SCS/NRCS unit hydrograph peak rate factor in design 1

[pinkpig]

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.

 

[psmart]

Although NEH discusses the possibility of using a different peak factor, the only common applications are a few regional unit hydrographs that have been around for a while. And these are custom UH tables - they're not true gamma curves as discussed in NEH.

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

http://www.hydrocad.net

 

[beej67]

Stretching my memory a bit here, but I think I used a 100 down in Miami-Dade once, but that was over a decade ago. SFWMD typically uses its own storm distributions, so your SCS mileage may vary.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[pinkpig]

I did a quick calc: for drainage area=1 sq. miles, TOC=60 min, Q-peak of standard SCS UH is 705 cfs and Q-peak under PRF of 300 is 437 cfs. This is a big difference. I am seriously thinking there should be some guide lines or researches that will deal with the selection of peak rate factor instead of just applying standard SCS UH everywhere for today's HH engineers.

Thoughts?

 

[psmart]

What's the location of your project? Have you spoken with the applicable stormwater agency? Stormwater management in the US tends to be at the state and county level, so those are the folks you need to convince.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Drew08]

From a pessimist:

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.

 

[pinkpig]

I totally agree with you Drew08 and I like your 12 fingers comment which made me laugh out.