Working through the rational method, I've calculated the Time of Concentration (Tc), then itterated using IDF (intensity-duration-frequency) charts until the duration = time of concentration (since the Tc is influenced by the corrisponding intensity at a given duration).
This really means to me that even though you can calculate this for a given statistical return period, i.e. 2 years, the duration of "24hrs" is badly worded and should only apply to run-off VOLUMES using the SCS methode after finding the run-off RATES with the rational method.
The weighted average Run-off Coefficent should then be used with the Intensity given at the corrisponding Tc as calculated using the Rational Method giving the required run-off RATES, but this still leaves me short for calculating the VOLUMES, which as previously stated, is usually done using American Charts based on American rainfall data.
I mean which CN number must I use if I'm building somewhere in Brazil? Does anyone have a concrete example of the complete set of calculations?
I know in America this is done by the Civil Engineer, but it does't alway fall into his camp in all parts of the world...even though maybe it should.