The peak factor is a characteristic of the Unit Hydrograph used in the runoff calculations. HydroCAD includes the standard SCS UH (with a peak factor or 484) and the Delmarva UH (PF=284).
Use of other peak factors is very rare, but can be accomplished by providing a custom unit hydrograph table. For details, click the Windows Start menu and select
Programs | HydroCAD | Unit Hydrograph Info.
Note that you cannot specify the peak factor directly. You must provide a complete unit hydrograph with the desired peak factor. Any regulations that specify a special peak factor will hopefully provide full details on the required UH curve.
Any agency that specifies a non-standard peak factor should also provide a complete UH table. The table is an integral part of the runoff hydrograph generation performed by HydroCAD, TR-20, or other programs that implement the SCS-UH procedure.
I would check the regulations carefully and see if they give any references for the UH data. If not, the regulations are incomplete, and you will need to request additional information from the agency.
However, I need a precise citation for the data. Is this from SFWMD? What publication? Date published? etc. We have to provide specific references for any data we distribute.
A preliminary evaluation of the values posted above indicates that the "323" data actually has a peak factor of 406. Without an authoratative source, this may be preliminary data that should not be relied on.
The "484" data has an actual peak factor of 475. And the data is clearly different than the standard SCS 484 UH. And why not just use the standard SCS UH? Again, we need to know more about the source of this information.
By way of example, SFWMD does have good published data for a 256 UH at
It should be noted that the derived peak factor for the published 300 table is actually 388. (You will see this value when you select the table in HydroCAD.) However, the file does contain the exact published data, so it should be consistent with other calculations based on the same table.
Also note that the Georgia 484 table is slightly different than the standard SCS 484 table. (The Georgia 484 baseline is four times the time-to-peak, while the SCS 484 baseline is five time the time-to-peak. The PF is also slightly different.)
For PF=323 there is no published table, and we must rely on equation 2.1.15 from the Georgia Stormwater Manual. However, this requires an "X" factor which is listed only for PF=484 and PF=300. There are no specifics at all for PF=323, which begs the question, why are you being asked to design for a peak factor that isn't mentioned in the Manual? Yes, you can create a custom UH file based on the data you received from the "City Engineer". If you would send an email to support@hydrocad.net we can send you a preliminary file. However, I would like to have more documentation before we publish this file for general use.
The subject regulatory agency typically prefers software such as ICPR, Haestad Methods (such as PondPack), or XP-SWMM due to the extensive modeling capabilities of interconnected ponds, and due to the fact that those softwares are approved by FEMA for conducting flood insurance studies. These softwares include various standard shape factors, including 323. The preference for the 323 shape factor is based on ICPR guidelines and training, and the range of shape factors therein is based on research by Hal Wilkening of the St. Johns River Water Management District in Florida. It is understood that 323 is also preferred by the City of Savannah, GA and regulatory agencies in southwest Florida which have similar topography to that of the subject regulatory agency.
The table for 323 provided is acceptable to the regulatory agency but may or may not be recommended for publication and general use. The table was derived based on the Georgia Stormwater Management Manual, and the corresponding X factor was interpolated to be 1.79. If it can be determined by HydroCAD that the 323 data provided may be inaccurate or not acceptable for general use, then HydroCAD should be able produce a 323 table that does accurately represent such. A review of the previously mentioned software may prove to be beneficial.
The subject regulatory agency does not plan to conduct any further hydrology research on the subject and does not feel obligated to adjust its regulations to accommodate all stormwater software or the shortcomings therein.
Either the 323 shape factor should be accommodated within HydroCAD via a custom unit hydrograph table file, or justification for the use of a different shape factor already supported by HydroCAD should be provided to the agency for consideration.
HydroCAD can accommodate any unit hydrograph for which tabular UH data is available. Although most applications will use the standard SCS or Delmarva unit hydrographs, HydroCAD will easily accommodate custom UH tables, in much the same manner as TR-20 or other applications. (Given the same input data, HydroCAD runoff calculations will match TR-20 results to within 1%, while also adding the ability to model interconnected ponds and many other situations.)
While any given unit hydrograph will have a "built-in" peak factor, a given peak factor does not uniquely define the unit hydrograph. Since there can be an infinite number of different unit hydrographs with the same peak factor, it is necessary to specify the entire table directly, and not just the peak factor. While some unit hydrographs may be approximated (or generated) by equations (as in the Georgia Stormwater Manual) there is no standardization to these procedures, hence the need to provide a specific table in each situation.
For any unit hydrograph, a dimensionless "shape factor" (K) can be readily calculated, equal to twice the fraction of the UH volume that occurs before the peak. This value is commonly expressed as a peak factor, given by:
PF = C K
Where C is a combined units conversion factor. For traditional SCS calculations, C=645.33, so a K value of 0.75 (for the standard SCS UH) yields a peak factor of 484.
Applying this calculation to the "323" unit hydrograph listed in a previous post gives an "actual" peak factor of 406, hence my questions about the origin of this data.
