In this article we will discuss about the determination of reservoir capacity with the help of hydrograph and mass-curve.
Determination of Reservoir Capacity with the Help of Hydrograph:
This method is not common in determining the capacity of the reservoir to be constructed. In the method the stream flow data at the site of the reservoir arc determined. In case of large reservoir monthly inflow rates are calculated, whereas in case of small reservoirs the daily inflow data are collected.
In case the exact inflow data at the site of reservoir are not available, the data at other points of the stream or on nearby streams are collected and adjustments are made for the exact site. The quantity of consumption (including losses) is determined for different months of the various years.
Now the quantities of the yield and the consumption (including losses) as calculated above are plotted to the same scale on a common graph as shown in Fig. 3.2. The curves so obtained are called Hydrograph.
If the yield curve is above the consumption curve, it indicates that inflow of water is more than the demand. It indicates surplus water which can be stored.
If the yield curve is below the consumption curve, it indicates the deficit, which is to be met out by storing the surplus water.
The maximum cumulative total deficit is determined from the hydrograph, which is equal to the minimum storage capacity of the reservoir.
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The exposed area of the water surface increases due to construction of the reservoir, which increases the evaporation losses. Sometimes the losses due to the evaporation and seepage are so huge that the every purpose of constructing the reservoir is not fulfilled. Seepage from the reservoir should also be added in the losses while determining the capacity of the reservoir.
Example:
The estimated hourly consumptions of water for a town for one day are given in table 3.2. ‘Determine the capacity of the distribution reservoir if the pump installed can supply the water in the reservoir at a uniform rate of 1.45 cu. m/sec.
Solution:
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Rate of pumping = 1.45 cu. m/sec.
= 1.45 x 60 x 60 x 1000 litres/hour
= 1.45 x 36/10 million litres/hour
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= 5.22 million litres/hour.
Water required from the storage reservoir is determined as shown in Table 3.3. The required capacity of the reservoir is the total sum of the hourly consumption taken from the reservoir as shown in column 4 of the Table 3.3.
Required capacity of the reservoir = 18.32 million litres. Ans.
Determination of Reservoir Capacity with the Help of Mass-Curve:
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The mass curve or consumption and yield can be used to determine the reservoir storage capacity. Fig. 3.3 shows the mass curve of consumption and yield. The demand lines drawn tangent to the high point A.B.C……… of the mass curve, represent the rate of withdrawal from the reservoir. Point P, Q, R. at which the demand line intersecting the mass curve of consumption denotes the points, at which the reservoir is full.
The vertical distance at any point between the demand line and the mass curve of consumption, denotes the water wasted over the spillway. The design of the spillway should be done such that, it should be sufficient to discharge the flood water.
When the total yield is more than the total consumption, the mass curve of yield remains above the mass curve of consumption and vice versa. The storage capacity or the requirement in dry weather will be found by adding the maximum ordinates of loops in the adjoining deficit and excess periods.
If the cumulative quantity of water yield is continuously in excess, both the mass both curves interact each other, which denotes that before this point the yield was less than the estimated consumption. For this period necessary provision of water is to be made.