This set of Irrigation Engineering Quiz focuses on “Energy Dissipation Below Overflow Spillway – 2”.
1. A troublesome and oscillating hydraulic jump is normally met in cases of _____________
a) weirs and barrages
b) overflow spillways of dams
d) large spillways
Explanation: When the Froude number lies in the range of 2.5 to 4.5, the jump is troublesome and oscillating as in case of weirs and barrages. There is an oscillating jet entering the jump bottom to the surface and back again which produces a large wave of irregular period doing unlimited damage.
2. A very steady and stable hydraulic jump is usually formed in the flows involving the approaching Froude number in the range of _____________________
a) less than 2.5
b) 2.5 to 4.5
c) 4.5 to 9.0
d) more than 9.0
Explanation: When the Froude number is in the range of 4.5 to 9.0, the jump performs at its best and is called steady jump. The length of the jump is almost constant and equal to 6 y2 where y2 is the post jump depth. The dissipation of energy ranges from 45 to 70%.
3. Standard USBR stilling basin-II is useful for energy dissipation at the bottom of the overflow structure, if the approaching Froude number is ________________
a) less than 4.5
b) more than 4.5
c) less than 2.5
d) more than 2.5
Explanation: USBR has standardized stilling basins for different Froude numbers. USBR stilling basin- II is recommended for large structures when the Froude number is more than 4.5. The dissipation of energy will be 45 to 85%.
4. When the Froude number is in the range of 2.5 to 4.5, the jump is ____________________
a) weak and energy loss is low
b) troublesome and oscillating
c) steady jump
d) strong jump
Explanation: When the Froude number is in the range of 2.5 to 4.5, the jump is troublesome and oscillating. This gives rise to the heavy waves on the surface and wave suppressors are needed in this range.
5. Which of the following stilling basin is applicable to only rectangular cross-sections?
a) U.S.B.R stilling basin-II
b) U.S.B.R stilling basin-IV
c) U.S.B.R stilling basin-I
d) U.S.B.R stilling basin-III
Explanation: USBR stilling basin-IV is used for Froude number varying between 2.5 and 4.5 which generally occurs in canal weirs, canal falls, diversion dams, etc. They are tried to be controlled by providing large chute blocks since oscillating waves are generated in this range. This is applicable only to rectangular cross-sections.
6. The Froude number of a hydraulic jump is 5.5. The jump can be classified as ____________
a) a weak jump
b) an oscillating jump
c) rough and choppy jump
d) steady jump
Explanation: When the Froude number is in the range of 4.5 to 9.0, the jump is steady. This is generally the case of dams and spillways. The oscillating jump occurs between the range of 2.5 to 4.5 and when Froude number is less than 2.5, the jump is weak and when it is more than 9.0, the jump is rough and choppy.
7. Standard stilling basin is provided at the toe of a dam spillway for energy dissipation is usually provided with auxiliary devices like chute blocks and dentated sills for the basic purpose of reducing the length of the stilling basin from about ________________
a) 6 y2 to 4 y2
b) 4 y2 to 2 y2
c) 6 y2 to 2 y2
d) 3 y2 to y2
Explanation: In the case of dams and spillways, the jump is steady and the length of the jump is almost constant and is equal to 6 y2 where y2 is the post jump depth. The length of the basin is related to Froude number as the economy in the length of the basin up to about 35% i.e. 4.3 y2 is thus obtained for auxiliary devices.
8. In the case of dam spillways, the approaching Froude number usually lies in the range of __________
a) less than 2.5 m
b) 2.5 – 4.5
c) 4.5 – 9.0
d) more than 9.0
Explanation: Froude number varying between 2.5 and 4.5 generally occurs in canal weirs, canal falls, diversion dams, etc. For dams and spillways, the Froude number is in between 4.5 and 9.0 i.e. steady jump and the jump is well balanced.
9. _____________________ are suitable only for low spillways or weirs.
a) Chute blocks
b) Sloping aprons
c) Baffle wall or friction blocks
d) Roller buckets
Explanation: Baffle piers or walls are blocks placed within the basin across the basin floor and they help in breaking the flow and dissipate the energy mostly by the impact. They generally give away under high-velocity jets due to their cavitation effects. It is unsuitable for large works.
10. Which of the following helps in dissipating the residual energy and to reduce the length of the jump or the basin?
a) Dentated sills
b) Chute blocks
c) Roller bucket
d) Baffle piers
Explanation: Chute blocks help in stabilizing the flow and thus improve the jump performance. Baffle piers help in breaking the flow and dissipate energy mostly by the impact. Dentated sills are generally provided at the end of the stilling basin and diffuse the residual portion of high-velocity jet reaching the end of the basin.
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