This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Streamflow – Velocity Measurement”.
1. What is the basic principle of a current meter used to measure stream velocity?
a) Deformation of spring element is directly proportional to stream velocity
b) Time period of oscillating element is directly proportional to stream velocity
c) Angular velocity of rotating element is directly proportional to stream velocity
d) Difference in level of manometric unit is directly proportional to stream velocity
View Answer
Explanation: Current meter is the most common device to use to measure stream velocity at a point. It consists of a rotating element rotated by the water current, for which the angular velocity is directly proportional to the stream velocity.
2. What the two main types of current meters?
a) Float and bubble meters
b) Recording and non-recording meters
c) Electrical and non-electrical meters
d) Horizontal and vertical axis meters
View Answer
Explanation: The current meters are mainly classified into two types depending on whether the main shaft with rotating element lies on the horizontal or vertical axis as horizontal axis and vertical axis current meters, respectively.
3. Which of the following is not an example of vertical-axis current meter?
a) Gurley current meter
b) Lynx current meter
c) Neyrtec current meter
d) Price current meter
View Answer
Explanation: Price, Gurley and Lynx current meters are typical examples of a vertical-axis current meter with a cup assembly. The Ott, Watt and Neyrtec current meters are types of horizontal-axis meters with a propeller.
4. What is the rotating element generally employed in a vertical-axis current meter?
a) Impeller
b) Propeller
c) Torsion spring
d) Conical cup assembly
View Answer
Explanation: Vertical-axis current meters consists of a cup assembly in the form of 6-8 conical cups mounted around a vertical axis. This arrangement is present in the forward most position of the current meter and is attached to a counting mechanism to record the signals due to the rotation of the assembly.
5. What is denoted by a vertical-axis current meter when it is lifted vertically in still water?
a) Zero velocity
b) Positive velocity
c) Negative velocity
d) Angular velocity
View Answer
Explanation: Since the cup assembly in a vertical-axis current meter rotates in the horizontal plane, it has the disadvantage that it cannot be used in cases when there is considerable vertical component of stream velocity. Hence, when it is lifted or lowered in still water, it does not show zero velocity.
6. Which of the following is true regarding accuracy of current meters?
a) Increases from threshold value to higher values of velocity for all current meters
b) Decreases from threshold value to higher values of velocity for all current meters
c) Increases with velocity for vertical-axis meters and decreases with velocity for horizontal-axis meters
d) Increases with velocity for horizontal-axis meters and decreases with velocity for vertical-axis meters
View Answer
Explanation: Every current meter has a threshold value of velocity below which it does not work. For vertical-axis meters, the accuracy is 1.5% at threshold value and improves to 0.3% for values more than 1 m/s. For horizontal-axis meters, the accuracy increases from 1% at threshold to 0.25% at velocities greater than 0.3 m/s.
7. What is the general relationship between stream velocity (v in m/s) and revolutions (N in revs/s) of the current meter? (a, b, c, k and n represent constants)
a) v = kN
b) v = Nc
c) v = aN + b
d) v = nN-1
View Answer
Explanation: The current meter works on the principle that the rotation of the meter is linearly dependent on the velocity of water at that point. So, a general relationship is given as,
v=aN+b , where ‘a’ and ‘b’ are current meter constants.
8. Let ‘m’ and ‘n’ be the multiplying constants of current meters A and B, respectively. Both A and B are cup-type current meters, but A has a significantly larger diameter cup assembly than B. What is the relationship between ‘m’ and ‘n’?
a) m > n
b) m < n
c) m = n
d) No relationship can be established
View Answer
Explanation: For a given stream velocity at a point, the smaller current meter (B) will undergo more rotations compared to the larger current meter (A), because of its smaller angular displacement per rotation. This implies that the multiplicative constant for the larger meter should be more than that of the smaller meter to get the same stream velocity.
9. The common components of a vertical-axis and horizontal-axis current meters are stabilizing fin, sounding weight and hoisting connection.
a) True
b) False
View Answer
Explanation: The vertical-axis and horizontal-axis current meters vary in size, assembly and working but have most of the components common between them. The only differentiating part is the rotating element, with vertical-axis meter having a cup assembly and horizontal-axis meter having a propeller.
10. A current meter has a constant 0.45. Find the velocity (in m/s) it measures if it undergoes 22 revolutions in 30 seconds?
a) 0
b) 0.17
c) 0.33
d) 0.78
View Answer
Explanation: Since only one constant is given the additive constant can be ignored. The velocity is given by,
v = aN + b = aN = 0.45 * \(\frac{22}{30}\) = 0.33 m/s
Sanfoundry Global Education & Learning Series – Engineering Hydrology.
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