This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Open Well and Dug Well”.
1. The rate of water contribution to the well increases with the increase in the depression head.
Explanation: The velocity of the percolating water depends on the depression head. If the amount of water withdrawn from the well is more, increasing the depression head, higher flow velocities will prevail in the vicinity of the well. At a certain stage, depression head may become so great that the soil particles also start coming with the percolating water.
2. The performance of a well is measured by its ________
a) specific capacity
b) specific yield
c) storage coefficient
d) permeability coefficient
Explanation: Specific capacity is the measure of the well performance indicating the rate of water percolation into a well. It is also defined as the yield of a well under the head of one meter. The specific capacity of the well is not constant but decreases as the discharge increases.
3. The safe depression head for open wells is generally taken to be x times the critical depression head when x is ______
Explanation: The depression head is kept equal to one-third of the critical depression head. The maximum or critical yield corresponds to the critical depression head and the maximum safe yield corresponds to the working head.
4. The yield of a well depends on ________________
a) permeability of soil
b) area of aquifer opening into the wells
c) actual flow velocity
d) permeability and actual flow velocity
Explanation: Yield of a well is the rate at which water percolates into the well under the safe maximum depression head or the critical depression head. It depends on the position of the water table, permeability, and porosity of the soil, rate of water withdrawal from the well, and amount of water storage in the well.
5. Which type of open well is suitable when the sub-soil is formed of gravel or coarse sand deposits?
a) Unlined wells
b) Wells with pervious lining
c) Wells with impervious lining
d) Temporary wells
Explanation: Permanent lining or wells with impervious lining is sunk in regions with alluvial soil formation. Unlined wells or temporary wells are of very shallow depths and can be constructed only when the water table is very near to the ground. Wells with pervious lining is generally suitable in the strata of gravel or coarse sand.
6. Depending upon the type of sub-soil formation, the construction of well is done in two ways i.e. Dug wells and Sunk wells. Dug wells are constructed in soft formations.
Explanation: Sunk wells are constructed in soft formations and the subsoil formation may be sandy or clayey. The lining is first constructed above the ground level and then it sunk in the sub-soil formation by putting a load on the lining, therefore, it is called sunk well.
7. A well 3 meters in diameter has its normal water level 3 meters below the ground level. By pumping water level in the well is depressed to 10 meters below the ground level. In 4 hours the water rises by 5 meters. Calculate the specific yield of the well.
a) 2.213 m3/hr
b) 5 m3/hr
c) 1.242 m3/hr
d) 3.224 m3/hr
Explanation: Specific yield (K) is calculated from the formula, K = 2.303 (A/T) log (H1/H2) where A is the area of the well, T is the total time of recuperation to bring the water level from depth H1 to H2.
Given values, A = 3.14/4 x 9 = 7.065 sq. m, T = 4 hours, H1 = 7m, H2 = 2m
K = 2.303 x 7.065 x 1/4 x log (7/2) = 2.213 m3/hr under a head of one meter.
8. An open well is called shallow well when ____________
a) the depth of well is small
b) the water table is high
c) it does not encounter mota formation
d) it finds the foundation in the mota formation
Explanation: A shallow well is that well which does not encounter mota formation. A deep well is that well which goes below the water table and finally finds its foundation in the mota formation.
9. Open well has big diameter than tube well because ____________
i. Open well has to irrigate more area.
ii. Water contribution to the well is natural and therefore, the percolation area has to be more.
iii. Storage of water has to be made before irrigation is done.
a) i and ii
b) i and iii
c) ii and iii
d) i, ii and iii
Explanation: Open wells have comparatively bigger diameter varying from 2-9 m and are suitable for low discharges in the order of 1-5 liters per second. The yield of the well is limited as the groundwater storage is limited. An open well irrigates small area and its cost per hectare is generally more than that of a tube well.
10. If the sub-soil formation is of fine sand, the rough value of the specific yield of well per unit area of the well (K/A) given by Marriot is ____________
Explanation: When the limit of critical velocity is not exceeded it may be rightly assumed that K/A is constant for a well. K/A is the specific yield of well per unit area of the well. The value for K/A given by Marriot for clay sub-soil is 0.25, for fine sand = 0.50 and for coarse sand, it is 1.0.
11. Which of the following method of lifting water is very suitable when the depth of well is from 12 to 20 meters?
c) Persian wheel
Explanation: Denkli is generally provided on the unlined wells which are supposed to irrigate only small fields. Churus is suitable for lined wells and can be conveniently used deep wells up to a maximum depth of 30 m. Persian wheel (also known as Rahat in some parts of India) is suitable when the depth of well is 12-30 m. The windmill is not very reliable.
12. Mota layer sometimes is also known as Marbarwa or Magasan is an impervious layer.
Explanation: Mota is an impervious patch of small thickness situated in the pervious soil mass. It gives structural support to the open well and is useful for unlined and partly lined wells. It can either be continuous or may be localized.
13. A well is to be constructed in a fine sandy sub-soil formation. The discharge of the well under the depression head of 4 m is 0.004 m3/sec. Calculate the diameter of the well.
a) 2 m
b) 2.5 m
c) 3 m
d) 3.5 m
Explanation: Given, Yield of well = 0.004 x 3600 = 14.4 m3/hour
Q or yield = K x H
14.4 = K x 4 or K = 3.6 m3/hour under head of 1 meter.
From Marriot’s table, the value of K/A for sandy sub-soil formation = 0.5
3.6/A = 0.5 or A = 7.2 m2
Diameter of well = (A x 4/3.14)1/2 = 3.07 m.
Sanfoundry Global Education & Learning Series – Irrigation Engineering.
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