This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Cross Drainage Work Design”.
1. What is Lacey’s equation for fixing the approximate value of the required waterway for the drain?
a) P = 4.75 Q
b) P = 4.57 Q
c) P = 4.75 Q1/2
d) P = 4.57 Q1/2
Explanation: The correct equation for fixing the waterway requirement for Aqueduct and Syphon-Aqueducts is given by P = 4.75 Q1/2 where P is the wetted perimeter in meters and Q is the total discharge in cumecs. The wetted perimeter may be taken equal to the width of the drain and equal to the waterway required in case of wide drains.
2. The maximum permissible reduction in the waterway from Lacey’s perimeter is _______
Explanation: The width of the perimeter should be so adjusted as to provide the required perimeter i.e. minimum value = 0.8P in case of small drains. From Lacey’s perimeter, the maximum permissible reduction in the waterway is 20% and the provision is made in a suitable number of bays or spans.
3. The permissible velocity through the barrels is generally limited to _____________
a) 1.5 to 2 m/sec
b) 2 to 3 m/sec
c) 2.5 to 3 m/sec
d) 1 to 2 m/sec
Explanation: The permissible velocity through the barrels is generally limited to 2 to 3 m/sec. A higher value of velocity requires higher and longer marginal banks since it causes quick abrasion of the barrel surfaces by the rolling grit and results in a higher amount of afflux on the U/s side of the aqueduct.
4. Which of the following method is applicable for the design of channel transition when the water depths of the flumed and unflumed section are the same or maybe different?
a) Mitra’s method of design of transitions
b) Chaturvedi’s method of design of transitions
c) Hind’s method of design of transitions
d) The general method of design of transitions
Explanation: Mitra’s method was derived on the basis that the rate of change of velocity per unit length remains constant (i.e. water depth). Chaturvedi’s method is used for the design of channel transitions when water depth remains constant.
5. In a siphon aqueduct, the worst condition of uplift on the floor occurs when ________
a) the canal is full and the drainage empty with the water table at drainage bed
b) the canal and drainage are flowing full
c) the canal is empty and the drainage full with the water table at drainage bed
d) the canal is full and the drainage empty with water table below the floor
Explanation: In siphon aqueducts, the uplift due to water table acts where the bottom floor is depressed below the drainage bed. The maximum uplift under the worst condition is when there is no water flowing in the drain and the water table has risen up to drain bed.
6. To reduce the cost of the CD works we resort to _____
b) blocking of drain
d) lifting of canal water
Explanation: Fluming is the contraction of the waterway of the canal. It reduces the length of the barrel or the width of the aqueduct. This is likely to produce an economy in certain cases.
7. The greater is the fluming, the greater is the length of upstream and downstream transition wings.
Explanation: The reduction in the width of the aqueduct saves cost which is balanced with the extra cost of the transition wings. The permissible head loss and the economy are the factors governing the extent of the fluming.
8. Which of the following wings protect the earthen slopes of the canal and also guide the drainage water?
a) Canal wings
b) Land wings
c) River wings
d) Side wings
Explanation: Canal wings or land wings provide a strong connection between sides of canal trough and earthen canal banks. Drainage wings or water wings or river wings provide a vertical cut-off for the water seeping from the canal into the drainage bed. It also protects the earthen slopes and guide the drainage water and join it to the guide banks.
9. The head loss through a siphon barrel is usually given by Unwin’s formula by neglecting velocity of approach as HL = [1 + F1 + F2. L/R] V2/2g, where F1 and F2 respectively represent the coefficient of head losses due to?
a) Barrel friction and entry
b) Entry and barrel friction
c) Barrel friction and exit
d) Exit and barrel friction
Explanation: The head loss through siphon barrels by Unwin’s formula is given as:
HL = [1 + F1 + F2. L/R] V2/2g
where F1 is coefficient of head loss at entry and F2 is coefficient such that loss of head due to surface friction is given by F2 = a[1+b/R], a and b are the values depending upon the material of the surface of the barrel.
10. The floor of a siphon aqueduct needs to be designed for conditions when?
i. The canal and drainage are flowing full
ii. The canal is full and there is no drainage discharge
iii. The canal is empty and drainage discharge is full
a) i only
b) ii only
c) i and ii
d) ii and iii
Explanation: It should be designed for:
I. full water load and dead weight with no uplift
II. full-uplift with no water load.
The floor should be designed for the summation of the total uplift caused due to the seepage and the static head.
11. A siphon aqueduct is constructed at a canal crossing site where drainage HFL was 212.2 m by allowing an afflux of 0.4 m at high flood discharge. At this site, the water level downstream of the crossing at the same high flood will be _________
a) 212.2 m
b) 212.6 m
c) 211.8 m
d) It cannot be predicted
Explanation: The D/s HFL of the drain remains the same by the construction of works and the U/s HFL can be obtained by adding afflux to the D/s HFL.
The D/s water level = 212.2 m
The U/s water level = 212.2 + 0.4 = 212.6 m.
12. The following data is available at the proposed site of a canal crossing:
The most appropriate and economical CD work at the above site will be:
a) An aqueduct
b) A super-passage
c) A siphon aqueduct
d) A siphon
Explanation: The bed of the natural drain 252.2 m is higher than the canal bed 248 by a margin of 4.2 m, the canal may be taken below the drain. The FSL of canal 253 m is above the drainage bed 252.2 m. the super-passage cannot be provided and the canal water has to be siphoned below the drainage.
13. Calculate the depth of cut-off for scour holes for an irrigation channel of F.S.Q equal to 354 cumecs and H.F.Q of natural drainage equal to 600 cumecs. Assume the value of friction factor as 1.
a) 7 m
b) 7.5 m
c) 10.5 m
d) 8 m
Explanation: Lacey’s normal depth of scour = R = 0.47 (Q/f)1/3 where Q = 354 cumecs and f = 1.
R = 0.47 (354/1)1/3 = 7 m
The depth of cut-off provided for scour holes = 1.5 R (on both sides)
= 1.5 x 7 = 10.5 m.
14. Calculate the value of hydraulic mean radius for a channel designed by Lacey’s theory which has a mean velocity of 1m/sec. The silt factor can be taken as unity.
a) 1 m
b) 0.5 m
c) 2 m
d) 2.5 m
Explanation: Using Lacey’s equation:
R = 5/2 (V2/f) where V = 1 m/sec and f = silt factor = 1.
R = 2.5 m.
Sanfoundry Global Education & Learning Series – Irrigation Engineering.
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