Quick Modified Rational Method Question

[joengr]

Hello,

I am reviewing some old work to get a handle on the rational and modified rational method for making hydrographs. The designer first used the rational method using the time of concentration and an IDF curve to find the rain fall intensity of a 10 yr storm to make a hydrograph.
The designer then made a second hydrograph with the same data using the modified rational method. In the second hydrograph, however, instead of using the same time of conentration as before to find the intensity, they used "storm duration" to find it, with their definition of storm duration consiting of 3 times original time of concentraion. Using an IDF curve this gave a much lower intensity. Does this make sense?

 

[psmart]

"Quick" questions are rarely so, especially when it comes to the Rational method.

The answer to your question depends on the intent of the second hydrograph. If it was intended to represent the runoff from the same rainfall event, then the duration and intensity should be the same, rather than 3 times the Tc.

However, a longer storm duration can also be used (with a correspondingly lower intensity) to predict the runoff for a longer event. That result is equally valid, albeit for a different rainfall duration.

So it really comes down to the intent and signficance of the longer "storm duration". One possibility is to simulate the larger volume that will be produced by the longer event, or perhaps to combine several subcatchments with different Tc values. But I see no reason why the longer duration would be specifically 3 times the Tc.

I suggest that you inquire as to the reason for using 3 times the Tc.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

note that the rational method, Tc and IDF curve "method" of generating a hydrograph is inappropriate and not an acceptable method. There is no need to use any variation of the rational method to generate a unit hydrograph. So my knee jerk reaction is that the hydrographs that were generated are probably not a reliable estimate of the actual conditions at the site.

 

[psmart]

As cvg says, using Rational method to generate hydrogrpahs is questionable at best. Unfortunately, there are many folks (and even regulatory agencies) that continue to use this approach. Of course, there has been much discussion of this issue in other threads, so I was trying to limit my comments to the immediate question at hand.


Peter Smart
HydroCAD Software

http://www.hydrocad.net