Download page Task 5. Create a Mixed Population Analysis Using the Summer and Winter B17C Results.
Task 5. Create a Mixed Population Analysis Using the Summer and Winter B17C Results
Now that you have fit an LPIII distribution to the summer and winter annual maximum series, you will create a Mixed Population analysis to create a flow-frequency curve using total probability theorem.
Select Analysis | New | Mixed Population Analysis.
Name the new analysis “MPA_imported_curves” and add an adequate description.
Check the boxes for Data Label and Data Units.
Add “Flow” and “cfs” within the label and units entry fields, respectively.
The General tab should resemble the following figure.
Move to the Frequency Curves
Change the name of Frequency Curve 1 and Frequency Curve 2 to Summer and Winter, respectively.
Select the Use Existing Analysis radio buttons for both Frequency Curves.
Using the drop down menus, select the Summer_Peaks_B17C and Winter_Peaks_B17C analyses for the appropriate Frequency Curves.
Click the Import button above the flow-frequency table for both Frequency Curves.
The Frequency Curves tab should resemble the following figure.
Clicking Plot Input FrequencyCurves will result in a plot that will resemble the following figure.
Close the plot.
Click the Compute button.
Move to the Results tab. The Results tab should resemble the following figure.
Question 2: Why do the results not include a 1-, 0.5-, and 0.2-percent chance exceedance flow? Hint: how would they be computed?
When using either imported results from an Existing Analysis or Manual Entry, the Mixed Population compute will not extrapolate input data unless told to. In this case, the Winter curve does not contain enough flow-frequency information to compute the 1.0-, 0.5-, and 0.2-percent chance exceedance flow rates.
Go to the General tab.
Click the radio button to Allow Extrapolation of Input Data.
Click Compute at the bottom of the editor to compute the Mixed Population analysis.
Move to the Results tab. Notice that the results now include ordinates for the 1-, 0.5-, and 0.2-percent chance exceedances, as shown in the following figure.
Now that you’ve computed a mixed population flow frequency curve using imported data from another analysis, you will create a new Mixed Population Analysis that uses the analytical distributions and parameters instead of the results.
Right click on the "MPA_imported_curves" analysis and select Save As....
Enter "MPA_analytical_curves" as the new Name and enter a description.
Click the OK button.
Open the new analysis and check that the inputs on the General tab are the same as the previous analysis.
Select the Frequency Curves tab.
Select the Analytical Distribution radio buttons for both Frequency Curves.
Ensure that the distribution drop down selection is set to Log Pearson type III.
Enter the LPIII parameters from each analysis within the Mean, Standard Deviation, and Skew entry field for both Frequency Curves.
Click the Compute button above the flow-frequency table for both Frequency Curves, as shown in the following figure.
The Frequency Curves tab should resemble the following figure.
Click Compute at the bottom of the editor to compute the Mixed Population analysis.
Move to the Results tab.
Notice that the results now include a complete mixed population flow-frequency curve without using the extrapolation option on the General tab. This is due to the use of the LPIII analytical distribution and input parameters. HEC-SSP is able to extrapolate the Summer and Winter flow-frequency curves for a complete probability range of (1, ~0). If you wish, you can compare the results of both Mixed Population Analyses graphically within HEC-SSP.
Hold the Ctrl key and select both analyses within the Study Explorer under Mixed Population Analysis.
Click Results | Graph.
You should see a plot of the MPA_imported_curves and MPA_analytical_curves analyses within the same plot. Click on the legend items to better identify which line style corresponds to which curve.
