## Tc and IDF Curves

## Tc and IDF Curves

(OP)

Modeling question: The ahj has a minimum Tc = 5 minutes requirement. The available rainfall data for the project site has IDF curves for a series of storms with a minimum duration of 10 minutes and higher. Have you encountered such a scenario? It would appear to me that I don't necessarily need to have an IDF for a 5 minute duration. I am working on confirming the storm duration requirement from the ahj. Your thoughts and input is greatly appreciated (My apologies if this seems as hydrology and hydraulics 101 questions but its a few years since I touched on this topic).

## RE: Tc and IDF Curves

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

## RE: Tc and IDF Curves

I am in the states but my project is in the middle east.

Hope others can chime in on this to get additional input.

## RE: Tc and IDF Curves

If you can get to the data (e.g. 5 min maximum annual daily precipitation or else), you could (re)perform the statistical analysis with it and make that IF curve for 5 min.

Less precise, but quick :

Plot the x (minutes) vs y (intensity) available and add a trend curve (power) and with the equation, estimate the value for the 5 minutes duration. I'd not design a dam with this procedure though.

## RE: Tc and IDF Curves

## RE: Tc and IDF Curves

Does it even rain in the Middle East anyway?

## RE: Tc and IDF Curves

What if all the points are 0,0 !?

Seriously if you use a duration-intensity series then don't plot the 0,0 in Excel as it won't match the rest and you won't be able to use the (power) trendline.

Just input the known values and with the equation y = Ax^-B calculate the y with a x of 5.

## RE: Tc and IDF Curves

## RE: Tc and IDF Curves

Peter Smart

HydroCAD Software

www.hydrocad.net

## RE: Tc and IDF Curves

seriously though, I have generally just extrapolated these, by hand with a pencil. Plot the curve at a large scale, sketch in the line and measure with ruler. statistics is probably unnecessary here as a Tc of 5 minutes indicates a very small watershed.