10 day snow-melt peak seems low...

[hydroponder]

I am modeling an industrial lot that will have three infiltration basins to manage stormwater. Per the watershed district, I have to analyze rates for the 100-year 5.9" rainfall and the 10-day snow melt (7.2") event. It seems to me that the peak runoffs should be a little closer than the rates I am seeing. Just wondering if I am missing something. The 100-year rate to the east is 20.90 cfs and the 10-day snow melt rate to the east is 1.97 cfs. Seems odd... Both HydroCAD models are attached.

 

[psmart]

For the snow-melt event you're using a 1-day/10-day PSH distribution. This has a fraction intensity of 0.065, which multiplied by your 7.2" depth gives a peak intensity of 0.47 in/hr. This is much lower than the intensity for most 24-hours storms. For example, a Type II storm has a fractional intensity 1.37, multiplied by your depth of 5.9" gives a peak intensity of 8.1 in/hr. So I would expect higher peak runoff for the Type II.

To view and compare the distributions in HydroCAD, click "View Storm" on the Rainfall tab of the calculation settings screen.

Peter Smart
HydroCAD Software

http://www.hydrocad.net