This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Cross-Section of an Irrigation Canal”.
1. The actual capacity of silt laiden water channel is worked out with 1/2 : 1 side slopes.
Explanation: Flatter slopes such as 1 : 1 and 1.5 : 1 are also constructed at the time of execution, but actual capacity of channel is worked out in 1/2 : 1 slope because even if we provide 1 : 1 or 1.5 : 1 slope the silt gets deposited on this slope and gives a new side slope 1/2 : 1. Therefore the actual capacity is calculated using 1/2 : 1 slope.
2. What is range of side slopes for a canal in cutting?
a) 1 H : 1 V to 1.5 H : 1 V
b) 1.5 H : 1 V to 1.5 H : 1 V
c) 1.5 H : 1 V to 1 H : 1 V
d) 1 H: 0.5 V to 1 H : 1 V
Explanation: The side slopes of a canal are designed such that they are stable, depending upon the type of soil present in canal area. A canal in cutting is provided with steeper slope than a canal in filing, as the stability of soil is more in case of canal in cutting than in canal in filing.
3. With what equation we can calculate the berm width?
a) (s1 – s2) d1
b) (s2 – s1) d1
c) (s2) d1
d) (s1) d1
Explanation: Berm is the horizontal distance between the toe of the bank and top edge of the cutting. If s2 : 1 is the slope in filling and s1 : 1 is the slope in cutting, and the initial depth of the berm from the bed of the canal is d1 then width of the berm is (s2 – s1) d1. As d1 varies, the width of the berm also varies. As siltation occurs the formula is (s2:1/2) y (y=over all new depth of the berm).
4. Deposition of silt to form the berms completely has lot of advantages.
Explanation: The silt which gets deposited is very fine and impervious therefore it acts as a good lining which reduces losses, leakage, breaches etc. This also gives strength to the banks and protects them against erosion and breaches. This deposition also provides scope for future widening of the canal.
5. What is the primary purpose of the banks?
a) Aesthetic View
b) Inspection Paths
c) Retaining Water
d) Means of Communication
Explanation: The main purpose of the banks is to retain water in the canal itself. This retaining action is always needed, in cases of low availability of water or during the period of floods. The other purposes of the banks include a means of communication and inspection paths.
6. What type of roads is provided for inspection purposes?
a) Side Road
b) Dirt Road
c) Service Roads
d) Toll road
Explanation: For inspection purposes of the canal, service roads are provided and they even serve as a means of communication in remote areas. Depending upon the size of the canal these roads are provided at a height of 0.4 m to 1 m above the fixed surface level.
7. What is provided as a measure of safety during driving?
c) Service Roads
Explanation: A structure of 0.3 m high and 0.3 to 0.6 m wide is provided along the banks, with side slopes of 1.5: 1 to 2: 1. This structure is known as dowlas. They also help in preventing soil erosion during rains.
8. Which structure is provided when the disposition of earth becomes costlier?
a) Borrow pits
b) Spoil Banks
c) Back Berm
Explanation: The disposition of earth becomes costlier when the earthwork in excavation exceeds earthwork in filling. To dispose this earth economically, it should be collected on the edge of the bank embankment. The soil is deposited on both banks or only on one bank in the form of heaps. Longitudinal drains are excavated to run along their sides to dispose rain water.
9. What type of structure is provided when earth has to be brought from somewhere else for canal construction?
b) Borrow Pits
c) Spoil Banks
Explanation: When the earthwork in filling exceeds the earthwork in excavation then the earth has to be brought from somewhere else to facilitate the construction of the canal. The pits or the places where the earth has been brought are called borrow pits.
10. Which type of borrow pits are preferred more?
a) External Borrow Pits
b) Internal Borrow Pits
c) Borrow pits
d) Spoil Banks
Explanation: The pits which are dug outside the canal are called external borrow pits and which are dug somewhere within the canal are called internal borrow pits. Internal borrow pits are preferred because bringing soil from distances is costlier than finding it near the canal itself. Moreover, there is a problem of mosquito nuisance in the external pits due rainwater collection.
Explanation: Clearly the diagram represents the excavation of pit of width less than half the width of the canal and the depth of the pit is less than 1 m. So, therefore according to this information we can say that the diagram represents a borrow pit. And more information we can obtain from the diagram is that the borrow pit is an internal borrow pit.
Explanation: From the diagram, we can observe that FSL is obtained in the canal and is provided with berms, drains, and service roads. So, therefore this represents a phenomenon where the earthwork of excavation exceeds the earthwork of filling. Therefore to economically dispose the earth we need construction of spoil banks with drains which the clearly represents. Therefore this diagram represents the spoil banks structure.
13. What is the condition for digging of borrow pits?
a) B/2 > b
b) Depth of pit 7 < 1 m
c) Should in the Centre
d) Dig only External Pits
Explanation: For digging up of borrow pits the main condition is that the width of the pit (b) should be less than half the value of width of the canal (B/2). Other secondary conditions include that the pit should be dug in the centre, depth of the pit should be < 1 m, and it should start from a point at a distance of 5m from toe for small canals and 10 m for large canals.
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