This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Instantaneous Unit Hydrograph (IUH)”.
1. For a D-hr unit hydrograph, as the value of D is reduced, what will be the change in peak of the unit hydrograph?
a) Increases
b) Decreases
c) Remains same
d) May increase or decrease
View Answer
Explanation: As the value of effective rainfall duration is decreased for a unit hydrograph of a particular catchment, the value of rainfall intensity should increase as the runoff volume of 1cm needs to be constant over the catchment. Hence, the peak of hydrograph keeps on increasing as the duration of rainfall decreases.
2. How many peaks does an instantaneous unit hydrograph have?
a) 0
b) 1
c) 2
d) More than 2
View Answer
Explanation: Instantaneous unit hydrograph assumes that a total volume of 1 cm effective rainfall falls over the catchment at one particular instant. This leads to a unit hydrograph with a single, very early and large peak.
3. What is the time base of an instantaneous unit hydrograph?
a) 0 unit
b) 1 unit
c) Infinite
d) Some finite value
View Answer
Explanation: For constructing an IUH, it is assumed that the rainfall duration approaches zero. Nevertheless, even in such a case, the runoff will eventually get discharged out of the catchment in some finite duration.
4. Which of the following is not associated with instantaneous unit hydrographs?
a) It is an imaginary concept
b) It has a finite peak discharge value
c) The area under the IUH represents unit runoff depth
d) It is dependent on the duration of effective rainfall
View Answer
Explanation: One of the main advantages of IUH is that it is independent of the duration of rainfall over the catchment. Whatever be the duration of effective rainfall, the IUH is constructed assuming that all the rain occurs in zero duration, and hence it has one less parameter than a normal unit hydrograph.
5. Which of the following hydrographs gives the best idea regarding catchment storage characteristics?
a) Direct runoff hydrograph
b) Unit hydrograph
c) Synthetic unit hydrograph
d) Instantaneous unit hydrograph
View Answer
Explanation: For a given catchment, the DRH, UH and SUH all depend on the rainfall characteristics and duration. On the other hand, the IUH is independent of the rainfall and is the best indicator of the catchment storage features.
6. Which of the following is the correct assumption for the plotting of an instantaneous unit hydrograph?
a) Unit rainfall excess in unit duration
b) Infinitely small rainfall excess in unit duration
c) Unit rainfall excess in infinitely small duration
d) Infinitely small rainfall excess in infinitely small duration
View Answer
Explanation: Since IUH is basically a unit hydrograph, it has to represent a runoff volume of unit depth over the catchment area. This total unit rainfall excess has to occur over the catchment in an instant.
7. An instantaneous unit hydrograph assumes that the rainfall excess occurs over an infinitely small area.
a) True
b) False
View Answer
Explanation: The IUH has no correlation with the area of the catchment over which rainfall occurs. The only assumption is that the rainfall excess should be of unit depth and be of infinitely small duration. The area consideration needs to be practical.
8. The following figure shows unit hydrographs of different duration occurring over a catchment. Which of them is most likely to be an instantaneous unit hydrograph?
a) A
b) B
c) C
d) Cannot say
View Answer
Explanation: The curve A has the highest peak out of all the three graphs, which means that it represents the rainfall of the lowest duration of the three. Hydrographs B and C have lower peaks and higher base times than A which means they cannot represent an IUH.
Sanfoundry Global Education & Learning Series – Engineering Hydrology.
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