Hydraflow Hydrographs v. HydroCAD - Rational Method

Yes; Hydraflow Hydrographs can do the Rational Method and can do routing; I've never used it to do routing through pipes and add hydrographs.

Hydraflow Hydrographs has more of a spreadsheet layout, though, so you'll have to keep your wits about you and work off a schematic on your desk using the same basin names.

While either program allows you to use the Rational Method and combine hydrographs, you may not want to use them that way. The Rational method does not generate a realistic hydrograph and is considered valid, if at all, only for small, urban watersheds. I, and my local City, limit its use to 20 acres or less. I use the so called modified Rational Method only if require to by some reviewing agency. Even then, for any really important project I would check the results by at least two other methods such as TR-55, Santa Barbara Urban Hydrograph, or Regression Equations. Tiny parking lot and similar "local" detention basins do not fit the description of "important".

good luck

I don't think Hydraflow Hydrographs does the Modified Rational method, just the rational method for a peak flow.

I agree with RWF7437. This is total misuse of the Rational method. Despite its heavy use, it was never intended to be used in a Q vs. Time relationship with multiple sub-basins as it suffers from the "Partial Area" affect. To offset this character, some practitioners employ, as Andrew Reese coined, "voodoo hydrology" in order to get around it such as "critical duration analysis".

If you have a PC and software that can employ more appropriate methods, such as SCS TR-20, why not use them?

BTW, Hydraflow Hydrographs does use the Standard, Modified and Dekalb Rational methods.

I don't like to recommend Rational method for anything, but if you MUST use it, you are faced with the decision of what rainfall duration to use. One way or another, you have to anwer this question.

For the textbook case of a single subcatchment, the duration is set equal to the time-of-concentration. This is the "critical duration": The duration that produces the highest peak flow. Longer durations will produce a lower peak because of the lower intensity from the IDF curve. Shorter durations will produce a lower peak because the rainfall doesn't last long enough for runoff from all parts of the watershed to reach the point of collection. So you just set Duration = Tc.

But when you combine multiple subcatchments, things are much more complex. In these cases, you REALLY should use a full hydrograph procedure such as the SCS/NRCS or SBUH method, as suggested by RWF7437. However, if you are compelled to use Rational Method, then you will have to use some approach for generating and combining the runoff hydrographs.

For HydroCAD users, the "critical duration analysis" will determine the duration that produces the highest flow (or pond storage) for a given point in the system. For details please see But remember that you're stretching the Rational method beyond its intended use, and that other methods are generally more appropriate for this type of analysis.