This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Streamflow – Area-Velocity Method”.

1. Which of the following is not true for regarding suitability of a gauging site for conducting streamflow measurements by area-velocity method?

a) The stream should have a well-defined cross section

b) The section of the stream should be straight

c) Backwater effects can be ignored

d) The stream cross-section should be constant throughout the year

View Answer

Explanation: The main criteria for selecting a suitable gauging site for area-velocity method are: stream cross-section should be well-defined and constant; easy access; section should be straight; completely free from backwater effects and should be stable.

2. Which of the following conditions does not require use of echo-depth recorder for stream depth measurements?

a) Shallow streams

b) High velocity streams

c) Streams with soft beds

d) Deep streams

View Answer

Explanation: An echo-depth recorder is a device used to determine the depth of streams quickly and accurately, using sound waves. It is most suitable in the case of rapidly flowing streams, deep streams, streams with mobile beds and other cases where sounding rods/weights are not applicable.

3. How is a particular cross-section of a stream divided to carry out area-velocity method?

a) Horizontal strips

b) Vertical strips

c) Grid

d) Horizontal or vertical strips depending upon the depth

View Answer

Explanation: To estimate the discharge by area-velocity method, the selected cross-section of the stream is divided into a number of sections in the vertical direction. These strips may or may not be of equal width. The average velocity and average depth of these sections are determined.

4. Which of the following is a disadvantage of having a large number of segments compared to a fewer segments of a cross-section in area-velocity method?

a) Better accuracy

b) Less expenditure

c) Less effort

d) More time required

View Answer

Explanation: Having a large number of vertical strips enables us to obtain a more continuous and uniform set of data regarding depths and velocities. This improves the accuracy of final discharge determination compared to the value obtained by having fewer vertical sections. However, more sections lead to larger effort, expenditure and time.

5. What is the minimum number of vertical sections a river cross-section should be divided into?

a) 5

b) 10

c) 15

d) 20

View Answer

Explanation: As a general guideline for selecting the number of vertical strips, it is given that the width of any individual strip should not be greater than 1/15

^{th}to 1/20

^{th}of the river width. For example, consider a width of 30m. The maximum strip width would be 30/15 = 2m. Considering all strips to be 2m wide, the minimum number of strips would 30/2 = 15 strips.

6. A stream cross-section of total discharge ‘Q’ is divided into 20 sections. If the velocity is constant throughout the cross-section, what will be maximum discharge allowed from the strip of largest area?

a) 0.05Q

b) 0.1Q

c) 0.25Q

d) 0.5Q

View Answer

Explanation: The number of strips should be selected in such a way that the individual discharge from any strip should be more than 10% of the total discharge from the cross-section.

7. A stream-cross section has N segments. Three consecutive segments A, B and C are selected at random. The average velocity is A is 0.52 m/s and average velocity is C is 0.74 m/s. What should be the velocity in segment B, for the selection of N segments to be valid?

a) 0.57 m/s

b) 0.60 m/s

c) 0.63 m/s

d) 0.67 m/s

View Answer

Explanation: As a general guidelines, the number of strips should be such that the difference of velocities in any two adjacent strips should be less than or equal to 20%.

Considering strip A,

80% of 0.52=0.416 m/s and 120% of 0.52=0.624 m/s

Considering strip C,

80% of 0.74=0.592 m/s and 120% of 0.74=0.888 m/s

From the above conditions the average velocity in segment B should lie in the range [0.416,0.624] and [0.592,0.888]. Which implies, considering the common range, the velocity in B should lie in [0.592,0.624]. The only option in the required range is 0.60 m/s. Now verifying the validity of 0.6 m/s,

Comparing with A, Error=\(\frac{0.6-0.52}{0.52}\)*100=15.4% < 20%

Comparing with C, Error=\(\frac{0.74-0.6}{0.74}\)*100=18.9% < 20%

Hence, verified.

8. The first and last segments in area-velocity method are considered as trapezoidal areas.

a) True

b) False

View Answer

Explanation: In general cases, the initial point and final point of the cross-section of stream are considered to be at zero depth. The first vertical and last vertical drawn have a given depth. Hence, for simpler calculations the first and last segments are assumed to be triangular in shape.

9. What is the correct formula for estimating average velocity by 3-point observation method? V_{0.2}, V_{0.6}, V_{0.8} represent the velocity at 0.2, 0.6 and 0.8 times the depth below the surface, respectively.

a) \(\frac{V_{0.2}+V_{0.8}}{2}\)

b) \(\frac{(V_{0.2}+V_{0.8})^2}{2V_0.6}\)

c) \(\frac{\left(\frac{V_{0.2}+V_{0.8}}{2}\right)+V_{0.6}}{2}\)

d) \(\frac{V_0.2+V_{0.6}+V_{0.8}}{3}\)

View Answer

Explanation: The velocity by 3-point method is calculated in two steps. The first step velocity is calculated as per 2-point method, where the velocity at 0.2 and 0.8 times the depth below the surface are averaged. Then, this averaged velocity is again averaged with the velocity at 0.6 times the depth below the surface to obtain the final average velocity.

10. If a stream cross-section is divided into N vertical sections, how many sections are used for discharge estimation?

a) N

b) N+1

c) N-1

d) 2N

View Answer

Explanation: The first and last triangular sections of the cross-section are considered along with half of their respective adjacent sections. Hence, the number of areas considered for discharge estimation is one less than the initial number of vertical sections.

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