In the SSP_Examples.ssp study, the input data for the FFA Test 4 Bulletin 17 analysis is the same as that for Example 4 in Appendix 12, Guidelines for Determining Flood Flow Frequency, Water Resources Council (WRC) Bulletin 17B.  FFA Test 4 illustrates the application to data that includes several zero flow years.

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 for this example is from Orestimba Creek in Newman, California. The period of record used for this example is from 1932 to 1973. To view the data in HEC-SSP, under the Data folder in the Study Pane, right-click on the data record labeled "Orestimba Creek-Newman, CA" and select Tabulate. The data will appear as shown in Figure 1.

Figure 1. Tabulation of the Peak Flow Data for Orestimba 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. The years with peak flows measuring zero are visible.

Figure 2. Plot of Orestimba Creek Data

A Bulletin 17 and General Frequency analysis have been developed for this example and can be viewed in HEC-SSP; however, this example only displays the results of the Bulletin 17 analysis. To open the Bulletin 17 Editor for FFA Test 4, from the Study Pane under the Bulletin 17 Analysis folder, double-click on the FFA Test 4 analysis. Alternatively, from the Analysis menu select Open and then select FFA Test 4 from the list of available analyses in the Open Analysis dialog box. When FFA Test 4 is opened, the Bulletin 17 Editor - FFA Test 4 will appear as shown in Figure 3.

Figure 3. Bulletin 17 Analysis Editor with FFA Test 4 Data Set

Figure 3 displays 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 -0.3 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, none of the available options for modifying the frequency curve were selected for this test example. These options include the Low Outlier Threshold and Historic Period Data. Additionally, the option to override the default Frequency Ordinates was not selected.

Figure 4. Bulletin 17 Editor with the Options Tab Selected for FFA Test 4

Once all of the General and Optional settings are set or selected, the user can press the Compute button (from any tab) 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 5.

Figure 5. Bulletin 17 Editor with the Results Tab Selected for FFA Test 4

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 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).  As noted earlier, there were six zero values in this record, and also a low outlier. A zero value causes difficulty because the first step in fitting a Log Pearson III distribution is computing the base-10 log of each flow value, which is undefined for zero. Bulletin 17B recommends removing the zero values (and the low outlier) from the systematic record to compute a preliminary frequency curve, and then adjusting that curve with the Conditional Probability Adjustment. The final frequency curve and statistics are shown in the table, and the preliminary calculations can be reviewed in the report file.

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 FFA Test 4 Results

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). The View Report button opens a text viewer window displaying the report file (e.g., FFA_Test_4.rpt). Shown in Figure 7 is the report file for FFA Test 4.  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 4