Hydraflow Hydrographs v. HydroCAD - Rational Method
Hydraflow Hydrographs v. HydroCAD - Rational Method
(OP)
I like HydroCAD because of the critical duration analysis function which allows me to analyse multiple drainage areas simultaneously using rational method.
Does hydraflow hydrographs do this?
Does hydraflow hydrographs do this?
RE: Hydraflow Hydrographs v. HydroCAD - Rational Method
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.
RE: Hydraflow Hydrographs v. HydroCAD - Rational Method
good luck
RE: Hydraflow Hydrographs v. HydroCAD - Rational Method
RE: Hydraflow Hydrographs v. HydroCAD - Rational Method
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.
RE: Hydraflow Hydrographs v. HydroCAD - Rational Method
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 www.hydrocad.net/rational.htm 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.