Power Plant Engineering

Table 11.2
Month
Mean monthly
Accumulative
Month
Mean monthly Accumulative
flow
flow second
flow
flow second
second-metre
metre-month
second-metre
metre-month
April
7710
7710
October
3300
23280
May
3850
11560
November
2330
25610
June
860
12420
December
3880
29490
July
1040
13460
January
5200
34690
August
2500
15960
February
6000
40690
September
4020
19980
March
12,100
52790
Let the flow demand be, 3000 sec-meter. Then the line X-Y may be called as `demand line' or
‘Use line’. If during a particular period, the slope of the mass Curve is greater than that of the demand
line, it means more water is flowing into the reservoir than is being utilized, so the level of water in the
reservoir will be increasing during that period and vice versa. Upto point X and beyond point Y the
reservoir will be overflowing. being full at both X and Y.
The capacity of the reservoir is given by the maximum ordinate between the mass curve and
the demand line. For the portion of mass curve between point X and Y, the storage capacity is about 4600

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348
POWER PLANT ENGINEERING
sec-meter-month. However, considering the entire mass curve, storage capacity will be about 15,400
sec-meter-months.
Note: The amount of water to be stored in a reservoir can also be determined with the help of hydrograph. A
horizontal line giving the average flow demand is drawn. The portion of the curve above this line will indicate the amount
of water that should be stored in the reservoir to be used during lean months. But, hydrograph for one year varies from year
to year, the storage capacity should be determined with the help of the mass curve.
56,000
48,000
40,000
32,000
24,000
16,000
8,000
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Time, Months
S
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age
Mass Curve
X
Y