I am using the SCS TR55 method, but every method I know of (I generally work in small watersheds(<500ac), as time of concentration goes up, peak flow goes down. Thanks so much for responding to my post. Good luck RE: Time of Concentration / Peak Runoff Rookie2 (Civil/Environmental) You'll find lots of other useful information along the way. For larger basins you are probably better off to use stream gage records or programs such as NFF the National Flood Frequency program available free on the net. It hasn't been shown to be accurate much beyond those original conditions. The Rational method was developed in 1873 in Rochester NY and is based on only a few, small urban drainge basins studied then. Where I live, Salem, Oregon, the City limits it to 100 acres or about 0.16 sq. Many local reviewing agencies limit it even more. FEMA limits its use to basins smaller than 1 square mile ( 640 acres). It never makes sense to use the Rational method for large drainage basins. My own opinion is that ten minutes is a reasonable minimum.ģ. It seldom, if ever, makes sense to divide a drainage basin into pieces which have a time of concentration of less than 5 or ten minutes. I-D-F curves are derived statistically from precipitation records.Ģ. Because of the shape of Intensity-Duration curves, the intensity goes down as duration increases. As time of concentration goes up, the Rational method assumes the storm duration also goes up.
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