Peak Time calibration

Hello,

I am using SWMM to model an area of 30 acre with a few ditches running through. I have collected flowrate measurements at those ditches and the corresponding precipitation records. During simulation, the time of peak flowrate I calculated is off 2-3 hours in a storm of 10 hours. And I was unable to change the peak time by adjusting the parameters in subcatchments. I wonder what parameters would affect the peak time significantly? I would appreciate any help from you.

Thanks,
Jason Chin-Cheng Sun, Ph.D.
jsun@sspa.com

>During simulation, the time of peak flowrate I calculated is off 2-3 hours in a storm of 10 hours. And I was unable to change the peak time by adjusting the parameters in subcatchments. I wonder what parameters would affect the peak time significantly? I would appreciate any help from you.

Sub-basin width is typically used to get the timing under control. But don't forget that the most sensitive parameter for timing and volume is rainfall. First, make sure that the rainfall and flow data are appropriate for your calibration events. I've been told that screening the measured data leads to understanding the measured data, which leads to understanding the stormwater system, which leads to better insight on how to model the stormwater system.

Also, you might want to check the hydraulic roughness in your ditches, particularly in the overbanks (if the flow gets up there during the calibration event). Are you underpredicting or overpredicting the peak? If you are underpredicting the timing and the flow does get up over the banks (into the floodplain), perhaps there's some storage that hasn't been represented. If you are overpredicting the timing, perhaps storage volume has been double-counted in the channel and as floodplain storage.

Michael A. Gregory, P.E.
Camp Dresser & McKee Inc.