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Example 4. Zero-Flood Years
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.
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.
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.
A Bulletin 17 and General Frequency analysis have been developed for this example and can be viewed in HEC-SSP; however, this manual 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 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.
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 of 0.05 (5 percent chance exceedance) and 0.95 (95% chance exceedance). 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.
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.
As shown in Figure 5, the Frequency Curve table contains the following results:
- Percent Chance Exceedance
- Computed Curve (Log-Pearson III results)
- Expected Probability Curve
- Confidence Limits (5% and 95% chance exceedance curves)
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.
The tabular and graphical results can be sent to the printer or the clipboard for transfer into another software application. To print the tabular results, select Print from the bottom of the analysis window. To copy the tabular results to the clipboard, highlight the desired data and then press the CTRL+C keyboard sequence. To print the graphical results, first bring up the graphical plot and then select Print from the File menu. To copy the graphic to the clipboard, select Copy to Clipboard from the File menu.
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.