Page 71 - Water and wastewater engineering

2-14 WATER AND WASTEWATER ENGINEERING

TABLE 2-8
Tabulated computations of annual minima for Troublesome Creek at Nosleep
3
3
Year Annual minima, m /s Rank Return period, years 5% of Minima, m /s
1992 0.67 1 22.00 0.034
1997 0.85 2 11.00 0.043
1988 1.08 3 7.33 0.054
1995 1.13 4 5.50 0.057
1990 1.15 5 4.40 0.058
1996 1.42 6 3.67 0.071
2000 1.42 7 3.14 0.071
1991 1.46 8 2.75 0.073
2004 1.49 9 2.44 0.075
1994 1.56 10 2.20 0.078
2001 1.61 11 2.00 0.081
2002 1.64 12 1.83 0.082
2005 1.69 13 1.69 0.085
1989 1.83 14 1.57 0.092
2003 1.90 15 1.47 0.099
1998 1.98 16 1.38 0.099
2007 2.15 17 1.29 0.108
2006 2.35 18 1.22 0.118
1993 2.38 19 1.16 0.119
1999 2.92 20 1.10 0.146
2008 3.14 21 1.05 0.157

theoretical relationship between an annual series and partial series is shown in Table 2-9 . The
partial series is approximately equal to the annual series for return periods greater than 10 years
(Langbein, 1949).
If the time period over which the event occurs is also taken into account, the analysis is
termed a partial-duration series. While it is fairly easy to define a flood as any flow that exceeds
the capacity of the drainage system, in order to properly define a drought, one must specify the
low flow and its duration. For example, if a roadway is covered with water for 10 minutes, one
can say that it is flooded. In contrast, if the flow in a river is below our demand for 10 minutes,
one certainly would not declare it a drought! Thus, a partial-duration series is particularly rel-
evant for low-flow conditions.

Low-Flow Duration. From an environmental engineering point of view, three low-flow
durations are of particular interest. The 10-year return period of seven days of low flow has
been selected by many states as the critical flow for water pollution control. Wastewater treat-
ment plants must be designed to provide sufficient treatment to allow effluent discharge without
driving the quality of the receiving stream below the standard when the dilution capacity of the
stream is at a 10-year low.
A longer duration low flow and longer return period are selected for water supply. In the
Midwest, drought durations of 1 to 5 years and return periods of 25 to 50 years are used in the