# Irrigation Engineering Questions and Answers – Capacity and Area Elevation Curves of a Reservoir Site

This set of Irrigation Engineering Question Bank focuses on “Capacity and Area Elevation Curves of a Reservoir Site”.

1. What is the most important physical characteristic of a reservoir?
a) Storage capacity
b) Annual Yield
c) Average yield
d) Reservoir water level

Explanation: The main function of a reservoir is to store water and thus to stabilize the flow of water therefore, the most important physical characteristic is its storage capacity. It is determined from the contour maps of the area. The planimeter is used to measure the area enclosed within each contour.

2. In general practice adopted for capacity computations is to actually survey the site contours only at vertical distances of __________________
a) 5 m or more
b) Less than 5 m
c) 0.5 m or more
d) Less than 0.5 m

Explanation: The area of the intervening contours at smaller intervals of 0.5 m or so and at vertical distances of 5 m or so. It is then interpolated by taking the square root of the surveyed contours. It is also assumed that the square root of the interpolated ones varies in exact proportion to their vertical distances.

3. The area-elevation curve when integrated will yield the capacity-elevation curve.
a) True
b) False

Explanation: The surveyed area at large intervals is plotted on a graph and area-elevation curve is drawn. The equation of this curve can be obtained by statistical methods. It can be integrated to obtain the equation of the capacity-elevation curve to obtain storage capacity.

4. Which of the following method does not give more than 3% error in the determination of the capacity of a reservoir?
a) Prismoidal formula
b) Trapezoidal method
c) Average formula
d) Integration method

Explanation: The capacity of the reservoir can be determined by surveying only a few contours by using this method. When it is cross-checked with the capacity worked out by surveying a large number of contours this method gives less than 3% error. Since the areas of contours are not very precise figures this error is not considered much.

5. The surcharge storage in a dam reservoir is the volume of water stored ________________
a) between the minimum and maximum reservoir levels
b) between the minimum and normal reservoir levels
c) between normal and maximum reservoir levels
d) below the minimum pool level

Explanation: Surcharge storage is uncontrolled storage which is the volume of water stored between the normal pool level and the maximum pool level. It is that portion above the crest of the dam’s spillway which cannot be regulated.
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6. The useful storage in a dam reservoir is the volume of water stored __________________
a) between the minimum and maximum reservoir levels
b) between the minimum and normal reservoir levels
c) between normal and maximum reservoir levels
d) below the minimum pool level

Explanation: Useful storage can be defined as the volume of water stored in a reservoir between the minimum pool and normal pool level. Conservation storage and flood-mitigation storage are the two sub-divisions of useful storage in a multi-purpose reservoir.

7. The dead storage in a dam reservoir is the available volume for collection of silt and sediment between _______________
a) bed level of the reservoir and minimum reservoir level
b) bed level of the reservoir and the silt level in the reservoir
c) bed level of the reservoir and the normal pool level
d) above the minimum pool level

Explanation: Useful storage can be defined as the volume of water stored in a reservoir between the minimum pool and normal pool level. Dead storage is the water stored in the reservoir below the minimum pool level. It is not of much use in the operation of reservoirs.

8. Bank storage in a dam reservoir _________________
a) increases the compound reservoir capacity
b) decreases the computed reservoir capacity
c) sometimes increases and sometimes decreases the computed reservoir capacity
d) has no effect on computed reservoir capacity

Explanation: The certain amount of water that seeps into the permeable reservoir banks when the reservoir is filled up is known as bank storage. This water comes out as soon as the reservoir gets depleted and it effectively increases the capacity of the reservoir.

9. The valley storage reduces the effective storage capacity of a reservoir.
a) True
b) False

Explanation: The effective storage for flood mitigation is equal to the useful storage plus surcharge storage minus valley storage i.e. storage corresponding to the rate of inflow in the reservoir. The actual net increase in the storage is equal to the storage capacity of the reservoir minus the natural valley storage. Thus, the available storage is reduced for flood mitigation reservoir.

10. The maximum level to which the water rises during the worst design flood is known as _________________________
a) full Reservoir level
b) maximum conservation level
c) minimum pool level
d) surcharge storage

Explanation: Normal pool level or maximum conservation level is the maximum elevation to which the reservoir water rises during normal operating conditions. Minimum pool level is the lowest water surface elevation which has to be kept under normal operating conditions. Surcharge storage can be defined as the volume of water stored between the normal pool level and the maximum pool level.

11. Which of the following formula can be used to determine the storage when three consecutive sections at equal height are taken?
a) Prismoidal formula
b) Trapezoidal method
c) Average formula
d) Integration method

Explanation: According to the Prismoidal formula,
Storage = h/6 [A1 + 4A2 + A3] where A1, A2, and A3 are the areas of succeeding contours and h is the vertical distance between two alternate contours.
It can be preferably used where three consecutive sections at equal height are taken.

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