This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Dilution Technique of Streamflow Measurement”.
1. Which of the following is not an example of a radioactive material used as a tracer?
a) Rhodamine-WT
b) Bromine-82
c) Sodium-24
d) Iodine-132
View Answer
Explanation: Bromine-82, Sodium-24 and Iodine-132 are examples of radioactive materials used as tracers in streamflow measurement. Rhodamine-WT is a common example of a fluorescent, xanthene dye.
2. Which of the following is not true with respect to use of radioactive materials as tracers for streamflow measurement?
a) It can be detected only upto concentrations of 0.5 ppb
b) It can be used as large-scale dilutions
c) Involves sophisticated instruments
d) Requires trained personnel
View Answer
Explanation: Radioactive materials as tracers are highly accurate being able to detect concentrations even in the range of picocuries per litre. This allows it to be used as large-scale dilutions. However, it requires the use of complex instruments which are to be used by trained personnel only.
3. What is the approximate range of concentration upto which common salt can be detected in streams as a tracer?
a) 0.01 ppm
b) 0.1 ppm
c) 1 ppm
d) 10 ppm
View Answer
Explanation: Common salt is an example of a chemical tracer which requires more amount compared to fluorescent or radioactive tracers. It can be detected upto concentrations of 10 ppm with an error of ±1%.
4. A 17 g/L solution of a tracer is being injected into a river at point A at a rate of 17 ml/s. At a point B, 20 km downstream of A, the tracer had an equilibrium concentration 0.17 ppm. If the background concentration of tracer in the river is zero, find the discharge of the river.
a) 1.7 m3/s
b) 17 cm3/s
c) 170 l/s
d) 17000 ml/s
View Answer
Explanation: Concentration of tracer injected, C1 = 17 g/L = 17000 mg/L
Concentration of tracer at point B, C2 = 0.17 ppm = 0.17 mg/L
Background concentration, C0 = 0
Rate of injection, Q1 = 17 ml/s = 17 x 10-6 m3/s
The discharge of the river by constant rate injection is given as,
\(Q=\frac{Q_1*(C_1-C_2)}{(C_2-C_0)}=\frac{17*10^{-6}*(17000-0.17)}{(0.17-0)}\)=1.6999 m3/s=1.7 m3/s
5. The mixing length for the tracer increases as the width of the river increases.
a) True
b) False
View Answer
Explanation: As per Rimmar empirical formula, the mixing length is directly proportional to the square of the width of the river. This means that the mixing length becomes large for wider rivers and this may be a problem for the application of dilution technique when the length of reach is limited.
6. A chemical tracer solution of 32 g/L is injected at a constant rate of 1 L/min into a river from a fixed point. At a point 3 km downstream from the point of injection, the tracer had achieved an equilibrium concentration of 0.9 ppm. Find the discharge of the river if the background concentration of the tracer was found to be 0.45 ppm.
a) 1.18 L/min
b) 1185 m3/sec
c) 71.11 m3/min
d) 71109 L/sec
View Answer
Explanation: Concentration of tracer injected, C1 = 32 g/L = 32000 mg/L
Concentration of tracer at downstream point, C2 = 0.45 ppm = 0.45 mg/L
Background concentration, C0 = 0.09 mg/L
Rate of injection, Q1 = 1 L/min = 10-3 m3/min
The discharge of the river by constant rate injection is given as,
\(Q=\frac{Q_1*(C_1-C_2)}{(C_2-C_0)}=\frac{10^{-3}*(32000-0.9)}{(0.9-0.45)}\)=71.11 m3/min
7. Estimate the minimum length the between the point of injection of tracer and sampling point to allow sufficient mixing in a stream of constant rectangular cross section of width 5 m and depth 4.5m. Take Chezy’s coefficient as 45 and g = 10 m/s2.
a) 31 m
b) 90 m
c) 116 m
d) 123 m
View Answer
Explanation: The mixing length (in m) given by Rimmar is,
\(L=\frac{0.13B^2 C(0.7C+2√g)}{gd}=\frac{0.13*5^2*45*(0.7*45+2√10)}{10*4.5}\)=122.93 m≅123m
Sanfoundry Global Education & Learning Series – Engineering Hydrology.
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