below the post jump depth curve, then the energy dissipation can be affected best by ___________________
a) a roller bucket
b) a ski-jump bucket
c) either roller or ski-jump bucket
d) a sloping apron
Answer: b
Clarification: Energy dissipation bucket called ski-jump bucket is used when the tail-water depth is insufficient or low at all discharge. It requires sound and rocky river bed. Water may shoot up out of the bucket and fail harmlessly into the river at some distance downstream of the bucket.
4. The device which does not help in energy dissipation at the bottom of a hydraulic structure over which water spills is ________________
a) chute block
b) dentated sill
c) morning glory
d) baffle piers
Answer: c
Clarification: 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. Chute blocks, dentated sills and baffle piers are all auxiliary devices which help in energy dissipation.
5. The formation of hydraulic jump at the foot of a spillway is one of the common methods of energy dissipation because ______________________
a) it destroys more than 90% of total energy by the turbulence produced in the jump
b) it reduces the kinetic energy by increasing the depth of flow
c) its action is not understood
d) it reduces the kinetic energy by decreasing the depth of flow
Answer: a
Clarification: Hydraulic jump is generally accompanied by large scale turbulence dissipating most of the kinetic energy of the super-critical flow. It is the most suitable method because the energy is lost in the impact of the water against water. Most of the kinetic energy is destroyed by creating a condition suitable for the hydraulic jump.
6. A ski-jump bucket is also known as _____________________
a) flip bucket
b) solid roller bucket
c) slotted roller bucket
d) flexible bucket
Answer: a
Clarification: A ski-jump bucket is also called flip bucket is used for energy dissipation when tail-water depth is insufficient or low at all discharge. A part of energy dissipation takes place by impact and some of the energy is dissipated in the air by diffusion or aeration.
7. The percentage of energy dissipation in a hydraulic jump increases with the increase in the Froude number.
a) True
b) False
Answer: a
Clarification: The energy dissipation in the jump depends upon the Froude number, if this Froude number is higher, the greater energy dissipation can take place.
| S NO. | Froude number | % loss in energy |
|---|---|---|
| 1. | 2.5 | 17 |
| 2. | 4.5 | 45 |
| 3. | 9.0 | 70 |
8. Which of the following stilling basin help in stabilizing the flow and improve the jump performance?
a) dentated sills
b) chute blocks
c) baffle piers
d) friction blocks
Answer: b
Clarification: Chute blocks are a row of small projections like teeth of saw and are provided at the entrance of the silting basin. It produces a shorter length of jump and stabilizes the flow. Hence, they improve jump performance.
9. What is the expected solution for the case when the T.W.C is lying above the J.H.C curve at all discharges?
a) By providing a simple concrete apron of length 5(Y1 – Y2)
b) By providing a sloping apron above the river bed
c) By providing a sloping apron below the river bed
d) Provision of a ski-jump bucket
Answer: b
Clarification: When the TWC is lying above the JHC at all discharges, the problem can be solved by-
1) By constructing a sloping apron above the river bed
2) By providing a roller bucket type of energy dissipator.
In this case, the jump is formed at the toe will be drowned by the tail-water and little energy will be dissipated.
10. A sloping apron is provided partly above the river bed and partly below the river bed in case of ____________________________
a) when TWC coincides with the JHC at all discharges
b) when TWC lies above the JHC at all discharges
c) when TWC lies below the JHC at all discharges
d) when TWC lies above the JHC at low discharges and below the JHC at high discharges
Answer: d
Clarification: At low discharges, the jump will be drowned and at high discharges tail-water depth is insufficient. When TWC lies above the JHC at low discharges and below the JHC at high discharges, the solution is the provision of sloping apron partly above and partly below the river bed. The horizontal apron and end-sill are also provided.
11. When the TWC lies below the JHC at all discharges, the problem can be solved by which of the following provisions?
i. Constructing a sloping apron above the river bed
ii. Provision of roller bucket type of energy dissipator
iii. Provision of a ski-jump bucket
iv. A sloping apron below the river bed
v. Construction of a subsidiary dam
vi. A sloping apron partly above and partly below the river bed
a) i, iii and v
b) i, ii and vi
c) iii, iv and v
d) i, iii, iv and v
Answer: c
Clarification: When TWC lies below the JHC at all discharges, the expected solution is –
i. Provision of a ski-jump bucket
ii. A sloping apron below the river bed of length 5 (y2 – y1)
iii. Construction of a subsidiary dam below the main dam.