| Product(s): | CivilStorm, SewerGEMS, PondPack |
| Version(s): | All |
What is the difference between using Unit Hydrograph and Rational Hydrograph in PondPack, CivilStorm or SewerGEMS? Which one is preferred?
The selection of catchment runoff method is a preference based on your engineering judgement and local standards. If you have a reviewing agency, first discuss and confirm with them.
Generally speaking the Rational Method tends to be used with smaller sites and utilizes an IDF rainfall curve, while the Unit Hydrograph method is typically for larger sites and uses time-depth or time-intensity rainfall tables (which can optionally be constructed based on dimensionless curves and a total depth) So, the choice of runoff method may depend on which type of rainfall data is available.
Also note that the "Rational Method" is a steady state/peak flow method and does not apply to unsteady simulations which incorporate the element of time. Instead, CivilStorm, SewerGEMS and PondPack have the ability to use the Modified Rational Method, which uses the same basic Q=CIA, but constructs a hydrograph based on a "Critical storm duration". It is important to fully understand how this works if you wish to utilize the Rational Method for pond design. See: Understanding the Modified Rational Method
You can see more about runoff methods in our Stormwater Conveyance Modeling and Design Book, and in the theory parts of the Stormwater fundamentals and advanced training courses.