Engineering Hydrology Questions and Answers – Synthetic Unit Hydrograph – Set 4

This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Synthetic Unit Hydrograph – Set 4”.

1. What is the ordinate of the dimensionless unit hydrograph developed by the US Soil Conservation Services?
a) Discharge
b) Ratio of the peak discharge to discharge values
c) Ratio of discharge values to the peak discharge
d) Time

Explanation: In the dimensionless unit hydrograph proposed by SCS, the ordinate represents the values of discharge expressed in the form of a ratio to the peak discharge, making the quantity a dimensionless one.

2. What is the abscissa of the dimensionless unit hydrograph developed by the US Soil Conservation Services?
a) Time
b) Ratio of time values to the time to peak
c) Ratio of time values to the lag time
d) Ratio of time values to the rainfall duration

Explanation: The abscissa of the SCS dimensionless unit hydrograph represents the time expressed as a ratio to the time to peak, thus making it a dimensionless quantity.

3. When t/tp = 1, then what will be the value of Q/Qp, if tp and Qp represent the time to peak and peak discharge respectively?
a) <1
b) >1
c) =1
d) Cannot be determined

Explanation: By definition of the SCS dimensionless unit hydrograph, the discharge ratio should be equal to 1 when the time ratio is equal to one. This coordinate represents the peak of the hydrograph.

4. In the SCS dimensionless UH, for t/tp values ranging from 0 to 1, what will be the corresponding variation in Q/Qp values?
a) Increasing
b) Decreasing
c) Increases then decreases
d) Remains constant

Explanation: The values of abscissa from 0 to 1 represents the rising limb of the dimensionless unit hydrograph from the origin point to the peak. The value of Q/Qp increases from 0 to 1 as the time quantity increases from 0 to 1.

5. For values of t/tp > 1, what will be the variation in Q/Qp in the SCS dimensionless unit hydrograph?
a) Remains constant and equal to 1
b) Decreases and approaches 0
c) Increases exponentially
d) Suddenly drops to 0

Explanation: The quantity Q/Qp can never be greater than 1 as the maximum value that can be attained by Q is Qp when t = tp. For t/tp > 1, the values of Q/Qp varies in such a way so as to represent the falling limb of the hydrograph.

6. The SCS dimensionless unit hydrograph is plotted using t/tp values upto 5 only.
a) True
b) False

Explanation: For t/tp values from 0 to 1, Q/Qp values increase from 0 to 1. For t/tp values from 1 to 5, Q/Qp values decrease from 1 to 0. For t/tp = 5. Q/Qp = 0.004 and further plotting of hydrograph with more values is insignificant as the discharge ratio approaches 0.

7. The peak discharge and time to peak values for SCS dimensionless unit hydrograph are determined from another hydrograph of what shape?
a) Bell-shaped
b) Triangular
c) Rectangular
d) Trapezoidal

Explanation: SCS has suggested a simplified model of a triangular hydrograph for determining the values of peak discharge and time to peak. The percentage of area under the rising limb of this triangular hydrograph is the same as that of the dimensionless hydrograph.

8. The figure shows a typical SCS triangular unit hydrograph. What is the ratio of peak time to recession time as per SCS?

a) 0.6
b) 0.67
c) 1.33
d) 1.67

Explanation: According to SCS, tr=1.67*tp
→ $$\frac{t_p}{t_r}=\frac{1}{1.67}$$=0.5988≅0.6

Sanfoundry Global Education & Learning Series – Engineering Hydrology.