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Probable Maximum Flood

Probable Maximum Flood

Probable Maximum Flood

(OP)
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,

RE: Probable Maximum Flood

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.  

 

RE: Probable Maximum Flood

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?   

RE: Probable Maximum Flood

(OP)
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

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