Analytical Frequency Function

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. However, synthetically-derived flow-frequency functions in 1.4.3 can be converted to flow-frequency functions in 2.0 by importing the ProbData.dbf file from the 1.4.3 study database into the 2.0 study. 

Steps to Creating Analytical Frequency Functions in HEC-FDA

To create an analytical frequency function, from the Study Tree right-click on Frequency Functions and select the create new option. Enter the name for the function and a description (optional, but recommended). Select Analytical, and provide the mean, standard deviation, skew, and equivalent record length. Click Save and Close to create the new function.

To edit the created frequency function, from the Study Tree right-click on the created function and click Edit Frequency Function.