SCS/NRCS unit hydrograph peak rate factor in design
SCS/NRCS unit hydrograph peak rate factor in design
(OP)
In NEH 630.1604 Page 16-14, it reads that 'This constant(Peak Rate Factor) has been known to vary from about 600 in steep terrain to 300 in very flat, swampy country.” Recent studies have shown the peak rate factor has a much wider range: from below 100 to more than 600. The standard 484 DUH was developed using graphical techniques and not an equation". I guess this is an interesting statement but it is hard to apply it during modeling and design (how steep is steep and how flat is flat? and except slopes, any other factors deserve a consideration?)
I am wondering in engineering practice, have you got involved in developing a SCS UH for rainfall-runoff transformation by using a non-standard peak rate factor (a value not equal to 484 - which is the normal SCS UH transformation method). A peak rate factor of 484 will generate a much larger peak flow rate than using a peak rate factor of 300 and 300 seems more appropriate in flat area according to the statement above from NEH.
I did some searches on google and found an interesting paper by Josha Crowley and Alan W Moore for a flood study in Cameron County, TX, in which they suggest to use a PRF of 200. (hydraulic model calibration on the coastal plan, a case study in Cameron County, Texas, Josha Drowley, and Alan W. Moore). You can review thi article from google book.
I am wondering in engineering practice, have you got involved in developing a SCS UH for rainfall-runoff transformation by using a non-standard peak rate factor (a value not equal to 484 - which is the normal SCS UH transformation method). A peak rate factor of 484 will generate a much larger peak flow rate than using a peak rate factor of 300 and 300 seems more appropriate in flat area according to the statement above from NEH.
I did some searches on google and found an interesting paper by Josha Crowley and Alan W Moore for a flood study in Cameron County, TX, in which they suggest to use a PRF of 200. (hydraulic model calibration on the coastal plan, a case study in Cameron County, Texas, Josha Drowley, and Alan W. Moore). You can review thi article from google book.





RE: SCS/NRCS unit hydrograph peak rate factor in design
256 SWFWMD (Southwest Florida Water Management District)
284 Delmarva (Delaware-Maryland-Virginia peninsula)
369 Michigan
484 Georgia (Same PF as SCS but slightly different shape, also used in FL)
If you're contemplating the use of a non-standard peak factor, the best justification would be if it provides a better match to the observed runoff for the existing conditions. But in most cases the runoff observations are not sufficiently accurate to justify using a non-standard PF, so the models generally use 484.
But I would certainly be interested to hear of other cases...
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: SCS/NRCS unit hydrograph peak rate factor in design
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: SCS/NRCS unit hydrograph peak rate factor in design
Thoughts?
RE: SCS/NRCS unit hydrograph peak rate factor in design
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: SCS/NRCS unit hydrograph peak rate factor in design
I'm convinced that when it comes to written rules and how those rules are enforced, constancy trumps accuracy because constancy is defendable, and idiot-proof trumps all because, to be honest, not all engineers are created equal. Standard methods are in place because experience shows that that combination of unit hydrograph (usually 484) and return period storm (usually 100 years) results in adequate level of flood protection regardless of the actual return period of the peak discharge. There is nothing magical about 100 years anyway. If we all had 12 fingers, we would be designing for the 120 year return period.
RE: SCS/NRCS unit hydrograph peak rate factor in design