In the SSP_Examples.ssp study, the FFA Test 6 Bulletin 17 example analysis demonstrates how to use historic information to improve a flow frequency analysis. A historic flood peak of 15,000 cfs which occurred in 1843 is included in the analysis. This value is the highest known value up to the present time (1974 for this example), even though the systematic record stopped in 1955.

This example uses Bulletin 17B procedures (Interagency Advisory Committee on Water Data, 1982).  Current Federal flood frequency guidance directs analysts to use Bulletin 17C procedures (England, et al., 2019).  Bulletin 17C examples can be found here.

The data used for the FFA Test 6 example is from Ridley Creek in Moylan, Pennsylvania. The period of record used for this example is from 1932 to 1955. To view the data in HEC-SSP, under the Data folder in the Study Pane, right-click on the data record labeled "Ridley Creek-Moylan, PA" and select Tabulate. The data will appear as shown in Figure 1.

Figure 1. Tabulation of the Peak Flow Data for Ridley Creek

To plot the data for this example, right-click on the data record and then select Plot. A plot of the data will appear as shown in Figure 2.

Figure 2. Plot of the Ridley Creek Data

A Bulletin 17 and General Frequency analysis have been developed for this example; however, this example only displays the results of the Bulletin 17 analysis.  To open the Bulletin 17 Editor for FFA Test 6, from the Study Pane under the Bulletin 17 Analysis folder, double-click on the FFA Test 6 analysis. Alternatively, from the Analysis menu select Open and then select FFA Test 6 from the list of available analyses in the Open Analysis dialog box. When FFA Test 6 is opened, the Bulletin 17 Editor - FFA Test 6 will appear as shown in Figure 3.

Figure 3. Bulletin 17 Analysis Editor for FFA Test 6

Shown in Figure 3 are the settings entered in the General tab that were used to perform this frequency analysis. As shown, the Generalized 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 Skew MSE. This selection requires the user to either look up a value from the generalized skew map of the United States, which is provided with Bulletin 17B, or develop a value from a regional analysis of nearby gages. In this example a value of 0.4 was taken from the generalized skew map of the U.S. from Bulletin 17B. Bulletin 17B suggests using a Regional Skew MSE of 0.302 whenever regional skew values are taken from the map.

The recommended procedure for estimating regional skew in Bulletin 17C is the Bayesian generalized least squares (B-GLS) method. Regional skew studies are available for many states through the Advisory Committee on Water Information, Subcommittee on Hydrology, Hydrologic Frequency Analysis Working Group website. Regional skew values presented in these reports supersede the values from the Bulletin 17B generalized skew map.

Also for this example, the Expected Probability Curve option was selected to be computed in addition to the Log Pearson III computed curve. The Median plotting position method was selected, as well as the default Confidence Limits.

Shown in Figure 4 is the Bulletin 17 Editor with the Options tab selected.  As shown in Figure 4, the Historic Period Data option has been selected to reflect a historical flood event of 15,000 cfs in 1843 and an analysis period from 1843 to 1974. Historic data is used to account for historic flood events large enough to be relevant to the analysis and not contained in the systematic data record. The additional information provided by historic data can improve the flood frequency analysis, especially when the data collection period for a given area is relatively short. Information for a Historic Flood Peak has been entered to account for a peak flow of 15,000 cfs in the 1843 water year. The Historic Period Start Year has been left blank. By default this value will be the earliest year found in the historic flood peak data or the systematic record. Therefore for this example, 1843 will automatically be used for the Start Year of the Historic Period. An End Year of 1974 has been entered. The systematic record for the gage ended in 1955, however when this analysis was performed in 1974, no other flood peaks of consequence had been observed between 1955 and 1974. Therefore, 1974 is set as the End Year for the historic period analysis.  Other features on the Options tab include the Low Outlier Threshold and the option to override the default Frequency Ordinates. Neither option is selected in this example.

Figure 4. Bulletin 17 Analysis Editor with Options Tab Shown for FFA Test 6

Once all of the General and Optional 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 from the analysis window. The analysis window should resemble Figure 5.

Figure 5. Bulletin 17 Editor with the Results Tab Selected for FFA Test 6

As shown in Figure 5, the Frequency Curve table contains the following results:

  • Percent Chance Exceedance
  • Computed Curve
  • Expected Probability Curve
  • Confidence Limits

On the bottom left-hand side of the Tabular Results tab is the Distribution Parameters table of Statistics for the observed station data (mean, standard deviation, station skew) and regional adjustment (regional skew, weighted skew, and adopted skew). Also on the bottom right-hand side side of the Tabular Results tab is the number of Events table showing the number of historic events used in the analysis, number of high outliers found, number of low outliers, number of zero or missing data years, number of systematic events in the gage record, and the historic record length (if historic data was entered).  The FFA Test 6 example reports one historical flood event, and a historical period of 132 years, between 1843 and 1974. The reported statistics reflect the use of the historical data adjustment outlined in Bulletin 17B Appendix 6. The report file (described below) shows the initial computation of the statistics and frequency curve before the historical data was used, and the resulting statistics and frequency curve after the historical data is taken into account.

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. A plot of the results for this example is shown in Figure 6.

Figure 6. Plot of the Frequency Curve Results for FFA Test 6

In addition to the tabular and graphical results, there is a report file that shows the order in which the calculations were performed. To review the report file, press the View Report button at the bottom of the Bulletin 17 Editor (Figure 5). When this button is selected a text viewer will open the report file and display it on the screen. Shown in Figure 7 is the report file for FFA Test 6.  The report file contains a listing of the input data, preliminary results, outlier and historical data tests, additional calculations needed, and the final frequency curve results. Different types and amounts of information will show up in the report file depending on the data and the options that have been selected for the analysis.

Figure 7. Report File for FFA Test 6