Now, you will repeat the previous steps to fit an LPIII distribution to the Mixed Snow and Rain-Only AMS.  First, 

Create a Mixed-Snow Bulletin 17 Analysis

  • Select Analysis | New | Bulletin 17 Flow Frequency.
  • Name the new analysis “MixedSnow_B17C” and add an adequate description.
  • Select the Mixed Snow AMS data set.
  • Ensure the 17C EMA method is selected within the Method for Computing Statistics and Confidence Limits panel.
  • 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).
  • 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.
  • Click Compute.
  • 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.

MixedSnow_B17C Analysis Plot

  • This curve more accurate portrays the behavior of rare mixed-snow floods, especially for rare AEP.

Create a Rain-Only Bulletin 17 Analysis

  • Select Analysis | New | Bulletin 17 Flow Frequency.
  • Name the new analysis “RainOnly_B17C” and add an adequate description.
  • Select the Rain-Only AMS data set.
  • Ensure the 17C EMA method is selected within the Method for Computing Statistics and Confidence Limits panel.
  • 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).
  • 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.
  • Click Compute.
  • 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.

RainOnly_B17C Analysis Plot

  • This curve more accurate portrays the behavior of rare rain-only floods, especially for rare AEP.

Create a TC/TSR General Frequency Analysis

  • Select Analysis | New | General Frequency.

TC/TSR floods do not occur each year within the Bald Eagle Creek watershed.  They are infrequent but important contributors to the overall assessment of flood frequency.  In order to accurately portray the expected flood frequency behavior of this flood mechanism, an empirical distribution should be fit to the available TC/TSR data.

  • Name the new analysis “TC_TSR_GenFreq” and add an adequate description.
  • Select the TC_TSR AMS data set.
  • Ensure the Graphical/Empirical method is selected within the Distribution panel.  All other General tab options should remain unchanged.
  • Move to the Options tab.  Enter the following ordinates within the Output Frequency Ordinates panel:


In addition to right-clicking within the table and selecting Insert Rows, you may Copy/Paste the desired ordinates.


AEP (%)
0.01
0.02
0.05
0.075
0.1
0.2
0.5
1
2
5
10
  • Move to the Graphical tab. 
    • Select the Flow option within the Data Type panel.
    • Within the User-Defined Graphical Curve panel, enter the following ordinates:

Copy/Paste the desired values within the User-Defined Graphical Curve panel.

AEP (%)Flow (cfs)
0.0190,000
0.0275,000
0.0561,000
0.07555,000
0.150,000
0.240,500
0.530,500
124,500
215,500
56,800
103,000
  • The Graphical tab should resemble the following figure:

TC_TSR_GenFreq Graphical Tab

  • Click Compute.
  • This curve more accurate portrays the behavior of rare TC/TSR floods, especially for rare AEP.
  • Click the Save button () to save the study.

Continue to Task 4. Create a Mixed Population Analysis and Create a Combined Flood Frequency Curve with Uncertainty.