Example 6 illustrates the computation of a peak flow frequency curve using EMA and Bulletin 17C procedures with an annual maximum series comprised of systematic data and historical flood information that can be used to extend the historical period. For this example, USGS gage 09480000 Santa Cruz River near Lochiel, AZ is used. The stream gage in question has an approximate 82.2 sq. mi. contributing watershed with an annual peak record consisting of 65 peaks beginning in water year 1949 and ending in water year 2013. A large flood event occurred on October 9, 1977. This flood is noted in the USGS Annual Water Data Report for this gage and there is historical information available for this large flood indicating that this flood is the largest since 1927 (Aldridge & Eychaner, 1984). Of the 65 annual peaks, the August 15, 1984 flood is equal to the October 1977 historic flood peak. Based on information contained within Aldridge and Eychaner (1984) for the October 1977 flood, this event can be used as a perception threshold to represent the 22 years of missing information from 1927 to 1946. The annual maximum series is plotted in Figure 1 and tabulated in Table 1.

Figure 1. Santa Cruz River at Lochiel, AZ Annual Peak Flow Record.
Table 1. Santa Cruz River at Lochiel, AZ Annual Peak Flow Record.

Date

Flow (cfs)

13 Sep 1949

1650

30 Jul 1950

4520

02 Aug 1951

2560

16 Aug 1952

550

14 Jul 1953

3320

22 Jul 1954

1570

06 Aug 1955

4300

17 Jul 1956

1360

09 Aug 1957

688

07 Aug 1958

380

14 Aug 1959

243

30 Jul 1960

625

08 Aug 1961

1120

29 Jul 1962

8

25 Aug 1963

2390

09 Sep 1964

2330

12 Sep 1965

4810

18 Aug 1966

1780

03 Aug 1967

1870

20 Dec 1967

986

05 Aug 1969

484

03 Aug 1970

880

10 Aug 1971

2830

16 Jul 1972

2070

30 Jun 1973

1490

04 Aug 1974

1730

22 Jul 1975

3330

22 Jul 1976

3540

05 Sep 1977

1130

09 Oct 1977

12000

25 Jan 1979

1060

30 Jun 1980

406

15 Jul 1981

1110

11 Aug 1982

2640

04 Mar 1983

1120

15 Aug 1984

12000

19 Jul 1985

850

29 Aug 1986

4210

10 Aug 1987

291

23 Aug 1988

804

04 Aug 1989

871

17 Jul 1990

3510

26 Jul 1991

17

01 Aug 1992

483

18 Jan 1993

4880

30 Aug 1994

478

12 Jul 1995

2020

10 Jul 1996

1860

11 Sep 1997

2970

07 Jul 1998

1110

28 Jul 1999

4870

06 Aug 2000

2240

22 Oct 2000

1080

04 Mar 2002

2

14 Aug 2003

22

05 Aug 2004

256

23 Aug 2005

73

08 Aug 2006

5940

19 Jul 2007

3060

23 Jul 2008

1180

01 Jul 2009

1530

31 Jul 2010

392

13 Aug 2011

95

28 Jul 2012

12

08 Sep 2013

612

A Bulletin 17 Analysis using EMA and Bulletin 17C procedures has been developed for this example. To open the analysis, either double-click on the analysis labeled "B17C Example 6" from the Study Explorer or from the Analysis menu select open, then select "B17C Example 6" from the list of available analyses. When "B17C Example 6" is selected, the Bulletin 17 analysis editor will appear as shown in Figure 2. As shown, the Skew option was set to use the Station Skew.

Figure 2. Bulletin 17 Analysis General Tab for B17C Example 6.
No changes to the Options tab are necessary. The EMA Data tab for this example is shown in Figure 3. This example uses an annual maximum series consisting of systematic data along with historical information. As was previously mentioned, historical information indicates that the October 1977 flood event was the largest since at least 1927. This information can be used within EMA to extend the historical period through the use of a perception threshold. Specifically, the first perception threshold should be modified to start at water year 1927. Then, a second perception threshold of 12000 – inf should be added for the period of missing data spanning water years 1927 through 1948, as shown in Figure 3. Once both perception thresholds have been entered, click the Apply Thresholds button to assign the complementary flow ranges for the periods of missing data. No further changes to the flow range table are necessary.

Figure 3. Bulletin 17 Analysis EMA Data Tab for B17C Example 6.
Once all of the General and EMA Data tab settings are set or selected, the user can press the Compute button to perform the analysis. Once the computations have been completed, a message window will open stating Compute Complete. Close this window and then select the Tabular Results tab. The analysis window should resemble Figure 4.

Figure 4. Bulletin 17 Analysis Tabular Results Tab for B17C Example 6.
In addition to the tabular results, a graphical plot of the computed frequency curves can be obtained by pressing the Plot Curve button at the bottom of the analysis window. The Log Pearson Type III distribution fit using EMA to the input annual maximum flow data set, the 5% and 95% confidence limits, and the computed plotting positions are shown in Figure 5.

Figure 5. Plotted Frequency Curves for B17C Example 6.
As shown in Figure 4, the Multiple Grubbs-Beck Test identified a low outlier threshold (critical value) of 380 cfs, which corresponds to the smallest peak that was retained. Ten annual peak flows were identified and censored, as shown in Figure 6. These annual peak flows were then recoded to have a flow interval of zero – 380. The perception thresholds for the years in which an annual peak flow was recorded were also adjusted to correspond with the Multiple Grubbs-Beck Test low outlier threshold (critical value). Consequently, these perception thresholds were changed from zero – inf to 380 – inf. The perception threshold for the years in which there is only historical nonexceedance data (i.e. 1927 – 1948) were left at 12,000 - inf, as shown in Figure 7.

Figure 6. Report File for B17C Example 6 showing censored low outliers.

Figure 7. Report File for B17C Example 6 showing recoded perception thresholds.