This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Diversion Head Works – Gravity and Non-Gravity Weirs”.
1. When the weight of the weir balances the uplift pressure caused by the head of water seeping below the weir, it is called as ____________
a) gravity weir
b) non-gravity weir
c) vertical drop masonry weir
d) concrete glacis weir
Explanation: In Gravity weir, the weight of the weir balances the uplift pressure caused by the seepage water. In non-gravity weirs, the weight of the concrete slab and the divide piers together keep the structure safe against the uplift.
2. Which weir is also called Dry Stone Slope Weir?
a) Masonry weir
b) Gravity weir
c) Rock-fill weir
d) Concrete weir
Explanation: Rock-fill weir is also called Dry Stone Slope Weir. It is the simplest type of construction and is economical only when the stone is easily available. This type of weir is also becoming obsolete with the development of concrete glacis weir.
3. Which type of weir is suitable for hard clay and consolidated gravel foundations?
a) Gravity weir
b) Non-gravity weir
c) Masonry weir with a vertical drop
d) Rock-fill weirs with sloping aprons
Explanation: This type of weir was used in all the old head-works and is particularly suitable for hard clay and consolidated gravel foundations. Masonry weir is becoming obsolete and all the modern constructions are done by the modern concrete weirs.
4. Which weir is of recent origin and their design is based on modern concepts of sub-surface flow?
a) Gravity weir
b) Non-gravity weir
c) Masonry dam with a vertical drop
d) Concrete weir with sloping glacis
Explanation: The design of concrete weir with sloping glacis is based on the modern concepts of sub-surface flow i.e. Khosla’s theory. Sheet piles are driven at sufficient depths at the upstream and downstream floor end. The hydraulic jump is formed on the downstream sloping glacis to dissipate the energy of the flowing water.
5. Which weir is now extensively used especially on pervious foundations?
a) Vertical drop masonry weir
b) Rock-fill weirs with sloping aprons
c) Concrete weirs with sloping glacis
d) Non-gravity weirs
Explanation: Modern concrete weirs with sloping downstream glacis are generally provided with low crest counter-balanced gates. This is now exclusively used on permeable foundations. Masonry weirs are suitable for hard clay and gravel foundations. Rock-fill weir is suitable for fine sandy foundations.
6. Which of the following statement is incorrect about Non-gravity weir?
a) The weir floor is designed continuous with the divide piers as a reinforced structure
b) The weight of the concrete slab balances the uplift pressure
c) The weight of the divide piers also keep the structure against uplift
d) Brick piers have to be used in place of RCC
Explanation: The weir floor and divide-pier are designed as a reinforced structure continuously. The RCC has to be used in place of brick piers and as the weight of the floor can be much less than that of gravity weir, considerable savings are obtained.
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