Peak Flow to Mean Flow (Average Flow or Qbar 2

[Fasil2011]

Hi There,

Would anyone explain to me how to convert the "Peak Flow" estimate you get, say from Rational method, to a "Mean Flow"? Or is it possible to know whether the peak flow estimated using Rational Method (or any other method) is a 2year return period flow, or a 50year or a 100year flow?

Thanks for your help.
Fasil

 

[francesca]

1. You can't. The Rational method calculates a peak flow. Unless you can get a "mean rainfall" depth, you're out of luck.
2. The return period is the same as the return period for the rainfall depth you chose.

 

[cvg]

it may be a bit more complicated than that. Mean flow of a stream is only partially dependant on storm runoff and may be influenced greatly by base flow, shallow groundwater flows, irrigation return flow, industrial discharges, snow melt etc. Mean flow typically varies throughout the year. It is best measured by gage data over a long enough period of time to allow statistical analysis to determine the mean flow for a 2-year or 100-year return period. USGS can help you find this, but the Rational method is of no use. This is covered in your basic hydrology textbook.