Irrigation Engineering Multiple Choice Questions on “Reservoir Losses”.
1. As the height of a proposed dam is increased, the cost per unit of storage initially increases and then decreases.
a) True
b) False
Answer: b
Clarification: The cost per unit storage of the dam initially decreases reaches to a certain minimum value and then increases when the height of the dam is increased. This lowest point on the curve gives the economical height for which the cost per unit storage is minimum.
2. The economical height of a dam is that height for which the ____________________
a) cost per unit of storage is minimum
b) benefit-cost ratio is maximum
c) net benefits are maximum
d) benefit-cost ratio is minimum
Answer: a
Clarification: The height of the dam corresponding to which the cost of the dam per unit storage is minimum is the economical height of the dam. The cost per unit storage of the dam initially decreases reaches to a certain minimum value and then increases when the height of the dam is increased.
3. If 20% of the reservoir capacity is earmarked for dead storage in a storage reservoir of 30 M.cum and the average annual silt deposition in the reservoir is 0.1 M cum, then the useful life of the reservoir will start reducing after ____________
a) 60 years
b) 120 years
c) 240 years
d) 300 years
Answer: a
Clarification: Given, Reservoir capacity = 30 M.cum
Dead storage is 20% of the reservoir capacity = 0.2 x 30 = 6 M.cum
Time during which 6 M cum is filled by sediment @0.1 M.cum/yr = 6/0.1 = 60 years.
4. According to the Central Water Commission, the average annual evaporation loss is estimated to be _____________
a) 100 cm
b) 125 cm
c) 225 cm
d) 250 cm
Answer: c
Clarification: The Central Water Commission in 1990 has estimated the average annual evaporation loss to be 225 cm on the basis of a review conducted on 130 sample reservoirs. The estimated total water loss from all the existing reservoirs to be 27000 Mm3 per annum.
5. Windbreakers, Integrated operation of reservoirs and Use of underground storage are all methods of ______________________
a) evaporation control
b) estimation of absorption losses
c) estimation of absorption losses
d) estimation of reservoir leakage
Answer: a
Clarification: The following are some of the methods of evaporation control such as Windbreakers, Reduction in the exposed water surface, Use of underground storage rather than surface storage, Integrated operation of reservoirs, and Use of chemical for retarding the evaporation rate from the reservoir surface. Use of Water Evapo-Retardants (WERs) is considered to be an only practical solution for the conservation of freshwater in spite of its limitations and disadvantages.
6. Which of the following is not used as WERs?
a) Hexa decanol
b) Octa decanol
c) Docosanol
d) Cyclohexanol
Answer: d
Clarification: Fatty alcohols like Cetyl alcohol (popularly known as Hexa decanol), Stearyl alcohol (also called Octadecanol), and Behenyl alcohol (also called docosanol) are found to be quite suitable. A mixture of these chemicals have also been generally used and the chemicals should be 99% pure for getting the desired properties of the monolayer.
7. Which of the following dam is situated in the seismologically inactive zone in Peninsular India?
a) Koyna dam
b) Hirakud dam
c) Tehri dam
d) Idukki dam
Answer: a
Clarification: A very strange phenomenon has been observed in several dams of the world, the reservoir basins which were seismologically inactive started showing seismic activities when the reservoir was filled up with water. Koyna dam (Maharashtra) is an important example of such a case.
8. Which of the following measure does not prevent or reduce the intensity of the earthquake?
a) Filling the reservoir up to a limited safe level
b) Draining out water from weaker adjoining rocks
c) Active exploration of the dam site for the absence of inactive faults before selecting the same
d) Increasing the pore pressure
Answer: d
Clarification: Earthquakes occur due to increased pore-pressure in the adjoining rocks which lowers their shearing strength resulting in the release of tectonic strain. Draining out water from weaker adjoining rocks actually reduces pore pressure.
9. Which of the following guideline is incorrect for the selection of a suitable site for a reservoir?
a) The geological formation should be such that it entails minimum leakage
b) The cost of the dam is a controlling factor
c) Too much silt-laden tributaries should be avoided as far as possible
d) The reservoir basin should have a shallow wide opening in the valley
Answer: d
Clarification: The site should be such that a deep reservoir is formed because of lower land cost per unit of capacity, less evaporation loss, and less possibility of weed growth as compared to shallow ones. The basin should have a narrow opening so that the length of the dam is minimum.