Huff Rainfall Distribution 1

[WILDWINGS27]

I need to provide the existing 100-year peak discharge from a 5 acre site. To do this, I usually use the SCS-Type II,24 hr. rainfall distribution in Hydraflow. Using=Cn:80,Tc=15, and 7.58in. rain.....I get a peak discharge of 28.82 cfs.

However, I need to provide the peak rainfall using the Huff rainfall distribution. I can not find any examples of how do do this. Can anyone help me out?

Thanks

 

[psmart]

Doesn't Hydraflow have an option for the Huff distribution (instead of the Type II storm)?

 

[WILDWINGS27]

It does, but when I run the 3rd. qrt.-24 hrs, the peak runoff is significantly less that the scs 24 hour. I guess I am just confused about how to use the Huff curves.

 

[psmart]

That's absolutely correct. The peak intensity of the 24-hour Type-II storm is more than ten times the peak of the 24-hour Huff 3rd quartile. Here are the mass curves (as shown by the HydroCAD rainfall viewer):

And here are the intensity curves, showing the extent of the difference:

 

[WILDWINGS27]

So, if I am required to find the maximum discharge from a 5 acre site using the Huff rainfall distribution, should I run numerous different time intervals in the 1-4 quatiles to find my maximum discharge? If I run the 1 hour-1st Quartile, the discharge seems high to me. I am just trying to understand what would be a reasonable methodology. Thanks for all your help in advance.

 

[psmart]

Your local stormwater regulations should provide specifics on the approriate duration and quartile to use. If they specify the use of the Huff distributions, they need to provide all the relevent details.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[WILDWINGS27]

Thank you for your help

 

[nwm94]

On small sites I typically check the 1, 2, 3, and 6 hour durations and take the worst case as my peak. You could keep checking all the way down to 24 hours, but your peak should fall off pretty quick. Generally, there is a relationship between time of concentration and the critical duration.

Huff's distribution produces lower peaks and is good for regulating the high frequency events (1, 2, 5, 10 yr) so that small storms don't blow right through detention ponds unrestricted. Peaks for the low frequency events are much higher using SCS.

 

[WILDWINGS27]

THANKS, THAT MAKES SENCE