Environmental Engineering FOURTH EDITION

116 ENVIRONMENTAL ENGINEERING

The variation in flow may be so great that even a small demand cannot be met
during dry periods, and storage facilities must be built to save water during wetter
periods. Reservoirs should be large enough to provide dependable supplies. However,
reservoirs are expensive and, if they are unnecessarily large, represent a waste of
community resources.
One method of estimating the proper reservoir size is use of a mass curve to
calculate historical storage requirements and then to calculate risk and cost using
statistics. Historical storage requirements are determined by summing the total flow
in a stream at the location of the proposed reservoir, and plotting the change of total
flow with time. The change of water demand with time is then plotted on the same
curve. The difference between the total water flowing in and the water demanded is
the quantity that the reservoir must hold if the demand is to be met. The method is
illustrated by Example 6.4.

EXAMPLE 6.4. A reservoir is needed to provide a constant flow of 15 cfs. The monthly

stream flow records, in total cubic feet, are

Month JFMAMJJASOND

Million@ ofwater %I 60 70 40 32 20 50 80 10 50 60 80

The storage requirement is calculated by plotting the cumulative stream flow as in

Fig. 6-8. Note that the graph shows 50 million ft3 for January, 60 + 50 = 110 million

ft3 for February, 70 + 110 million ft3 for March, and so on.

The demand for water is constant at 15 cfs, or

ft3 S h days ft3

15 x lo6 - x 3600 - x 24 - x 30 - = 38.8 x lo6 -

S h day month month ’

This constant demand is represented in Fig. 6-8 as a straight line with a slope of

38.8 x lo6 @/month, and is plotted on the curved supply line. Note that the stream flow

in May was lower than the demand, and this was the start of a drought lasting until June.

In July the supply increased until the reservoir could be filled up again, late in August.

During this period the reservoir had to make up the difference between demand and

supply, and the capacity needed for this time was 60 x lo6 ft3. A second drought, from

September to November required 35 x lo6 ft3 of capacity. The municipality therefore

needs areservoir with a capacity of 60 x lo6 ft3 to draw water from throughout the year.

A mass curve like Fig. 6-8 is not very useful if only limited stream flow data are

available. Data for one year yield very little information about long-term variations.

The data in Example 6.4 do not indicate whether the 60 million cfs deficit was the worst

drought in 20 years, or an average annual drought, or occurred during an unusually

wet year.