IDF Curve Adjustment

[Anonymous]

My local jurisdictional requirements usually require me to check conveyance at the 25-yr and 100-yr events. I use IDF curves so that I can analyze the same model for multiple events. The typical version of the rainfall intensity equation I am used to is

I = B / [P*A*(Tc)]^E

- B and E are return period specific factors

-P is the site-specific rainfall intensity (in/hr)

-Tc is the time of concentration (I usually use TR-55)

in SSA the equation used is very similar

I = B / [(Tc+D)]^E

My question is about the variable D. The SSA user manual says that D is used to "straighten" the curves when plotted on a log-log axis. I noticed that this has the effect of reducing flows. This variable is not included in my local drainage manual. Can someone explain the theoretical justification for the purpose of this variable?

Thanks,

John

[Timothy_Sean_Hulbert]

Greetings John,

Thank you for your recent forum post.  My name is Sean and I am more than happy to help you with this issue. I understand you have questions about the origins of the Rainfall Intensity formula used in Autodesk Storm and Sanitary Analysis. This is known as the “Chicago Method” developed by Keifer and Chu (1957). The constants are dependent on units used and return frequency. Generally, these values are provided by the storm water management agency in your area.

I have subscribed to the thread though and will continue to monitor it should you need additional assistance. If you would like to communicate directly, you may do so at any time by sending me a private message in the forum.

[Anonymous]

The IDF curve does not read correctly with respect to return periods..My model is flooded for a return period of 5 years while for10 years return period, there's no flooding, which is an incorrect scenario. When i checked on the calculated values for one of the basins, Rainfall Intensity(5 years) is 462mm/hr and Rainfall Intensity (10 years) is 391mm/hr. There must be something wrong with my customized idf attached below. Please help me figure out the issue. thank you very much.

[Anonymous]

is the time for soil saturation and filled of depressions, use to be 5 mins