# Irrigation Engineering Questions and Answers – Tank Irrigation

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This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Tank Irrigation”.

1. The depth of water stored in an irrigation tank rarely exceeds ______________
a) 4 m
b) 8 m
c) 12 m
d) 16 m

Explanation: Most of the existing tanks possess a maximum depth of 4.5 m, only a few tanks are as deep as 7.5 to 9 m. Only in the exceptional cases, the depth exceeds 11 m in depth. The tank is generally referred to as a reservoir when the depth of the tank exceeds 12 m or so.

2. The storage created on the upstream side of a smaller stream by the construction of low earthen bunds is technically known as _________________
a) a reservoir
b) a tank
c) a lake
d) a pond

Explanation: The storage irrigation scheme utilizes the stored water on the upstream side of a smaller earth dam i.e. bund. These earthen bund reservoirs are called as tanks. Such works are very common in South India but this terminology is limited to India only.

3. The provision of suitable breaching sections becomes more important in irrigation tanks which are isolated.
a) True
b) False

Explanation: A breaching section is a length kept lower and weaker than the remainder of the bound so as to localize a breach in that length only. A breach occurring in a tank supplements the surplus power to a reasonable degree to the greater security both of the tank itself and those below it in the group.

4. The cubic meters of water that can be stored in an irrigation tank between full tank level and sill level of the lowest supply sluice is known as __________________
a) the gross capacity
b) effective capacity
c) the specific capacity
d) total capacity

Explanation: The cubic content of water stored in the tank up to F.T.L is called the gross capacity of the tank. The cubic content of water stored between F.T.L and the bottom or sill level of the lowest supply sluice is the effective capacity of the tank. These capacities can be computed by using the contour plan of the area of the water spread.

5. If A1 represents the area of a tank bed at bottom and A2 is the area of the tank at F.T.L, then the storage capacity of the tank of height H is given as ________________________
a) H [A1 + A2 + A1.A2]
b) H/2 [A1 + A2 + A1.A2]
c) H/3 [A1 + A2 + A1.A2]
d) 2H [A1 + A2 + A1.A2]

Explanation: The effective cubic content of the tank may be computed as H/3 [A1 + A2 + A1.A2] where H is the height or difference in elevations between F.T.L and the sill level of the lowest sluice. This is based on the assumption that the water stored is in the shape of a frustum of a cone.

6. The top-level of a tank weir in a tank bund scheme is kept at ________
a) Maximum Water Level (M.W.L)
b) Full Tank Level (F.T.L)
c) Either M.W.L or F.T.L
d) Full Supply Level

Explanation: The top-level i.e. crest level of the tank weir is kept at F.T.L. The extra water is discharged over the surplus escape weir when the tank is full up to F.T.L. The design of the length or capacity of this surplus escape weir is such that the water level in the tank never exceeds the M.W.L.

7. The usual arrangement made in a tank-bund scheme to spill the surplus excess water is known as ________________________
a) tank spillway
b) tank sluice
c) tank weir
d) tank bunds

Explanation: To avoid overtopping of the tank bund in case of all dam reservoir projects, tanks are provided with arrangements for spilling the excess surplus water. These arrangements may be in the form of a surplus escape weir provided in the body or at one end of the tank bund. In the case of earth dam projects, some other arrangements like a siphon spillway can be provided.

8. If A1 and A2 represent the areas enclosed between two successive contours, then the cubic content between these contours is roughly taken as _________________
a) (A1 + A2) x h/2
b) A1.A2 x h/2
c) A1 x h/2
d) A2 x h/2

Explanation: The cubic content between two successive contours is computed as (A1 + A2) x h/2. The required storage capacity of the tank is the summation of all cubic contents between the successive contours.

9. The most adopted earthen section for tank bunds is of the type ___________________
a) homogeneous embankment type
b) zoned embankment type
c) diaphragm type
d) both diaphragm and zoned type embankment

Explanation: Homogeneous embankment has been constructed with the soils excavated from pits in the immediate vicinity of each bund and carried by the head load to the bund. Most of the tank bunds of South India belong to this type.

10. What is the approximate value of side slope of the bund for smaller tanks with water depths not exceeding 2.5 m in favorable soils such as red and white gravel, red and black loams?
a) 2 H : 1 V
b) $$\frac{1}{2}$$ H : 1 V
c) 2 $$\frac{1}{2}$$ H : 1V
d) 1 $$\frac{1}{2}$$ H : 1 V

Explanation: For smaller tanks with water depths not exceeding 2.5 m in favorable soils such as red and white gravel, red and black loams, the side slope may be taken as $$\frac{1}{2}$$ : 1 and 2 : 1 for larger ones up to 5 m in depth. The slopes may be kept in between 2 : 1 and 2 $$\frac{1}{2}$$ : 1 in light sandy, black cotton or clayey soils. 