This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Canal Falls – Types”.
1. Canal drop or canal fall is needed construction in the canal bed.
Explanation: If the available natural slope is steeper than the bed slope of the canal, then this is adjusted by the construction of vertical falls or drops in the canal bed at suitable intervals. But such a drop will not be stable, therefore in order to maintain this drop a masonry structure is constructed. This is known as canal fall or canal drop.
2. On what term does the construction of a fall in the case of the main canal depend?
a) Discharge Capacity of the Canal
b) Length of the Canal
d) Cost of Excavation and Filling versus Cost of fall
Explanation: The main does not irrigate any area directly, so therefore the site of the fall is based on the considerations of economy in cost of excavation and filling versus cost of fall. The excavation and filling on both sides of the fall should be balanced because unbalanced work causes extra cost.
3. On what factor in case of branch canals, the construction site for a fall depends?
a) Cost of Excavation
c) Commanded Area
d) Cost of fall
Explanation: By considering the commanded area of a branch canal or a distributary canal the location for the falls is decided. The procedure is to fix the FSL needed at the head of the off taking channels and outlets. Thus the FSL can be marked at all commanded points and hence deciding appropriate locations for the falls in canal FSL and therefore in canal beds.
4. How many types of falls are present?
Explanation: Since the idea of the construction of falls various types of falls have been designed. Some important falls are ogee falls, rapids, trapezoidal notch falls, simple vertical drop type and sarda type falls, straight glacis falls, Montague type falls and inglis falls.
Explanation: The diagram gives clear image that in this fall a high crested fall takes place into the water cushion below. There is no clear hydraulic jump or energy dissipation, as the velocity jet enters the deep pool of downstream. Hence it is a simple vertical drop type fall.
Explanation: The diagram shows that water is gradually led down the convex and concave curves. It also shows the draw down on the upstream side which results in lower depths, higher velocities and subsequent soil erosion. The curves provide smooth surfaces which results in less energy dissipation, which again causes soil erosion. Hence the diagram is ogee fall type.
7. What number of major defects does the ogee fall have?
Explanation: Ogee fall has two major defects. Heavy drawdown occurs on the upstream side, which results in lower depths, higher velocities and bed erosion. The kinetic energy of the flow will not be dissipated due to smooth transition, causing erosion on downstream bed and banks.
8. In which fall the depth discharge relationship is unaffected?
a) Trapezoidal Notch Fall
b) Ogee Fall
d) Straight Glacis Fall
Explanation: In this fall the notches could be designed to maintain the normal water depth in the upstream channel at any two discharges, as the intermediate values do not vary much. Therefore the depth discharge relationship of the channel is not affected by the introduction of the fall.
9. Which type of fall is suitable for 60 cumecs discharge and 1.5 m drop?
a) Montague Type Fall
c) Straight Glacis Fall
d) Baffle Fall
Explanation: In this type of modern fall, a straight glacis is provided after the raised crest. The hydraulic jump is made to happen on this glacis causing sufficient energy dissipation. If not flumed this fall gives good performance. This fall is suitable for 60 cumecs discharge and 1.5 m drop.
10. What is the reason for the construction of baffle wall in baffle fall?
a) To Maintain Uniform Velocity Flow
b) To Ensure Formation of Jump
c) To Reduce Soil Erosion
d) To Ensure Uniform Discharge of the Flow
Explanation: The baffle wall is provided at a calculated height and a calculated distance from the toe of the glacis to ensure proper formation of the jump on the baffle platform. This type of fall is suitable for all discharges and for drops which are more than 1.5 m.
11. Which type of fall is not adopted in India?
a) Ogee Fall
c) Inglis Fall
d) Montague Type Fall
Explanation: In this type of fall the energy dissipation is incomplete on a straight glacis due to the vertical component of velocity remains unaffected. So, therefore due to this reason the straight glacis is replaced by a parabolic curve known as Montague profile. This curved glacis is difficult to construct and therefore is costlier. Hence it is not adopted in India.
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