Once the analysis name has been entered, the user can begin defining the analysis. Contained on the Curve Combination Analysis editor are four tabs. The tabs are labeled General, Frequency Curves, and Graphical. The first tab contains general settings for performing the curve combination analysis (Figure 1). These settings include:

  • Number of Curves
  • Output Labeling
  • Log Transform
  • Confidence Limits
  • Output Frequency Ordinates

Figure 1. Curve Combination Analysis Frequency Curves Tab.

Number of Curves

This option, shown in Figure 2, lets the user choose the number of frequency curves to combine. The Frequency Curves tab will update based on the number of curves.

Figure 2. Number of Frequency Curves to Combine.

Output Labeling

This option, shown in Figure 3, allows the user to enter labels for data contained in the output tables and plots. The user must enter the name of the data as well as the data units. The output labeling does not result in the conversion of data from one unit system to another; it only affects what is displayed on table headings and the y-axis of the results plot.

Figure 3. Output Labeling Options.

Log Transform

There are two options contained within the Log Transform setting, as shown in Figure 4: Use Log Transform and Do not use Log Transform. This setting only affects the y-axis of subsequent plots. If the user selects Use Log Transform, the y-axis of all plots will use a logarithmic scale (base 10). If the user selects Do Not Use Log Transform, the y-axis of all plots will use a linear scale. This option does not affect any of the computations. The default setting is Do Not Use Log Transform.

Figure 4. Log Transform Options.

Confidence Limits

The computed frequency curve is only an estimate of the probability distribution of the larger parent population. Confidence limits can be used to provide a measure of the uncertainty of the estimated exceedance probability of a selected discharge, stage, precipitation depth, etc or a measure of the uncertainty of the discharge, stage, precipitation depth, etc at a selected exceedance probability.

By default, confidence limits will not be computed within a Curve Combination Analysis. However, the user has the option to compute confidence limits using the order statistics method (Figure 5), which is discussed in ETL 1110-2-537. The Equivalent Years of Record must be defined when using this method.

When computing confidence limits, the default interval will be 90 percent confidence (i.e. the 5% and 95% confidence limits). The confidence limits must be entered in decimal form (i.e. 95% = 0.95, and 5% = 0.05). The user has the option to override the default values and enter whatever values they would like for the confidence limits.

Figure 5. Confidence Limit Options.

Output Frequency Ordinates

This option allows the user to change or add to the frequency ordinates for which the resulting frequency curve and confidence limits are computed (Figure 6). The default values listed in percent chance exceedance are 0.2, 0.5, 1, 2, 5, 10, 20, 50, 80, 90, 95, and 99. Check the box next to Use Values from Table below to change or add additional values. Once this box is checked, the user can add/remove rows and edit the frequency values. To add or remove a row from the table, select the row(s), place the mouse over the highlighted row(s) and click the right mouse button. The shortcut menu contains options to Insert Row(s), Append a Row, and Delete Row(s). The program will use the default values, even if they are not contained in the table, when the Use Values from Table below option is not checked. Finally, all values in the table must be between 0 and 100. Note that these values have no impact on the computed frequency curve, but rather only the values of the curve that are reported.

Figure 6. Output Frequency Ordinate Options.