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# Probable Maximum Flood

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 LeoDMD (Civil/Environmental) 24 Mar 10 9:38
 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]] = CSMI have used Excel to illustrate my understanding and include that in PDF format.Thanks for any assistance,
 Drew08 (Civil/Environmental) 30 Mar 10 16:48
 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 (Civil/Environmental) 30 Mar 10 16:53
 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 (Civil/Environmental) 30 Mar 10 18:52
 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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