Page 74 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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2.4 Design Storage 37

Column 2: These are observed flows for the Westfield Little River, near Springfield, MA.
2
2
Column 3: The values 27, 30, 33, and 36 MG/mi 0.89, 1.1, 1.09, and 1.18 MGD/mi ,
2
respectively, for 30.4 day/month. For a total flow of 462 MG/mi in 12 months, the average
2
2
flow is 462>365 1.27 MGD/mi , and for a total draft of 360 MG/mi the development is
100 360>462 78%.
Column 4: Positive values are surpluses; negative values, deficiencies.
Column 5: P 1 is the first peak, and T 1 is the first trough in the range P 1 P 2 , where P 2 is the sec-
ond higher peak; similarly T 2 is the second trough in the range P 2 P 3 .
Column 6: The required maximum storage S m max(P j T j ) P m T m P 1 T 1
2
2
177 1 176 MG/mi in this case. The fact that P 2 T 2 279 103 176 MG/mi also
implies that there is seasonal rather than over-year storage. Storage at the end of month i is
S i min{S m , [S i–1 (Q i D j )]}; for example, in line 2, S m 176 and [S i–1 (Q i D j )] 67 95
162, or S i 162; in line 3, however, S m 176 and [S i–1 (Q i D j )] 162 95 257 or S i
S m 176.
Column 7: The flow wasted W i max{0, [(Q i D i ) (S m S i-1 )]}; for example, line 3,
(Q i D i ) (S m S i–1 ) 15 (176 162) 1 or W i 1; in line 3 of the second series, how-
ever, (Q i D i ) (S m S i–1 ) 15 (176 176) 15. There is no negative waste.
Solution 2 (SI System):
3
An SI or metric system solution can be obtained using these conversation factors: 1 MG 3,785 m
3
3
3
3.785 ML; 1 ML 1,000 m ; 1 MGD 3,785 m /day 3.785 MLD 0.0438 m /s 43.8 L/s;
2
2
2
2
2
1 mi 2.59 km ; 1 MG/mi 1.461 ML/km ; 1 MGD/mi 1.461 MLD/km 2
For variable drafts and inclusion of varying allowances for evaporation from the water
surface created by the impoundage, the analytical method possesses distinct advantages
over the graphical method. The principal value of the mass diagram method, indeed, is not
for the estimation of storage requirements, but for determining the yield of catchment areas
on which storage reservoirs are already established.
2.4 DESIGN STORAGE
Except for occasional series of dry years and very high-density housing and industrial develop-
ments, seasonal storage generally suffices in the well-watered regions of the United States.
Water is plentiful, stream flows do not vary greatly from year to year, reservoirs generally refill
within the annual hydrologic cycle, and it does not pay to go in for advanced or complete devel-
opment of catchment areas. In semiarid regions, on the other hand, water is scarce, stream flows
fluctuate widely from year to year, runoff of wet years must be conserved for use during dry
years, and it pays to store a large proportion of the mean annual flow. In these circumstances,
operational records of adequate length become important, along with computational aids.
Given a series of storage values for the flows observed or generated statistically, the en-
gineer must decide which value he will use. Will it be the highest on record, or the second,
third, or fourth highest? Obviously, the choice depends on the degree of protection to be af-
forded against water shortage. This must also be considered in terms of drought experience,
which is a function of the length of record examined. To arrive at a reasonable answer and
an economically justifiable storage design, the engineer may resort to (a) a statistical analy-
sis of the arrayed storage values and (b) estimates of the difficulties and costs associated
with shortage in supply. Storage values equaled or exceeded but once in 20, 50, or 100
years, that is, 5%, 2%, and 1% of the years, are often considered. For water supply, Hazen
(1956) suggested employing the 5% value in ordinary circumstances. In other words, design
storage should be adequate to compensate for a drought of a severity not expected to occur