Task 2. Fit an LPIII Distribution Using Bulletin 17C techniques to the All-Season Annual Maximum Series

Create an All-Season Bulletin 17 Analysis

Now that you've imported several time series, you will create a Bulletin 17 analysis and fit an LPIII distribution to the all-season AMS.

Select Analysis | New | Bulletin 17 Flow Frequency.
Name the new analysis “Mixed_B17C” and add an adequate description.
Select the All-Season AMS data set.
Ensure the 17C EMA method is selected within the Method for Computing Statistics and Confidence Limits panel.
- This will automatically select other features including the Multiple Grubbs-Beck low outlier test and Hirsch/Stedinger Plotting Positions.
- The General tab should resemble the following figure.

Mixed_B17C Analysis General Tab

Move to the Options tab.
Within the Output Frequency Ordinates panel, enter 4 new ordinates representing the 0.01, 0.02, 0.05, and 0.1% annual exceedance probabilities (AEP), as shown in the following figure.

Right-click within the table and select Insert Rows.

Mixed_B17C Analysis Options Tab

Move to the EMA Data tab.
Ensure that the Flow Ranges table contains a low and high value for every single year in the analysis period. The completed EMA Data tab should resemble the following figure.

Mixed_B17C Analysis EMA Data Tab

Click Plot Curve. This will result in the computed curve, 5- and 95-percent confidence limits, and observed events being plotted. The plot should resemble the following figure.

Examine the right-hand tail of the flood frequency curve. The largest two flood events were caused by TC/TSR (Agnes and Ivan) while the next two largest flood events were caused by Mixed/Rain-on-Snow (January 1996 and December 2010), as shown in the following image:

This disparity in flood mechanism leads to inaccurate estimation of flood frequency for rare AEP, like those required as part of this study.
Click the Save button () to save the study.