Engineering Hydrology Questions and Answers – Derivation of Unit Hydrographs

This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Derivation of Unit Hydrographs”.

1. A number of isolated storm hydrographs of short spell rainfall excess are selected which have largely varying durations to derive unit hydrograph of each.
a) True
b) False

Explanation: In case of short spell rainfall excess, a number of isolated storm hydrographs are collected from the study of continuously gauged runoff to the stream. The duration of such hydrographs is almost same and are within the range of 0.9D-h to 1.1D-h. Unit hydrographs are derived from these storm hydrographs.

2. Which of the following is evaluated from the area and volume of direct runoff hydrograph?
a) Depth of direct runoff
b) Duration of storm
c) Baseflow
d) Depth of direct runoff and duration of storm

Explanation: When the storm hydrographs of short spell rainfall excess are obtained then firstly, the baseflow is deducted from it. The area of each direct runoff hydrograph is obtained. The volume of water in each hydrograph is found out. The volume of water in each hydrograph is divided by area of the catchment to obtain the depth of direct runoff.

3. The ordinates of direct runoff hydrograph are divided by effective rainfall depth to obtain __________
a) Ordinates of hyetograph
b) Ordinates of duration
c) Ordinates of volume of hydrograph
d) Ordinates of unit hydrograph

Explanation: When the direct runoff hydrograph is obtained. First of all, its area and volume are found out. Then, the volume of direct runoff hydrograph is divided by the area of the catchment to obtain depth of effective rainfall. Now, the ordinates of direct runoff hydrograph are divided by effective rainfall depth to obtain the ordinates of unit hydrograph.

4. The flood hydrographs selected for deriving unit hydrographs should have which one of the following features?
a) Each storm should occur individually
b) Each storm should be of different durations
c) Each storm must have same rainfall depth
d) Each storm should have same rainfall excess

Explanation: Flood hydrographs are selected to derive unit hydrographs. The flood hydrographs should meet certain features for this purpose. The storms should occur individually that is the storms should be isolated having short spell of rainfall excess.

5. The flood hydrographs used for deriving unit hydrographs should depict non uniformity of rainfall over the entire catchment area during specific duration.
a) True
b) False

Explanation: The flood hydrographs used for deriving unit hydrographs, should be of isolated storms. Moreover, the rainfall should be almost uniformly spread over the entire catchment area for the duration of storm.

6. For flood hydrographs used for deriving unit hydrograph should have the duration of rainfall under which of the following ranges?
a) 1/5 to 1/3 of the basin lag
b) 1/6 to 1/3 of the basin lag
c) 1/4 to 1/3 of the basin lag
d) 1/2 to 1/3 of the basin lag

Explanation: The unit hydrographs can be derived from flood hydrographs. In this case, the flood hydrograph should have some features. One of the features is the range of duration of storm of the flood hydrographs. The storm should have a duration of 1/5 to 1/3 of the basin lag.

7. The range of effective rainfall depth values preferred for flood hydrographs used for deriving unit hydrographs is __________
a) 1 to 2 cm
b) 1 to 3 cm
c) 2 o 5 cm
d) 1 to 4 cm

Explanation: The flood hydrographs used for deriving unit hydrographs have high rainfall excess or effective rainfall depth. The effective rainfall depth of 1 cm to 4 cm is sometimes preferred.

8. The unit hydrographs derived from the flood hydrographs of same durations will be identical.
a) True
b) False

Explanation: The unit hydrographs derived from flood hydrographs of same duration are not identical. The unit hydrographs will vary due to variation of rainfall in both space and time. The unit hydrographs of same duration are plotted on the same graph and the mean curve is selected as the unit hydrograph of that specific duration.

9. For catchments of area more than 250 km2, which of the following duration is considered satisfactory?
a) 2 hours
b) 3 hours
c) 6 hours
d) 8 hours

Explanation: The effective rainfall depth should be uniform over the entire catchment for the duration of storm. Therefore, little fluctuations of the rainfall intensity should not affect the rainfall excess in the entire catchment for the duration of storm. So, the catchment has a damping effect on the rainfall intensity. This indicates that larger durations are for larger catchments. For the catchment area of more than 250 km2, the duration of 6 hours is satisfactory.

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