Probable Maximum Flood

[LeoDMD]

I have a 1979 Army Corps of Eng Phase 1 Inspection Report that states the analysis conditions for dam to be 1 PMF equals 1250 CSM for 24 hour rainfall of 21.4 inches over an 8.42 sq-mi drainage area... My opinion is that this can not be correct. Either the 1250 CSM is wrong or the 21.4" of rain is wrong.

My question: Is this the proper equation?
Gross inflow = [(Rain [ft]) * (Drainage area [sq-ft] )] /
[(Storm duration [seconds]) * (Drainage area [sq-mi]] = CSM

I have used Excel to illustrate my understanding and include that in PDF format.
Thanks for any assistance,

 

[Drew08]

Are you sure peak flow is not reported in CMS (cubic meters per second) as appose to CSM (cfs per square mile)?

Without knowing anything about the watershed, I just threw typical numbers into the NRCS graphical peak discharge method (Tc = 1.5 hour & CN = 80) and got a result of 43,000 cfs = 1,200 CMS.

 

[Drew08]

I'm not sure I really understand your spreadsheet, but it seems like you're calculating average inflow instead of peak inflow which would be a function of the hyetograph also. Does the USACE report state the PMF is a peak or average?

 

[LeoDMD]

Yes I am sure that the Units of measure are CSM which is = to cubic feet per second (CFS) per square mile..

And no where does the Army Corps of Eng define the inflow as average or peak... I believe it defines the rate of inflow for the duration of the test storm being studied.

Regards,
Leo