Engineering 101: The FAA Equation – Time of Concentration

Let us continue the Engineering 101 series of blog entries.  Over the past few months, we have seen few posts on some basic engineering tasks associated with the engineering study of hydrology.  I would like to make this a recurring series.  If you have any topics that you would like to be covered before, during or after the next set of Civil 3d 2010 topic,  comment away.

Let’s recap the series so far…  

The Rational Method post kicked the series off back in July.  Mark’s Time of Concentration post quickly followed.   Mark gave me the opportunity to post to the series for two additional posts; one on Storm Design and then another on Rainfall Intensity and Precipitation.  

The Rational Method,  Q = CIA, is a simple calculation that solves for the Peak Flow. 

Finding the data to solve this equation typically involves a lot of trial and error.  To find the intensity, one needs to solve for the time of concentration.  While many methods exist for solving the time of concentration, the NRCS provides a great Technical Note No N4 here. [A word document]    This methods are outlined and documented in the National Engineering Handbook, buried somewhere in this site [Its a Technical Release TR-210-55, titled Urban Hydrology for Small Watersheds (second edition) (6/1986)] 

But, what if you don’t know all of the hydraulics of your watershed?

Back in 1965, the U.S. Federal Aviation Agency [The FAA] developed a simple estimation of Tc to be used with the Rational Method.  The FAA method, as it is commonly referred too, calculates the Time of Concentration over a uniform slopes and surface cover.  Think airfields.  The simplistic method it uses is:

Tc = [ 1.8 * (1.1 – c) L ^ 1/2] / S ^ 1/3

where C is the cover factor,  L is the hydraulic length in feet, and S is the slope in Percent.  Since this is a very simple calculation, it is widely used in many urban areas. 

imageAll of the AutoCAD Civil 3d Hydraflow Extensions – Hydrographs, Storm Sewers, and Express allow the use of the FAA method in the calculations when computing the Rational Method. 


  1. Wesley Ashworth says:

    How about talking about the need to integrate H&H with civil 3D? It is mind numbing having to switch back and forth. the xml import/export works poorly to say the least. And having usable pipe/structure tables that are dynamic would be just wonderful.

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