Example 10 exhibits the use of a regional skew coefficient and MSE with systematic and historical data. The regional skew and MSE were estimated for this location in conjunction with several other large subbasins within the Delaware River watershed (Goldman & Konieczki, 2009).

Examples of additional regional skew studies can be found through online searches, from the U.S. Army Corps of Engineers, and USGS, amongst others. Several regional skew studies have been compiled and made available at the following location: https://acwi.gov/hydrology/Frequency/b17c/supplementary-materials/reports.html.

The data for this example comes from the USGS gage 01470500 Schuylkill River at Berne, PA. The period of record used for this example contains 70 peaks beginning in 1942 and ending in 2016. A single period of missing data exists within this record. Also, a historic flood event occurred outside the period of gaging record in April 1942. The annual maximum series is plotted in Figure 1 and tabulated in Table 1.

Figure 1. Schuylkill River at Berne, PA Annual Peak Flow Record.
Table 1. Schuylkill River at Berne, PA Annual Peak Flow Record.

Date

Flow (cfs)

30 Apr 1942

26900

08 Nov 1947

6500

30 Dec 1948

10400

23 Mar 1950

5430

04 Dec 1950

24200

11 Mar 1952

17000

22 Nov 1952

19900

07 Dec 1953

8290

19 Aug 1955

29400

15 Oct 1955

8580

06 Apr 1957

10700

21 Dec 1957

13100

03 Sep 1959

7710

20 Sep 1960

8290

26 Feb 1961

10400

12 Mar 1962

6010

10 Nov 1962

4720

25 Jan 1964

11700

08 Feb 1965

12000

14 Feb 1966

5850

06 Mar 1967

3220

31 May 1968

8640

02 Aug 1969

15000

02 Apr 1970

17300

13 Feb 1971

15600

22 Jun 1972

42800

08 Nov 1972

8290

21 Dec 1973

15400

26 Sep 1975

9920

26 Jan 1976

18300

09 Oct 1976

12100

26 Jan 1978

12300

25 Jan 1979

21700

21 Mar 1980

8780

11 Feb 1981

8110

03 Feb 1982

6600

16 Apr 1983

25300

14 Dec 1983

20100

12 Feb 1985

7850

15 Mar 1986

13400

13 Sep 1987

13000

19 May 1988

15500

06 May 1989

17600

16 Nov 1989

11500

04 Dec 1990

10000

27 Mar 1992

8280

11 Apr 1993

10500

05 Dec 1993

7210

28 Nov 1994

4820

19 Jan 1996

22000

19 Oct 1996

21000

12 May 1998

4720

16 Sep 1999

18700

22 Mar 2000

9530

17 Dec 2000

14500

15 Apr 2002

3230

21 Jun 2003

11300

18 Sep 2004

24900

28 Nov 2004

12700

28 Jun 2006

39900

17 Nov 2006

19900

05 Mar 2008

15900

12 Dec 2008

9720

14 Mar 2010

4210

08 Sep 2011

23900

18 Sep 2012

7240

21 Dec 2012

7220

16 May 2014

7240

01 Jul 2015

7040

25 Feb 2016

12300

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 10" from the Study Explorer or from the Analysis menu select open, then select "B17C Example 10" from the list of available analyses. When "B17C Example 10" is selected, the Bulletin 17 analysis editor will appear as shown in Figure 2 As shown, the Skew option was set to use the Weighted Skew. To use the weighted skew option, the user must enter a value for the Regional Skew and the Regional MSE. In this example, a regional skew of 0.001 was used along with a Regional Skew MSE of 0.064.

Figure 2 Bulletin 17 Analysis General Tab for B17C Example 10.
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 both systematic data and an historical event in April 1942. Also, the record is broken with a period of missing data spanning 1943 – 1947. Since 17C EMA requires a non zero – inf perception threshold for all periods of missing data, two perception thresholds are required. In this case, the peak discharge estimate associated with the April 1942 event can be used to inform the perception threshold for the period of missing annual peak flow data. The use of a perception threshold of 26000 – inf for this period of missing data implies that had a flood event occurred with a peak flow greater than 26,000 cfs, someone would have measured and recorded it. Since the April 1942 event was not directly measured, the perception threshold should span 1942 – 1947. Once the perception thresholds have been entered as shown in Figure 3, click the Apply Thresholds button to assign the complementary flow ranges for the periods of missing data. Finally, the April 1942 event should be set to the historical data type, as denoted by a USGS peak flow rate qualification code of "7".

Figure 3. Bulletin 17 Analysis EMA Data Tab for B17C Example 10.
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 10.
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 10.
As shown in Figure 6, the 1942 – 1947 period used a perception threshold of 26000 – inf. Since the 1943 – 1947 period was missing, the complementary flow range of 0 – 26000 cfs was assigned and used within the computations.

Figure 6 Report File for B17C Example 10 showing EMA Representation of Data.