# Energy Engineering Questions and Answers – Rainfall and its Measurement – 2

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This set of Energy Engineering Question Bank focuses on “Rainfall and its Measurement – 2”.

1. How many years of minimum rainfall is required to design hydroelectric projects?
a) 10
b) 20
c) 25
d) 35

Explanation: In the design of hydro-electric projects, the rainfall record of nearly 35 years is required. This period is accepted as giving a reasonably accurate assessment of mean annual rainfall. Many times, it is not possible to wait many years for rainfall data and in such cases, shorter periods are accepted by design engineers with reserve and allowance is made for possible error.

2. There is requirement for wind shelter to reduce wind effect at rain gauge area.
a) True
b) False

Explanation: A wind shelter should be provided to reduce the effect of wind. The gauge should be erected on a level ground. The distance of the gauge from every object should not be less than twice the height of the object above the gauge.

3. What is the primary intention of rain gauging in hydro electric plant?
a) To determine force of storm
b) To determine monthly rainfall
c) To determine annual rainfall
d) To determine monthly and annual rainfall

Explanation: The primary intention of the rain gauging in hydroelectric work is to determine the monthly and annual rainfalls over catchment area. When the rain-gauging stations are more than one for a particular basin, calculation of average rainfall depending upon the area of the basin is done.

4. How many methods are used to determine the average rainfall depending upon area of basin in rain gauging?
a) 2
b) 3
c) 4
d) 5

Explanation: When there are more basins, three of the following methods are used to determine the average of rainfall depending upon the area of basin. They are Arithmetic mean method, Thiesson method and iso-hyetol method.

5. In which of the following rain gauging methods are the values of rainfalls of all stations added?
a) Arithmetic mean method
b) Thiesson method
c) Iso-hyetol method
d) Recording type

Explanation: In arithmetic mean method, the values of rainfalls of all the stations are added and the sum is divided by the number of gauging stations. This can be represented in the mathematical term as:
ha = $$\frac{h1+h2…….hn}{n} = \frac{Ʃh}{n}$$.

6. Which among the following method is very accurate of rain gauging?
a) Arithmetic mean method
b) Thiesson method
c) Iso-hyetol method
d) Recording type

Explanation: Thiesson method is very accurate compared to other methods and is generally used when the catchment area lies between 500 to 5000km square. Assume that there are stations in the given basin names=d as 1, 2, 3, 4, 5, 6, 7, and 8. It is very important to divide the total basin area in such a way that each located station in the basin represents that area in the true.

7. Thiesson method is better result than arithmetic mean method because _________
a) It is shorter time consuming
b) Gives better detail about records
c) It is highly accurate
d) Small procedure

Explanation: The mean rainfall of the basin in thiesson method is given by:
ha = $$\frac{h1A1 + h2A2 + h3A3 + h4A4 + h5A5 + h6A6 + h7A7 + h8A8}{A1+A2+A3+A4+A5+A6+A7+A8}$$
This method gives better result than arithmetic mean method because it is very accurate and each point on a perpendicular bisector of the line joining the two gauging stations will be equidistant from both the stations. If one moves slightly this or that side of the bisector, the position will distinctly fall in the domain of that station of which ones position is now near.

8. In which of the following rain gauging method is contour joining the points of equal rainfall is done?
a) Arithmetic mean method
b) Thiesson method
c) Iso-hyetol method
d) Recording type

Explanation: In iso-hyetol method, contour joining the points of equal rainfall in the given catchment area is done. Iso-hyetes have following properties:
i) Two different iso-hyetes do not cross each other.
ii) Each iso-hyete closes on itself.
iii) Iso-hyete of higher value indicates the higher rainfall.

9. Two successive Iso-hyetes in the basin drawing of catchment area are measured using _______
a) Planimeter
b) Manometer
c) Mass flow meter
d) Chronometer

Explanation: Generally interval of iso-hyete is one cm. after drawing the iso-hyetes; the area between two successive iso-hyetes is measured by using a Planimeter. The rest of the procedure of finding out the mean rainfall is similar to that described in the second method.

10. Iso-hyete method is commonly used for the basin area above 500sq.km.
a) True
b) False

Explanation: Iso-hyete method is commonly used for the basin above 500sq.km. The analysis of rainfall for power generation purposes is usually more concerned with dry years rather than wet years. For the assessment of storage requirements, it is often necessary to determine the driest period for which provision must be made.

11. Study of shape and features of earth surface is called __________
a) Vegetation
b) Geography
c) Anatomy
d) Topography

Explanation: The study of the shape and features of the surface of the earth is known as topology and it is a very important factor which is affecting the runoff and also the nature of catchment area depends on it.

12. Vegetation affects the runoff.
a) True
b) False

Explanation: The transpiration and interception losses depend on the nature and extent of vegetation including crops. Vegetation, particularly of forest, has considerable effect upon the runoff. It consumes a proportion of the rainfall, causes interception losses and provides physical obstruction for runoff.

13. Rocky area gives higher runoff than sandy area.
a) True
b) False

Explanation: The geology of catchment area is of fundamental importance in the consideration of runoff. Rocky area gives higher runoff than sandy area. Even the evaporation losses increase with the increase in temperature.

14. The runoff calculated from the rainfall data available for sufficiently long period by multiplying with a coefficient is known as _________
a) Runoff coefficient
b) Rainfall coefficient
c) Topographic coefficient
d) Geo coefficient

Explanation: The runoff calculated from the rainfall data available for sufficiently long period by multiplying with a coefficient is known as runoff coefficient. The value of runoff coefficient is decided by considering all the factors which affect runoff.

15. The maximum rate of flow through the river is calculated by _________
a) Energy curves of flow
b) Force of flow
c) Pressure of flow

Explanation: The maximum rate of flow through the river is calculated by energy curves. Flood marks are also used to get the maximum flow rate. The following curves include Dickens formula, Inglis formula and ryve’s formula.

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