Engineering 101: Modified Rational Method and SDF

There was a post in the Stormwater Discussion Blog regarding the Modified Rational Method and the Hydraflow Hydrographs program.  The question focused on the SDF value in the Rational Method Dialog box.  For those not familiar with the term, the SDF is the Storm Duration Factor.


As we’ve discussed before, Rational Method is Q = CIA, and is great and simple estimation of the peak runoff.   Built into the Rational method is the simply conclusion that the peak flow reaches the outlet at the time equal to the time of concentration. 

The Modified Rational Method simply provides a way to extend a uniform rainfall over a longer duration as a way to estimate storm water storage.  The method is an attempt to reach a simple hydrograph.   The area under this hydrograph is the volume of runoff. 

If you enter a Qtarget, Hydraflow Hydrographs seeks out a critical duration to provide the best answer for your required storage.  Its an estimate and not a true pond routing.  You will have to add that later.

The DG poster got flummoxed with what he saw as a limitation.  The SDF factor peaked out a 9.9 for him.  His site was a nice forested site that will soon become a parking lot.  The regulations only allow a 85% pre-development release as the Qtarget. By his hand calculation the critical duration is significantly large and his SDF should be about 50 times his 6-minute Tc. 

The limitation is the rainfall intensity curve.


60-minutes is as far as Modified Rational will take you.  The flat top hydrograph pulls the intensity based on the duration and the equations and table stops at 60 minutes.  Then again, why should we assume that it will rain uniformly for any time longer than 60 minutes?

2 responses to “Engineering 101: Modified Rational Method and SDF”

  1. Civil3D Stuff « Percol8

    [...] Engineering 101: Modified Rational Method and SDF [...]

  2. Tony Perez


    I would like to know more about your software that’s create hydrograph for the modified rational mehod. What’s the price?.


    Best Regards

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