I'm not sure what you're asking. There are several ways to read your question.

(1) Since the Tc typically is defined as "the travel time from the most remote point in the catchment to the point of analysis", it is standard practice to use the same Tc for all events. The Tc is a physical characteristic of the watershed which is independent of the rainfall. (While there may be some variation with rainfall intensity, it is generally ignored.)

(2) If you're using to the Rational method, you may be referring to the practice of setting the rainfall duration equal to the Tc. That's an entirely different issue about correct use of the Rational Method. For details please see www.hydrocad.net/rational.htm

(3) If you're asking about the HydroCAD "Critical Duration report", it operates by automatically varying the rainfall duration in order to determine the rainfall duration (and the corresponding intensity) which produces the greatest flow or WSE at a given node. Again, for details please see www.hydrocad.net/rational.htm

Peter Smart
HydroCAD Software
www.hydrocad.net

I understand that critical duration for undetained peak flow from a watershed is usually the same as the time of concentration. That critical duration is usually different from the rainfall duration that produces the highest water surface elevation in a downstream detention pond. We calculate that critical duration by trial and error to peak out the WSE in the pond. When doing the duration trials, do we also change the Tc of the tributary watershed to match the duration of that trial?

When you talk about "duration trials", I assume you're aware that the HydroCAD critical duration report does this automatically. All you have to do is open the duration report for the desired node and click the "Update" button.

HydroCAD automatically recalculates the contributing subcatchments for each duration so that all nodes are being evaluated under identical rainfall conditions.

Again, remember that the Tc for each subcatchment remains the same - it's the rainfall duration that changes. You should NOT change the Tc.

Peter Smart
HydroCAD Software
www.hydrocad.net

Thank you. You've addressed our question.

Mark