Page 72 - Water and wastewater engineering

GENERAL WATER SUPPLY DESIGN CONSIDERATIONS 2-15

TABLE 2-9
Theoretical relationship between partial
series and annual series return periods in years
Partial series Annual series
0.5 1.18
1.0 1.58
1.45 2.08
2.0 2.54
5.0 5.52
10.0 10.5
50.0 50.5
100.0 100.5
Source: W. B. Langbein, “Annual Floods and Partial Duration
Series,” Transactions of the American Geophysical Union,
vol. 30, pp. 879–881, 1949.

design of water-supply reservoirs. Where water supply is by direct draft (withdrawal) from a
river, the duration selected may be on the order of 30 to 90 days with a 10-year return period.
When alternate sources are available, shorter return periods may be acceptable.

When to Use Which Series. The probability of occurrence (l/ T ) computed from an annual
series will not be the same as that found from a complete series. There are many reasons for this
difference. Among the most obvious is the fact that, in computing an annual series, 1/12 of the
data is treated as if it were all of the data when, in fact, it is not even a representative sample. It is
only the extreme end of the possible range of values.
The following guidelines can be used to decide when to use which analysis:

1. Use a complete series to determine the long-time reliability (yield) for water supply.
2. Use an annual minima series to determine the need for storage.
3. Use a partial-duration series to predict low-flow conditions.

In practice the complete-series analysis can be performed to decide whether or not it is worth
doing a partial series for water supply. If the complete series indicates that the mean monthly
flow will not supply the demand, then computation of an annual minima series to determine the
need for storage is not worth the trouble, since it would be impossible to store enough water to
meet the demand.

Volume of Reservoirs. The techniques for determining the storage volume required for a res-
ervoir are dependent both on the size and use of the reservoir. This discussion is limited to the
simplest procedure, which is quite satisfactory for small water-supply impoundments. * It is called
the mass diagram or Rippl method (Rippl, 1883). The main disadvantage of the Rippl method is

*It is also useful for sizing storm-water retention ponds and wastewater equalization basins.