Randy McCollum | Applications Expert
Starting with the 2020 release of Storm and Sanitary Analysis, Autodesk has made it possible to alter the Peak Rate Factor (PRF) when using TR-20 or TR-55 hydrology. Below is an explanation of what the Peak Rate Factor is and how is can be applied to your workflow.
What is the Peak Rate Factor?
The PRF is a factor applied to the ordinates of the SCS Dimensionless Unit Hydrograph that alters the hydrograph’s shape while maintaining the total volume of runoff. The default PRF (and the set PRF for each version of SSA prior to 2020) is 484. The main driver of this value is the ratio of time to peak over time of recession (Tp/Tr). Applying a peak rate factor of a lower value will extend the value of Tr, while reducing the peak runoff Qp. Conversely, a larger PRF will increase Qp while shortening Tr.
Where Can I Apply the Peak Rate Factor?
Peak rate factors are only available when using TR-20 or TR-55 Hydrology. From the Menu bar, go to Input>Project Options. Next to Hydrology Method, click the drop-down menu and select either TR-20 or TR-55. Click OK to accept and close the dialog.
Now that you are using one of the hydrology methods with an SCS Unit Hydrograph, select a subbasin within your model and open the subbasin properties. The peak rate factor will be in the top-righthand part of the Subbasins dialog window. If this value is greyed-out, it means that your hydrology method in project options is not set correctly.
As of the current version, when importing subbains from GIS data, I have noticed the PRF will be set to 0 (zero) after import. I have tested this with the default PRF of 484 and the results were identical, meaning when the PRF is 0 (zero) SSA will default to 484 for calculations. However, the subbasin report will show 0 (zero) instead of 484. If you are using any of the reports within SSA, you will need to correct the PRF for each subbasin prior to running your analysis and printing the report.