Engineering 101: Rainfall Intensity

This morning while driving to work in a minor mist, I thought about and Twittered about posting a new blog entry here.  I have finally found the time.  Let us talk about the next 101 topic: Rainfall Intensity. Today’s intensity and the mud-pit that we are turning into a parking lot has me thinking about rainfall intensity, rather the lack of rainfall intensity.  As with everything at this time of year, finding time to write about topics like this get far fewer.   Mark Scacco touched on the topic with his early July post on the Rational Method.


AutoCAD’s Civil 3d Hydraflow Storm Sewer, Express and Hydrograph Extensions available for AutoCAD Civil 3d can all utilize the same Rainfall Intensity-Duration-Frequency file to select the appropriate rainfall amount.  The Rational Method formula to calculate Q is to multiply the surface Coefficient by the Rainfall Intensity by the Area in question.    Check out the original 101 post for more details.  

Rainfall is a very interesting study. 

The image here is the same Federal Highway Administration IDF Curve.   How do was this curve developed?  Your answer comes from here – the FHWA HEC-12 Document “Drainage of Highway Pavements” – This is a hefty PDF document but is a great reference to have.  The Hydraflow program uses the steps outlined in Appendix A to convert rainfall total amounts into very nice curves and a couple of Coefficients.  I think that is a little backwards, the coefficients come first. 

If you are lucking enough, your local regulatory agency might have actually computed coefficients for the storms and return periods that they are concerned with.  For those of us stuck in the Midwest who utilize the Bulletin 70 or 71 rainfall, the B,D, and E coefficients are probably not appropriate but do get used.  A 3rd degree polynomial expression should be used and is available in Hydraflow – but this requires us to prepare an Excel spreadsheet to calculate the coefficients based on your location.  Hydraflow does NOT calculate your 3rd degree polynomials from entered rainfalls.

After you have established your curve by one method (B, D, E methods or 3rd Degree), this file can be used in any of the extensions.   I would also point out that you must make a choice – you can not store both in one file.  Be sure of the units – especially if you are using metric rainfall as the program is looking for mm – not cm!

Rainfall  makes everything flow downhill and there is much more to talk about.  Until next time.

Join me in Las Vegas at Autodesk University in a few weeks!


  1. […] While I have not had a chance to run through the extensions’ functionality personally, a recent post over at covers some of the basics of rainfall data and how they relate to the AutoCAD extensions.  While […]

  2. […] few months ago, I posted about the Rainfall Intensity and some of the information you will need to generate an intensity-duration-frequency curve.  […]