Quick Modified Rational Method Question
Quick Modified Rational Method Question
(OP)
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?
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?





RE: Quick Modified Rational Method Question
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
www.hydrocad.net
RE: Quick Modified Rational Method Question
RE: Quick Modified Rational Method Question
Peter Smart
HydroCAD Software
www.hydrocad.net