# Engineering Hydrology Questions and Answers – Use and Limitation of Unit Hydrograph

This set of Engineering Hydrology Multiple Choice Questions & Answers (MCQs) focuses on “Use and Limitation of Unit Hydrograph”.

1. The approximate minimum catchment area that enables us to plot a fairly accurate unit hydrograph is _________
a) 2 hectares
b) 2 sq.km.
c) 20 hectares
d) 20 sq.km.

Explanation: For catchment areas smaller than 2 sq.km. or 200 hectares, the relationship between the rainfall and its resulting excess is dependent on a number of factors, which leads to an unreliable unit hydrograph that cannot be used for the prediction of the direct runoff hydrograph.

2. The general maximum catchment area beyond which, the plotting of unit hydrograph would give undesired and inaccurate results is _______
a) 5000 sq.km.
b) 5000 hectares
c) 7500 sq.km.
d) 7500 hectares

Explanation: The use of the unit hydrograph is limited by the size of the catchment over which the storm has been recorded. Generally, for areas larger than 5000 sq.km., the point location of each different storm may vary which gives different peak DRH values for similar storm characteristics.

3. For a given 6 hour storm, which of the following assumptions should be necessarily true in order to plot a unit hydrograph?
a) Plane catchment area and one-way drainage only
b) Hydrograph has a base time of 6 hours
c) Uniform rainfall distribution and constant rainfall intensity for 6 hours
d) No base flow must be present in the catchment

Explanation: The two major assumptions for plotting a unit hydrograph is that the rainfall is distributed uniformly over the catchment and the rainfall intensity is constant for the duration of runoff. But in reality, these assumptions are rarely satisfied.

4. The following statements are the steps of plotting the DRH for a very large basin using unit hydrograph.
A. Compound DRH obtained at catchment outlet.
B. Catchment area is divided into smaller subbasins.
C. Each DRH is routed through its individual channel to the outlet.
D. A DRH is plotted for each of the subbasins using unit hydrograph.
What is the correct sequence of carrying out the above steps?
a) C→B→D→A
b) B→C→D→A
c) B→D→C→A
d) C→D→B→A

Explanation: Very large catchment areas can be conveniently studied by dividing it into smaller preferably equal areas and analyzing the rainfall-runoff pattern for each area individually. All the individual UH are converted to DRH and them combined to obtain the final compounded DRH for the complete basin.

5. Which of the following types of precipitation can be represented by a unit hydrograph?
a) Rain only
b) Rain and snow
c) Rain and hail
d) Rain, snow and hail

Explanation: The excess precipitation due to the melt of snow, hail and other forms of solid precipitation cannot be accurately depicted in a unit hydrograph. It is a limitation of UH that the precipitation must be rainfall only.

6. A basin with a large pond does not affect the unit hydrograph plotted for a storm over the basin.
a) True
b) False

Explanation: Basins with large storage areas like water tanks, flood banks, ponds, etc. affects the linear relationship between storage and discharge at a point which in turn affects the unit hydrograph values for the area at that point.

7. Which of the following is not a common use of unit hydrographs?
a) Estimation of time of concentration
b) Design of hydraulic structures
c) Extending flood flow records based on rainfall
d) Flood forecasting and warning systems

Explanation: The time of concentration cannot be directly calculated from the unit hydrograph but can be depicted as the time from the end of excess rainfall to the inflection point on the recession limb. The time of concentration is calculated from empirical equations like Kirpich equation.

8. A unit hydrograph for a storm over a catchment was plotted by four different people. The UH peaks obtained by the four people P, Q, R, S were 29.0 m3/s, 31.2 m3/s, 35.6 m3/s and 33.5 m3/s respectively. The expected value of the peak discharge as estimated from past data of similar storms over the catchment is 32.5 m3/s. The result obtained by which person is unacceptable as per the expected peak discharge value?
a) P
b) Q
c) R
d) S

Explanation: Due to the limitations of unit hydrograph, the variation in peak discharge values by ±10% of the expected value is considered acceptable. Therefore, the reasonable limit of peak values in the above problem is (32.5 x 0.9) to (32.5 x 1.1), i.e., 29.25 m3/s to 35.75 m3/s. Since the reading obtained by P, which is 29.0 m3/s lies outside this range, it is therefore unacceptable.

Sanfoundry Global Education & Learning Series – Engineering Hydrology.