This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Risk, Reliability and Safety Factor”.

1. A hydrologic structure is designed for a flood of return period of X years for a life of Y years. When is the structure most likely to fail?

a) If a flood of return period greater than X years occurs in Y years

b) If a flood of return period lesser than X years occurs in Y years

c) If a flood of return period greater than Y years occurs in X years

d) If a flood of return period lesser than Y years occurs in X years

View Answer

Explanation: A structure designed for a particular expected life for a particular flood considers that it is very likely for a flood of higher magnitude to occur even once during the structures design life. A flood or greater or equal magnitude occurring at least once within the design life may lead to failure.

2. Hydrologic risk is defined as which of the following probabilities?

a) Occurrence of flood smaller than design flood at least once in a period of successive years

b) Occurrence of flood greater than design flood at least once in a period of successive years

c) Non-occurrence of flood smaller than design flood at least once in a period of successive years

d) Non-occurrence of flood greater than design flood at least once in a period of successive years

View Answer

Explanation: Hydrologic risk is defined as the probability of occurrence of a flood event of magnitude greater than or equal to the design magnitude at least once in a period of successive years. It is estimated by subtracting the probability of non-occurrence of such an event from 1.

3. What is the relationship between risk (R) and reliability (r)?

a) R.r=1

b) \(\frac{R}{r}\)=1

c) R+r=1

d) R-r=1

View Answer

Explanation: Reliability is the probability of non-occurrence of an event for a period of successive years. Hence it is related to risk as, R = 1 – r , which implies R + r = 1.

4. The hydrologic design risk to be used in practice for a project is governed only by cost-benefit considerations.

a) True

b) False

View Answer

Explanation: The risk for which a flood should be chosen for the design of a hydrologic structure is governed mainly by the economic considerations of the project and policy consideration of the particular area.

5. The actual value of a parameter used in design of a hydrologic structure is F_{act} and the value of the same parameter based on hydrologic considerations is F_{hyd}. What is the safety factor for this parameter?

a) F_{act}-F_{hyd}

b) F_{hyd}-F_{act}

c) \(\frac{F_{act}}{F_{hyd}}\)

d) \(\frac{F_{hyd}}{F_{act}}\)

View Answer

Explanation: The safety factor for any parameter is defined as the ratio of its actual value used for design to its value obtained as per hydrological considerations only. The safety factor can be used to represent parameters like flood magnitude, river stage, reservoir capacity, etc.

6. The actual value of a parameter used in design of a hydrologic structure is K_{act} and the value of the same parameter based on hydrologic considerations is K_{hyd}. What is the safety margin for this parameter?

a) K_{act}-K_{hyd}

b) K_{hyd}-K_{act}

c) \(\frac{K_{act}}{K_{hyd}}\)

d) \(\frac{K_{hyd}}{K_{act}}\)

View Answer

Explanation: The safety margin for any parameter is defined as the difference of its actual value used for design and its value obtained as per hydrological considerations only. The safety margin can be used to represent parameters like flood magnitude, river stage, reservoir capacity, etc.

7. The hydrologic design flood as estimated by Gumbel’s method is found to be 68400 m^{3}/s and the actual flood magnitude used in the design is 74500 m^{3}/s. What is the safety factor related to the flood discharge?

a) 0.92

b) 1.09

c) 1.19

d) 1.9

View Answer

Explanation: The safety factor is given as,

\(Safety \, factor=\frac{Actual \, value \, used}{Hydroloigc \, design \, value} = \frac{74500}{68400}\)=1.0892≅1.09

8. What is the safety margin of a structure with respect to flood discharge, if the hydrologic design flood was 8000 m^{3}/s but it was actually designed for 1.2 times that value?

a) 800 m^{3}/s

b) 1000 m^{3}/s

c) 1200 m^{3}/s

d) 1600 m^{3}/s

View Answer

Explanation: The safety margin is given by,

Safety margin = Actual value used – Hydrologic design value = (1.2*8000)-8000=1600 m

^{3}/s

9. A structure has a flood magnitude safety margin of 24000 m^{3}/s for a hydrologic design magnitude of 101000 m^{3}/s. What is the flood magnitude safety factor of this structure?

a) 0.81

b) 1.24

c) 1.31

d) 1.62

View Answer

Explanation: Safety margin = Actual value used – Hydrologic design value

So, Actual value used = Hydrologic design value + Safety margin = 101000 + 24000 = 125000 m

^{3}/s

∴\(Safety \, factor=\frac{Actual \, value \, used}{Hydroloigc \, design \, value} = \frac{125000}{101000}\)=1.2376≅1.24

10. A structure is to be designed for a flood magnitude safety margin of 2220 m^{3}/s and a flood magnitude safety factor of 1.44. What is the magnitude of actual design flood (in m^{3}/s) considered?

a) 5000

b) 5046

c) 7200

d) 7266

View Answer

Explanation: Let X be the actual flood and Y be the hydrologic design flood.

So, Safety margin = X – Y = 2220 ………(1)

And Safety factor = X/Y = 1.44 ………(2)

Substitute (2) in (1),

1.44Y-Y=2220 ⇒ 0.44Y=2200 ⇒Y=5045.45 m

^{3}/s

⇒X=1.44Y=1.44*5045.45=7265.45 m

^{3}/s≅7266 m

^{3}/s

**Sanfoundry Global Education & Learning Series – Engineering Hydrology.**

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