Analytical frequency functions are used for flow-frequency functions only.
Analytical Approaches
There is one approach to creating an analytical frequency function in HEC-FDA Version 2.0: "Parameter Entry" creates the frequency function based on the first three moments of the estimated Log Pearson Type III (LP3) distribution (mean, standard deviation, and skew). The equivalent record length is required to reflect the uncertainty in the relationship. The "Fit to Flows" approach is not recommended to be used because HEC-FDA does not provide as elaborate of a distribution as other programs, such as HEC-SSP
Where Can I Find Flow Data for the Analytical Approach?
Hydrologic engineers can use HEC-HMS, HEC-SSP, or some other hydrologic statistics software to estimate the three moments needed to create a Log Pearson Type III distribution.
Working with Flow Data for the Analytical Approach in HEC-FDA
Differences Between HEC-FDA Versions 1 and 2
There are two differences between HEC-FDA Versions 1 and 2. First, the new procedure in HEC-FDA Version 2.0 for quantifying the uncertainty about flow-frequency is consistent with USGS Bulletin 17C - HEC-FDA Version 1.4.3 used USGS Bulletin 17B procedures. Second, synthetically deriving the LP3 parameters will not be supported for new HEC-FDA studies.
Steps to Creating Analytical Frequency Functions in HEC-FDA
Right click on Frequency Functions in the Study Tree and select Create New Frequency Function...
Enter a name, a description, and the equivalent record length
Choose Analytical as the type of frequency function
Define the frequency function using "Parameter Entry": enter the mean, standard deviation, and skew
Click Save and Close. See below image of finished Analytical Frequency Function.
