IDF Curves

[bank]

The City has a set of IDF curves covering rainfall frequencies from two years through 100 years. However, the LEED SS6.1 template asks for 1- and 2-year runoff amounts.

Is there a way to generate b, d, and e coefficients for a 1-year frequency if 2-, 3-, 5, 10, 25, and 100-year coefficients are known?

 

[psmart]

The coefficients are usually derived by fitting to the individual IDF curve for each return period. I'm not aware of a procedure for deriving one curve from another, be it the raw curve or the coefficients.

You'll probably have to use local rainfall data from the NOAA PFDS or another similar source.

http://hdsc.nws.noaa.gov/hdsc/pfds/

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[gbam]

Have you compared the City's IDF curves with NOAA's curves? Maybe they are close and you can justify using NOAA?

 

[beej67]

USGBC is loose in their review. Use national maps for your submittal to them and local info for your civil design.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cht13er]

One local municipality uses what they call a "1-year" storm - which is actually just a 3-hour, 25mm storm event using the same b and c coefficients that the 2-year storm (with 45mm or so depth) uses.

In I = a / [(tc + b) ^c ] it's the "a" that can be modified to directly change the maximum intensity and thus total rainfall depth. So what we do is just decrease this parameter until we get a total rainfall depth of 25mm.

HTH