Based on your post, it appears that you may be manually applying the entire rainfall to the dimensionless unit hydrograph (UH), which is incorrect. There are actually two different curves involved: The UH and the temporal rainfall distribution. The rainfall distribution determines how the rainfall is distributed over time, and specifies the magnitude and timing of the peak rainfall. The UH represents the runoff from a single burst of rainfall of relatively short duration (related to the Tc). To generate a runoff hydrograph, the rainfall is divided into bursts of equal duration, the (SCS) runoff equation is used to determine the volume of runoff for each burst, and the UH is used to generate a runoff hydrograph for each burst. Adding all the individual hydrographs gives the total runoff. As such, the peak runoff is dependent on the characteristics of the UH and the rainfall distribution. Performing this calculation by hand is a non-trivial process.

Peter Smart
HydroCAD Software
www.hydrocad.net

i did the same thing as you mentions. i take the 24 hr distribution from HEC-HMS and apply the formula as present in sheet. kindly go through it

• http://files.engineering.com/getfile.aspx?folder=43b009c4-41e6-4936-9c72-e7

There may be other issues, but using a 1-hour time step is going to significantly understate the peak rainfall compared to the 1/10 hour time step commonly used. Even with your fairly long Tc of 1.22 hrs I wouldn't use a time step of more than 1/2 hour.

Retract that - I overlooked the fact that you're using a 6-hour storm, so the time increment may be OK.

HOWEVER, the peak runoff for a 6-hour Type II storm should be roughly between 3 and 4 hours - not 15 hours as shown in your calculations. I would recheck the spreadsheet.

Peter Smart
HydroCAD Software
www.hydrocad.net