# Engineering Hydrology Questions and Answers – Flood Frequency Studies

This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Flood Frequency Studies”.

1. What is the set of data comprising yearly peak flood values of a catchment for a many successive years known as?
a) Annual series
b) Recurrence series
c) Flood flow series
d) Hydrologic risk series

Explanation: The annual flood peak discharge data from a catchment over a large number of consecutive years is known as annual series or annual flood series. This data is very useful in conducting flood frequency studies for the catchment.

2. For a flood frequency analysis, the discharge data of the annual flood series is arranged in what order?
a) Chronological
b) Reverse chronological
c) Increasing
d) Decreasing

Explanation: After the peak flood data for a number of years is collected, it is arranged from the highest discharge value to the lowest, assigning a rank to each value. Consequently, the probability of each flood is calculated by suitable formulae.

3. If ‘T’ represents the return period of a flood peak of magnitude ‘Q’ in a catchment, what does ‘T’ signify?
a) A flood peak of magnitude Q occurs once in T years
b) A flood peak of magnitude less than or equal to Q occurs once in T years
c) A flood peak of magnitude more than or equal to Q occurs once in T years
d) A flood peak of magnitude (Q±0.1Q) occurs once in T years

Explanation: Return period ‘T’ of a flood peak ‘Q’ represents the average time interval between occurrence of a flood or magnitude more than or equal to ‘Q’ in that catchment. However, it does not imply that once in ‘T’ years such an event is necessarily bound to occur.

4. What is the relation between return period ‘R’ of a flood and its dependability ‘D’?
a) R∝D
b) R∝$$\frac{1}{D}$$
c) R∝Dn, n is a constant
d) R∝$$\frac{1}{D^n}$$, n is a constant

Explanation: The dependability of a flood is nothing but its probability of occurrence or exceedance. The probability of occurrence of a flood and its return period (in years) follow a simple inverse relationship.

5. As per Weibull formula, what is the probability of occurrence of a flood ranked ‘p’ in a descending annual flood series out of a total of ‘N’ peak values?
a) $$\frac{p}{N}$$
b) $$\frac{p+1}{N}$$
c) $$\frac{p}{N+1}$$
d) $$\frac{N}{p}$$

Explanation: Weibull’s formula is a common plotting-position formula used to find the probability of a particular flood being equaled or exceeded from a ranked list of peak flood values. The rank of the given flood value is divided by the total samples plus one.

6. Which of the following is not a distribution function for the estimation of peak flood?
a) Casagrande’s logarithmic distribution
b) Log normal distribution
c) Gumbel’s extreme value distribution
d) Log-Pearson type III distribution

Explanation: It was found that many frequency distribution functions used in hydrologic studies and analysis could be applicable to flood studies also. Some of these functions that can be used to predict peak flood values are Gumbel’s extreme value, log normal and Log-Pearson distributions.

7. Which of the flowing represents reliability of a hydrologic structure designed for a flood of return period ‘T’ years for a design life of ‘n’ years?
a) $$\frac{1}{T^n}$$
b) $$(1-\frac{1}{T})^n$$
c) $$1-\frac{1}{T^n}$$
d) $$1-(1-\frac{1}{T})^n$$

Explanation: Reliability is defined as the probability of flood or given magnitude or greater not occurring at all in given number of successive years. Let p be the probability of occurrence of a flood and q be the probability of not occurring. Then,
p=$$\frac{1}{T}$$ and q=1-p
∴Realiability=qn=(1-p)n=$$(1-\frac{1}{T})^n$$.

8. For a given annual flood series, which of the following represents the general equation for frequency analysis for XT, which is a general variate X of return period T years? $$\overline{X}$$ is the mean of the variate, σ is the standard deviation of the variate and K is a frequency factor.
a) $$X_T=\overline{X}+Kσ$$
b) $$X_T=\overline{X}-Kσ$$
c) $$X_T=\frac{\overline{X}}{Kσ}$$
d) $$X_T=\overline{X}+\frac{K}{σ}$$

Explanation: Prediction of extreme flood events is done using frequency analysis. The required statistical parameters are calculated by assuming specific extreme value distributions. From this, a general equation of hydrologic frequency analysis was arrived at, given as,
$$X_T=\overline{X}+Kσ$$.

Sanfoundry Global Education & Learning Series – Engineering Hydrology.