The user can view output from the flow frequency analysis directly from the Bulletin 17 Editor. The output consists of tabular results, a frequency curve plot, and a report documenting the data and computations performed.

Tabular Output

Once the computations for the flow frequency analysis are completed, the user can view tabular output by opening the Tabular Results tab. When this tab is pressed, the results will be displayed as shown in Figure 1.
Figure 1. Bulletin 17 Editor with Tabular Results Tab Active.

Output on the results tab consists of three tables: Frequency Curves, Distribution Parameters, and Events. The Frequency Curve output table contains the percent chance exceedance ordinates, the computed Log Pearson III frequency curve quantiles, the expected probability adjusted frequency curve quantiles (if selected on the General tab), the variance of each quantile (if using 17C EMA), and the computed confidence limits (0.05 and 0.95, by default). Data in the frequency curve table can be sorted in either ascending or descending order by clicking in the column header (two mouse clicks are required the first time). The percent chance exceedance ordinates and frequency curves will sort so that the lowest values are on top or the highest values are on top.

The Distribution Parameters table contains (in log space) the mean, standard deviation, and station skew of the data, the user entered regional skew (if entered), weighted skew (weighted between station skew and regional skew), and the adopted skew for the analysis. This table also contains the EMA Estimate of MSE (G at site) and Grubbs-Beck Critical Value.

The Events table tabulates the number of historic events, high outliers (not calculated when using 17C EMA), low outliers and zero flows, missing values, systematic events, the number of years in the historic period, and an equivalent record length in years (if using 17C EMA). In an effort to be conservative, the equivalent record length is computed as the number of systematic events plus the number of historical events minus the number of low outliers (an additional equivalent record length computation is detailed within the Report file).

The tabular results can be printed by using the Print button at the bottom of the Bulletin 17 Editor. When the print button is pressed, a window will appear giving the user options for how they would like the table to be printed.

Graphical Output

Graphical output of the frequency curves can be obtained by pressing the Plot Curve button. When the Plot Curve button is pressed, a frequency curve plot will appear in a separate window as shown in Figure 2. The user can modify the plot properties by selecting the Edit | Plot Properties menu option. A plot properties window will open that lets the user change the line style for each data type, change the axis labels, modify the plot title, and edit other plot properties. The user can also edit line styles by placing the mouse on top of the line or data point in the plot or legend and clicking the right mouse button. Then select the Edit Properties menu option in the shortcut menu. Both the Y and X-axis properties can be edited by placing the mouse on top of axis and clicking the right mouse button. Then select the Edit Properties menu option in the shortcut menu. For example, the user can turn on minor tic marks for the y-axis and modify the minimum and maximum scale for the x-axis.

Figure 2. Example Frequency Curve Plot.

The frequency curve plot can be sent to the printer by selecting the Print option from the File menu at the top of the window. Additional printing options available from the File menu are Page Setup, Print Preview, and Print Multiple (used for printing multiple graphs on the same page). The graphic can also be sent to the Windows Clipboard by selecting Copy to Clipboard from the File menu. Additionally, the plot can be saved to a file by selecting the Save As option from the File menu. When the Save As option is selected, a window will appear allowing the user to select a directory, enter a filename, and select the format for saving the file. Currently, four file formats are available for saving the graphic to disk, windows metafile, postscript, JPEG, and portable network graphic.

The data contained within the plot can also be tabulated by selecting Tabulate from the File menu. When this option is selected, a separate window will appear with the data tabulated. Additional options are available from the File menu for saving the graphics options as a template (Save Template) and applying previously saved templates to the current graphic (Apply Template).

The Edit menu on the graphic window contains several options for customizing the graphic. These options include Plot Properties, Configure Plot Layout, Default Line Styles, and Default Plot Properties. Also, a shortcut menu will appear with further customizing options when the user right-clicks on a line on the graph or the legend.

The graphic customizing capabilities within HEC-SSP are very powerful, but are also somewhat complex to use. The code used in developing the plots in HEC-SSP is the same code that is used for developing graphics in HEC-DSSVue and several other HEC software programs. Please refer to the HEC-DSSVue User's Manual for details on customizing plots.

Viewing the Report File

When the Bulletin 17 computations are performed, a report file documenting the statistical computations is created. The report file lists all of the input data and user settings, plotting positions of the data points, intermediate results, each of the various statistical tests performed (i.e. high and low outliers, historical data, etc…), and the final results. This file is often useful for understanding how the software arrived at the final frequency curve.

Press the View Report button at the bottom of the Bulletin 17 Analysis editor to view the report file. When this button is pressed a window will appear containing the report as shown in Figure 3.

Figure 3. Example of the Bulletin 17 Report File.

Plots, tables and reports can also be created by selecting menu options from the Results menu. At least one Bulletin 17 Analysis must be selected in the study explorer before selecting one of the menu options on the Results menu. Results from multiple analyses are combined in one graph if they are selected in the study explorer when the Graph menu option is selected. The Results | Summary Report menu option will create a summary table of statistics and frequency curve ordinates for the selected analyses as shown in Figure 4.

Figure 4. Summary Table for Selected Bulletin 17 Analyses.